Water-assisted peptide bond formation between two double amino acid molecules in the gas phase.

The gas phase mechanism of the peptide bond formation between two double amino acid (DAA) molecules described by the (NH2)2C(COOH)2 formula is investigated in the presence of a water molecule. Formations of trans and cis DAA-DAA dipeptide products along both concerted and stepwise mechanisms have been studied at the CCSD(T)/aug-cc-pVDZ//MP2/aug-cc-pVDZ level. The results indicate that the activation energy barriers estimated for the water-assisted mechanisms are significantly reduced in comparison to the corresponding uncatalyzed reactions. The trans DAA-DAA isomer is expected to dominate in the final product due to its larger stability compared to the cis DAA-DAA product.