MO‐Calculation of the Optical Activity of Oligopeptides. I. Computational Procedure and Application to Small Helices

Abstract

Socitte suisse de chimie, BLle -Societa svizzera di chimica, Basilea Nachdruck verboten -Tous droits rCservts -Printed by Birkhauser AG, Basel, Switzerland Erscheint 9mal jahrlich -Parait 9 fois par an ISSN 0018-019X SummaryAll-valence electron calculations on the chiroptic properties of oligopeptides rapidly become intractable, as the size of the molecule increases. A Frozen Core procedure is here proposed, which is inspired by the PPP method. It takes explicitly into account only the (pseudo) 7c electrons and oxygen n electrons of the amide moieties, as well as the 7c electrons of eventual long-wavelength absorbing sidegroups. Local n-71 interaction is taken into account by an adjustable parameter. Other parameters are calibrated on the isolated amide and carbonyl chromophores. The method then follows the usual SCF-CI procedure. Rotatory strengths and f-values for the transitions are computed without further approximations, as previously described. Comparisons with results from exciton calculations and experimental data on polypeptides show the computed quantities to have a consistent and satisfactory order of magnitude. General introduction. -Twenty years ago M o f j t t [ 11 [3] and Fittscomputed by the exciton model the optical activity of the z-n* bands of a-helical polypeptides. Mofitt, using periodic boundary conditions, arrived at definite selection rules and clearcut predictions: If one considers the electric dipoleelectric dipole interaction (,ui-,uj mechanism) of the individual long-wavelength z-z* transitions (A-190 nm) in each amide chromophore, the composite system, i.e. the helix, shows two transitions: One transition at longer wavelength (A-210 nm) polarized parallel to the axis of the helix, another degenerate one at shorter wavelength (A -190 nm) polarized perpendicularly. In the righthanded helix the rotatory strength of the former is negative, that of the latter positive (see Fig. I ) . It was then shown that Moflitt's approach contained an inconsistency [3]: namely, that of assuming periodic boundary conditions -suited for quasi-infinite helices -, while at the same time using the nonretarded spatially averaged expression for the rotatory strength applicable to randomly oriented molecules of much smaller size than the wavelength of light. Later investigations have dealt with this question in detail [4-61. Mofjtt's original predictions, however, appeared to agree essentially ') 2, 120a

Circular Dichroism of Chromopeptides from Phycocyanin

“…The crude Born-Oppenheimer wavefunctions thus obtained were used to calculate the oscillator and rotatory strengths by published procedures [23][24][25].…”