Hydrogen bonding lifetime dynamics

This bimodal dynamics of hydration water is intriguing. A model based on dynamic equilibrium between quasi-bound and free water molecules on the surface of biomolecules (or self-assembly) predicts that the orientational relaxation at a macromolecular surface should indeed be biexponential, with a fast time component (few ps) nearly equal to that of the free water while the long time component is equal to the inverse of the rate of bound to free transition [4], In order to gain an in depth understanding of hydration dynamics, we have carried out detailed atomistic molecular dynamics (MD) simulation studies of water dynamics at the surface of an anionic micelle of cesium perfluorooctanoate (CsPFO), a cationic micelle of cetyl trimethy-lainmonium bromide (CTAB), and also at the surface of a small protein (enterotoxin) using classical, non-polarizable force fields. In particular we have studied the hydrogen bond lifetime dynamics, rotational and dielectric relaxation, translational diffusion and vibrational dynamics of the surface water molecules. In this article we discuss the water dynamics at the surface of CsPFO and of enterotoxin. 

APPENDIX 3.B QUANTIFICATION OF HYDROGEN-BOND LIFETIME DYNAMICS... 

However—and this is the third aspect—the characteristic lifetime of a hydrogen bond is very short (between 10 and 10 s) and this is why viscoelastic properties of a gel structure will never be observed even in short characteristic time experiments. The explanation of such a short time is that hydrogen bond lifetimes are determined by the proton dynamics [3,8]. In particular, large-amplitude librational movements take easily the proton from the region, between two oxygens, where the energy of the bond is sufficiently large. 

Hydrogen-bond lifetime analysis revealed that HBs between the polar head groups of the micelle and the water molecules are much stronger than those between two water molecules in bulk water and thus exhibit much slower dynamics - almost 13 times slower than that of bulk water. This result indicates the presence of quasibound water molecules on the surface. 

The dynamic nature of hydrogen bonding and the effects of temperature and density on the persistence of H-bonds in supercritical water were recently studied in MD simulations by Mountain (1995) using the ST2 and RPOL intermolecular potentials, and by Mizan et al. (1996) using the SPC potential and its flexible version. These authors use two different approaches to the estimation of the hydrogen bonding lifetime, but both of... 

The information presented in Sections 5.1.1.1-5.1.1.4 and Table 5.1, although construed to pertain to the effects of ions on the structure of the solvent, in the sense of whether it is enhanced or loosened by the presence of ions, actually reflects the effects on the dynamics of the solvent in the immediate neighborhood of the ions. The mean residence times of water molecules in the vicinity of ions are indirectly measures of the effect of the ions on the structure of the water as described in Section 5.2.1. There are aspects of solvent dynamics that are not covered by these effects, such as the orientational relaxation rate and hydrogen-bond lifetimes. Two experimental methods have mainly been employed for obtaining such information ultrafast mid-infrared and dielectric relaxation spectroscopy on the fs to ps time scales. Some slower processes were studied by NMR relaxation studies. Computer simulations added additional information, since it could be applied to individual ions rather than salts. As for the ion effects dealt with in the previous sections, the vast majority of the studies dealt with ions in aqueous solutions and only few ones considered ions in nonaqueous solvents... 

These molecular capsules self-assemble through a range of forces such as metal-ligand interactions, hydrophobic interactions, hydrogen bonding, and dynamic covalent chemistry (Figure 8.2). Their lifetimes vary from milliseconds to days. 

The values of rc of the solvation shells are surprisingly long in comparison to the value of rc of 500 100 fs of the O-H- -O hydrogen bond in bulk liquid water, but are quite comparable to the recently calculated residence time of 18 ps of water in the solvation shell of Br- [10]. However, one should be very careful with this comparison since the characteristic time of the fluctuations of the hydrogen bond is not the same as the residence time in the solvation shell because the breaking of the hydrogen bond does not automatically mean that the water molecule really leaves the shell. The narrow width and long rc of the O-H- Y absorption component imply that the first solvation shell forms a stable and well-defined structure. The solvation shells of F and of the cations likely show similar dynamics, but unfortunately these dynamics could not be measured because the O-H stretch vibrational lifetime of the water molecules in these solvation shells is comparable to that of bulk HDO D20. 

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