Martin Volk

Bio: Martin Volk is an academic researcher from University of Liverpool. The author has contributed to research in topics: Photosynthetic reaction centre & Protein folding. The author has an hindex of 25, co-authored 53 publications receiving 1764 citations. Previous affiliations of Martin Volk include University of Pennsylvania & University of New Orleans.

The detection sensitivity of commonly used singlet oxygen probes in aqueous environments.

Abstract: The sensitivity for singlet oxygen (1O2) of two convenient 1O2 probes, 1,3-diphenylisobenzofuran (DPBF) and 9,10-Anthracenediyl-bis(methylene)dimalonic acid (ABDA), has been investigated in different aqueous environments. Both probes are commercially available at reasonable cost and can be used with standard UV-vis spectrometers. Although DPBF is not soluble in neat water and is not specific to the detection of 1O2, it has very high, essentially diffusion-limited, reactivity towards 1O2; it can trap up to 50% of all 1O2 created in alcohol/water or micellar solution, and even more when replacing H2O by D2O, which makes it highly useful when the process under investigation does not yield much 1O2. On the other hand, ABDA has a much lower reactivity, reacting with only 2% of the singlet oxygen generated in H2O, as well as a smaller extinction coefficient, resulting in a much smaller spectroscopic response, but is soluble in neat water and is specific for 1O2, allowing for discrimination from other reactive oxygen species. The results presented here not only allow a comparative assessment of the usefulness of the two 1O2 probes, but also provide a reference for an accurate absolute quantification of the amount of 1O2 generated in an experiment from the observed absorbance bleach.

Inhomogeneity of Radical Pair Energies in Photosynthetic Reaction Centers Revealed by Differences in Recombination Dynamics of P+HA- When Detected in Delayed Emission and in Absorption

Abstract: Nonuniform radical pair recombination reactions originating from energetic inhomogeneity can be revealed by comparing the kinetics monitored in transient absorption and in delayed emission. The dynamics of the high-energy tail of a distribution (weighted by the Boltzmann factor for repopulation of the emitting state) determines the kinetics observed in emission, while in absorption the bulk average is reflected. In reaction centers of Rb. sphaeroides we found the recombination dynamics of the radical pair P[sup +]H[sub A][sup [minus]] to be faster and their magnetic field dependence to exhibit a significantly broader resonance line width when detecting in emission than when monitoring in absorption. This points to larger values of both the singlet and triplet recombination rates for the high-lying radical pair states. The observed increase of both rates with increasing energy can be understood only if superexchange coupling via P[sup A]B[sub A][sup [minus]] is prevailing for these recombination reactions, since its energy denominator decreases for the high-lying states. The observed weak temperature dependence of the average amplitude of the delayed fluorescence as compared to the amplitude of the prompt emission can be modelled by averaging over a Gaussian distribution of the free-energy differences with a maximum at [Delta]G[sub 0] = 0.25more » eV and a width of 2[sigma] [approx equal] 0.1 eV. 66 refs., 4 figs., 2 tabs.« less

Peptide Conformational Dynamics and Vibrational Stark Effects Following Photoinitiated Disulfide Cleavage

Abstract: Photoinitiation of relaxation of two peptides (labeled 1 and 2) and spectroscopic studies of the ensuing dynamics have led to new information about peptide conformational dynamics. Following photolysis of the aryl disulfide chromophore that constrains a peptide to be distorted from its equilibrium form, the S−S bond is broken in <200 fs, and the liberated thiyl radicals either undergo geminate recombination or diffuse apart to allow the peptides to change conformation. From anisotropy measurements, overall peptide rotation is on the time scale of 600 ps. At an even earlier time (ca. 100 ps), transient IR measurements show a bleaching of the amide I‘ region, arising from a vibrational Stark effect produced upon ring opening of peptide 2. We did not detect any significant shift in the amide I‘ region up to 2 ns, suggesting no significant helix formation in this time domain. Thiyl radicals arising from peptide 2 recombine with a power law rate over the time range from picoseconds to microseconds signaling an...

Inflicting controlled nonthermal damage to subcellular structures by laser-activated gold nanoparticles.

Abstract: We show that low-intensity laser irradiation of cancer cells containing endosomal gold nanoparticles leads to endosome rupture and escape of the nanoparticles into the cytosol without affecting the cells’ viability. The low light intensity of our experiments allows us to rule out photothermal effects as the underlying mechanism, and we present results that suggest photoinduced radicals as the photogenerated active species. This nonthermal mechanism may also be important in the context of cell death at higher laser intensities, which had been reported previously.

Singlet Oxygen Generation by Laser Irradiation of Gold Nanoparticles

Abstract: The formation of singlet oxygen by irradiation of gold nanoparticles in their plasmon resonance band with continuous or pulsed laser light has been investigated. Citrate-stabilized nanoparticles were found to facilitate the photogeneration of singlet oxygen, albeit with low quantum yield. The reaction caused by pulsed laser irradiation makes use of the equilibrated hot electrons that can reach temperatures of several thousand degrees during the laser pulse. Although less efficient, continuous irradiation, which acts via the short-lived directly excited primary “hot” electrons only, can produce enough singlet oxygen for photodynamic cancer therapy and has significant advantages for practical applications. However, careful design of the nanoparticles is needed, since even a moderately thick capping layer can completely inhibit singlet oxygen formation. Moreover, the efficiency of the process also depends on the nanoparticle size.

The random walk's guide to anomalous diffusion: a fractional dynamics approach

Abstract: Fractional kinetic equations of the diffusion, diffusion–advection, and Fokker–Planck type are presented as a useful approach for the description of transport dynamics in complex systems which are governed by anomalous diffusion and non-exponential relaxation patterns. These fractional equations are derived asymptotically from basic random walk models, and from a generalised master equation. Several physical consequences are discussed which are relevant to dynamical processes in complex systems. Methods of solution are introduced and for some special cases exact solutions are calculated. This report demonstrates that fractional equations have come of age as a complementary tool in the description of anomalous transport processes.

Infrared spectroscopy of proteins.

Abstract: This review discusses the application of infrared spectroscopy to the study of proteins. The focus is on the mid-infrared spectral region and the study of protein reactions by reaction-induced infrared difference spectroscopy.

Synthetic molecular motors and mechanical machines.

Abstract: The widespread use of controlled molecular-level motion in key natural processes suggests that great rewards could come from bridging the gap between the present generation of synthetic molecular systems, which by and large rely upon electronic and chemical effects to carry out their functions, and the machines of the macroscopic world, which utilize the synchronized movements of smaller parts to perform specific tasks. This is a scientific area of great contemporary interest and extraordinary recent growth, yet the notion of molecular-level machines dates back to a time when the ideas surrounding the statistical nature of matter and the laws of thermodynamics were first being formulated. Here we outline the exciting successes in taming molecular-level movement thus far, the underlying principles that all experimental designs must follow, and the early progress made towards utilizing synthetic molecular structures to perform tasks using mechanical motion. We also highlight some of the issues and challenges that still need to be overcome.

DLS and zeta potential - What they are and what they are not?

Abstract: Adequate characterization of NPs (nanoparticles) is of paramount importance to develop well defined nanoformulations of therapeutic relevance. Determination of particle size and surface charge of NPs are indispensable for proper characterization of NPs. DLS (dynamic light scattering) and ZP (zeta potential) measurements have gained popularity as simple, easy and reproducible tools to ascertain particle size and surface charge. Unfortunately, on practical grounds plenty of challenges exist regarding these two techniques including inadequate understanding of the operating principles and dealing with critical issues like sample preparation and interpretation of the data. As both DLS and ZP have emerged from the realms of physical colloid chemistry - it is difficult for researchers engaged in nanomedicine research to master these two techniques. Additionally, there is little literature available in drug delivery research which offers a simple, concise account on these techniques. This review tries to address this issue while providing the fundamental principles of these techniques, summarizing the core mathematical principles and offering practical guidelines on tackling commonly encountered problems while running DLS and ZP measurements. Finally, the review tries to analyze the relevance of these two techniques from translatory perspective.

Structure and function of DNA photolyase and cryptochrome blue-light photoreceptors.

Abstract: 1. Electron Donor 2223 2. Electron Transfer Path 2225 3. Electron Transfer Mechanism 2226 4. Physiological Relevance 2226 D. Radical Reactions in Photolyase 2226 E. “Dark Function” of Photolyase 2227 F. Regulation of Photolyase 2227 III. (6−4) Photolyase 2228 IV. Cryptochromes 2230 A. Structure 2231 B. Function 2231 1. Circadian Rythm 2231 2. Mammalian Circadian System 2231 3. Cryptochromes and the Circadian Clock 2232 V. Perspectives 2234 VI. Acknowledgment 2235 VII. References 2235