Conformational analysis of helical poly(beta-L-aspartate)s by IR dichroism

Poly(beta-l-aspartate)s are known to take up helical conformations reminiscent of the alpha-helix of polypeptides. The isobutyl, n-butyl, and 2-methoxyethyl esters have been examined by polarized ir spectroscopy in order to discriminate between the left (1L) and right (2R)-handed conformations, which are known to be compatible with the 13/4-helix adopted by these polyamides when crystallized in the hexagonal form. Dichroic ratios obtained from samples stretched in poly(ethylene oxide) together with orientation measurements made by x-ray diffraction were used to estimate the transition moment directions of amide A, I, and II bands with respect to the fiber axis. These were compared to those calculated by modeling simulations to conclude that the right-handed conformation consisting of 14-membered hydrogen-bonded rings is the correct model for the 13/4-helix. These results give definite support to earlier molecular mechanics calculations, which had shown that the 2R model is energetically favored over the 1L by about 2.5 kcal/(mol residue).