Water NMRD in diamagnetic systems

The relatively low number of available nuclear species may entail some ambiguity on studies of water by spin relaxation. Those spins are due to  

As the essentials of this book are devoted to paramagnetic systems, we shall, for the sake of completeness, conclude this introductory chapter by a related discussion about diamagnetic systems and more precisely about water NMRD in protein solutions. 

At ambient temperature, H, and 0 relaxation is in the extreme narrow range and dispersion curves are perfectly flat (see Fig. 9 bottom) precluding any correlation time determination. Furthermore as inter- and intramolecular contributions to proton relaxation cannot be easily separated and as the deuterium and 0 quadrupole coupling constants are not known with sufficient accuracy, there is a real problem for determining a meaningful correlation time. This problem was solved only in the early 1980s by resorting to the cross-relaxation rate which is purely intramolecular 

The proton longitudinal relaxation rate of deoxygenated water is 0.3 s at 25°C, with about 25% of this value being attributed to intermolecular dipolar relaxation. In that case, of course, no dispersion occurs. However, for water in equilibrium with air, due to paramagnetic molecular oxygen, the relaxation rate increases by 0.1 at low fields and exhibits a dispersion around 40 MHz (47). 

There exists an enormous amount of experimental results and the reader is referred to several reviews containing extensive data along with tentative interpretations (48-50). In addition to the information obtained by NOESY (see section I.D) about residue directly interacting with water by crossrelaxation (51), many conclusions have been drawn from the dispersion 

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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... 

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