Improved accuracy in measuring one-bond and two-bond 15N,13Ca coupling constants in proteins by double-inphase/antiphase (DIPAP) spectroscopy

An extension to HN(CO-a/b-N,Ca-J)-TROSY (Permi and Annila in J Biomol NMR 16:221–227, 2000) is proposed that permits the simultaneous determination of the four coupling constants 1JN'(i)Ca(i), 2JHN(i)Ca(i), 2JCa(i-1)N'(i), and 3JCa(i-1)HN(i) in 15N,13C-labeled proteins. Contrasting the original scheme, in which two separate subspectra exhibit the 2JCaN' coupling as inphase and antiphase splitting (IPAP), we here record four subspectra that exhibit all combinations of inphase and antiphase splittings possible with respect to both 2JCaN' and 1JN'Ca (DIPAP). Complementary sign patterns in the different spectrum constituents overdetermine the coupling constants which can thus be extracted at higher accuracy than is possible with the original experiment. Fully exploiting data redundance, simultaneous 2D lineshape fitting of the E.COSY multiplet tilts in all four subspectra provides all coupling constants at ultimate precision. Cross-correlation and differential-relaxation effects were taken into account in the evaluation procedure. By applying a four-point Fourier transform, the set of spectra is reversibly interconverted between DIPAP and spin-state representations. Methods are exemplified using proteins of various size.