Comparison of alpha-helix stability in peptides having a negatively or positively charged residue block attached either to the N- or C-terminus of an alpha-helix: the electrostatic contribution and anisotropic stability of the alpha-helix

An estimation of the thermodynamic effects of a charged random coil, which is attached either to the N- or C-terminus of polyalanine, upon alpha-helix stability is attempted. A temperature-induced helix-coil transition of Ala20Lys20Phe and Lys20Ala20Phe was studied under various conditions of salt concentration and pH. By combining the results with previous ones for Ala20Glu20Phe and Glu20Ala20Phe, which have opposite electric charges to the present system [S. Ihara et al. (1982) Biopolymers 21, 131-145], the free energy of the coil to helix transition of the polyalanine block could be separated into two terms--one term for the electrostatic interaction of electric charges in the random-coil block with the alpha-helix dipole, and a second term for the intrinsic stability of the helix. The first term indicates the significance of the helix dipole-charge interactions, which affects the helix stability depending on the attaching side of the charged block and on the sign of the charges. This clearly shows the anisotropic stability of the alpha-helix. Furthermore, analysis of the dependence of these thermodynamic quantities on salt concentrations showed, assuming that the effect of the attached electric charges was symmetric (in other words, the absolute values of the electrostatic interaction terms were independent of the sign of electric charges), that the intrinsic stability of the alpha-helix was dependent on which side of the helix was attached to the random coil: a random coil attached to the N-terminus of the alpha-helix had little effect while that attached to a C-terminal significantly destabilized the helix.