Theoretical π‐π* absorption and circular dichroic spectra of polypeptide β‐structures

Absorption and circular dichroic (CD) spectra of the π‐π* transition near 200 nm are calculated for poly(Gly), poly(Ala), poly(Abu), and poly(Val) in the βP (paralle) and βA (antiparallel) pleated‐sheet structures using the dipole interaction model and including interactions among all atoms; optical parameters were obtained from previous studies of related molecules. Most of the calculations are for structures with one or three chains of six residues each. The oscillator strength and splitting of the π‐π* modes are found to be affected only to a small extent by variations in side‐chain structure and conformation, whereas the CD spectrum is very sensitive to these variations. Poly(Gly) and poly(Ala) β‐structures in uniform, planar lattices do not show sufficient rotational strength to account for observed CD spectra. Poly(Abu) and poly(Val) β‐structures in uniform, planar lattices show rotational strengths comparable to experiment when χ1 is near −60° for βA‐structures or in a broad range near 140° for βP‐structures. Poly(Ala) in uniformly twisted βA‐ and βP‐structures or in irregular βA‐structures corresponding to regions of the crystal structure of concanavalin A also show enhanced rotational strengths in the principal π‐π* CD band. Absorption and CD spectra calculated for poly(Abu) in uniform βA‐ and βP‐structures are compared with experimental data on poly(Lys) in the β‐form, assuming side‐chain conformations in Abu that maximize the intensity of the principal CD band. The calculations for the βA‐form show the better agreement with experiment for both types of spectra.

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