A MODEL FOR DIFFUSIVE TRANSPORT THROUGH A SPHERICAL INTERFACE PROBED BY PULSED-FIELD GRADIENT NMR

In biological systems, because of higher intracellular viscosity and/o r the restriction of the diffusion space inside cells, the (apparent) diffusion coefficient of an intracellular species (e.g., water) is gen erally smaller than when it is in the extracellular medium. This diffe rence affects the spin-echo signal attenuation in the pulsed field gra dient NMR experiment and thus affords a means of separating the intrac ellular from the extracellular species, thereby providing a basis for studying transmembrane transport. Such experiments have commonly been analyzed using the macroscopic model of Karger (see Adv. Magn. Reson. 21:1-89(1998)). In our previous study, we considered a microscopic mod el of diffusive transport through a spherical interface using the shor t gradient pulse approximation (J. Magn. Reson. A114:39-46 (1995)). Th e spins in the external medium were modeled with the ''partially absor bing wall'' condition or as having a small but finite lifetime. In the present paper, we extend our treatment to the case in which there is no limitation upon the lifetime in either medium. We also consider a s imple modification of Karger's model that more properly accounts for t he restricted intracellular diffusion. Importantly, it was found that the exact solution within the short gradient pulse approximation devel oped here and the modified Karger model are in close agreement in the (experimentally revealed) long-time limit. The results of this study s how that when there is no limitation upon the lifetime of the transpor ted species in either phase, the spin-echo attenuation curve is very s ensitive to transport.