Effect of pressure on helix-coil transition of an alanine-based peptide: an FTIR study

Effects of pressure and temperature on the helix-coil transition of an alanine-based peptide (Ac- AA(AAKAA)(3)AAY-NH(2)) have been investigated using CD and FTIR spectroscopy. From the correlation between CD and FTIR data, we showed that the change in infrared intensity of the amide I' band at 1633 cm(-1) is almost identical to the change in the helical content calculated from the CD result. Thus, we monitored the amide I' band intensity at 1633 cm(-1) to determine the helical content at high pressures. We determined free energy, enthalpy, and volume changes upon unfolding of the alpha-helix. The obtained volume change (0.98 +/- 0.04 cm(3) mol(-1) res(-1) at 25.4 degrees C) is not consistent with a recent molecular dynamics simulation study by Pascheck et al. who used temperature-pressure replica exchange methods (Paschek, Gnanakaran, and Garcia, Proc Natl Acad Sci USA 2005;102:6765-6770). They reported a small negative volume change upon unfolding of the alpha-helix, indicating that pressure induced the peptide to unfold. Pressure dependence of the band-width of the amide I' band also supported the present experimental results in which pressure induces the peptide to fold, which is also apparently inconsistent with the pressure-induced protein unfolding that is generally observed. We propose a hypothesis to unravel the paradox of pressure-induced peptide folding and protein unfolding.