MO‐Calculation of the Optical Activity of Oligopeptides. II. Open‐chain conformations. Comparison with some cyclic systems

Abstract

Using the Frozen Core (FC) MO procedure described in a previous communication, we have computed the long‐wavelength chiroptic properties of oligopeptides in the parallel‐chain (PC) pleated sheet conformation, in the poly (L‐proline) I and poly (L‐proline) II conformations. The main features of the computed results are: (a) In the PC pleated sheet conformation the π–π* transition with the highest positive rotatory strength appears at shorter wavelength than the π–π* transition with highest negative rotatory strength and is polarized mainly perpendicularly to the chain axis. There is an analogy between the computed PC pleated sheet spectrum and that calculated for a cyclohexapeptide of low symmetry, in a conformation which is probably stabilized by intra‐annular hydrogen bonds. (b) In the polyproline I conformation the computed π–π* transition with highest positive rotatory strength is also the longest‐wavelength π–π* transition and is polarized mainly along the helix axis. There is an analogy between this spectrum and that calculated for cyclo(tri‐L‐proline) of symmetry C₃. (c) The poly‐proline II CD. spectrum may be qualitatively reproduced only by invoking very strong n – π interaction. The local n –π mixing parameter Λ must take on values ⪖3 eV, which is of the order of a nearest‐neighbor core resonance integral between atomic 2p orbitals of same spatial orientation. The question concerning the structural reasons for this situation is raised. As far as comparisons with experimental data are possible, qualitative agreement is obtained.