Chemical activation of the cyanobacterial orange carotenoid protein

The effects of the Hofmeister series of ions on the activation of the orange carotenoid protein (OCP) from the inactive orange form to the active red form were tested. Kosmotropes led to lower OCP activation, whereas chaotropes led to greater OCP activation. Concentrations of thiocyanate exceeding 1.5 M dark activate the orange carotenoid protein to its red form. This chemically activated OCP was studied by UV–vis and circular dichroism spectroscopies. The chemically-activated OCP quenches the fluorescence of phycobilisomes in vitro, to a level comparable to that of the light-activated OCP.