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⁺, Mg ²⁺and Mn²⁺ over a temperature range 5-100°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.