Studies of Molecular Motions of Ocotillol-Type Saponins in Solution by ~(13)C Nuclear Magnetic Relaxation

In this paper, the fully anisotropicoverall tumbling motions and side groups internal rotation of ocotillol-type saponins separated from the leaves of Panax Quidquefolium L. are investigated by ~(13)C nuclear magnetic relaxation. The fully anisotropic overall tumbling motion model with methyl conformation jumps internal rotation among three equivalent sites is presented, and the spectral density function of this model is derived. The rotation rates for overall tumbling motions to ocotillol-type saponins (OTS) are computed by Woessner's fully anisotropic overall tumbling motion model, and the internal rotation rate and barrier for side groups in OTS are calculated using free diffusion internal rotation model, restriction diffusion internal rotation model and conformation jumps internal rotation model, respectively.     

Quantum mechanical theory of dynamic nuclear polarization in solid dielectrics

Microwave driven dynamic nuclear polarization (DNP) is a process in which the large polarization present in an electron spin reservoir is transferred to nuclei, thereby enhancing NMR signal intensities. In solid dielectrics there are three mechanisms that mediate this transfer--the solid effect (SE), the cross effect (CE), and thermal mixing (TM). Historically these mechanisms have been discussed theoretically using thermodynamic parameters and average spin interactions. However, the SE and the CE can also be modeled quantum mechanically with a system consisting of a small number of spins and the results provide a foundation for the calculations involving TM. In the case of the SE, a single electron-nuclear spin pair is sufficient to explain the polarization mechanism, while the CE requires participation of two electrons and a nuclear spin, and can be used to understand the improved DNP enhancements observed using biradical polarizing agents. Calculations establish the relations among the electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) frequencies and the microwave irradiation frequency that must be satisfied for polarization transfer via the SE or the CE. In particular, if δ, Δ < ω(0I), where δ and Δ are the homogeneous linewidth and inhomogeneous breadth of the EPR spectrum, respectively, we verify that the SE occurs when ω(M) = ω(0S) ± ω(0I), where ω(M), ω(0S) and ω(0I) are, respectively, the microwave, and the EPR and NMR frequencies. Alternatively, when Δ > ω(0I) > δ, the CE dominates the polarization transfer. This two-electron process is optimized when ω(0S(1))-ω(0S(2)) = ω(0I) and ω(M)~ω(0S(1)) or ω(0S(2)), where ω(0S(1)) and ω(0S(2)) are the EPR Larmor frequencies of the two electrons. Using these matching conditions, we calculate the evolution of the density operator from electron Zeeman order to nuclear Zeeman order for both the SE and the CE. The results provide insights into the influence of the microwave irradiation field, the external magnetic field, and the electron-electron and electron-nuclear interactions on DNP enhancements.     

On-the-fly, electric-field-driven, coupled electron-nuclear dynamics

An on-the-fly, electric field driven, coupled electron-nuclear dynamics approach is developed and applied to model the photodissociation of water in the A((1)B1) excited state. In this method, a quantum propagator evolves the photon-induced electronic dynamics in the ultrafast time scale, and a quasi-classical surface hopping approach describes the nuclear dynamics in the slower time scale. In addition, strong system-field interactions are explicitly included in the electronic propagator. This theoretical development enables us to study rapid photon-induced bond dissociation dynamics and demonstrates the partial breakdown of electronic coherence as well as electronic population trapping in the excited state when the molecular vibrations detune the system with respect to the applied field. The method offers a practical way to use on-the-fly dynamics for modeling light-molecule interactions that lead to interesting photochemical events.     

Investigation of the rotational dynamics in liquids by time-resolved cars

The asymptotic decay time τ_an of the anisotropic vibrational excitation in liquids has been measured for the first time in time-resolved CARS. The use of different polarization geometries also allows us to determine the orientational relaxation time τ_R. Experimental data are presented for totally symmetric modes of liquid CS₂ and CH₃I, where τ_R describes the “tumbling” motion of the symmetry axis.     

Probing the dynamic guest-host interactions in sol-gel films using single molecule spectroscopy

Organic dyes usually exhibit enhanced photostability when trapped inside sol-gel silicates. The enhanced photostability is attributed to the reduction of intramolecular motions that facilitate photodegradation. We report the simultaneous detection of mobility and photostability of sol-gel encapsulated didodecyl-3,3,3',3'-tetramethylindocarbocyanine (DiI) using single molecule spectroscopy. Fluorescence from DiI was resolved into parallel and perpendicular polarization components and separately detected. On the basis of the calculated fluorescence polarization, single DiI molecules were classified into "tumbling" and "fixed". Out of 212 molecules investigated, 52% were found to be fixed. For the first time, the mobility of a guest molecule in sol-gel silicate can be directly correlated with its own photostability. Both tumbling and fixed molecules have shown to exhibit nonuniform photostability, indicative of the very heterogeneous guest-host interactions within each subgroup. The survival lifetimes for the majority of the tumbling and fixed molecules were found to be 4.3 and 13.1 s, respectively, demonstrating unequivocally that fixed molecules exhibit a higher photostability than tumbling molecules. These results are in accordance with a recent study on rhodamine B encapsulated in dried sol-gel silicates.     

Parametrization, molecular dynamics simulation, and calculation of electron spin resonance spectra of a nitroxide spin label on a polyalanine alpha-helix

The nitroxide spin label 1-oxyl-2,2,5,5-tetramethylpyrroline-3-methyl-methanethiosulfonate (MTSSL), commonly used in site-directed spin labeling of proteins, is studied with molecular dynamics (MD) simulations. After developing force field parameters for the nitroxide moiety and the spin label linker, we simulate MTSSL attached to a polyalanine alpha-helix in explicit solvent to elucidate the factors affecting its conformational dynamics. Electron spin resonance spectra at 9 and 250 GHz are simulated in the time domain using the MD trajectories and including global rotational diffusion appropriate for the tumbling of T4 Lysozyme in solution. Analysis of the MD simulations reveals the presence of significant hydrophobic interactions of the spin label with the alanine side chains.     

Molecular mechanisms of a single H2O diffusion on palladium surfaces

Surface diffusion of a water monomer is not as simple as previously imagined. Using state-of-the-art density functional theory, we have obtained important molecular insights relating to the elementary steps of atop-to-atop diffusion of a water monomer. We provide theoretical evidence for an anisotropic effect of rotation-jump coupling on Pd{100}; the preference of H-down tumbling motion along the nearest-neighbor direction of the square lattice was identified. The tumbling motion is attributed to quantum-tunneling-assisted diffusion, while the next-neighbor motion on the square lattice as well as the nearest-neighbor motion on Pd{111} favors molecular-axis-guided classical hopping motion whenever possible. The physical origin of the classical diffusions is discussed in the framework of the electronic structure. Our study gives useful direction for further studies on molecular couplings in the elementary steps.     

Molecular dynamics in paramagnetic materials as studied by magic-angle spinning 2H NMR spectra

A magic-angle spinning (MAS) 2H NMR experiment was applied to study the molecular motion in paramagnetic compounds. The temperature dependences of 2H MAS NMR spectra were measured for paramagnetic [M(H2O)6][SiF6] (M=Ni2+, Mn2+, Co2+) and diamagnetic [Zn(H2O)6][SiF6]. The paramagnetic compounds exhibited an asymmetric line shape in 2H MAS NMR spectra because of the electron-nuclear dipolar coupling. The drastic changes in the shape of spinning sideband patterns and in the line width of spinning sidebands due to the 180 degrees flip of water molecules and the reorientation of [M(H2O)6]2+ about its C3 axis were observed. In the paramagnetic compounds, paramagnetic spin-spin relaxation and anisotropic g-factor result in additional linebroadening of each of the spinning sidebands. The spectral simulation of MAS 2H NMR, including the effects of paramagnetic shift and anisotropic spin-spin relaxation due to electron-nuclear dipolar coupling and anisotropic g-factor, was performed for several molecular motions. Information about molecular motions in the dynamic range of 10(2) s(-1)相似文献   

Conformational flexibility of the group B meningococcal polysaccharide in solution

To elucidate the role of secondary structure in the immune response against alpha(2-->8)-linked polysialic acid, the capsular polysaccharide of Group B meningococci, we have investigated its solution dynamics by using specific models of molecular motion and hydrodynamic modeling to interpret experimental NMR data. (13)C-[(1)H] NMR relaxation times and steady-state NOE enhancements were measured for two aqueous solutions of alpha(2-->8)-linked sialic acid polysaccharides. Each contained a unique distribution of polysaccharide chain lengths, with average lengths estimated at 40 or 400 residues. Models for rigid molecule tumbling, including two based on helical conformations proposed for the polysaccharide,(31) could not explain the NMR measurements. In general for these helices, the correlation times for their overall tumbling that best account for the NMR data correspond to polysaccharide chains between 9 and 18 residues in length, far short of the average lengths estimated for either solution. The effects of internal motions incorporated into these helices was modeled with an effective correlation time representing helix tumbling as well as internal motion. This modeling demonstrated that even with extreme amounts of internal motion, "flexible helices" of 25 residues or more still could not produce the NMR measurements. All data are consistent with internal and segmental motions dominating the nuclear magnetic relaxation of the polysaccharide and not molecular tumbling. Statistical distributions of correlation times have been found specifically for the pyranose rings, linkage groups, and methoxy groups that can account for the measured relaxation times and NOE enhancements. The distributions suggest that considerable flexibility attends the polysaccharide in solution, and the ranges of motional frequencies for the linkage groups and pyranose rings are comparable. We conclude that the Group B meningococcal polysaccharide is a random coil chain in solution, and therefore, does not have antigenic epitopes dependent upon a rigid, ordered conformation.     

Cation migration between two unequivalent sites in 4-cyano pyridine alkali metal ion pairs

The ESR spectra of 4-cyanopyridine alkali metal ion pairs were recorded at different temperatures. The different hyperfine components are selectively broadened by a dynamic process which affects the linewidths. The analysis of the linewidth parameters has been accomplished as due to a jumping process of the cation between two unequivalent positions. An ab initio calculation of the potential energy surface of the radical anion shows that the motion should take place between two positions near the two nitrogen atoms in the molecular plane. Modulation of the anisotropic interactions by the tumbling motion of the ion pair cannot explain the observed data.     

Sensitive through-space dipolar correlations between nuclei of small organic molecules by partial alignment in a deuterated liquid solvent

Through-space residual dipolar correlations in NMR spectra can be measured between nuclei of small organic molecules by partially aligning them with respect to the magnetic field in a pure deuterated liquid solvent, 4-pentyl-4'-cyanobiphenyl. A simple temperature change of this liquid phase enables spectra to be compared between samples under isotropic tumbling conditions and weakly oriented anisotropic states. This should provide access of a number of small nonpolar molecules to more sensitive through-space nuclear correlations than possible through NOE experiments, depending on the net orientation of specific nuclear pairs with respect to the magnetic field and the specific coherence transfers employed.     

Electron spin polarization of free radicals induced by electron-nuclear cross relaxation

Transient radicals ^•C (CH₃)₂X are generated in solution by laser flash photolysis, and chemically induced electron polarization (CIDEP) is investigated by time-resolved ESR spectroscopy. The sign reversal of multiplet polarization at longer times, observed for t-butyl and 2-hydroxy-2-propyl radicals is studied as a function of temperature and solvent and is explainable by efficient electron-nuclear cross relaxation due to modulation of the nonplanarity at the radical centre.     

Simulating electron spin resonance spectra of nitroxide spin labels from molecular dynamics and stochastic trajectories

Simulating electron spin resonance spectra of nitroxide spin labels from motional models is necessary for the quantitative analysis of experimental spectra. We present a framework for modeling the spin label dynamics by using trajectories such as those from molecular dynamics (MD) simulations combined with stochastic treatment of the global protein tumbling. This is achieved in the time domain after two efficient numerical integrators are developed: One for the quantal dynamics of the spins and the other for the classical rotational diffusion. For the quantal dynamics, we propagate the relevant part of the spin density matrix in Hilbert space. For the diffusional tumbling, we work with quaternions, which enables the treatment of anisotropic diffusion in a potential expanded as a sum of spherical harmonics. Time-averaging arguments are invoked to bridge the gap between the smaller time step of the MD trajectories and the larger time steps appropriate for the rotational diffusion and/or quantal spin dynamics.     

Hopping dynamics and diffusion of atoms,molecules, and ions in nanoporous solids by exchange NMR spectroscopy

Molecular diffusion in nanoporous materials can be understood as series of dynamic hopping or exchange motions of molecules between different discrete sites. Exchange NMR offers the spectral resolution to distinguish between these different sites, based on isotropic and anisotropic NMR interactions that manifest differences in the local chemical or structural environments of molecules at different sites or their local orientations. Such interactions facilitate the observation of distinct adsorption environments and provide insights on the number and distributions of distinct types of environments and the geometries and motional correlation times of local hopping events between different sites. The temporal range accessible by exchange NMR is governed by the time required for the observation of the NMR signal (<?1 ms) and the return of the nuclear magnetic polarization to thermal equilibrium (typically several seconds). Over such timescales, this permits slow molecular exchange processes between local environments to be probed in great quantitative detail. The resulting insights on dynamic exchange or hopping of atoms, molecules, or ions in nanoporous solids provide a basis for understanding processes that occur over longer length and time scale, which ultimately account for their macroscopic diffusion properties.

     

Background-Free Detection of Spin-Exchange Dynamics at Ultra-Low Magnetic Field

Ultra-low field nuclear magnetic resonance spectroscopy (NMR) and imaging (MRI) inherently suffer from a low signal-to-noise ratio due to the small thermal polarization of nuclear spins. Transfer of polarization from a pre-polarized spin system to a thermally polarized spin system via the Spin Polarization Induced Nuclear Overhauser Effect (SPINOE) could potentially be used to overcome this limitation. SPINOE is particularly advantageous at ultra-low magnetic field, where the transferred polarization can be several orders of magnitude higher than thermal polarization. Here we demonstrate direct detection of polarization transfer from highly polarized ¹²⁹Xe gas spins to ¹H spins in solution via SPINOE. At ultra-low field, where thermal nuclear spin polarization is close to background noise levels and where different nuclei can be simultaneously detected in a single spectrum, the dynamics of the polarization transfer can be observed in real time. We show that by simply bubbling hyperpolarized ¹²⁹Xe into solution, we can enhance ¹H polarization levels by a factor of up to 151-fold. While our protocol leads to lower enhancements than those previously reported under extreme Xe gas pressures, the methodology is easily repeatable and allows for on-demand enhanced spectroscopy. SPINOE at ultra-low magnetic field could also be employed to study ¹²⁹Xe interactions in solutions.     

Oxygen accessibility to ribonuclease a: quantitative interpretation of nuclear spin relaxation induced by a freely diffusing paramagnet

The nuclear spin relaxation induced by a freely diffusing paramagnetic center provides a direct measure of intermolecular accessibility. A number of factors are involved in a quantitative interpretation of relaxation data including excluded volume effects, solvation differences, and the details of the electron spin relaxation in the paramagnetic center. In the case where the electron relaxation time is short compared with correlation times describing the electron-nuclear coupling, the nuclear spin relaxation rates may be related to the effective local concentration of the paramagnetic center at different locations about the solute of interest. The local concentrations may in turn be related to differences in the local free energies of interaction between the diffusing paramagnet and the cosolute. We demonstrate this approach for the case of ribonuclease A and deduce surface free energy differences for a large number of protein proton sites. We find that the oxygen accessibility is poorly represented by hard-sphere models such as computed solvent or steric accessibility. There is a distribution of local intermolecular interactions with a width of the order of RT that dominates the report of the intermolecular exploration of the protein by this simple solute.     

Ligand protons in a frozen solution of copper histidine relax via a T1e -driven three-spin mechanism

Davies electron-nuclear double resonance spectra can exhibit strong asymmetries for long mixing times, short repetition times, and large thermal polarizations. These asymmetries can be used to determine nuclear relaxation rates in paramagnetic systems. Measurements of frozen solutions of copper(L-histidine)(2) reveal a strong field dependence of the relaxation rates of the protons in the histidine ligand, increasing from low (g( parallel)) to high (g( perpendicular)) field. It is shown that this can be attributed to a concentration-dependent T(1e)-driven relaxation process involving strongly mixed states of three spins: the histidine proton, the Cu(II) electron spin of the same complex, and another distant electron spin with a resonance frequency differing from the spectrometer frequency approximately by the proton Larmor frequency. The protons relax more efficiently in the g( perpendicular) region, since the number of distant electrons able to participate in this relaxation mechanism is higher than in the g( parallel) region. Analytical expressions for the associated nuclear polarization decay rate Tau(een) (-1) are developed and Monte Carlo simulations are carried out, reproducing both the field and the concentration dependences of the nuclear relaxation.     

Electron-nuclear correlations for photo-induced dynamics in molecular dimers

Ultrafast photoinduced dynamics of electronic excitation in molecular dimers is drastically affected by the dynamic reorganization of inter- and intra- molecular nuclear configuration modeled by a quantized nuclear degree of freedom. The dynamics of the electronic population and nuclear coherence is analyzed by solving the chain of coupled differential equations for population inversion, electron-vibrational correlation, etc. Intriguing results are obtained in the approximation of a small change of the nuclear equilibrium upon photoexcitation. In the limiting case of resonance between the electronic energy gap and the frequency of the nuclear mode these results are justified by comparison to the exactly solvable Jaynes-Cummings model. It is found that the photoinduced processes in the model dimer are arranged according to their time scales: (i) Fast scale of nuclear motion, (ii) intermediate scale of dynamical redistribution of electronic population between excited states as well as growth and dynamics of electron-nuclear correlation, (iii) slow scale of electronic population approach to the quasi-equilibrium distribution, decay of electron-nuclear correlation, and decrease of the amplitude of mean coordinate oscillation. The latter processes are accompanied by a noticeable growth of the nuclear coordinate dispersion associated with the overall nuclear wave packet width. The demonstrated quantum relaxation features of the photoinduced vibronic dynamics in molecular dimers are obtained by a simple method, applicable to systems with many degrees of freedom.     

Isotropic and anisotropic Raman scattering study in liquid p-dioxane

The isotropic and anisotropic profiles of the 835 and 2965 cm⁻¹ Raman lines of p-dioxane in the neat liquid and in solution have been studied as a function of temperature and concentration. From the correlation functions obtained by Fourier inversion of band contours, the possible interaction responsible for the vibrational dephasing of the oscillators and their reorientational relaxation are considered. It is shown that the p-dioxane molecule tumbling about the C₂ axis in the molecular plane perpendicular to the oxygen-oxygen direction proceeds by small-step Brownian diffusion associated with an Arrhenius activation energy of 9.0 kJ mol⁻¹. The vibrational relaxation mechanism of the two modes is interpreted in terms of pure dephasing due to weak collisions.     

甲基氰乙基纤维素／二氯乙酸液晶溶液的红外研究

本文用FTIR研究了甲基氰乙基纤维素/二氯乙酸溶液相转变过程中分子间相互作用的变化。发现溶液中分子之间的相互作用随溶液结构的改变而变化。随着浓度的增大,溶液由各向同性态向各向异性态转变,分子产生有序排列,导致某些氢键被增强或削弱,使基团的红外吸收谱带发生位移。当加热液晶溶液时,各向异性态向各向同性态转变,也使基团的红外吸收谱带位移。