A quantum-mechanical study on the geometry of a hexacyclopeptide dichotomin A

The geometry of dichotomin A, cyclo(-Gly-Thr-Phe-Leu-Tyr-Val-) is optimized at the density functional theory (DFT) with B3LYP parameterization, Hartree-Fock (HF) and our own N-layered integrated molecular orbital and molecular mechanics (ONIOM) levels of theory. The gauge-including atomic orbital (GIAO) methods at both the HF and DFT levels of theory are performed to calculate the 13C chemical shifts of the three optimized geometries. The resulting data show the computing time with HF and ONIOM is much less than DFT and the optimized geometry obtained from HF is the most similar to experimental structure. In the three optimized geometry two β-turns and two hydrogen bonds are formed, which may be the primary factors affecting its activities. Comparing to experiment, the RMS errors for 13C chemical shifts relative to TMS determined using HF with the 6-31G(d) basis set are the smallest among all the methods used here.