EFFECT OF DIPOLAR CROSS-CORRELATION ON MODEL-FREE MOTIONAL PARAMETERSOBTAINED FROM C-13 RELAXATION IN AX(2) SYSTEMS

The importance of dipolar cross correlation in C-13 relaxation studies of molecular motion in AX(2) spin systems (A = C-13, X = H-1) was exa mined. Several different models for the internal motion, including two restricted-diffusion, and two-site jump models, the Kinosita model [K . Kinosita, Jr., S. Kawato, and A. Ikegami, Biophys. J. 20, 289 (1977) ], and an axially symmetric model, were applied through the Lipari and Szabo [J. Am. Chem. Soc. 104, 4546 (1982)] formalism to calculate err ors in C-13 T-1, obtained from inversion-recovery measurements under p roton saturation, and NOE when dipolar cross correlation is neglected. Motional parameters in the Lipari and Szabo formalism, tau(m), S-2, a nd tau(e), were then determined from T-1 and NOE (including the errors ) and compared with parameters initially used to simulate the relaxati on data. The resulting differences in the motional parameters, while m odel dependent, were generally small for plausible motions. At larger S-2 values (greater than or equal to 0.6), the errors in both tau(m) a nd S-2 were <5%. Errors in tau(e) increased with S-2 but were usually less than 10%. Larger errors in the parameters were found for an axial ly symmetric model, but with tau(m) fixed even those were >5% only for the tau(m) = 1 ns, tau(e) = 10 ps case. Furthermore, it was observed that deviations in a given motional parameter were mostly of the same sign, which allows bounds to be set on experimentally derived paramete rs. Relaxation data for the peptide melittin synthesized with gly enri ched with C-13 at the backbone cu position and with lys enriched with C-13 in the side chain were examined in light of the results of the si mulations. All in all, it appears that neglect of dipolar cross correl ation in C-13 T-1 (With proton saturation) and NOE measurements in AX( 2) systems does not lead to major problems in interpretation of the re sults in terms of molecular motion. (C) 1995 Academic Press, Inc.