Tuning the Electronic Absorption of Protein-Embedded All- trans -Retinal

Abstract

Seeing the Light Rhodopsins respond to a range of electromagnetic radiation—allowing visual perception over a broad wavelength range in animals and facilitating light-driven ion transport and phototaxis in microorganisms. All rhodopsins contain an embedded retinal chromophore in which absorbance is tuned by the protein environment. To gain insight into how the protein tunes absorbance, Wang et al. (p. 1340 ; see the Perspective by Sakmar ) turned to a smaller soluble protein, cellular retinol binding protein II. They engineered the protein to fully encapsulate and covalently bind all- trans -retinal as a Schiff base. From this starting point, they used rational mutagenesis to vary the absorption maximum over a range of more than 200 nanometers by altering the electrostatic environment of the protein-binding pocket.