ASSESSMENT OF PROTEIN SOLUTION VERSUS CRYSTAL-STRUCTURE DETERMINATIONUSING SPIN-DIFFUSION-SUPPRESSED NOE AND HETERONUCLEAR RELAXATION DATA

A spin-diffusion-suppressed NOE buildup series has been measured for E . coil thioredoxin. The extensive C-13 and N-15 relaxation data previo usly reported for this protein allow for direct interpretation of dyna mical contributions to the H-1-H-1 cross-relaxation rates for a large proportion of the NOE cross peaks. Estimates of the average accuracy f or these derived NOE distances are bounded by 4% and 10%, based on a c omparison to the corresponding X-ray distances. An independent fluctua tion model is proposed for prediction of the dynamical corrections to H-1-H-1 cross-relaxation rates, based solely on experimental structura l and heteronuclear relaxation data. This analysis is aided by the dem onstration that heteronuclear order parameters greater than 0.6 depend only on the variance of the H-X bond orientation, independent of the motional model in either one- or two-dimensional diffusion (i.e., 1 - S-2 = 3/4 sin(2) 2 theta(sigma)). The combination of spin-diffusion-su ppressed NOE data and analysis of dynamical corrections to H-1-H-1 cro ss-relaxation rates based on heteronuclear relaxation data has allowed for a detailed interpretation of various discrepancies between the re ported solution and crystal structures.