Uncovering the source for the hysteresis of Poly-L-Proline’s temperature dependent solubility
This experiment characterizes the hysteresis of the heat-induced precipitation of poly-L-proline (PLP). PLP is a polypeptide polymer composed from the naturally occurring amino acid, proline. There are a number of proteins in which proline constitutes a high proportion of the total amino acids such as collagen, a component of connective tissue and spider silk. PLP is highly soluble under cold aqueous conditions and precipitates when warmed. However, cooling to re-dissolve heat-precipitated PLP is not path reversible and a significant hysteresis is observed. To gain better understanding of the mechanism behind this hysteresis, we used Fourier transform infrared spectroscopy, Raman spectroscopy and x-ray powder diffraction. Vibrational spectra showed little to no difference between the starting PLP to the precipitated PLP and are consistent with literature reports. However x-ray diffraction of the two samples show large differences including the appearance of diffraction peaks seen at d values of 7.16, 6.11, and 4.25 Å not previously reported in literature. We suggest that differences between these two samples maybe the result of a cis-trans isomerization of the peptide bond between the two proline amino acids. Further research is required to determine if and where in the polypeptide chain the isomerization occurs using polarimetry.
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