Theoretical UV circular dichroism of aliphatic cyclic dipeptides

Four cyclic dipeptides (piperazine-2,5-diones), cyclo(L-Pro-Gly), cyclo(L-Pro-L-Leu), cyclo(L-Ala-L-Ala), and cyclo(L-Pro-L-Ala), were modeled from crystal structure data. Conformations resulting from energy minimization using molecular mechanics were compared with traditional ab initio and density functional theory geometric optimizations for each dipeptide. In all computational cases, the gas phase was assumed. The pi-pi transition feature of the UV circular dichroic (CD) spectra was predicted for each peptide structure via the classical dipole interaction model. The dipole interaction model predicted CD spectra that qualitatively agreed with experiment when MP2 or DFT geometries were used. By coupling MP2 or DFT geometric optimizations with the classical physics method of the dipole interaction model, significantly better CD spectra were calculated than those using geometries obtained by molecular mechanics. Thus, one can couple quantum mechanical geometries with a classical physics model for calculation of circular dichroism.