Superslow backbone protein dynamics as studied by 1D solid-state MAS exchange NMR spectroscopy

Superslow backbone dynamics of the protein barstar and the polypeptide poly glycine was studied by means of a solid-state MAS 1D exchange NMR method (t ime-reverse ODESSA) that can detect reorientation of nuclei carrying anisot ropic chemical shift tensors. Experiments were performed on carbonyl C-13 i n polyglycine (natural abundance) and backbone N-15 nuclei in uniformly N-1 5-enriched barstar within a wide range of temperatures in dry and wet powde rs for both samples. Two exchange processes were observed in the experiment s: molecular reorientation and spin diffusion. Experimental conditions that are necessary to separate these two processes are discussed on a quantitat ive level. It was revealed that the wet protein undergoes molecular motion in the millisecond range of correlation times, whereas in dry protein and p olyglycine molecular reorientations could not be detected. The correlation time of the motion in the wet barstar at room temperature is 50-100 ms; the activation energy is about 80 kJ/mol. Previously, protein motions with suc h a long correlation time could be observed only by methods detecting chemi cal exchange in solution (e.g., hydrogen exchange). The application of soli d-state MAS exchange spectroscopy provides new opportunities in studying sl ow biomolecular dynamics that is important for the biological function of p roteins. (C) 1999 Academic Press.