Evaluation of the water environments in deoxygenated sickle cells by longitudinal and transverse water proton relaxation rates

Abstract

The longitudinal and transverse water proton relaxation rates of oxygenated and deoxygenated erythrocytes from both normal adults and individuals with sickle cell disease were measured as a function of temperature at two different frequencies. The simplest model which fits all of the data consists of three different environments for water molecules. The majority of the water (98%) has a correlation time indistinguishable from bulk water (3 × 10 −11 sec). Secondly, there is a small amount of water (1.3–1.5%) present which has a correlation time of 2–4 × 10 −9 sec and is apparently independent of the erythrocyte sample studied. Presumably this water is the hydration sphere around the hemoglobin molecules and its correlation time is significantly slower than bulk water. The third environment contains approximately 0.2% of the water present and has a correlation time≥ 10 −7 sec. This third environment is considered tightly bound to the hemoglobin because the water proton correlation time is very similar to the expected rotational correlation time for the hemoglobin molecules. The value of the transverse relaxation rate, f b ( T 2 b ) −1, for the tightly bound water fraction decreases in oxy ( S S ), deoxy ( A A ), and oxy ( A A ) erythrocyte samples as the temperature is increased as expected for a rotational correlation time process. In dramatic contrast, f b ( T 2 b ) −1 increases almost linearly as the temperature is increased over the whole 4 ° to 37 °C temperature range in samples of deoxy ( S S ) erythrocytes. The observation suggests a continual increase in the formation of deoxyhemoglobulin S polymers rather than a sudden transition from a homogeneous solution of deoxyhemoglobin S molecules to a solid gel.