ANALYSIS OF ROTATIONAL RESONANCE MAGNETIZATION EXCHANGE CURVES FROM CRYSTALLINE PEPTIDES

Rotational resonance is a solid-state NMR method for restoring homonuc lear dipolar couplings in magic angle spinning (MAS) experiments. Meas urements of dipolar couplings can be used to determine internuclear di stances which in turn provide direct constraints on molecular structur e. The dipolar coupling between two spins is estimated from the observ ed intensity changes in a magnetization exchange experiment carried ou t while spinning the sample at a rotational resonance condition, i.e. when Delta = n omega(r), where Delta is the difference in chemical shi ft of two dipole-coupled spins, omega(r) is the rotational frequency i n the MAS experiment, and n is a small integer corresponding to the ro tational resonance order. Rotational resonance NMR data and calculatio ns are presented for a crystalline 11-residue peptide incorporating pa irs of C-13 labels separated by 3.7, 4.5, 4.8, 5.1, and 6.8 Angstrom. We discuss the critical parameters in generating and interpreting the magnetization exchange curves that are used to relate the observed int ensity changes in the rotational resonance spectra to dipolar coupling s. The accuracy of internuclear distance estimates from these experime nts depends on the precision of the measurements as well as correctly accounting for natural abundance background signals, the effects of pr oton B-1 field strengths, and inhomogeneous broadening of the dipole-c oupled NMR resonances. For the crystalline 11-residue peptide, the pre cision and accuracy of the RR distance measurements are on the order o f 0.1 and 0.2-0.3 Angstrom, respectively. On the basis of these studie s, we outline approaches for determining internuclear distances in bot h crystalline and non-crystalline solid-state samples.