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

✍ Scribed by Jon Applequist

• Publisher: Wiley (John Wiley & Sons)
• Year: 1982
• Tongue: English
• Weight: 993 KB
• Volume: 21
• Category: Article
• ISSN: 0006-3525
• DOI: 10.1002/bip.360210405

✦ Synopsis

Absorption and circular dichroic (CD) spectra of the T -T * transition near 200 nm are calculated for poly(Gly), poly(Ala), poly(Abu), and poly(Va1) in the Pp (parallel) and PA (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 T -T * 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(G1y) and poly(A1a) 0-structures in uniform, planar lattices do not show sufficient rotational strength to account for observed CD spectra. Poly(Abu) and poly(Va1) @-structures in uniform, planar lattices show rotational strengths comparable to experiment when x 1 is near -60" for @*-structures or in a broad range near 140' for pp-structures. Poly(A1a) in uniformly twisted PAand pp-structures or in irregular @A-structures corresponding to regions of the crystal structure of concanavalin A also show enhanced rotational strengths in the principal T -T * CD hand. Absorption and CD spectra calculated for poly(Abu) in uniform PAand pp-structures are compared with experimental data on poly(Lys) in the 0-form, assuming side-chain conformations in Ahu that maximize the intensity of the principal CD hand. The calculations for the PA-form show the better agreement with experiment for both types of spectra.