Infrared spectra of the N‐acetyl‐N′‐methylamides of glycine, L‐alanine, and L‐leucine in dilute solutions of chloroform and carbon tetrachloride

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AbstractWe have examined the NH stretching frequencies of N‐acetyl‐N′‐methyl‐L‐alanineamide (blocked Ala), N‐acetyl‐N′‐methylglycineamide (block Gly), and N‐acetyl‐N′‐methyl‐L‐leucineamide (blocked Leu) in chloroform using irspectroscopy. Their spectrum of blocked Leu in carbon tetrachloride was also obtained. A major absorption band at 3450 cm−1 is attributed to the unperturbed NH stretching frequency. Another major band at 3437 cm−1 (for Ala) or 3432 cm−1 (for Leu) is attributed to conformations in which the NH stretching frequency is perturbed by the spatial proximity of the Cβ atom. An absorption band between 3300 and 3370 cm−1, which has in the past been assigned to the intramolecular hydrogen‐bonded NH in the C conformation, was found to be concentration dependent and could not be observed below 5 × 10−4M in chloroform; thus we find no evidence for a strongly hydrogen‐bonded NH in the C conformation in chloroform. An absorption band at 3416 cm−1 was observed in chloroform solutions of blocked Gly, and a similar absorption appeared as a shoulder on the 3437‐ and 3432‐cm−1 bands of blocked Ala and blocked Leu, respectively, in the same solvent. These bands, occurring near 3416 cm−1, may be assigned to extended (C5) conformations [Avignon et al., Biopolymers 8, 69 (1969)]. In CCl4 the spectrum of blocked Leu remained concentration dependent below 2.8 × 10−4M, with the 3300–3370‐cm−1 band progressively weakening and shifting to higher frequencies on dilution from higher concentrations. Analysis of the spectra indicates that there is considerable flexibility in the blocked single residues, in agreement with the results of conformational energy calculations.