Quantitative analysis of helix–coil transition of block copolypeptide, Glu12–Ala12, by combined use of CD and NMR spectroscopy
✍ Scribed by Toshimasa Yamazaki; Hidemine Furuya; Takeshi Watanabe; Sayaka Miyachi; Yuji Nishiuchi; Hideki Nishio; Akihiro Abe
- Publisher
- Wiley (John Wiley & Sons)
- Year
- 2005
- Tongue
- English
- Weight
- 454 KB
- Volume
- 80
- Category
- Article
- ISSN
- 0006-3525
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✦ Synopsis
Abstract
To investigate helix–coil transition mechanisms, conformations of Glu~12~–Ala~12~, EA, in aqueous solution have been studied in detail over the pH range from 2 to 8 and the temperature range from 20 to 60°C using CD and NMR spectroscopy. The 750‐MHz NMR spectra displayed excellent dispersion of the backbone amide proton signals, and permitted essentially complete sequence‐specific resonance assignments. These assignments, together with short‐ and medium‐range nuclear Overhauser effect (NOE) constraints and coupling constants, enable us to analyze conformational characteristics of all the residues in the EA peptide individually. A combined use of CD and NMR techniques reveals that the EA peptide assumes a stable α‐helix from Glu12 to Ala19 in 0.1M NaCl solution at 20°C above pH 7. The α‐helix is getting longer as decreasing pH. Below pH 4, the peptide assumes the longest α‐helix from Glu3 to Ala23. The important observation of the present study is that the helix–coil transition occurs stepwise, residue by residue, from both the N‐ and C‐termini of the α‐helix. No conformational equilibrium between the helical and random‐coil states is detected for the residues in the central region of the α‐helix. Quantitative analysis of temperature‐induced helix‐to‐coil transitions at various pHs provides a pH‐independent residual enthalpy change ΔH~r~ = 0.95 kcal res^−1^. Similar values have been reported for a 50‐residue alanine‐rich peptide (1.2 kcal res^−1^), poly‐L‐glutamate (1.1 kcal res^−1^), poly‐L‐lysine (1.1 kcal res^−1^), and poly‐L‐alanine (0.86 kcal res^−1^). Those investigations, along with our present result, suggest that ΔH~r~ is mainly determined by the transformation of the backbone associated with the disruption of the intramolecular hydrogen bond. These results should increase our understanding of the helix–coil transition. © 2005 Wiley Periodicals, Inc. Biopolymers (Pept Sci), 2005