Water hydrogen bond lifetime

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. 

Fig. 3 Hydrogen bond lifetime correlation function for bound water molecules in the CsPFO micellar solution. Inset The same for bulk water. Note the much slower decay of the bound water species.

Thus, Shb(i) decays as soon as the bond breaks for the first time while Chb(1) allows bond breaking at intermediate times. In Fig. 3, we show the water-surfactant hydrogen bond lifetime correlation functions, SnB(t) and ChbM- The decay of SnB(t) for the bound species is much slower than the corresponding decay for the water-water hydrogen bond in pure water [8]. 

It is well known [54,270] that the macroscopic dielectric relaxation time of bulk water (8.27 ps at 25°C) is about 10 times greater than the microscopic relaxation time of a single water molecule, which is about one hydrogen bond lifetime [206,272-274] (about 0.7 ps). This fact follows from the associative structure of bulk water where the macroscopic relaxation time reflects the cooperative relaxation process in a cluster of water molecules. 

The hydrogen bond lifetime tjq, a fundamental parameter in our picture for water, has also been considered by other authors, some of whonf suggested the use of depolarized Rayleigh scattering eiqiaiments to measure this quantity. The observed frequency spectrum consists of two distinct Lorentzians, the linewidth of the broader one being interpreted by these authors as the inverse of Tiq. 

The values of the hydrogen bond lifetime x, for confined water are close to those of bulk water [69] they have an Arrhenius temperature dependence (Figure 7b), whereas the residence time Xq does not exhibit such a behavior (Figure 7a). 

Figure 7 (a) Arrhenius plot of the residence time Xq for different levels of hydration of water at the surface of H20-hydrated d-CPC protein (empty symbols) contained in hydrated Vycor (solid symbols) as compared with bulk water (empty circles) [49]. (b) Arrhenius plot of the hindered rotations characteristic time, x,. This time can be associated with the hydrogen bond lifetime [69]. 

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... 

The rotational retardation factor of hydration shell water came from terahertz spectroscopy applied by Ebbinghaus et al. [143], yielding hydrogen bond lifetimes, up to 2.4ps at 0.2mn distance from the protein surface, that were larger than those in bulk water, 1.6pm, and were still detected up to a distance of 0.6mn. The whereabouts of sites on the surfaces of the proteins lysozyme and staphylococcal nuclease. 

The excited-state lifetime calculated for TINS in PVA film is found to be 1.3 + 0.1 ns compared with 44 4 ps found in the case of water (H). This supports the earlier proposal that complexation, which is proposed to occur in protic, hydrogen-bonding solvents, does not occur in this polymer. In the PVP film an intense fluorescence and a long excited-state lifetime, similar to that found for TINS in PVA, is observed and is consistent with the ESIPT process being prevented in this aprotic medium. 

---