Sodium exchange between two sites. The binding of sodium to halotolerant bacteria.

The longitudinal and transverse relaxation curves of sodium undergoing exchange between two sites are presented. When the two sites are 'bound' and 'free' sodium respectively, the relaxation curves are, in general, not exponential. It is shown that in some cases only one exponential decay could be detected experimentally though the true decay curve is much more complicated. In such cases where the population of 'free' and 'bound' sodium are equal, only 40-70% of the total intensity would be detected, depending on the lifetime of sodium in the two sites. It is also shown that the fast exchange approximation, usually applied in the interpretation of sodium relaxation curves, might lead to wrong conclusions. Measurements of sodium relaxation times in halotolerant bacteria show that T1 and T2 are different and frequency-dependent. The intensity of the sodium sugnal is 40% of the total sodium concentration. It was possible to simulate the relaxation behaviour and intensity measurements by applying the following model. There are three types of sodium, the extracellular sodium (A) which exchanges with part of the intracellular sodium (B) and a fraction (C) which is bound but does not exchange with the extracellular sodium. It was possible to estimate the physical properties of sodium at site B. The quadrupole coupling constant (e2qQ/H)B = 9.10(6) rad/s, the correlation time taucB = 5.5.10(-7) s and the lifetime of sodium at site B, tauB = 6.10(-4) s.