| Abstract
| - We have measured the kinetics of the helix-coil transition for the synthetic 21-residue peptide Ac-WAAAH+(AAAR+A)3A-NH2 initiated by nanosecond laser temperature jumps. This peptide was designed with tryptophanin position 1 and histidine in position 5 so that the side chains interact when the backbone of residues 1−5is α-helical. Histidine, when protonated, efficiently quenches tryptophan fluorescence providing a probe forthe presence of helical structure. The kinetics measured throughout the melting transition are well-describedby a single-exponential relaxation, with a rate of 3.3 × 106 s-1 at 301 K, the midpoint of the helix−coiltransition. The rate increases with increasing temperature with an apparent activation energy of approximately8 kcal/mol. To interpret these results we have fitted the equilibrium and kinetic data with the statisticalmechanical model of Muñoz et al. (Proc. Natl. Acad. Sci. U.S.A. 1998, 95, 5872−5879). This model includesboth variable helix propensities and side chain−side chain interactions. The model accounts for the single-exponential kinetics by predicting that approximately 90% of the change in the tryptophan fluorescence resultsfrom melting of stretches of helix which include residues 1−5 by passage over a nucleation free energybarrier. The measured temperature dependence is reproduced by introducing damping from solvent frictionand an activation barrier for the individual helix propagation and melting steps. This barrier is somewhatlarger than that which results from the loss in conformational entropy or breaking of hydrogen bonds. Themodel provides a description of the kinetics of the helix-coil transition which is consistent with the results ofother experimental studies as well as molecular dynamics simulations.
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