Azido Gauche Effect on the Backbone Conformation of β-Azidoalanine Peptides

To study the azido gauche effect on the backbone conformation of beta-azidoalanine (Aza) dipeptide (AAD, Ac-Aza-NHMe) and tripeptide (AAT, Ac-Aza-Aza-NH2), we used spectroscopic methods in combination with quantum chemistry calculations and molecular dynamics (MD) simulations. From the H-1 NMR coupling constants and H-1,H-1 NOESY experimental data, we found that AAD in water mainly adopts a seven-membered cyclic (C-7) rather than polyproline II (P-II) backbone conformation and prefers the gauche (g(-)) side-chain conformer. From the amide 1 IR absorption and circular dichroism (CD) spectra, the backbone conformation of AAD in water is found to deviate from P-II but is rather close to C-7. Thus, the backbone conformation of AM) differs from that of alanine dipeptide (AD, Ac-Ala NHMe), which is mainly P-II in water. The underlying origin of the backbone conformational difference between AAD and AD in water was elucidated by quantum chemistry calculations with density functional theory (DFT). It was found that the C-7/g(-) conformer is the lowest energy structure of an isolated AAD. Here, the beta-azido group forms intramolecular electrostatic interactions with two neighboring peptide bonds, which are facilitated by the azido gauche effect. Thus, the beta-azido group appears to be responsible for directing the peptide backbone conformation toward the C-7 structure. The quantum mechanical/molecular mechanical (QM/MM) MD simulations show that AAD in water adopts neither P-II nor right-handed alpha-helix (alpha(R)) and prefers the g(-) conformer. Thus, the intramolecular electrostatic interactions between the beta-azido group and two nearby peptide bonds are also found even in the aqueous solution structure of AAD. Consequently, the beta-azido group appears to be an effective (C-7-conformation-directing element, which may also be useful for tuning the structures of other amino acids and polypeptides.