Side-chains in native and random coil protein conformations. Analysis of NMR coupling constants and chi1 torsion angle preferences.

The behaviour of amino acid side-chains in proteins in solution has been characterised by analysing NMR 3JHalphaH beta coupling constants and crystallographic chi1 torsion angles. Side-chains both in the core of native folded proteins and in situations where there is an absence of close packing including the random coil state have been considered. An analysis of experimental 3JHalphaH beta coupling constant data for ten proteins shows that in the core of native proteins a very close similarity is observed between the chi1 conformations adopted in solution and in crystals. There is clear evidence, however, for significant motional averaging about the chi1 torsion angles in solution. Using a model of a Gaussian distribution about the average torsion angles the extent of these fluctuations has been quantified; the standard deviation for the motion is 26 degrees, the fluctuations about chi1 in the protein core being similar in size to those found for main-chain phi torsion angles in solution. From the distribution of chi1 torsion angles in a data base of protein crystal structures, torsion angle populations and coupling constants have been predicted for a random coil polypeptide. Significant variations in the chi1 distributions for different amino acids give differences in the predicted coupling constants; for 3JHalphaH 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 excellent agreement with the predictions, indicating that the overall chi1 distributions in protein crystals reflect the local preferences of the amino acids. Predictions from the protein data base therefore provide an important framework for interpreting experimental data for non-native protein conformations and for residues on the surface of folded proteins.