Understanding how proteins fold is one of the central problems in biochemistry. To investigate the mechanism of protein folding on the previously inaccessible submillisecond time scale, we have developed photochemical triggering, nanosecond laser temperature-jump, and ultrarapid mixing methods. These experiments provide the first glimpse of elementary motions in protein folding, including alpha-helix, beta- hairpin, and loop formation, as well as collapse from the random coil state. Hairpins form in a few microseconds, about 10-fold slower than helices. This result can be explained in terms of a simple model, in which hairpin formation is a continuously uphill process in free energy until favorable interactions are formed between amino acid side chains on opposite strands. Our data and analysis lead to the striking result that formation of a 16 amino acid beta-hairpin exhibits most of the basic features of protein folding, including stabilization by both hydrogen bonds and hydrophobic interactions, two-state kinetic and thermodynamic behavior, and a funnel-like, partially rugged energy landscape. We have used this beta hairpin model to calculate the rates of folding of 18 two-state proteins from their three-dimensional structure with remarkable success, suggesting that folding rates are largely determined by the strength and distribution of inter-residue interactions that are present in the native structure. This result indicates that the underlying physics determining protein folding rates is very much simpler than previously thought. - proteins, folding, dynamics, kinetics, lasers, T-jump
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