An exploratory ab initio study on the conformations of ethylguanidine in its neutral [CH3-CH2-NH-C(=NH)NH2] and protonated [CH3-CH2-NH-C(NH2)2] forms

To explore the conformation intricacies of the guanidine group in the arginine side chain, ab initio computations have been carried out with ethylguanidine and the ethyl guanidinium ion. HF computations have been performed using 3-21G and 6-31G basis sets and DFT calculations were carried out at the B3LYP/6-31G(d) level of theory. The ethyl guanidinium ion has a single isomer due to its internal symmetry, although this structure has at least three conformations. However, several structures were found and optimized for ethylguanidine, involving the endo- and exo- orientation of the lone NH and torsional angle χ6, as well as the torsional modes associated with χ4 and χ5. Torsional angle χ5 gives rise to s-cis and s-trans structures.Key words: ethylguanidine, ethylguanidinium ion, ab initio MO, arginine side-chain, conformational analysis.