Studies of Poly-β-benzyl-<small>L</small>-aspartate Helix. III. Infrared Spectra of Copolymers of β-Benzyl-<small>L</small>-aspartate with β-<I>p</I>-Methyl, Chloro, Cyano, or Nitrobenzyl-<small>L</small>-aspartate in a Chloroform Solution

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  • Studies of Poly-β-benzyl-<scp>l</scp>-aspartate Helix. III. Infrared Spectra of Copolymers of β-Benzyl-<scp>l</scp>-aspartate with β-<i>p</i>-Methyl, Chloro, Cyano, or Nitrobenzyl-<scp>l</scp>-aspartate in a Chloroform Solution

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Infrared studies of the copolymers in a chloroform solution have revealed that, in each series of copolymers, the transition from the left- to the right-handed α-helix simultaneously brings about sharp changes in the characteristic amide frequencies. The frequencies of the NH-stretching, amide I, and amide II bands of the copolymers in the left-handed α-helix are higher by ca. 13 cm−1, ca. 10 cm−1, and ca. 5 cm−1 respectively than those in the right-handed α-helix. The amide bands of the copolymers in the right-handed form lie in the normal frequency ranges as established for α-helical polypeptides. On the contrary, the amide frequencies of poly-β-benzyl-L-aspartate or the copolymers in the left-handed form are anomalous. The anomalous amide frequencies of the lefthanded helix are to be ascribed to the reversal in the helical sense, but not to the particular aspartate group, since the copolyaspartate esters in the right-handed helix exhibit normal amide frequencies. The transition in the helical sense is reflected also in the C=O stretching bands of the side-chain ester groups.

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