Anomalous proton NMR relaxation behavior of cell wall materials from Chinese water chestnuts

Abstract

Proton T1 NMR relaxation of the cell wall materials from Chinese water chestnuts (CWC) is an order of magnitude more efficient than those from potatoes, even though both the major relaxation pathways and the relative proton population of the relaxation groups are similar. This anomalous behavior of CWC can be explained by the detection of a strong ESR signal, which is an order of magnitude larger than that in samples from potatoes. The estimated free radical concentration is about 60 µM. Spectral properties of the radicals (g = 2.00599, linewidth 10.75 G) are consistent with that of semiquinone radicals derived from wall-bound polyphenols in cell wall materials. Hydration led to the drastic reduction of the ESR signal; it virtually disappeared when as little as 13% water was added to CWC in the vapor phase. Redrying over phosphorus pentoxide under vacuum followed by exposure to air resulted in some recovery of the free radical signals. Paradoxically, 1H T1 relaxation of a wet CWC (21% D2O) showed sustained efficiency at T > 300 K. T1ρ relaxation was only moderately affected by the presence of free radicals and transverse relaxation showed no effects. All the relaxation behavior could be accounted for by a spin diffusion model involving low concentration of unpaired electron spins. Copyright © 2000 John Wiley & Sons, Ltd.