Molecular Self-Assembling of -Methylacetamide in Solvents

The self-association of -methylacetamide (NMA), which is one of the most simple compound having a peptide bond, in various solvents such as carbon tetrachloride (CCl4), chloroform, dichloromethane, and acetonitrile was studied through the near-infrared (NIR) spectroscopic observation at various temperatures and concentrations. An analysis assuming a successive association processes for the NMA molecules was applied to the sharp 1470-nm band (the first-overtone band of NH stretching vibration mode attributed to free NH group of NMA monomer and partly to the free, terminal NH group of NMA aggregate); the mean association number for NMA in CCl4 increases with increasing concentration and decreases with increasing temperature. Comparisons of the association number of NMA in various solvents indicate that the degree of association is in the following order: chloroform ≒ dichloromethane acetonitrile. Interestingly, the association number of NMA in CCl4 is thought to be larger than that in its pure liquid. 1. Introduction Near-infrared (NIR) spectroscopy is a useful analytical method for practical materials [1] and has been applied to various industrial and agricultural problems [2, 3]. The applications, however, have been mostly based on mathematical treatments of the NIR spectra but not on analytical knowledge which the NIR spectra should give. Its applicants to basic chemical problems are still very limited probably because detailed spectral analyses of standard compounds have not been well carried out. On the other hand, NIR spectroscopy has such important technical merits that a liquid quartz cell with the path length of 10？mm or more can be used, remote spectroscopy is applicable, and so on. The use of the long-path-length cell makes us able to obtain the more reliable results especially in concentration. To make full use of the advantages, it is absolutely important to utilize the analytical information which NIR absorptions possess. By the way, hydrogen-bonding interactions are now known to be important in determining the structural properties of proteins. In particular, hydrogen bonding between the carboxyl oxygen and amide hydrogen atoms in the protein backbone help to stabilize the β-sheet and other motifs [4]. Among the forces that contribute to the stabilization of configuration of protein molecules in solution, intramolecular hydrogen bonds have often been assigned a key role. Furthermore, hydrogen bonding between peptide groups has indeed been definitely demonstrated in proteins and polypeptides, as well as in model amino acids and small