Chemical modification of biliprotein chromophores.

The reaction of biliproteins with sodium dithionite has been studied. The reagent is selective towards the chromophores. In denatured phycocyanin from Spirulina platensis, all three chromophores react to form yellow "phycorubin", whereas only 1/3 of the chromophores react in native phycocyanin in a non-statistical manner. From reversion experiments, it can be shown, that the thermodynamic stability of the chromophores towards reaction with dithionite is increased in the native pigment. Similarly, native phytochrome in its Pr form reacts only partially to a pigment absorbing at both 420 and 660 nm. The same product is formed from native Pfr, indicating both a reversion to Pr and a partial reduction.