Alanine polypeptide helix-coil transition

The rate-determining step is thought to accoimt for the relaxation time of 160 ns measured with a laser-induced temperatme jiunp from 280 K to 300 K in an alanine-rich polypeptide containing 21 residues. It is thought that the limitation on the rate of the helix-coil transition in this peptide arises from an activation energy barrier of 1.7 kj mol associated with initial events of the form. .. hhhh hhch... in the middle of the chain. Therefore, initiation is... 

Stability of a-helix has been studied extensively both from an experimental as well as from a theoretical point of view. Work on the stability of alanine based polypeptides has been pushed forward mainly by Baldwin and his collaborators [69-71]. Poly-alanine itself is not soluble in aqueous buffers but, when hydrophilic residues such as Glu, Gin, Asp, Asn, and Lys are inserted in every fifth or so position, the resulting polypeptides are readily soluble. All these alanine based polypeptides form a-helical conformation with a moderately high propensity as determined by CD spectrum. For relatively short polypeptides, the temperature induced transition between helical and random coil states is rather a gradual one. In aqueous mixture with trifluoroethanol (TFE), these polypeptides achieve progressively higher content of a-helical conformation. 

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