Is the parallel or antiparallel beta-sheet more stable? A semiempirical study

The geometry and energy of parallel and antiparallel peptidic P-sheets have been calculated using AM1. beta-Sheets composed of two peptide chains of up to 11 amino acid residues (Ala and Gly) and the dimers of cyclooctapeptides are used as model systems. The enthalpic difference between the parallel and the antiparallel arrangement is calculated to be very small, as it is found experimentally for the cyclic systems. The coordinates of the calculated structure of the cyclooctapeptide dimer 1 (cyclo-D,L-(Ala)(8)) have an rms deviation of only 0.223 Angstrom to the coordinates of the corresponding cyclopeptide obtained by X-ray analysis.