Additive Effects of Chlorin E6 and Metal Ion Binding on the Thermal Stability of Rhodopsin In Vitro

Zinc (Zn2+) deficiency causes retinal dysfunctions such as night blindness and neurodegeneration. Because Zn2+ binds directly to the photoreceptor rhodopsin and alters its stability, the stabilization of rhodopsin may be key to prevention and treatment of retinal dysfunctions. In this paper, we investigated if not only trace metals but also other nutrients may stabilize rhodopsin structure in vitro. Detailed studies of the thermal stability of secondary and tertiary structure of rhodopsin in the presence and absence of the chlorophyll derivative chlorin e6 alone and together with bivalent metal ions Zn2+, Cu2+, Fe2+, Ni2+, Mg2+ and Mn2+ over a temperature range 5-100 degrees C were conducted using circular dichroism and fluorescence spectroscopy. When both chlorin e6 and Zn2+ are present, a pronounced increase in the thermal stability of overall secondary structure content is observed compared to either compound alone. This additive capacity is also noted with Cu2+, but not when other metal ions and chlorin e6 are combined.