Conformational transitions in phosvitin with pH variation. Vibrational circular dichroism study

The vibrational circular dichroism (VCD) spectra of metal-free phosvitin are presented as a function of pH and analyzed both qualitatively and by using a factor analysis approach referenced to a protein data set. The qualitative pattern of both the IR and VCD changes is consistent with a coil-to-sheet transition occurring as pH is progressively decreased to values lower than 3. A similar transition was seen in commercial preparation of phosvitin which still contained metal ions, but there the transition was more gradual and occurred at somewhat different pH values. Such a gradual change is also evident in the solution phase absorption band profile but is made clearer using Fourier deconvolution. Based on VCD results, the low pH transition appears to occur with two distinct manifestations of the beta-sheet form. However, at the lowest pH values the sample may precipitate. These two forms are not distinguishable with Fourier transform infrared alone and may be due to a twist of the beta-sheet form or to aggregation.