抗原变异可使得多种致病微生物易于逃避宿主免疫应答。表达在感染红细胞表面的恶性疟原虫红细胞表面蛋白1（PfPMP1）与感染红细胞、内皮细胞、树突状细胞以及胎盘的单个或多个受体作用，在黏附及免疫逃避中起关键的作用。每个单倍体基因组var基因家族编码约60种成员，通过启动转录不同的var基因变异体为抗原变异提供了分子基础。

疟原虫var基因转换速率变化导致抗原变异[英]／Paul H, Robert P, Christodoulou Z, et al//Proc Natl Acad Sci U S A

In the present review, aerogels designate dried gels with a very high relative pore volume. These are versatile materials that are synthesized in a first step by low-temperature traditional sol-gel chemistry. However, while in the final step most wet gels are often dried by evaporation to produce so-called xerogels, aerogels are dried by other techniques, essentially supercritical drying. As a result, the dry samples keep the very unusual porous texture which they had in the wet stage. In general these dry solids have very low apparent densities, large specific surface areas, and in most cases they exhibit amorphous structures when examined by X-ray diffraction (XRD) methods. In addition, they are metastable from the point of view of their thermodynamic properties. Consequently, they often undertake a structural evolution by chemical transformation, when aged in a liquid medium and/or heat treated. As aerogels combine the properties of being highly divided solids with their metastable character, they can develop very attractive physical and chemical properties not achievable by other means of low temperature soft chemical synthesis. In other words, they form a new class of solids showing sophisticated potentialities for a range of applications. These applications as well as chemical and physical aspects of these materials were regularly detailed and discussed in a series of symposia on aerogels,1-5 the last of them being held in Albuquerque in 2000.6 Reviews were also regularly published, either on both xerogels and aerogels7 or more focused on the applications of aerogels.8-13 The particularly interesting properties of aerogels arise from the extraordinary flexibility of the solgel processing, coupled with original drying techniques. The wet chemistry is not basically different for making xerogels and aerogels. As this common basis has been extensively detailed in recent books,14 it does not need to be reviewed. Compared to traditional xerogels, the originality of aerogels comes from * To whom all correspondence should be addressed. † Institut de Recherches sur la Catalyse. ‡ Laboratoire d’Application de la Chimie à l’Environnement. 4243 Chem. Rev. 2002, 102, 4243−4265

https://is.muni.cz/el/1431/podzim2006/C7780/um/Read/2711172/aerogel_Pajonk_ChRev02_4243.pdf

Chemistry of Aerogels and Their Applications

https://escholarship.org/content/qt16g3d6mk/qt16g3d6mk.pdf

Nuclear magnetic resonance of laser-polarized noble gases in molecules, materials, and organisms.

The nuclear spin polarization of the noble gas isotopes (3)He and (129)Xe can be increased using optical pumping methods by four to five orders of magnitude. This extraordinary gain in polarization translates directly into a gain in signal strength for MRI. The new technology of hyperpolarized (HP) gas MRI holds enormous potential for enhancing sensitivity and contrast in pulmonary imaging. This review outlines the physics underlying the optical pumping process, imaging strategies coping with the nonequilibrium polarization, and effects of the alveolar microstructure on relaxation and diffusion of the noble gases. It presents recent progress in HP gas MRI and applications ranging from MR microscopy of airspaces to imaging pulmonary function in patients and suggests potential directions for future developments.

MRI of the lungs using hyperpolarized noble gases.

Restricted diffusion is a common feature of many physicochemical, biological, and industrial processes. Nuclear magnetic resonance techniques are often used to survey the atomic or molecular motion in confining media by applying inhomogeneous magnetic fields to encode the trajectories of spin-bearing particles. The diversity and complexity of diffusive NMR phenomena, observed in experiments, result from the specific properties of reflected Brownian motion. Here the focus is on the mathematical aspects of this stochastic process, their physical interpretations, and their practical applications. The main achievements in this field, from Hahn’s discovery of spin echoes to present-day research, are presented in a unified mathematical language. A long-standing problem of restricted diffusion under arbitrary magnetic field is reformulated in terms of multiple correlation functions of the reflected Brownian motion. Many classical results are retrieved, extended, and critically discussed.

https://pmc.polytechnique.fr/pagesperso/dg/publi/2007_01.pdf

NMR survey of reflected brownian motion

Spin polarized liquid helium three can be obtained by liquefaction of a gas polarized by laser optical pumping. We show that this method can lead to nuclear polarizations exceeding 40% in a liquid at about 400 mK and almost zero pressure. The relaxation time of the orientation is a few minutes, comparable to the relaxation time of weakly polarized bulk liquid. The NMR signals given by the liquid are strongly shifted in frequency; this provides a precise and convenient way to measure the polarization and to follow the motion of the liquid inside the experimental cell Obtention de polarisations nucleaires superieures a 40% dans un liquide a ∼400 mK et pression presque nulle. Le deplacement en frequence des signaux RMN permet de mesurer de facon precise la polarisation nucleaire et de suivre le mouvement du liquide le long des parois de la cellule

/pdf/obtaining-polarized-liquid-helium-3-from-optically-oriented-1jiydyg13v.pdf

Obtaining polarized liquid helium 3 from optically oriented gas

We present NMR measurements of the diffusion of hyperpolarized helium-3 in the human lung performed at fields much lower than those of conventional MRI scanners. The measurements were made on standing subjects using homebuilt apparatus operating at 3 mT. Oxygen-limited transverse relaxation (T2 up to 15-35 s) could be measured in-vivo. Accurate global diffusion measurements have been performed in-vivo and in-vitro. 1D ADC mapping with high SNR demonstrates the real possibility of performing quality lung imaging at extremely low fields.

In-Vivo NMR of Hyperpolarized Helium-3 in the Human Lung at Very Low Magnetic Fields

Spin polarized gases such as H↑ or 3He↑ can propagate damped spin waves at low temperatures; their origin is the so-called « identical spin rotation effect » in collisions between identical particles. We report here the observation of such effects in 3He↑ in the temperature range 2 < T < 6 K, and preliminary measurements of the coefficient μ which characterizes the quality factor of the spin oscillations. The measurements are in agreement with the numerical calculations of C. Lhuillier.

/pdf/spin-rotation-effects-and-spin-waves-in-gaseous-3he-3nvstc2ieo.pdf

Spin rotation effects and spin waves in gaseous 3He

We report the first in vivo NMR experiments performed with hyperpolarised 3He gas at 0.1 T. Hyperpolarisation is achieved by laser optical pumping of 3He atoms in the metastable triplet state. Longitudinal relaxation times of nuclear magnetisation of the order of 35 s are measured in human lungs. Very long transverse relaxation times of a few seconds are also observed. This work establishes the possibility of magnetic resonance imaging of human lung airways at low field.

Low-field 3He nuclear magnetic resonance in human lungs

The structure of the excited 23S and 23P triplet states of 3He and 4He in an applied magnetic field B is studied using different approximations of the atomic Hamiltonian. All optical transitions (line positions and intensities) of the 1 083 nm 23S- 23P transition are computed as a function of B. The effect of metastability exchange collisions between atoms in the ground state and in the 23S metastable state is studied, and rate equations are derived, for the populations these states in the general case of an isotopic mixture in an arbitrary field B. It is shown that the usual spin-temperature description remains valid. A simple optical pumping model based on these rate equations is used to study the B-dependence of the population couplings which result from the exchange collisions. Simple spectroscopy measurements are performed using a single-frequency laser diode on the 1 083 nm transition. The accuracy of frequency scans and of measurements of transition intensities is studied. Systematic experimental verifications are made for B = 0 to 1.5 T. Optical pumping effects resulting from hyperfine decoupling in high field are observed to be in good agreement with the predictions of the simple model. Based on adequately chosen absorption measurements at 1 083 nm, a general optical method to measure the nuclear polarisation of the atoms in the ground state in an arbitrary field is described. It is demonstrated at B∼ 0.1 T, a field for which the usual optical methods could not operate.

Geneviève Tastevin

Papers

Obtaining polarized liquid helium 3 from optically oriented gas

In-Vivo NMR of Hyperpolarized Helium-3 in the Human Lung at Very Low Magnetic Fields

Spin rotation effects and spin waves in gaseous 3He

Low-field 3He nuclear magnetic resonance in human lungs

Magnetic field effects on the 1 083 nm atomic line of helium