Zwitterions are the most stable form for neutral arginylglycine in gas phase: Clear theoretical evidence

The canonical and zwitterionic conformers of gaseous dipeptide arginylglycine, ArgGly, were thoroughly researched. A large number of trial structures were generated by considering all combinations of internal single bond rotamers of three ArgGly tautomers. The structures were successively optimized with the methods of PM3, HF/3-21G*, BHandHLYP/6-31G* and BHandHLYP/6-31+G**. The conformational energies were finally calculated by the approaches of DFT/BHandHLYP, DFT/B3LYP, DFT/B97D and MP2 using the basis set of 6-311++G (2df, 2p). The results of the four methods are quite different on various aspects. However, all the computational approaches find that the most stable structure of ArgGly is zwitterionic. The properties of the low energy conformers such as the rotational constants, dipole moments, vertical ionization energies, temperature dependent conformational distributions and IR spectra are analyzed. These data should be helpful for understanding the experiments as well as for the differentiation of the four theoretical approaches by the experiments.