Attenuation of cross-peak intensities in QUIET-BIRD-NOESY experiments

The buildup curves in QUIET-BIRD-NOESY experiments, which are designed to i solate two-spin subsystems within macromolecules, are attenuated by transve rse relaxation and evolution under homonuclear couplings during the bilinea r rotation decoupling (BIRD) pulse sandwich. If the signals of both source and target spins are attenuated equally (uniform damping), this is readily accounted for by normalizing the cross peaks with respect to the diagonal p eaks. However, unequal attenuation of source and target spins (differential damping) affects the initial buildup slopes and hence leads to apparent cr oss-relaxation rates that are significantly distorted from their true value s. A simple method for recognizing this situation and extracting accurate c ross-relaxation rates is presented. (C) 1999 Academic Press.