Comparison of ROESY and EXSY Methods Using Bloch Equations

Abstract

A comparison of longitudinal and transverse frame magnetization transfer methods is provided using modified Bloch equations for a two-spin system representative of a methylene group exchanging between two chemically distinct sites. This simple formalism incorporates chemical shift, auto- and cross-relaxation terms, chemical exchange first-order rate constants and r.f. field terms for ROESY experiments. Overhauser and exchange magnetization transfer is emulated at three limits of molecular reorientation time (τc). The calculated build up curves suggest a parasitic effect of ROE on chemical exchange magnetization transfer. In addition, for molecules harboring spin systems exchanging between sites such that the exchange rate approximates the transverse cross-relaxation rate, and reorienting in solution at rates slower than that defined by the CAMELSPIN limit (ω0τc≈1.118), the calculations suggest observation of exchange or Overhauser cross peaks may be compromised. Finally, the effect of off-resonance spin locking r.f. power level is explored upon exchange or Overhauser build-up curve profiles. The results suggest off-resonance approaches that at low r.f. spin locking power may adversely affect ROE or exchange build-up rates and may compromise either exchange rate determination or internuclear distance estimation.