The interpretation of multi-exponential water proton transverse relaxation in the human and porcine eye lens

We report results of 1H NMR transverse relaxation experiments on human and porcine eye lenses. Several authors have reported that transverse relaxation is not mono-exponential when observed by the Carr-Purcell-Meiboom-Gill (CPMG) sequence and have interpreted the results by postulating the presence of "pools" of water molecules in different binding environments that do not exchange rapidly on the NMR timescale. We have compared CPMG data for intact lenses with results for lens homogenates and have combined a CPMG spectroscopic pulse train with NMR micro-imaging to study the nature of the transverse relaxation process in human and porcine lenses. Fast exchange of water protons with the lens proteins (crystallins) leads to an enhanced transverse relaxation rate that varies linearly with protein concentration. At the resolution of NMR micro-imaging the transverse relaxation process is mono-exponential. The results show that the multi-exponential CPMG data observed spectroscopically for whole lenses reflect spatial variations in crystallin content through the lens rather than the presence of distinct "bound" and "free" water pools.