Water rotational relaxation

M. Marchi, F. Sterpone, M. Ceccarelli, Water rotational relaxation and diffusion in hydrated lysozjmie, J. Am. Chem. Soc. 124 (2002) 6787-6791. 

FigureBl.5.16 Rotational relaxation of Coumarin 314 molecules at the air/water interface. The change in the SFI signal is recorded as a fimction of the time delay between the pump and probe pulses. Anisotropy in the orientational distribution is created by linearly polarized pump radiation in two orthogonal directions in the surface. (After [90].)...

Castro A, Sitzmann E V, Zhang D and Eisenthal K B 1991 Rotational relaxation at the air-water interface by time-resolved second-harmonic generation J. Phys. Chem. 95 6752-3... 

TABLE 4 Rotational Relaxation of Octadecylrhodamine B at Toluene-Water Interface... 

A second approach with respect to anisotropic flavin (photo-)chemistry has been described by Trissl 18°) and Frehland and Trissl61). These authors anchored flavins in artificial lipid bilayers by means of C18-hydrocarbon chains at various positions of the chromophore. From fluorescence polarization analysis and model calculations they conclude, that the rotational relaxation time of the chromophore within the membrane is small compared to the fluorescence lifetime (about 2 ns74)). They further obtain the surprising result that the chromophore is localized within the water/lipid interface, with a tilt angle of about 30° (long axis of the chromophore against the normal of the membrane), irrespective of the position where the hydrocarbon chain is bound to the flavin nucleus. They estimate an upper limit of the microviscosity of the membrane of 1 Poise. 

The water proton NMRD of the pseudooctahedral Co(H20)g (reported in Fig. 13) shows almost field-independent water proton relaxation rate values in the 0.01-60 MHz region (47). Therefore, the (Os c = 1 a nd of course the co/Cc = l dispersions must occur at fields higher than 60 MHz. This provides an upper limit value for Tig equal to 4 x 10 s. Such a low Tig value is consistent with the low water proton relaxation rate values. By using the SBM theory, Tie at 298 K can be estimated to be about 10 s. It can be larger, if the presence of a probable static ZFS is taken into account (47). When measurements are performed in highly viscous ethyleneglycol the observed rates are similar to those obtained in water. This suggests that Tig is also similar and, therefore, it is rotation-independent (47). 

Water exchange reaction mechanism 332 Water NMRD in diamagnetic systems 33-9 Water protein relaxation rate 149 Wigner rotation matrices 65, 67 Wild type azurin 122... 

The value of kp obtained in this way for the phenanthreneonium ion is not far from the limit set by the rotational relaxation of water. For such fast reactions, Richard has pointed out that azide trapping could be influenced by preassociation.6 Preassociation has been well characterized in a number of nucleophilic reactions of reactive carbocations with water6 but its impact on deprotonation has not been fully clarified.5,6 In so far as preassociation... 

The frequencies of rotational transitions are much smaller than vibrational frequencies, which means that the rotational motion is slower than the vibrational one. For a free molecule, the period of rotational motion is within 10 12-10 9 s. In condensed media the rotational motion is even slower, its period being respectively greater. At this stage it is more correct to speak of the relaxation time of the molecules. The latter essentially depends on the phase state of the medium. For example, in liquid water the relaxation time of molecular dipoles in an external electric field is about 10 11 s, whereas in ice (at 0°C) it is — 1 () 5 s. 

Figure 4.1. Arrhenius dependence of different characteristics of lysozyme (1) enzyme activity (2) relative fluorescence intensity (3) water proton relaxation in the presence of the sample spin labelled by His-lS (4) partial spin capacity (5) H-D exchange, (6,7) spin labels rotation and (8) the lysozyme globule rotation. Likhtenshtein et al., 2000. Reproduced with permission.

Table 9 The self-diffusion constant for water and the ubiquitin molecule D p in aqueous solutions as calculated with different force fields for the water. In addition, i and T2 are rotational relaxation times for the ubiquitin molecule. Where possible, the results are compared with experimental values. All results are from ref. 36... 

With local information given by INM analysis in mind, we next see the character of rotational relaxation in liquid water. The most familiar way to see this, not only for numerical simulations [76-78] but for laboratory experiments, is to measure dielectric relaxation, by means of which total or individual dipole moments can be probed [79,80]. Figure 10 gives power spectra of the total dipole moment fluctuation of liquid water, together with the case of water cluster, (H20)io8- The spectral profile for liquid water is nearly fitted to the Lorentzian, which is consistent with a direct calculation of the correlation function of rotational motions. The exponential decaying behavior of dielectric relaxation was actually verified in laboratory experiments [79,80]. On the other hand, the profile for water cluster deviates from the Lorentzian function. As stated in Section III, the dynamics of finite systems may be more difficult to be understood. 

For the mixture of [bmim]Br and D20, anomalous dynamics were found utilizing diffuson NMR as well as lH and 81Br NMR relaxation measurements [41]. The addition of water here also was found to lead to a network of hydrogen bonding and very dilute samples behaved in a special manner (whatever that may mean). The strength of solvation for D20 and C6D6 mixtures with [bmim]Cl and -PF6 was investigated via the rotational correlation time (cf. Sect. 2.2) [42]. Water rotated two times slower in the Cl-IL as compared with the PF6-IL while no difference in... 

Here we proposed a physically based approximation to get around this problem. We separate the coordinate set X into two domains X " and where slow and fast are with respect to the rate of approaching equilibrium. For example, we argue that the translation and rotation degrees of freedom of a bulk water molecule relax to equilibrium more rapidly than the protein dihedral angles of an amino acid i. 

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