Low-temperature CD spectra indicate a shift in population to that conformer associated with a positive n-p* transition CD band. The hydrogen-bond donor solvents methanol and water favor a shift in population to the conformer producing a negative n-p* CD band.
Molecular mechanics minimization both in a vacuum and with the explicit inclusion of solvent water molecules was performed using the AMBER and CHARMM force fields. Theoretical CD spectra were calculated for the low energy conformations and Boltzmann averaged spectra were generated.
The theoretical CD spectrum of the AMBER vacuum force field predicts negative ellipticity in the region of the n-p* transition which is in contrast to experimental spectra in acetonitrile. The CHARMM theoretical spectrum also predicts negative ellipticity in this region. Theoretical spectra from solvated molecular mechanics calculations indicate a positive CD band due to the n-p* transition for both AMBER and CHARMM. This is in contrast to experimental spectra which indicate negative ellipticity due to the n-p* transition in polar, hydrogen-bond donor solvents.
Theoretical CD spectra were calculated with the n-p* transition energy assigned values of 220 and 225 nm. based on the conformation of the dipeptide. The CD spectra generated were based on the predicted molecular mechanics vacuum conformations, however they show qualitative agreement with experimental spectra from 210 through 250 nm. in hydrogen-bond donor solvents.
Molecular dynamics simulations were carried out on the dipeptide in vacuo at 300K and 143K, and with solvent water molecules included at 300K. The rate of conformational interchange at the two temperatures indicates an energy barrier to interchange which is consistent with molecular mechanics results.