FTIR 2D correlation spectroscopy of α₁ and α₂ fractions of an alkali-pretreated gelatin.

An alkali-pretreated gelatin (pI~4.9) was fractionated by means of alcohol coacervation and semi-preparative gel chromatography. The thermal responses of the isolated α fractions, the coacervate and the total gelatin were investigated by 2D-correlation FTIR spectroscopy in the amide I band region (1600-1700 cm⁻¹). The gelation temperature was the same for all examined samples (24.5°C) while the melting temperature of the α₂ fraction was lower (30°C) than that of the other samples (32.5°C). The 2D COS plots indicate that on cooling (gelation) the core sequence of conformational changes is the same for all samples. On heating, however, the α₂ fraction deviates from the α₁-containing samples and shows an earlier disappearance of the triple helix signal in the event sequence. The lower melting temperature (less thermostable gelatin gel) of the α₂ fraction thus results from a different conformational cascade of the α₂ chains upon melting. In all samples the initial conformational changes take place in the β-turns, providing further evidence for the models proposed previously.