An investigation of “bound” water in frozen erythrocytes by proton magnetic resonance spin-lattice, spin-spin, and rotating frame spin-lattice relaxation time measurements

Abstract

The water proton spin-lattice relaxation time data for frozen normal and sickle erythrocytes at temperatures between −20 and −80°C can be adequately described by a log-normal distribution of correlation times for water proton motion with a single peak centered at approximately 10 −9 sec at −35°C. However, the dependence of the rotating frame spin-lattice relaxation time ( T 1 ϱ ) on the strength of the radio-frequency field suggests that water proton motions with correlation times greater than 10 −7 sec are present in this system. The magnitude of the spin-spin relaxation time for these frozen erythrocytes (0.1 msec at −55°C) is consistent with the T 1 ϱ data, suggesting that a bimodal distribution of correlation times may be a better model for all of the low-temperature relaxation time data. We find that such a model with peaks centered at about 10 −9 sec at −35°C and about 10 −6 sec at −55°C fits all of the relaxation time data obtained at low temperatures. This bimodal distribution of correlation times for the water motion in frozen erythrocytes is discussed in terms of a model in which the peak centered at 10 −9 sec may represent a “loose” hydration shell and that centered at 10 −6 sec may represent a “tight” hydration shell, and/or water interactions with protein sidechains.