Improved sensitivity in blue-membrane bacteriorhodopsin films.

We show that D85N/V49A, a mutant bacteriorhodopsin in which two residues, aspartate (D) at position 85 and valine (V) at position 49, have been replaced with asparagine (N) and alanine (A), respectively, has significantly improved optical properties compared with other forms of blue-membrane Bacteriorhodopsin. Absorption studies of the mutant in gels show that it forms the P(490) state at light levels that are comparable with M-state formation in wild-type films. Theoretical calculations based on Kramers-Kronig transformation of light-induced absorption data predict that the refractive index is three times larger than that of mutant D85N. Holographic measurements performed on gelatin-based films confirm that the sensitivity is improved by a factor of 50 over that of D85N.