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Theoretical π‐π* absorption and circular dichroic spectra of polypeptide β‐structures
Author(s) -
Applequist Jon
Publication year - 1982
Publication title -
biopolymers
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.556
H-Index - 125
eISSN - 1097-0282
pISSN - 0006-3525
DOI - 10.1002/bip.360210405
Subject(s) - chemistry , dichroic glass , antiparallel (mathematics) , oscillator strength , spectral line , crystallography , absorption spectroscopy , dipole , planar , absorption (acoustics) , cotton effect , molecule , molecular physics , circular dichroism , optics , materials science , nanotechnology , physics , computer graphics (images) , organic chemistry , quantum mechanics , astronomy , magnetic field , computer science
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 β P (paralle) and β A (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 χ 1 is near −60° for β A ‐structures or in a broad range near 140° for β P ‐structures. Poly(Ala) in uniformly twisted β A ‐ and β P ‐structures or in irregular β A ‐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 β A ‐ and β P ‐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 β A ‐form show the better agreement with experiment for both types of spectra.