Conformational Preference of Alanine Dipeptide in the Gas Phase and in Solutions

We report here the results on N-acetyl-N'-methylamide of alanine (Ac-Ala-NHMe) calculated using the ab initio molecular orbital method with the self-consistent reaction field (SCRF) theory at the HF level with the 6-3l+G(d) basis set to investigate the conformational preference of alanine depending on the backbone torsion angles $\square$ and$\square$ in the gas phase, chloroform, and water. There are seven local minima (LM) in the gas phase and two additional LM are found in chloroform and water. These two additional LM A (an $\square$-helical structure) and F (a polyproline structure) are stabilized only in solutions. In the gas phase, the lowest LM is the conformation C with a C$\sub$7/ intramolecular hydrogen bond and the relative conformational energies range from 0.3 to 6.0 ㎉/mol. In chloroform, the lowest LM is the conformation E (an extended structure) and the relative conformational energies range from 0.7 to 4.9 ㎉/mol. In particular, we identified 14 possible transition states connecting between seven LM in the gas phase. The search for transition states probable in chloroform and water is now in progress.