Amide I bands of terminally blocked alanine in solutions investigated by infrared spectroscopy and density functional theory calculation: hydrogen-bonding interactions and solvent effects.

Structural aspects of terminally blocked alanine trans-N-acetyl-L-alanyl-trans-N'-methylamide (Ac-Ala-NHMe) in several different solvents were compared by attenuated total reflection infrared (ATR-IR) spectroscopy and density functional theory (DFT) calculations. The amide I bands between 1600 and 1700 cm(-1) appeared to change depending on media, indicating dissimilar hydrogen-bonding interactions among the peptides and solvent molecules. The minimum energy geometry in the isolated gas phase and aqueous environments were calculated at the B3LYP/6-311++G** theoretical level. In the solid state, Ac-Ala-NHMe is assumed to have an extended beta-stranded structure (C5), whereas it is assumed to have a cyclic structure (C7eq or alphaL) in a nonpolar tetrahydrofuran (THF) solvent. The optimized backbone dihedral angles (Phi, Psi) of Ac-Ala-NHMe plus four explicit water molecules were estimated to be -94 degrees and +133 degrees, respectively, indicating the polyproline II structure (PII). The energy differences between the most stable conformers were predicted to be larger for Ac-Ala-NHMe, which implies that more conformational ensemble structures should coexist for the gas phase than for the aqueous medium with explicit water molecules.