Corrections for off-resonance effects and incomplete saturation in conventional (two-site) saturation-transfer kinetic measurements

Abstract

The effects of off-resonance irradiation and incomplete saturation on saturation-transfer measurements of chemical-exchange rates are discussed. With off-resonance effects there is no exact formula for the exchange rate constant from spin D to spin E, k(DE), in terms of observable signal intensities and relaxation rates. However, k(DE) can be estimated by measuring the effective spin-lattice relaxation rate constant of spin D when spin E is saturated, *R1, plus signal intensities with no RF irradiation, M(OD) and M(OE); with irradiation of a control position, M'(D) and M'(E); and with saturation of spin E, *M(D) and *M(E). Several formulas are compared and the best formula for calculating k(DE) appears to be either k(DE) = *R1 [(M'(D)- *M(D))/M(OD)]/[(M'(E) - *M(E))/M(OE)], or the same formula with M(OD) and M(OE) replaced by M'(D) and M'(E). These formulas are exact with incomplete saturation and no off-resonance effects, and are better than previously published formulas when off-resonance effects are present. More accurate formulas are available if signal intensities and relaxation rates can be measured while the exchange process is stopped. (C) 2000 Wiley-Liss, Inc.