NMR study of role of the cross-relaxation effect in the cortex and the nucleus rabbit lens fragments

We have experimentally studied the coupling between the longitudinal magnet ization of macromolecular protons and water protons in the cortex and the n ucleus rabbit lens fragments (cross-relaxation). The NMR lines of water pro tons in the cortex and the nucleus of rabbit lens are inhomogenously broade ned and relative narrow. We have assumed that the NMR line of crystalline p rotons in the lens is homogeneously broadened, very broad (the halfwidth or der of 25 kHz) with small intensity. To detect the effect of cross-relaxati on we adopted concept of the magnetization transfer used in magnetic resona nce imaging. We irradiated the sample 20 kHz off resonance of the water pro tons with a train of low power RF pulses forming a selective saturation pul se and then after variable delay excited with a high-power 90 degrees, dete ction pulse. Finally we have measured the sample NMR line intensity as a fu nction of duration of selective pulse for several fixed delays between sele ctive and detection pulses. We have found the significant difference betwee n time constants of decays for the cortex and the nucleus. We attributed th is difference to longer time of hydration water molecules in the cortex spe nd within proximity of the protein surface than in the nucleus. We have int erpreted these finding as an effect of chemical protons exchange between ma cromolecular and water protons rather then cross-relaxation effects, and it is discussed in terms of the Levy walk mechanism. (C) 1999 Elsevier Scienc e B.V. All rights reserved.