N.m.r. investigation of water structure in frozen tropomyosin samples

Abstract

Results are presented of deuteron and proton magnetic resonance investigations on frozen samples of aqueous dispersions of tropomyosin. In general, the deuteron resonance spectra are complex, showing that the D₂O molecules exist in a number of phases. In two of these, the D₂O molecules are essentially static and show spectra characteristic of ice, but the observation of two separate electric quadrupole interactions indicates the occurrence of two strengths of intermolecular hydrogen bond. A third phase produces a narrow deuteron resonance line characteristic of a liquid, and observable down to a temperature of –80°C. Its intensity has a complex temperature dependence showing a transition and hysteresis behaviour over the temperature range –20° to –40°C. Arrhenius-type plots give activation energies of 23 and 6 kcal/mol for the temperature regions above and below this transition point. Proton resonance of the non-deuterated analogue shows evidence of similar multiphase behaviour, but because of the large intensity of the narrow component the equivalent static phases have not been observed directly. A fourth phase observed by deuteron resonance produces and electric quadrupolar interaction which suggests that some D₂O molecules undergo a rapid, but anisotropic motion, or alternatively have a preferential alignment.