Force-field dependence of Boltzmann weighting factors on predicted pi-pi* circular dichroic spectra of cyclo (Gly-Pro-Gly)2.

Semi-empirical energy calculations were performed for published conformations of cyclo (Gly-Pro-Gly)2 using different force fields (DISCOVER cvff and cff91, AMBER, and CHARMM). The resulting potential energies were then used to create Boltzmann weighting factors for an ensemble of cyclo(Gly-Pro-Gly)2 structures. The dipole interaction model was used to predict pi-pi* circular dichroic spectra (CD) for the individual structures of cyclo(Gly-Pro-Gly)2. The Boltzmann weighting factors were applied to the individual spectra so that a composite spectrum was constructed to represent a CD arising from a collection of different structures in solution. Weighting factors determined from different force fields were compared. Boltzmann-weighted spectra better resembled the experimental CD than any calculated spectrum using only a single conformation of cyclo (Gly-Pro-Gly)2. The structures most heavily weighted contained at least one type I beta-turn.