Fast cleavage of phycocyanobilin from phycocyanin for use in food colouring

- Cleavage of phycocyanobilin from phycocyanins was achieved by alcoholysis.
- Two phycocyanobilin isomers and different solvent adducts were formed.
- Two competitive reaction mechanisms are proposed.
- The use of temperatures higher than the boiling point increased the reaction rate.
- Heating by microwave irradiation resulted in product degradation.

Phycocyanins from cyanobacteria are possible sources for new natural blue colourants. Their chromophore, phycocyanobilin (PCB), was cleaved from the apoprotein by solvolysis in alcohols and alcoholic aqueous solutions. In all cases two PCB isomers were obtained, while different solvent adducts were formed upon the use of different reagents. The reaction is believed to take place via two competing pathways, a concerted E2 elimination and a SN2 nucleophilic substitution. Three cleavage methods were compared in terms of yield and purity: conventional reflux, sealed vessel heated in an oil bath, and microwave assisted reaction. The sealed vessel method is a new approach for fast cleavage of PCB from phycocyanin and gave at 120 °C the same yield within 30 min compared to 16 h by the conventional reflux method (P < 0.05). In addition the sealed vessel method resulted in improved purity compared to the other methods. Microwave irradiation increased product degradation.