We determined the onset of helical structures in solution for oligopeptides by means of the decrease in intensity of the 190 mμ band (hypochromism), and related these findings to optical rotatory dispersion data. Measurements of ultraviolet absorption spectra in trifluoroethanol show that helical conformations commence at about the tetradecamer for peptides derived from β‐methyl‐L‐aspartate. In the γ‐methyl‐L‐glutamate series the hypochromic effect starts at the nonamer, increasing in magnitude with peptide chain length. Essentially the same results were obtained by using optical rotatory dispersion. By dividing the b0 (Moffitt‐Yang constant) for the oligopeptides into helical and nonhelical components, we have better indications that in previous work for the onset of helicity. We have also studied the far ultraviolet spectra and optical rotatory dispersion for high polymers derived from β‐methyl‐L‐aspartate and γ‐methyl‐L‐glutamate. Assuming complete helicity for these high polymers, we assigned helical contents to the oligopeptides. In addition, ultraviolet spectra were measured for the polymer from γ‐methyl‐DL‐glutamate in order to determine its helical content.
SynopsisInfrared spectra of polypeptides were measured in the region of 1800-400 cm-1. For the a-helical form, disordered form, and antiparallel-chain &form, amide V bands arising from N-H out-of-plane bending modes were observed at 610-620, around 650, and 700-705 cm-1, respectively, and amide V' bands arising from N-D oubof-plane bending modes were observed at 455-465, around 510, and 515-530 cm-1, respectively. These correlations are useful for conformation diagnoses, particularly for copolyamino-acids or proteins which are not oriented. The nature of low-frequency amide bands are discussed with reference to potential energy distributions calculated for the a-helical form and p form.
Polypeptides or poly-a-amino acids have been known to take various chain conformations. The conformational changes may be followed by the measurements of the characteristic amide bands. It was first remarked by Ambrose and Elliott' that the frequencies and dichroism of the amide I (1650 cm.-l) and I1 bands (1550 cn-1) were different for the a and fi conformations. Recently the correlations between the conformations and the amide I and I1 bands of polypeptides were analyzed theoretically* and certain new correlations were found, allowing one to distinguish between the ~r helical conformation, the parallel-chain fi conformation, the antiparallel-chain fi conformation, and the random coil.* If, however, several conformations coexist as in the case of copolymers of various amino acids or in the case of proteins, the fractions of various conformations may not necessarily be estimated by the measurements only of the amide I and I1 bands. I n these circumstances, characteristic amide bands in the low frequency region have been studied for the hope of obtaining additional frequencyconformation correlations. For N-methylacetamide (liquid), containing one CONH group, three characteristic bands have already been observed a t 627 cm.-1 (amide IV, C=O in-plane bending), 720 cm.-1 (amide V, N-H out-of-plane bending), and 600 cm.-1 (amide VI, C=O out-of-plane bending).'I n the present study the infrared spectra of polypeptides, (-CHR-CO-NH-)p,, in the solid film were measured in the region 800-400 cm.-l. Spectral changes upon deuteration of the CONH group were also observed. It is expected that the vibrations localized in the R group will not be shifted whereas the bands arising from the CONH group or the backbone chain will be shifted more or less upon deuteration. I n Table I are shown the amide bands of polypeptides in the 01 helical conformation. The band of poly-ymethyl-L-glutamate (PMLG) a t 620 em.-' shifts to 465 cm.-1 on deuteration,
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