Water molecules in hydrophobic regions are suspected to connect segments of the proton transport pathway in bR. In earlier 2D solid-state proton NMR studies an exchange between hydrophilic (chemical shifts downfield from 4.8 ppm) and hydrophobic regions (chemical shifts upfield from 4.8 ppm) was detected, suggesting the presence of water molecules in hydrophobic pockets. In addition, in solid state deuterium NMR spectra we observed a signal with anomalous motional averaging at low temperatures which we suspected was due to non-hydrogen bonded water. To verify this interpretation, we studied D2O in polycarbonate and organic solvents, which also exhibit 1H water resonances upfield from 4.8 ppm. The quadrupole echo sequence was utilized for obtaining the deuterium lineshapes at temperatures between -90~C to -100~C. We found that water which could easily be dissolved in the organic phase of solvents such as dimethyl sulfoxide, acetonitrile, pyridine and 1,4-dioxane, showed the characteristic behavior of ice (i.e. reorientation with tetrahedral jumps) at low temperatures. In cases when only minimal amounts of water could be incorporated, i.e. polycarbonate and benzene, an extremely weak signal at the isotropic peak position could be observed at short acquisition times. In the intermediate case, when small amounts of water were dissolved in the organic phase of chloroform and nitromethane, a strong narrow signal (a very high extent of motional averaging) and a low intensity broad ice-like feature were detected. Thus, we conclude that the lower the polarity of the solvent (as estimated by the solubility of the water) the more motional averaging is found at low temperature. Our future work includes studies of water in solvents similar to chloroform and nitromethane and a detailed study of the quadrupole echo lineshapes of D2O in chloroform in a very broad temperature range.
Showing the most recent 10 out of 12 publications
