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 stabi...