Poly(β-amino acid) Helices. Theoretical π−π* Absorption and Circular Dichroic Spectra

Abstract

Absorption and circular dichroic (CD) spectra of the π-π * transition near 190 nm are predicted for helical structures proposed for poly(β-amino acids) in accordance with the hydrogen-bonding schemes of Fernandez-Santin et al. We consider four helical structures which are sterically unhindered when alkyl substituents on C α and C β are in R configurations, including two right-handed forms and two left-handed forms. Calculations are made for polymers of β-alanine, (R)-3-amino-2-methylpropanoic acid, (R)-3-aminobutanoic acid, (2R,3R)-3-amino-2-methylbutanoic acid, and (1R,2R)-2-aminocyclohexanecarboxylic acid. The predicted absorption and CD spectra are qualitatively similar for the four helices, showing some sensitivity to backbone structure and relatively little sensitivity to side-chain substitutions. The most notable features of the spectra are (i) CD band couplets centered near 200 nm with a +/-pattern, (ii) large hyperchromicities of 48-115% in the absorption spectra, and (iii) linear dichroic spectra dominated by a strong parallel dichroism in one of the right-handed helices and strong perpendicular dichroism in the left-handed helices. It is suggested that poly[(1R,2R)-2-aminocyclohexanecarboxylic acid] would be a good candidate for experimental study because one of the helices should be particularly stable for this case. Available experimental absorption and CD spectra are qualitatively consistent with the predictions for any of the four helices, though the observed CD spectra are less intense.