Measurement of (15)N- (1)H coupling constants in uniformly (15)N-labeled proteins: Application to the photoactive...

A modified HNHB experiment is presented that allows thedetermination of J(NH) coupling constants directly from the ratio ofcross-peak to diagonal-peak intensities. The experiment was applied to thephotoactive yellow protein (PYP) and yielded the magnitude of 117(3)J(NH(beta)) coupling constants. In addition, 29(3)J(NH(alpha(i-1))) coupling constantscould be measured, providing information about the backbone angle psi.These data, in conjunction with the magnitudes of the(3)J(H(N)H(alpha)) coupling constantsobtained from the HNHA spectrum, effectively discriminate the twopossibilities for the stereospecific assignment of theH(alpha) resonances in glycine residues. For all eight glycineresidues in PYP that were not subject to conformational averaging and hadnon-degenerate H(alpha) resonance frequencies, the J-couplingdata, together with limited NOE data, yielded the stereospecific assignmentof the H(alpha) resonances for these residues. In addition,reliable and precise phi,psi dihedral constraints were also derived forthese residues from the J-coupling data.