Conformational study on poly [gamma-(alpha-phenethyl)-L-glutamate] using vibrational circular dichroism spectroscopy

Area of Science:

• Spectroscopy
• Polymer Chemistry
• Chiroptical Methods

Background:

• Vibrational Circular Dichroism (VCD) and IR spectroscopy are powerful tools for analyzing molecular structure.
• Polypeptides exhibit secondary structures like alpha-helices, which can be studied using chiroptical methods.
• Chirality in side chains can influence the overall spectral properties.

Purpose of the Study:

• To investigate the secondary structure of poly[gamma-((R)-alpha-phenethyl)-L-glutamate] (PRPLG) and poly[gamma-((S)-alpha-phenethyl)-L-glutamate] (PSPLG).
• To differentiate between enantiomeric forms of polypeptides based on their VCD spectra.
• To explore the relationship between side-chain chirality and VCD spectral features.

Main Methods:

• Obtaining VCD and IR absorption spectra in chloroform solution.
• Analyzing characteristic amide bands (A, I, II) for secondary structure.
• Examining VCD spectra in the CH stretching region to probe side-chain chirality.

Main Results:

• VCD patterns of amide bands indicate a right-handed alpha-helix for both PRPLG and PSPLG, similar to poly[gamma-benzyl-L-glutamate] (PBLG).
• Distinct VCD spectral patterns in the CH stretching region were observed for PRPLG and PSPLG, correlating with their opposite side-chain chirality.
• The additivity of optical activity contributions from the main chain and side chains explains the observed spectral differences.

Conclusions:

• PRPLG and PSPLG predominantly form right-handed alpha-helical structures.
• VCD spectroscopy, particularly in the CH stretching region, is effective for distinguishing local chirality differences in polypeptide side chains.
• VCD difference spectra serve as a valuable diagnostic tool for elucidating chirality in complex molecules.