Pressure-induced helix - Coil transition of poly-l-glutamic acid in water

Abstract

The pressure effect on the helix-coil transition of aqueous poly-L-glutamic acid sodium salt (PLGA-Na) solutions was investigated by FT-IR spectroscopy. From the analysis of the amide I' band, the peak at 1637 cm-1 corresponding to the solvated α-helix structure increases with increasing pressure. The solvated α-helix structures of oligopeptides and proteins are stabilized upon compression. On the other hand, the peak intensities at 1710 and 1565 cm-1 corresponding to the C=O stretching mode of the COOH group and the C - O stretching mode of the COO-group of the side chain do not change with the pressure. According to high-pressure Raman study for the condensed aqueous CH3COONa solution, the pressure-induced dissociation of CH3COONa is found to be small. This result is qualitatively consistent with that of the side chain of PLGA-Na. The small pressure dependences of the side chain of PLGA-Na may be due to the high salt concentration in the solutions We have determined the difference in the partial molar volumes (PMV) between the α-helix and random coil structures of PLGA-Na at pD 5.0 to be +10.68±0.46 cm3/mol from the pressure dependence of the band intensities. The positive volume change of helix-coil transition of PLGA-Na is qualitatively consistent with the recent result of the volume change accompanying the helix - coil transition of PLGA (Noudeth G D et al, 2003 Biopolymers 70 563.). © 2008 IOP Publishing Ltd.