Kinetic optimization of bacteriorhodopsin films for holographic interferometry

The M-lifetime of bacteriorhodopsin films for optical recording is a key pa rameter in obtaining a high light sensitivity, a high contrast ratio, and a high contrast decay time. An increase of the M-lifetime causes a proportio nal reduction of the light intensity required for optical applications. In bacteriorhodopsin variants such as BR-D96N, the M-lifetime can be tuned ove r several orders of magnitude by simply changing the pH value with respect to the proton availability in the matrix of the films. At low humidities, t he proton transport steps linked to the photocycle limit the overall kineti cs. A proton-diffusion-limited, two-state model (PDL2 model) for bacteriorh odopsin is introduced which allows us to model mathematically the optical e xcitation and thermal relaxation processes for both high and low humidities in bacteriorhodopsin films. Films containing wildtype bacteriorhodopsin an d the variant D96N are compared in dependence on the pH value and the relat ive humidity at 20 degrees C. Of the investigated materials, only BR films containing BR D96N can be used for recording at low light levels of 100 mu W/cm(2). In a holographic interferometry experiment-a typical application w here a high light sensitivity is a key issue-it is demonstrated to what hig h extent the water content in the films affects their suitability for recor ding at low light levels. Kinetically optimized bacteriorhodopsin films yie ld a more-than-30-fold improvement of sensitivity in holographic interferom etry compared to dry bacteriorhodopsin films.