Separation and evaluation of the cis and trans isomers of hydroxyprolines: effect of hydrolysis on the epimerization.

A procedure has been developed which can detect the hydroxyproline isomers trans-4-hydroxyproline (Hyp), trans-3-hydroxyproline, cis-4-hydroxyproline, and cis-3-hydroxyproline present in hydrolysates of collagens. The method involves hydrolyzing collagen, and reacting the primary amino acids with o-phthaladehyde (OPA) and the hydroxyprolines and proline with 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole (NBD-Cl) which combines specifically with secondary amino acids. The proline and hydroxyprolines are then separated by thin-layer chromatography and quantified by using a scanning spectrofluorometer. The method was used to show that both trans-4-L-hydroxyproline and trans-3-L-hydroxyproline were epimerized as a function of hydrolysis time to the cis isomers. An appreciable amount of trans-3-Hyp was degraded. Hydrolysis with 6 N HCl in the presence of 6% trichloroacetic acid gave greater epimerization than the 6 N HCl alone. Alkaline hydrolysis in 0.2 M Ba(OH)2 caused more epimerization of trans-4-Hyp and trans-3-Hyp compared with acid hydrolysis but less degradation, so that alkaline hydrolysis is proposed for the evaluation of trans-3-Hyp, provided that the total of the cis and trans isomers be considered in this case.