Differential Ordering of the Protein Backbone and Side Chains during Protein Folding Revealed by Site-Specific Recombinant Infrared Probes

The time scale for ordering of the polypeptide backbone relative to the side chains is a critical issue in protein folding. The interplay between ordering of the backbone and ordering of the side chains is particularly important for the formation of beta-sheet structures, as the polypeptide chain searches for the native stabilizing cross-strand interactions. We have studied these issues in the N-terminal domain of protein L9 (NTL9), a model protein with mixed alpha/beta structure. We have developed a general approach for introducing site-specific IR probes for the side chains (azide) and backbone ((13)C=(18)O) using recombinant protein expression. Temperature-jump time-resolved IR spectroscopy combined with site-specific labeling enables independent measurement of the respective backbone and side-chain dynamics with single residue resolution. We have found that side-chain ordering in a key region of the beta-sheet structure occurs on a slower time scale than ordering of the backbone during the folding of NTL9, likely as a result of the transient formation of non-native side-chain interactions.