Primary Reaction of Sensory Rhodopsin II Mutant D75N and the Influence of Azide

Abstract

The early steps in the photocycle of sensory rhodopsin II mutant D75N are investigated in a comprehensive study using femtosecond visible pump/probe spectroscopy. An overall slower response dynamics after photoexcitation is observed compared to wild-type sensory rhodopsin II, which is assigned to changed electrostatics and an altered hydrogen-bonding network within the retinal binding pocket. Furthermore, the influence of azide on the primary reaction is analyzed. The addition of azide accelerates the sub-10 ps dynamics of the D75N mutant nearly to reaction rates found in wild-type. Moreover, a further reaction pathway becomes observable in the investigated time range, which is assigned to a previously described K(1) to K(2) transition. The specific acceleration of the early steps seems to be a unique feature of the D75N mutant as similar azide effects do not emerge in analogous azide measurements of wild-type sensory rhodopsin II, bacteriorhodopsin, and the bacteriorhodopsin mutant D85N.