Tilt angle dependence of cross-relaxation in off-resonance ROESY

We present an efficient experimental method to evaluate whether the effecti ve cross-relaxation rate between a pair of spins vanishes when applying an off-resonance spin-lock field. It is shown that the cross-relaxation rate c an be made to vanish even when the two spins concerned resonate at differen t offsets and experience significantly different tilt angles of their respe ctive spin-lock fields. This is verified experimentally using a sample of N -15-labeled human ubiquitin, through selective excitation of chosen amide p rotons. The results are relevant for the quantitative interpretation of off -resonance ROESY experiments. (C) 1999 Academic Press.