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

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) I1 conformations. The main features of the computed results are: a) In the PC pleated sheet conformation the nn* transition with the highest positive rotatory strength appears at shorter wavelength than the nn* 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 nn* transition with highest positive rotatory strength is also the longest-wavelength nn* 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 C,. c) The poly-proline I1 CD. spectrum may be qualitatively reproduced only by invoking very strong nn interaction. The local n-n mixing parameter A 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. Introduction.We are exploring the possibility of describing the long-wavelength chiroptic properties of oligopeptides by a simplified molecular orbital procedure, taking explicitly into account only the pseudo n electrons of the amide moieties (and of eventual unsaturated side-groups) and the nonbonding amideoxygen electrons. As shown in a previous report [l], this Frozen Core (FC) procedure is suggested by the results of semiempirical all-valence SCF-CI calculations of the

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