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.
Lee
Maeng-Eun
ME
CAE Team, R&D Center, Samsung SDI Company, Limited, Yongin-si, Gyeinggi-do 446-577, Korea.
Lee
So Yeong
SY
Joo
Sang-Woo
SW
Cho
Kwang-Hwi
KH
D000409:Alanine / Q000031:analogs & derivatives* / Q000737:chemistry; D005740:Gases / Q000737:chemistry; D006860:Hydrogen Bonding; D011506:Proteins / Q000737:chemistry; D011789:Quantum Theory; D012996:Solutions; D012997:Solvents / Q000737:chemistry*; D013055:Spectrophotometry, Infrared; D013997:Time Factors; D014867:Water / Q000737:chemistry
D005740:Gases; D011506:Proteins; D012996:Solutions; D012997:Solvents; D014867:Water; C024314:N-acetylalanyl-N-methylamide; D000409:Alanine