MINIMIZING SENSITIVITY LOSSES IN GRADIENT-SELECTED N-15-H-1 HSQC SPECTRA OF PROTEINS

It is shown that in certain circumstances the sensitivity of nitrogen-15-proton gradient-selected HSQC spectra of proteins dissolved in H2O is compromised due to exchange between the NH and the water protons. The loss in sensitivity arises because, at the start of the relaxation delay, the magnetization from the water is dephased. Exchange of magnetization between the saturated water protons and the NH protons slows down the recovery of the NH equilibrium magnetization. Thus, in a typical relaxation delay, the magnetization from the NH protons has only partly recovered. It is shown that it is possible to ensure that the water magnetization is at equilibrium at the start of the relaxation delay by making simple modifications to the conventional pulse sequences. Such changes result in a significant increase in the signals observed from the NH protons, as their return to equilibrium is no longer retarded by exchange with the water; indeed the rate of return to equilibrium is enhanced. (C) 1994 Academic Press, Inc.