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Microscopic Dynamics of Liquid-Liquid Phase Separation and Domain Coarsening in a Protein Solution Revealed by X-Ray Photon Correlation Spectroscopy

TL;DR: In this article, the authors used x-ray photon correlation spectroscopy to determine the liquid-liquid phase separation dynamics of a model protein solution upon low temperature quenches and find distinctly different dynamical regimes.
Abstract: While the interplay between liquid-liquid phase separation (LLPS) and glass formation in biological systems is highly relevant for their structure formation and thus function, the exact underlying mechanisms are not well known. The kinetic arrest originates from the slowdown at the molecular level, but how this propagates to the dynamics of microscopic phase domains is not clear. Since with diffusion, viscoelasticity, and hydrodynamics, distinctly different mechanisms are at play, the dynamics needs to be monitored on the relevant time and length scales and compared to theories of phase separation. Using x-ray photon correlation spectroscopy, we determine the LLPS dynamics of a model protein solution upon low temperature quenches and find distinctly different dynamical regimes. We observe that the early stage LLPS is driven by the curvature of the free energy and speeds up upon increasing quench depth. In contrast, the late stage dynamics slows down with increasing quench depth, fingerprinting a nearby glass transition. The dynamics observed shows a ballistic type of motion, implying that viscoelasticity plays an important role during LLPS. We explore possible explanations based on the Cahn-Hilliard theory with nontrivial mobility parameters and find that these can only partially explain our findings.
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Microscopic Dynamics of Liquid-Liquid Phase Separation and Domain Coarsening
in a Protein Solution Revealed by X-Ray Photon Correl ation Spectroscopy
Anita Girelli ,
1
Hendrik Rahmann,
2
Nafisa Begam ,
1
Anastasia Ragulskaya,
1
Mario Reiser,
2,3
Sivasurender Chandran ,
1,4
Fabian Westermeier ,
5
Michael Sprung,
5
Fajun Zhang ,
1,*
Christian Gutt ,
2,
and Frank Schreiber
1,
1
Institut für Angewandte Physik, Universität Tübingen, Auf der Morgenstelle 10, 72076 Tübingen, Germany
2
Department Physik, Universität Siegen, Walter-Flex-Strasse 3, 57072 Siegen, Germany
3
European X-Ray Free-Electron Laser XFEL, Holzkoppel 4,22869 Schenefeld, Germany
4
Department of Physics, Indian Institute of Technology Kanpur, Uttar Pradesh 208016, India
5
Deutsches Elektronen-Synchrotron DESY, Notkestraße 85, 22607 Hamburg, Germany
(Received 19 October 2020; accepted 23 February 2021; published 2 April 2021)
While the interplay between liquid-liquid phase separation (LLPS) and glass formation in biological
systems is highly relevant for their structure formation and thus function, the exact underlying mechanisms
are not well known. The kinetic arrest originates from the slowdown at the molecular level, but how this
propagates to the dynamics of microscopic phase domains is not clear. Since with diffusion, viscoelasticity,
and hydrodynamics, distinctly different mechanisms are at play, the dynamics needs to be monitored on the
relevant time and length scales and compared to theories of phase separation. Using x-ray photon
correlation spectroscopy, we determine the LLPS dynamics of a model protein solution upon low
temperature quenches and find distinctly different dynamical regimes. We observe that the early stage
LLPS is driven by the curvature of the free energy and speeds up upon increasing quench depth. In contrast,
the late stage dynamics slows down with increasing quench depth, fingerprinting a nearby glass transition.
The dynamics observed shows a ballistic type of motion, implying that visco elasticity plays an important
role during LLPS. We explore possible explanations based on the Cahn-Hilliard theory with nontrivial
mobility parameters and find that these can only partially explain our findings.
DOI: 10.1103/PhysRevLett.126.138004
Recent work suggests that structure formation in biology
can take place, inter alia, through liquid-liquid phase
separation (LLPS) [13]. Phase separation in crowded
environments thus represents a mechanism for intracellular
organization via the formation of biomolecular condensates
[4]. The biological functions of these condensatesinclud-
ing steering biochemical reactions rates, sensing, or signal-
ingare being intensely investigated [3]. LLPS is also
associated with a variety of diseases caused by a loss and/or
change of function of the condensates [5,6].
The state of the condensates depends on the dynamic
processes during their formation, often involving non-
equilibrium processes over a hierarchy of length and time
scales [2,7]. A case in point is the slowdown of the
dynamics on molecular length scales caused by concen-
tration and its influence on the dynamical and structural
properties of the condensate on mesoscopic length scales.
Ultimately, such a microscopic slowdown can lead to the
arrest of LLPS on larger length scales accompanied by the
formation of bicontinuous gel network structures [810].
The kinetics of arrested phase separations in protein
solutions have been studied successfully in the past,
demonstrating that the ensemble-averaged structure factor
ceases to develop further in q position and intensity upon
low temperature quenches [1115]. However, the dynamics
of protein solutions en route to an arrested LLPS is largely
unknown, mainly because of the requirement to monitor an
exceptionally broad range of time and length scales
simultaneously. This, in turn, prevented the experimental
validation of models of the dynamics of critical phenomena
during LLPS, such as the Cahn-Hilliard equation and
related models, especially in the vicinity of glass-gel
transitions displaying large dynamical asymmetries
between the species involved [2].
LLPS is a general phenomenon, which is relevant not
only for protein systems but also in many other fields of
science [16]. LLPS domains have been studied with
different microscopy techniques [17,18]. Their macro-
scopic properties such as turbidity and viscosity [1921]
were monitored, as well as their molecular properties
[20,2227]. Other scattering techniques were used to
access the kinetics of the phase separation [14,15,19,21].
X-ray photon correlation spectroscopy (XPCS) employ-
ing coherent x-rays can resolve the collective dynamics on
the required length scales, ranging simultaneously from
nanometers to microns and timescales from microseconds
to hours [2835]. Here, we demonstrate that a combination
of scanning techniques, large beams, and long sample
detector distances [3638] allows us to reduce the required
x-ray doses to values below the critical dose of many
PHYSICAL REVIEW LETTERS 126, 138004 (2021)
0031-9007=21=126(13)=138004(7) 138004-1 © 2021 American Physical Society

protein systems [39]. More details on the experimental
parameters are provided in the Supplemental Material (SM)
[40]. With this approach, we are able to follow the
dynamics during an LLPS of γ globulin (Ig) in a concen-
trated aqueous polyethylene glycol (PEG) solution. The
experimental results are compared to simulations based on
the Cahn-Hilliard equation, taking the gel transition into
account (in the spirit of model C according to Ref. [2]; see
the SM) [2,45,46]. Our work paves the way for future
XPCS experiments exploring the full spatiotemporal win-
dow of LLPS, allowing one also to benchmark and guide
computer simulations of complex protein dynamics in
crowded environments.
XPCS experiments were conducted at the P10 Coherence
Applications Beamline at PETRA III, Deutsches Electronen-
Synchrotron, employing an x-ray beam of photon energy
8.54 keV, a size of 100 × 100 μm
2
, and a maximum photon
density of 10
7
photons=s=μm
2
. The key for performing low
dose XPCS experiments is to make use of large beams with a
sufficient degree of coherence. Time series of coherent
diffraction patterns were collected with an EIGER 4-mega-
pixel detector covering a q range from 0.003 to 0.05 nm
1
.
Samples of Ig with PEG and NaCl have been prepared as
described in [15] and in the SM and quenched from 37 °C to
different quench temperatures T
q
below the binodal line.
Figure 1(a) displays the temporal evolution of the
scattering intensity as a function of scattering vector q
for a quench temperature of T
q
¼ 10 °C [for other quench
temperatures, see Figs. S1(a) and S1(b) in the SM],
capturing the LLPS process during the first 60 s. The
x-ray intensity increases rapidly during the early time of the
phase separation, and the position of the spinodal peak q
max
shifts to smaller values, indicating an increase in length
scales of the concentration fluctuations. Appropriately
normalizing the intensity and wave vectors by the respec-
tive peak intensities Iðq
max
Þ and positions q
max
, we obtain a
master plot as expected for spinodal decomposition [47]
[see inset, Fig. 1(a) and Figs. S1(c) and S1(d)]. Iðq
max
Þ
shows a rapid increase during the early stage, with a rate
determined by the quench depth, while at around 10 s the
growth starts to slow down considerably [Fig. 1(b)]. This
slowdown is more pronounced when quenched to lower
temperatures, while at higher temperatures, IðqÞ continues to
grow even beyond 40 s, albeit at a slower rate. Insights into
the dynamics during the LLPS are obtained by analyzing a
time series of the coherent x-ray speckle patterns. For this, we
calculate two-time correlation functions (TTCs) for specific
scattering vectors q and different quench depths via
Cðt
1
;t
2
;qÞ¼
Iðt
1
Þ
¯
Iðt
1
Þ½Iðt
2
Þ
¯
Iðt
2
Þi
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
½
I
2
ðt
1
Þ
¯
I
2
ðt
1
Þ½I
2
ðt
2
Þ
¯
I
2
ðt
2
Þ
q
; ð1Þ
with h·i being an average over detector pixels corresponding
to a specific range q δq (calculated for q from 5 to 11 μm
1
and δq ¼ 0.3 μm
1
) and
¯
Iðt
1
Þ¼hIðt
1
Þi. From this quantity,
the time dependent g
2
ðt; t
w
;qÞ¼Cðt þ t
w
;t
w
;qÞ have
been extracted by horizontal cuts along t
1
, starting at the
diagonal of the respective TTC [48].
Figure 2 shows in the upper panel the TTCs and in the
lower panels the correlation function g
2
ðt; t
w
;qÞ for quench
temperatures of T
q
¼ 15, 8, and 4 °C (from left to right),
respectively. We identify three stages of the dynamics
during LLPS: the first stage appears directly after the
quench in the first 20 s, and its very fast dynamics is visible
only by a thin line in the TTC and by the final part of the
decay in the correlation function. After this early stage, the
TTC shows a pronounced slowing down of the dynamics
accompanied by a rising background level visible by the
appearance of a squarelike feature. This corresponds to a
second relaxation mode with a much slower relaxation time
appearing in the g
2
functions. The contribution of this
relaxation channel to the overall decay is increasing rapidly.
After ca. 40 s, the fast process in the LLPS is coming to an
end, as evidenced by the disappearance of the first decay.
(a)
0.005 0.01 0.015 0.02
q (nm
-1
)
0
100
200
300
400
I(q) (a.u.)
0
10
20
30
40
50
t
w
(s)
123
q/q
max
0
0.5
1
I(q)/I(q
max
)
(b)
FIG. 1. (a) Intensity as a function of scattering vector q for a
quench temperature T
q
¼ 10 °C; in the inset, the rescaled
intensity. The different colors correspond to different waiting
times t
w
as indicated in the color bar. The time t
w
¼ 0 is the time
at which the quench temperature T
q
was reached. (b) The
intensity at the peak position as a function of time for different
temperatures. The error bars are within the symbol size if they are
not visible.
FIG. 2. In the upper panels, the two-time correlation function
for T
q
¼ 15, 8, and 4 °C (left to right) at q ¼ 0 .005 nm
1
are
displayed, and the lower panels show the corresponding g
2
functions at different waiti ng times t
w
.
PHYSICAL REVIEW LETTERS 126, 138004 (2021)
138004-2

In the third stage, later also called the late stage, the TTC
evidences a second slowing down process, with the
relaxation time depending on the final quench temperature
with slower dynamics visible for lower temperatures (from
T
q
¼ 15 °C to T
q
¼ 4 °C, on the top row). The g
2
ðt; t
w
;qÞ
functions have been fitted by a sum of Kohlrausch-
Williams-Watts (KWW) functions [49], resulting in the
following equation:
g
2
ðt; t
w
;qÞ¼A
1
exp
2
t
τ
1
γ
1
þ A
2
exp
2
t
τ
2
γ
2
;
ð2Þ
with decorrelation times τ
1
and τ
2
, relaxation amplitudes A
1
and A
2
, and KWW exponents γ
1
and γ
2
. We note that for
some g
2
functions a third exponential decay with small
amplitude and slow decay is needed to describe additional
tails in g
2
at very long timescales. However, due to the low
statistics of the third decay, we evaluate only the two
leading decays here.
Figure 3(a) displays the relaxation times as a function of
waiting time t
w
. We identify an exponential increase of the
relaxation time in the early stage of the LLPS and faster
dynamics with increasing quench depths [see Fig. 3(c) and
inset of Fig. 3(a)]. At this early phase of the spinodal
decomposition, the dynamics is driven by the curvature of
the free energy
2
F=c
2
, which increases in magnitude
with deeper quenches (lower temperatures) and thus speeds
up the dynamics. This picture reverses when the slow
relaxation sets in [τ
2
in Fig. 3(c)]. Now the relaxation times
for the lower temperatures are considerably larger than the
high temperature quenches.
The transition from dynamics dominated by the fast
decay to dynamics dominated by the slow decay is
quantified by the nonergodicity parameter [50], here
defined as f ¼ A
2
=ðA
1
þ A
2
Þ. A rather sharp increase of
the nonergodicity parameter around t
w
¼ 30 s is observed
[Fig. 3(b)], with a later rise time and slower increase for
lower temperatures, suggesting that the transition to coars-
ening dynamics is already slowed down by the lower
mobility at low temperatures.
These three stages in the dynamics can be linked to the
evolution of the LLPS. The first corresponds to the early
stage of a spinodal decomposition, displaying an enhance-
ment of the concentration fluctuations and the formation of
an interface between two different phases. The third one is
the late stage in which the domains are growing. For our
system, this seems to be controlled by the mass transport
mechanism and by the mobility of the involved compo-
nents. In other cases in condensed matter physics, different
domain growth mechanisms have been identified [51]. The
second stage is a transition between the early and late stage.
Important for the assessment of the form of the dynamics
and relevant for computing quantities such as diffusion and
transport coefficients during the LLPS are the relaxation
rate ΓðqÞ¼1=τðqÞ and the KWW exponents, including
their q dependence [32]. We find in the early phase of the
LLPS a linear relationship Γ q [Fig. S2(a)] and γ
1
around
1.4 to 1.7 (Fig. S2) with no pronounced q dependence. In
contrast, the dynamics during the late coarsening stage also
displays a linear (Γ q) behavior but now with a pro-
nounced q dependence of the KWW exponents, which are
decreasing from values of γ
2
¼ 2 at small q values to
γ
2
¼ 1 at large values of q [Fig. S2(d)]. Our results clearly
show that the coarsening dynamics of the protein droplets
at this length scale are not governed by Brownian dynamics
but instead by a ballistic and partially cooperative motion of
the protein droplets driven by the spinodal decomposition.
Similar super diffusive ballistic types of dynamics have
been observed frequently in soft matter systems, e.g., in the
late stage of colloidal gelation processes [5254], during
spinodal decomposition of colloidal systems [27], and in
the framework of MD simulations of metallic glasses [55]
with strongly interconnected clusters moving together.
Phenomenological models of single microcollapse events
(a)
10
1
10
2
t
w
(s)
10
0
10
1
10
2
(s)
5 10152025
2
4
6
8
(b)
0 1020304050
t
w
(s)
0
0.2
0.4
0.6
0.8
1
Non-ergodicity parameter
T
q
=15
o
C
T
q
=12
o
C
T
q
=10
o
C
T
q
=8
o
C
T
q
=6
o
C
T
q
=4
o
C
(c)
5
10
15
1
(s)
4 6 8 10121416
T
q
(
o
C)
100
200
300
2
(s)
t
w
=22s
t
w
=96s
FIG. 3. (a) Decorrelation time for q ¼ 0.005 nm
1
as a function of waiting time t
w
. The open symbols correspond to the slow mode
(τ
2
) and the filled symbols to the fast one (τ
1
). Inset: linear representation. (b) Nonergodicity parameter f ¼ A
2
=ðA
1
þ A
2
Þ as a function
of t
w
. The legend also applies to (a). (c) Decorrelation time of the fast and slow modes as a function of quench temperature T
q
,
respectively.
PHYSICAL REVIEW LETTERS 126, 138004 (2021)
138004-3

have been put forward as possible explanations [56]
underlying such unusual dynamics and subsequently
extended to series of such intermittent events to account
for both q dependence of the relaxation rate and the KWW
exponents [57,58]. The connection of these phenomeno-
logical models to the dynamics during an LLPS is not
obvious, especially with regard to typical field theories
such as the Cahn-Hilliard equation (CHE) used to model
spinodal decomposition. A shared feature between colloi-
dal gels and the LLPS, investigated here, is the presence of
elastic deformations. These can arise in an LLPS from the
viscoelastic properties, which are caused by a dynamical
coupling of diffusion and stresses, and could explain
the similarities between these observations [57]. In fact,
the dynamical asymmetry of the two phases present in the
system can lead to viscoelastic phase separation [59].
To obtain more insight into the underlying dynamical
mechanisms, we compare the experimental data to numeri-
cal simulations of the temporal evolution of the CHE
[60,61] (here in 2D). The dynamics of the ordinary CHE
shows faster dynamics for quenches to lower temperatures.
As this is not what we observe in the experimental data, we
introduce a dependence of the mobility μ on protein density
ρðr; tÞ, representing the slowdown of the dynamics when the
density of the dense phase is increasing and its mobility
freezes out [45] in the spirit of model C according to Ref. [2].
The parameter c
g
sets the concentration at which the mobility
decreases (for details, see the SM). The simulations then
yield a time dependent real space configuration ρðr; tÞ of the
protein density [Fig. 4(c)], which is converted into an x-ray
speckle pattern by means of its Fourier components jρðq; tÞj
2
and analyzed with the same time correlation methods as
applied to the experimental data [45,46].
Figure 4(a) displays the temporal evolution of the
spatially averaged mobility ¯μðt
w
Þ in the dense phase and
Fig. S10 the spatially averaged density
¯
ρðt
w
Þ of the dense
and dilute phases for different values of c
g
. A point in the
2D image was considered part of the dense phase if its
concentration was higher than the initial concentration. The
mobility of the dense phase drops quickly during the LLPS
upon lowering the c
g
value. The densities in the dense and
dilute phases do not reach their equilibrium values any-
more, which is considered an arrest of the LLPS [45,46].
Typical real space configurations are shown in Fig. 4(b)
with the LLPS visible via the formation of domains of the
diluted phase (blue) in a host matrix of the dense phase
(red). The corresponding density profiles (see Fig. S9)
show the typical hallmarks of the spinodal decomposition,
with density fluctuations developing quickly and reaching
rather smooth profiles at the end of the LLPS for a mobility
that is independent of protein density and equal to 1. In
contrast, for c
g
¼ 0.6, we observe that the density fluctua-
tions become immobile when reaching the threshold value,
leading to smaller domains.
TTC and g
2
functions were computed following the
procedure of experimental data. Figure 4(c) displays the
TTC for the simulations and the corresponding g
2
func-
tions. We identify a fast dynamic process during the early
time of the LLPS, which represents the dynamics of
spinodal interface formation between the dense and diluted
phases. The dynamics quickly slows down with a sudden
appearance of a second much slower relaxation process
when the dense phase has formed. The relaxation times
during the first stage are determined by the quench depth
(smaller for deeper quenches) because the curvature of the
free energy is here the main driving force for the velocity of
interface formation. The corresponding KWW values γ
1
and γ
2
[Figs. S11(c) and S11(d)] are between 1.5 and 2.5,
which is in good agreement with our experimental values.
Comparing the real space data and the parameters
describing the dynamics, it is possible to see that the rise
(a)
10
2
10
3
t
w
0
0.2
0.4
0.6
0.8
1
(b)
(r,t
w
=10)
(r,t
w
=70)
(r,t
w
=90)
(r,t
w
=130)
0
0.5
1
(c)
FIG. 4. The simulated ¯μ of the dense phase (a) as a function of waiting time. In brown, the case with constant mobility and the quench
depth for this case was set to T
q
¼ 0.2 T
c
, with T
c
being the critical temperature [62,63]. In red, c
g
¼ 0.8 and T
q
¼ 0.12 T
c
, and in
yellow, c
g
¼ 0.6 and T
q
¼ 0.08 T
c
. (b) Examples of real space configurations for T
q
¼ 0.14 T
c
and mobility at different waiting times
with the densities indicated by the color bar. (c) Upper panel: two-time correlation function of the simulated data for q ¼ 1. From left to
right, the parameters T
q
and c
g
of the simulations are T
q
¼ 0.2 T
c
and c
g
¼ , which correspond to a constant mobility set to 1,
T
q
¼ 0.12 T
c
and c
g
¼ 0.8, and T
q
¼ 0.08 T
c
and c
g
¼ 0.6. (c) Lower panel: corresponding g
2
functions at different waiting times t
w
.
PHYSICAL REVIEW LETTERS 126, 138004 (2021)
138004-4

of the nonergodicity parameter starts during the onset of
domain and coarsening dynamics. In this short transition
time (see Fig. 3), domain coarsening is progressing in
parallel with the final interface formation. After this
transition, the domain coarsening is taking place. The
dynamics seems to be composed of three processes: the
coalescence of different domains, domain growth, and
the spatial movement that is guided by the surface tension
and precedes the coalescence of two domains. Its decorre-
lation time is larger for deeper quenches because of the
lower transition concentration of the mobility [Fig. 4(c) and
Fig. S11], representing a situation in which the temper-
ature-dependent glass line of the dense host phase intersects
the spinodal phase region. For the lower value of c
g
, the
diluted phase is essentially trapped inside the frozen host
matrix [red in Fig. 4(b)].
Based on the simulation r esults, we can conclude that
thekineticsofthesystemsiscapturedbymodelCas
suggested before [2]. The d ynamics, however, is only
partially reproduced. While the slowdown of the late s tage
dynamics with increasing quench depth is correctly
described, as well as the presence of two relaxation
modes, model C does not reproduce the Γ q behavior
observed in the experiment or the q dependence of the
KWW values of the slow dynamics (Fig. S11). We
speculate that this is due to the neglect of the viscoelastic
properties of the system in the CHE, which would lead to
elastic deformation and motion inducing a linear
dispersion relation.
In conclusion, we demonstrated the technique of low
radiation dose XPCS and used it to study protein dynamics
during an LLPS. The method delivers simultaneously
information on the collective dynamics via XPCS and
the structural evolution via the ensemble averaged scatter-
ing IðqÞ. The two-time correlation maps provide a high
level of detail of the dynamics during a spinodal decom-
position of Ig in solution with PEG. We identify distinctly
different dynamical regimes of the LLPS with different
temperature behaviors. The early stage dynamics reflects
concentration fluctuation and interface formation and is
faster for lower temperatures, reflecting stronger quench
depths. In contrast, the later stage of coarsening is slower
for lower temperatures, which is caused by the reduced
mobility of the slowed down proteins comprising the host
matrix. With simulations, we were able to identify a
concentration and time dependence of the molecular-scale
mobility that connects the dynamics of the condensate to
molecular-scale quantities.
The authors acknowledge discussions with M. Oettel,
financial support from the DFG and the BMBF
(05K20VTA), and the allocation of beamtime by DESY.
N. B. acknowledges the Alexander von Humboldt-Stiftung
for a postdoctoral fellowship. A. R. acknowledges the
Studienstiftung des Deutschen Volkes for a Ph.D.
fellowship. C. G. acknowledges BMBF (Grants
No. 05K19PS1 and No. 05K20PSA) for financial support.
*
fajun.zhang@uni-tuebingen.de
christian.gutt@uni-siegen.de
frank.schreiber@uni-tuebingen.de; https://publons.com/
researcher/2502617/frank-schreiber/
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138004-5

Citations
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TL;DR: X-ray photon correlation spectroscopy (XPCS) as mentioned in this paper enables the study of sample dynamics between micrometer and atomic length scales, and it benefits from the increased brilliance of the next-generation synchrotron radiation and Free-Electron Laser (FEL) sources.
Abstract: X-ray photon correlation spectroscopy (XPCS) enables the study of sample dynamics between micrometer and atomic length scales. As a coherent scattering technique, it benefits from the increased brilliance of the next-generation synchrotron radiation and Free-Electron Laser (FEL) sources. In this article, we will introduce the XPCS concepts and review the latest developments of XPCS with special attention on the extension of accessible time scales to sub-μs and the application of XPCS at FELs. Furthermore, we will discuss future opportunities of XPCS and the related technique X-ray speckle visibility spectroscopy (XSVS) at new X-ray sources. Due to its particular signal-to-noise ratio, the time scales accessible by XPCS scale with the square of the coherent flux, allowing to dramatically extend its applications. This will soon enable studies over more than 18 orders of magnitude in time by XPCS and XSVS.

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TL;DR: In this paper, the authors measured the kinetics and dynamics of a pressure-induced liquid-liquid phase separation (LLPS) in a water-lysozyme solution and showed that the protein solution gels upon reaching the phase boundary.
Abstract: Employing X-ray photon correlation spectroscopy we measure the kinetics and dynamics of a pressure-induced liquid-liquid phase separation (LLPS) in a water-lysozyme solution. Scattering invariants and kinetic information provide evidence that the system reaches the phase boundary upon pressure-induced LLPS with no sign of arrest. The coarsening slows down with increasing quench depths. The $g_2$-functions display a two-step decay with a gradually increasing non-ergodicity parameter typical for gelation. We observe fast superdiffusive ($\gamma \geq 3/2$) and slow subdiffusive ($\gamma < 0.6$) motion associated with fast viscoelastic fluctuations of the network and a slow viscous coarsening process, respectively. The dynamics age linear with time $\tau \propto t_\mathrm{w}$ and we observe the onset of viscoelastic relaxation for deeper quenches. Our results suggest that the protein solution gels upon reaching the phase boundary.

8 citations

Journal ArticleDOI
TL;DR: In this paper, the authors used X-ray photon correlation spectroscopy to study the dynamics and kinetics of a protein solution undergoing liquid-liquid phase separation (LLPS) and demonstrated that in the early stage of spinodal decomposition, the kinetics relaxation is up to 40 times slower than the dynamics.
Abstract: Microscopic dynamics of complex fluids in the early stage of spinodal decomposition (SD) is strongly intertwined with the kinetics of structural evolution, which makes a quantitative characterization challenging. In this work, we use X-ray photon correlation spectroscopy to study the dynamics and kinetics of a protein solution undergoing liquid-liquid phase separation (LLPS). We demonstrate that in the early stage of SD, the kinetics relaxation is up to 40 times slower than the dynamics and thus can be decoupled. The microscopic dynamics can be well described by hyper-diffusive ballistic motions with a relaxation time exponentially growing with time in the early stage followed by a power-law increase with fluctuations. These experimental results are further supported by simulations based on the Cahn-Hilliard equation. The established framework is applicable to other condensed matter and biological systems undergoing phase transitions and may also inspire further theoretical work.

7 citations

Journal ArticleDOI
TL;DR: In this paper, the authors studied the dynamics on the molecular level under strongly varying solution conditions and observed a strong decrease in antibody diffusion, while internal flexibility persists to a significant degree, thus ensuring the movement necessary for the preservation of molecular function.
Abstract: Antibody therapies are typically based on high-concentration formulations that need to be administered subcutaneously. These conditions induce several challenges, inter alia a viscosity suitable for injection, sufficient solution stability, and preservation of molecular function. To obtain systematic insights into the molecular factors, we study the dynamics on the molecular level under strongly varying solution conditions. In particular, we use solutions of antibodies with poly(ethylene glycol), in which simple cooling from room temperature to freezing temperatures induces a transition from a well-dispersed solution into a phase-separated and macroscopically arrested system. Using quasi-elastic neutron scattering during in situ cooling ramps and in prethermalized measurements, we observe a strong decrease in antibody diffusion, while internal flexibility persists to a significant degree, thus ensuring the movement necessary for the preservation of molecular function. These results are relevant for a more dynamic understanding of antibodies in high-concentration formulations, which affects the formation of transient clusters governing the solution viscosity.

7 citations

Journal ArticleDOI
TL;DR: In this paper , the authors measured the kinetics and dynamics of a pressure-induced liquid-liquid phase separation (LLPS) in a water-lysozyme solution and showed that the protein solution gels upon reaching the phase boundary.
Abstract: Employing X-ray photon correlation spectroscopy, we measure the kinetics and dynamics of a pressure-induced liquid-liquid phase separation (LLPS) in a water-lysozyme solution. Scattering invariants and kinetic information provide evidence that the system reaches the phase boundary upon pressure-induced LLPS with no sign of arrest. The coarsening slows down with increasing quench depths. The g2 functions display a two-step decay with a gradually increasing nonergodicity parameter typical for gelation. We observe fast superdiffusive (γ ≥ 3/2) and slow subdiffusive (γ < 0.6) motion associated with fast viscoelastic fluctuations of the network and a slow viscous coarsening process, respectively. The dynamics age linearly with time τ ∝ tw, and we observe the onset of viscoelastic relaxation for deeper quenches. Our results suggest that the protein solution gels upon reaching the phase boundary.

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References
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TL;DR: In this article, it was shown that the thickness of the interface increases with increasing temperature and becomes infinite at the critical temperature Tc, and that at a temperature T just below Tc the interfacial free energy σ is proportional to (T c −T) 3 2.
Abstract: It is shown that the free energy of a volume V of an isotropic system of nonuniform composition or density is given by : NV∫V [f 0(c)+κ(▿c)2]dV, where NV is the number of molecules per unit volume, ▿c the composition or density gradient, f 0 the free energy per molecule of a homogeneous system, and κ a parameter which, in general, may be dependent on c and temperature, but for a regular solution is a constant which can be evaluated. This expression is used to determine the properties of a flat interface between two coexisting phases. In particular, we find that the thickness of the interface increases with increasing temperature and becomes infinite at the critical temperature Tc , and that at a temperature T just below Tc the interfacial free energy σ is proportional to (T c −T) 3 2 . The predicted interfacial free energy and its temperature dependence are found to be in agreement with existing experimental data. The possibility of using optical measurements of the interface thickness to provide an additional check of our treatment is briefly discussed.

8,720 citations

Journal ArticleDOI
TL;DR: In this article, the empirical dielectric decay function γ(t)= exp −(t/τ 0)β was transformed analytically to give the frequency dependent complex dielectrics constant if β is chosen to be 0.50 in the range log(ωτ0) > −0.5.
Abstract: The empirical dielectric decay function γ(t)= exp –(t/τ0)β may be transformed analytically to give the frequency dependent complex dielectric constant if β is chosen to be 0.50. The resulting dielectric constant and dielectric loss curves are non-symmetrical about the logarithm of the frequency of maximum loss, and are intermediate between the Cole-Cole and Davidson-Cole empirical relations. Using a short extrapolation procedure, good agreement is obtained between the empirical representation and the experimental curves for the α relaxation in polyethyl acrylate. It is suggested that the present representation would have a general application to the α relaxations in other polymers. The Hamon approximation, with a small applied correction, is valid for the present function with β= 0.50 in the range log(ωτ0) > –0.5, but cannot be used at lower frequencies.

3,675 citations

Journal ArticleDOI
22 Sep 2017-Science
TL;DR: The findings together suggest that several membrane-less organelles have been shown to exhibit a concentration threshold for assembly, a hallmark of phase separation, and represent liquid-phase condensates, which form via a biologically regulated (liquid-liquid) phase separation process.
Abstract: BACKGROUND Living cells contain distinct subcompartments to facilitate spatiotemporal regulation of biological reactions. In addition to canonical membrane-bound organelles such as secretory vesicles and endoplasmic reticulum, there are many organelles that do not have an enclosing membrane yet remain coherent structures that can compartmentalize and concentrate specific sets of molecules. Examples include assemblies in the nucleus such as the nucleolus, Cajal bodies, and nuclear speckles and also cytoplasmic structures such as stress granules, P-bodies, and germ granules. These structures play diverse roles in various biological processes and are also increasingly implicated in protein aggregation diseases. ADVANCES A number of studies have shown that membrane-less assemblies exhibit remarkable liquid-like features. As with conventional liquids, they typically adopt round morphologies and coalesce into a single droplet upon contact with one another and also wet intracellular surfaces such as the nuclear envelope. Moreover, component molecules exhibit dynamic exchange with the surrounding nucleoplasm and cytoplasm. These findings together suggest that these structures represent liquid-phase condensates, which form via a biologically regulated (liquid-liquid) phase separation process. Liquid phase condensation increasingly appears to be a fundamental mechanism for organizing intracellular space. Consistent with this concept, several membrane-less organelles have been shown to exhibit a concentration threshold for assembly, a hallmark of phase separation. At the molecular level, weak, transient interactions between molecules with multivalent domains or intrinsically disordered regions (IDRs) are a driving force for phase separation. In cells, condensation of liquid-phase assemblies can be regulated by active processes, including transcription and various posttranslational modifications. The simplest physical picture of a homogeneous liquid phase is often not enough to capture the full complexity of intracellular condensates, which frequently exhibit heterogeneous multilayered structures with partially solid-like characters. However, recent studies have shown that multiple distinct liquid phases can coexist and give rise to richly structured droplet architectures determined by the relative liquid surface tensions. Moreover, solid-like phases can emerge from metastable liquid condensates via multiple routes of potentially both kinetic and thermodynamic origins, which has important implications for the role of intracellular liquids in protein aggregation pathologies. OUTLOOK The list of intracellular assemblies driven by liquid phase condensation is growing rapidly, but our understanding of their sequence-encoded biological function and dysfunction lags behind. Moreover, unlike equilibrium phases of nonliving matter, living cells are far from equilibrium, with intracellular condensates subject to various posttranslational regulation and other adenosine triphosphate–dependent biological activity. Efforts using in vitro reconstitution, combined with traditional cell biology approaches and quantitative biophysical tools, are required to elucidate how such nonequilibrium features of living cells control intracellular phase behavior. The functional consequences of forming liquid condensates are likely multifaceted and may include facilitated reaction, sequestration of specific factors, and organization of associated intracellular structures. Liquid phase condensation is particularly interesting in the nucleus, given the growing interest in the impact of nuclear phase behavior on the flow of genetic information; nuclear condensates range from micrometer-sized bodies such as the nucleolus to submicrometer structures such as transcriptional assemblies, all of which directly interact with and regulate the genome. Deepening our understanding of these intracellular states of matter not only will shed light on the basic biology of cellular organization but also may enable therapeutic intervention in protein aggregation disease by targeting intracellular phase behavior.

2,432 citations

Journal ArticleDOI
26 Jun 2009-Science
TL;DR: It is shown that P granules exhibit liquid-like behaviors, including fusion, dripping, and wetting, which is used to estimate their viscosity and surface tension, and reflects a classic phase transition, in which polarity proteins vary the condensation point across the cell.
Abstract: In sexually reproducing organisms, embryos specify germ cells, which ultimately generate sperm and eggs In Caenorhabditis elegans, the first germ cell is established when RNA and protein-rich P granules localize to the posterior of the one-cell embryo Localization of P granules and their physical nature remain poorly understood Here we show that P granules exhibit liquid-like behaviors, including fusion, dripping, and wetting, which we used to estimate their viscosity and surface tension As with other liquids, P granules rapidly dissolved and condensed Localization occurred by a biased increase in P granule condensation at the posterior This process reflects a classic phase transition, in which polarity proteins vary the condensation point across the cell Such phase transitions may represent a fundamental physicochemical mechanism for structuring the cytoplasm

2,134 citations

Journal ArticleDOI
TL;DR: The theory of phase separation from a single phase fluid by a spinodal mechanism is given in this paper, where the predicted structure may be described in terms of a superpositioning of sinusoidal composition modulations of a fixed wavelength, but random in amplitude, orientation, and phase.
Abstract: The theory of phase separation from a single phase fluid by a spinodal mechanism is given. The predicted structure may be described in terms of a superpositioning of sinusoidal composition modulations of a fixed wavelength, but random in amplitude, orientation, and phase. Sections through a calculated structure are shown. These show that the structure has many of the geometrical features found in phase separable glasses, in particular the high degree of connectivity among particles of each phase.

1,591 citations