Solution-Phase Conformations of N-Acetyl-N‘-methyl-l-alaninamide from Vibrational Raman Optical Activity

Abstract

Experimental and ab initio theoretical vibrational Raman optical activity (VROA) spectra are presented for N-acetyl-N‘-methyl-l-alaninamide (NANMLA), in order to determine the predominant conformations of this molecule in the solution phase. The experimental spectra were obtained in three different solvents, CHCl3, H2O, and D2O. The ab initio VROA spectra were predicted for nine different conformations of NANMLA optimized with the 6-31G* basis set. The vibrational frequencies and normal modes for all nine conformers were also obtained with the 6-31G* basis set. From a comparision of the predicted and observed VROA spectral patterns, it is suggested that there is reasonable support for the presence of C7,eq−C5, C7,eq, and αR conformers in aqueous solution and for C7,eq−C5 and αR conformers in chloroform solutions.

Solvation effects on alanine dipeptide: A MP2/cc‐pVTZ//MP2/6‐31G** study of (Φ, Ψ) energy maps and conformers in the gas phase, ether, and water

“…In 1980, based on NMR and CD measurements, Madison and Kopple 31 concluded that P II -like (corresponding to the ␤-conformer in this article) and ␣ R conformers were the dominant species in water. In contrast, based on the experimental and computational studies of vibrational Raman optical activity of AD in CHCl 3 , H 2 O, and D 2 O, Deng et al, 27 in 1996, proposed the presence of C7 eq OC 5 (i.e., ␤ in the present article), C7 eq , and ␣ R in aqueous solution and C7 eq -C 5 and ␣ R in chloroform. The Weisshaar group 7,30 found that the structure of ␤ ϩ 4H 2 O, computed by Han et al, 11 fits to their dipolar coupling data best, and concluded that the P II -like conformer may be the primary species.…”

“…7,[27][28][29][30][31] As we will detail in the Results and Discussion section, the experimental conclusions were not consistent. Although the alanine dipeptide is an important test system, there has been no cutting-edge study on it, especially with regard to the effects of solvation.…”

Temperature and pressure effects on conformational equilibria of alanine dipeptide in aqueous solution

“…The four peaks of 1660, 1640, 1618, and 1604 cm Ϫ1 were determined by the second derivative analysis in a similar manner as the skeletal stretching mode. From the results by Deng et al 4 and Han et al, 5 the peaks around 1660 and 1640 cm Ϫ1 were assigned to the ␣ R and P II conformations, respectively.…”

“…The four peaks of 842, 859, 885, and 924 cm Ϫ1 were determined by the second derivative analysis of the original spectra. Deng et al 4 and Han et al 5 have studied on the conformation of AAlaMA in some model systems of aqueous solution by the ab initio MO method. They assigned the peaks around 860 and 925 cm Ϫ1 to the P II and the ␣ R conformations, respectively.…”

Vibrational spectroscopy and DFT calculations of di‐amino acid peptides: α‐ and β‐N‐acetyl‐L‐Asp‐L‐Glu in the solid state

“…The conformation-sensitive bands in the spectra of polypeptides in water are amide I vibrations, [10,11] amide II vibrations, [12,13] NH stretching vibrations (amide A), [14,15] C α D vibrations, [16] and skeletal vibrations observed in Raman spectra. [17,18] However, the most frequently used bands for structural determination are the amide I and amide III bands.…”