Nuclear magnetic cross-relaxation spectroscopy

Abstract

Magnetic cross relaxation is studied in a coupled two-spin population system. Equations for the magnetization of one spin system in the presence of partial or total saturation of the second spin system are developed. These equations predict the dependence of the signal intensity of the observed spin population on the relaxation parameters of the second coupled spin population and the frequency and amplitude of the RF field. These equations may be used to gain a qualitative understanding of the factors affecting the Z spectrum, i.e., the spectrum acquired by observing the intensity of one resonance as a function of the frequency of a saturating preparation pulse applied selectively to the other. The contributions of the different relaxation and magnetization transfer rates are analyzed analytically and demonstrated experimentally in samples of cross-linked bovine serum albumin. The data demonstrate that the predominant mechanism for the coupling is the magnetic exchange or cross relaxation between the protein and the solvent rather than the chemical exchange of exchangeable protons on the protein.