Polypeptide conformational space. Dynamics by solution NMR disorder by X-ray crystallography.

A symmetric dimer of a polypeptide in an organic solvent is shown to be a useful molecular system for observing the effects of crystal packing and solution dynamics on structure determination by X-ray crystallography and solution NMR methods. Organic solvents are not ideal models of a lipid environment, but they represent a better model environment than does water, in fact, for modeling some properties of the lipid environment, organic solvents work very well. In particular, it is a good model for assessing the influence of lipids on local polypeptide dynamics and conformational rearrangements. The solution NMR structure of gramicidin in the mixed solvent of benzene and ethanol is compared to the crystal structure formed from a benzene/ethanol azeotrope. High resolution structural characterization leads to unique correlations of disorder in the crystal to dynamics in solution. Furthermore, crystal packing effects lead to significant asymmetries in both the polypeptide backbone and in the side-chains.