DETECTION OF NITROGEN-NITROGEN J-COUPLINGS IN PROTEINS

Methods are described which allow the measurement of geminal and vicin al N-15-N-15 coupling constants in isotopically labeled proteins. The experiments rely on a quantitative J-correlation strategy and include either N,N-COSY or N,N-TOCSY magnetization transfer steps. The pulse s equences are demonstrated with a 16-kDa protein. Although the (3)J(NN) couplings are almost vanishingly small, they give rise to visible cro ss peaks for 56% of the amino acid residues. A dependence of the vicin al coupling constants on the backbone torsion angle psi is observed. H owever, only in favorable cases can the (3)J values be translated into local structural information. In arginine side chains, correlations b etween epsilon- and eta-nitrogens via two-bond scalar couplings are ob tained that can be exploited for the resonance assignment of the guani dino groups. (C) 1998 Academic Press.