Spectroscopic properties of tetrapyrroles on denatured biliproteins

Four biliproteins (phycoerythrin 545, phycocyanin 612, phycocyanin 645, and C-phycocyanin) were denatured by a high concentration of urea and then studied by absorption spectroscopy. Low pH and high protein concentrations conserved the tetrapyrroles' color, and mercaptoethanol and dithiothreitol promoted bleaching. One of these tetrapyrroles, cryptoviolin, appeared not to be hypochromic in the presence of depleting phycocyanobilin, but its absorbance did decay when phycocyanobilin is absent. The product from the treatment of phycocyanobilin with mercaptoethanol or dithiothreitol overlapped spectrally with cryptoviolin and gave the false appearance of maintaining a constant cryptoviolin concentration. Failure to note this effect could result in erroneous cryptoviolin/phycocyanibilin ratios.