A natural cyanobacterial protein C-phycoerythrin as an HS^- selective optical probe in aqueous systems

A naturally fluorescent cyanobacterial protein C-phycoerythrin (CPE) was investigated as a fluorescent probe for biologically and environmentally important hydrosulphide (HS^-) ion. It was selective for HS amongst a large anion screen and the optical response was rapid. Sequential UV-visible titration showed considerable peak shift and attenuation with increasing [HS^-] while fluorescence titration proved that HS^- quenched CPE fluorescence in a concentration dependent manner. The linear response range was 0-2 mM HS^- while the Stern Volmer curve was non-linear and the limit of detection was 185.12 μM. Except bicarbonate and glycine, no anion or biomolecule interfered with the detection even at 10 times the concentration of HS^-. It was also free of influences from other sulphur forms like sulphite, sulphate and thiosulphate. CPE reliably detected HS^- in freshwater and effluent samples, though some under- and over - estimation was evident. The % recovery ranged from ~96 to 105% (RSD ~ 0.035-0.188%). FTIR analysis showed significant changes in the amide I and II regions of CPE, along with minor modifications in the amide III region as well, showing that HS^- was able to influence the protein secondary structure at higher concentrations.