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Conformations of model compounds of proteins II. Infrared spectra of n‐acetyl‐amino acid methylamides
Author(s) -
Koyama Yasushi,
Shimanouchi Takehiko,
Sato Motomu,
Tatsuno Takashi
Publication year - 1971
Publication title -
biopolymers
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.556
H-Index - 125
eISSN - 1097-0282
pISSN - 0006-3525
DOI - 10.1002/bip.360100612
Subject(s) - chemistry , amide , norvaline , norleucine , infrared spectroscopy , stereochemistry , alanine , deuterium , infrared , crystallography , leucine , phenylalanine , amino acid , medicinal chemistry , organic chemistry , biochemistry , physics , quantum mechanics , optics
N‐Acetyl‐amino acid methylamides CH 3 CONHCHRCONHCH 3 were prepared from L ‐ and DL ‐alanine, L and DL ‐α‐amino‐n‐butyric acid, L ‐ and DL ‐norvaline, DL ‐norleucine, L ‐ and DL ‐methionine, L ‐ and DL ‐leucine, L ‐aspartic acid and DL ‐phenylalanine. The deuterium homologs of the type CH 3 CONDCHRCONDCH 3 , CD 3 CONHCHRCONHCH 3 , and CH 3 CONHCHRCONHCD 3 were also prepared. The infrared spectra of these compounds were measured down to 300 cm −1 in the crystalline state. The infrared spectra of N‐isopropylacetamide CH 3 CONHCH(CH 3 ) 2 , N‐methylisobutyramide (CH 3 ) 2 CHCONHCH 3 and their deuterium homologs were also measured. The CO in‐plane and out‐of‐plane bending vibration bands of the CH 3 CONHC α group (amide IVa and VIa) and those of the –C α CONHCH 3 group (amide IVb and VIb) were assigned from these data. Two crystalline modifications, form I and form II, were found for the compounds prepared from L ‐alanine, DL ‐leucine, L ‐aspartic acid and DL ‐phenylalanine. The two forms show quite different skeletal vibrations, which suggest, rotational isomerism. Two distinct patterns were found as to the positions of the amide IVa and VIa bands for the above compounds. The two amide bands were found near 630 and 600 cm −1 in form I, whereas they were found near 600 cm −1 in form II. The crystals of the remaining compounds were also classified into form I or form II on the basis of the arrangement of the amide bands. The X‐ray structure analyses suggest that these two forms have different hydrogen‐bond structures.