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

Abstract

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.

UV Resonance Raman and DFT Studies of Arginine Side Chains in Peptides: Insights into Arginine Hydration

“…We calculated the effect of changing the torsion angle of the C13–C10–N3–C2 linkage on the energy of EG using DFT. The calculated potential energy surface along the C13–C10–N3–C2 torsion angle (Figure ) is consistent with a previous report that indicated the trans-conformation (180°) is the global energy minimum. Two local minima corresponding to ±90° conformations exist 2.4 kJ·mol –1 above the global minimum.…”

Prospects in computational molecular medicine: a millennial mega-project on peptide folding

“…The upper portion of Fig. 16 shows the potential energy curve E Ex 5 of structure VIII calculated at HF/6-31G [31].…”