SIDE-CHAINS IN NATIVE AND RANDOM COIL PROTEIN CONFORMATIONS - ANALYSIS OF NMR COUPLING-CONSTANTS AND CHI(1) TORSION ANGLE PREFERENCES

The behaviour of amino acid side-chains in proteins in solution has be en characterised by analysing NMR (3)J(H alpha H beta) coupling consta nts and crystallographic chi(1) torsion angles. Side-chains both in th e core of native folded proteins and in situations where there is an a bsence of close packing including the random coil state have been cons idered. An analysis of experimental (3)J(H alpha H beta) coupling cons tant data for ten proteins shows that in the core of native proteins a very close similarity is observed between the chi(1) conformations ad opted in solution and in crystals. There is clear evidence, however, f or significant motional averaging about the chi(1) torsion angles in s olution. Using a model of a Gaussian distribution about the average to rsion angles the extent of these fluctuations has been quantified; the standard deviation for the motion is 26 degrees, the fluctuations abo ut chi(1) in the protein core being similar in size to those found for main-chain phi torsion angles in solution. From the distribution of c hi(1) torsion angles in a data base of protein crystal structures, tor sion angle populations and coupling constants have been predicted for a random coil polypeptide. Significant variations in the chi(1) distri butions for different amino acids give differences in the predicted co upling constants; for (3)J(H alpha H beta), for example, values of 5.1 and 5.7 Hz are predicted for serine compared with 4.9 and 9.9 Hz for leucine. Experimental data for short unstructured peptides show an exc ellent agreement with the predictions, indicating that the overall chi (1) distributions in protein crystals reflect the local preferences of the amino acids. Predictions from the protein data base therefore pro vide an important framework for interpreting experimental data for non native protein conformations and for residues on the surface of folded proteins. (C) 1998 Academic Press.