Stabilization of Neutral NH2-R-COOH Form of the Antihypertensive Peptides L-Valyl-L-Prolyl-L-Proline and L-Isoleucyl-L-Prolyl-L-Proline

Spectroscopic and structural elucidation of the peptides L-Valyl-L-Prolyl-L-Proline (1) and L-Isoleucyl-L-Prolyl- L-Proline (2) are reported on the basis of experimental linear-polarized IR-spectroscopy in solid-state, 1H-NMR data and DFT. Curiously, the experimental data shown that both peptides stabilized in solution and in solid-state neutral H2N-RCOOH form. Conformational analysis made, shown two strong intramolecular NH2…O=C-N(Amide) and O=C-OH…NH2 hydrogen bonds with lengths of 2.979 Å and 2.475 Å in (1) and 2.599 Å and 2.507 Å in (2) respectively. The presence of the Pro-Pro fold resulted to strong steric effect leading to the stabilization of free COOH and NH2 groups. The Erel values of zwitterion form are significant higher than the neutral forms with a difference of 1.2 and 0.9 kJ/mol. The manner of interaction of the peptides with angiotensin-I converting enzyme is proposed.