Experimental investigation of initial steps of helix propagation in model peptides

It is not certain whether the helix propagation parameters s(n) (i.e., the equilibrium constants between (n - 1)- and n-residue long alpha-helices) determined from numerous studies of rather long model peptides are applicable for description of the initial steps of the helix formation during the protein folding process. From fluorescence, NMR, and calorimetric studies of a series of model peptides, containing the La3+-binding sequence nucleating the helix (Siedlecka, M., Goch, G., Ejchart, A., Sticht, H., and Bierzynski, A. (1999) Proc. Natl. Acad. Sci. U.S.A. 96, 903-908), we have determined, at 25 degreesC, the average values of the enthalpy DeltaH(n) and of the helix growth parameters s(n) describing the first four steps of helix propagation in polyalanine. The absolute values of the C-cap parameters, describing the contribution of the C-terminal residues to the helix free energy, have also been estimated for alanine (1.2 +/- 0.5) and NH2 group (1.6 +/- 0.7). The initial four steps of the helix growth in polyalanine can be described by a common propagation parameter s = 1.54 +/- 0.04. The enthalpy DeltaH(n) is also constant and equals -980 +/- 100 cal mol(-1).