Fully extended poly(β-amino acid) chains:: Translational helices with unusual theoretical π-π* absorption and circular dichroic spectra

Calculations of ultraviolet absorption and circular dichroic (CD) spectra in the pi-pi* band for fully extended chains of poly(beta-amino acids) yield simple spectra dominated by a single normal mode, using our classical dipole interaction theory with parameters developed previously for peptides and proteins. These molecules represent the unusual case of a purely translational helix, in which the array of amide chromophores is achiral. The CD spectra are thus unusual in deriving all of their rotational strength by "borrowing" from the nonchromophoric atoms in the molecule. Calculations are presented also for parallel and antiparallel sheet structures, whose existence is known, and for both single chains and sheets in the presence of solvent, using a lattice-filled cavity model for the surrounding medium. The molecules treated are poly(beta-alanine), poly(beta-aminobutyric acid), poly(beta-aminoisobutyric acid), poly(3-amino-2-methylbutanoic acid), and poly(2-aminocyclohexanecarboxylic acid).