Theoretical Study On The Microscopic Mechanism Of Molecular Dynamics Of Complex Aqueous System

Among the different species of non-freezing interfacial water,the water on the protein surface is probably the most concerned.In the hydrated state,the protein exhibits a kinetic behavior transition from a rigid,resonant state to a flexible,non-resonant state at around 200-260 K.This transition is associated with the activation of the function of various proteins,such as myoglobin and ribonuclease,which exhibit an active state above the kinetic transition temperature and are able to maintain their function.We found that the conversion of water-water hydrogen bonds on the protein surface is a rather surface-independent process.By further demonstration it is known that hydrated water drives the rapid relaxation of the underlying material by transferring its intrinsic energy barrier to the latter.But fast relaxation exhibits very different rates,depending on the structure of the material and its interactions.For DMSO-H₂O Binary mixtures,the microstructure deviated significantly from the ideal behavior and microscopic non-homogeneity was observed by different experimental methods.Nonhomogeneity has high practical applications,such as regulating the kinetics of chemical reactions,using as cryoprotectants and affecting the structure and function of proteins.We combined ultrafast two-dimensional infrared spectroscopy and molecular dynamics simulations to study and compare the kinetic behavior of two probes（SCN^-and CH₃SCN）with widely different hydrophobicity in mixtures.The two probes showed significant differences in IR spectrograms,vibrational relaxation kinetics and rotational kinetics.In the low concentration region(0<X_DMSO<0.35),the two probes have essentially the same rotation behavior.In the medium concentration region,(0.35<X_DMSO<0.7),due to the accumulation of more dimethyl sulfoxide around the methylthiocyanide,the dimethyl sulfoxide is weakly coupled to the probe rotation and the methylthiocyanide rotation relaxation time is faster than that of the hydrophilic thiocyanide.In the high concentration region(0.7<X_DMSO<1),the two probes also showed significantly different rotational behavior,with little change in the relaxation time of hydrophobic methylthiocyanine rotation due to the different microscopic environments in which the two probes are located,with DMSO-DMSO appearing to aggregate and a plateau around the CH₃SCN probe with the presence of highly stable DMSO aggregates.In contrast,the relaxation time of thiocyanate ion rotation decreases significantly with increasing concentration.Their different solventization kinetic behaviors are attributed to their different hydrophobicity and microscopic non-homogeneity of the mixtures.