ANALYSIS OF CHI-1 ROTAMER POPULATIONS FROM NMR DATA BY THE CUPID METHOD

The theoretical foundations of CUPID, a new method for determining the rotamer probability distributions from NMR spin-spin coupling constants and NOE data, has been presented (Dzakula, Z.; Westler, W. M.; Edison, A. S.; Markley, J. L. J. Am. Chem. Soc., accompanying paper in this issue). Here we apply the method to find thc distribution of rotamers about the chi-1 angle in L-leucine (cation and anion) on the basis of six measured coupling constants [Fischman, A. J.; Wyssbrod, H. R.; Agosta, W. C.; Cowburn, D. J. Am. Chem. Soc. 1978, 100, 54-58] between various pairs of spins across the C(alpha)-C(beta) bond. We also test the ability of the method to reproduce simulated rotamer probability distributions from calculated spin-spin coupling and proton-proton cross-relaxation parameters with and without simulated errors. In all cases CUPID reproduced error-free distributions to the precision of the numerical procedures used. Data with simulated errors were reproduced quite well, indicating that given sufficient quality and quantity of experimental data, CUPID is able to construct accurate rotamer probabilities from experimental data. Guidelines for CUPID's implementation and its accuracy as a function of experimental errors are presented.