Raman spectra of D and L amino acid copolymers. Poly-DL-alanine, poly-DL-leucine, and poly-DL-lysine

✍ Scribed by B. G. Frushour; J. L. Koenig

• Publisher: Wiley (John Wiley & Sons)
• Year: 1975
• Tongue: English
• Weight: 882 KB
• Volume: 14
• Category: Article
• ISSN: 0006-3525
• DOI: 10.1002/bip.1975.360140210

✦ Synopsis

Abstract

The Raman spectrum of poly‐DL‐alanine (PDLA) in the solid state is interpreted in terms of the disordered chain conformation, in analogy with the spectrum of mechanically deformed poly‐L‐alanine. The polymer is largely disordered with only a small α‐helical content in the solid state. When PDLA is dissolved in water, the spectra suggest that short α‐helical segments are formed upon dissolution. These helical regions might be stabilized by hydrophobic bonds between side‐chain methyl groups. Addition of methanol to the aqueous PDLA solutions results in a Raman spectrum resembling that of solid PDLA. This result suggests that the methanol disrupts the helical regions by breaking the hydrophobic bonds.

The Raman spectra of poly‐DL‐leucine (PDLL) and poly‐L‐leucine (PLL) are compared and only slight differences are observed in the amide I and III regions, indicating that PDLL does not have an appreciable disordered chain content. Significant differences are observed in the skeletal regions. The 931‐cm^−1^ lines in the PLL and PDLL spectra are assigned to residues in α‐helical segments of the preferred screw sense, i.e., L‐residues in right‐handed segments and D‐residues in left‐handed segments (in PDLL). On the other hand, the 890‐cm^−1^ line in the spectrum of PDLL is assigned to residues not in the preferred helical sence, i.e., L‐residues in left‐handed segments and D‐residues in right‐handed ones. The Raman spectra of poly‐DL‐lysine and poly‐L‐lysine in salt‐free water at pH 7.0 are compared. The Raman spectra of the two polymers are very similar. However, this does not negate the hypothesis of local order in poly‐L‐lysine because the distribution of the residues in poly‐DL‐lysine probably tends towards blocks, and the individual blocks may take up the 3~1~ helix.