RESEARCH COMMUNICATIONS CURRENT SCIENCE, VOL. 85, NO. 11, 10 DECEMBER 2003 1587 # For correspondence. (e-mail: laser@aero.iisc.ernet.in) Reversible photobleaching of bacteriorhodopsin molecules exposed to 570 nm laser light Gopalkrishna M. Hegde and K. P. J. Reddy* ,# Electronics and Computer Engineering Division, School of Engineering, Ngee Ann Polytechnic, Singapore 599 489 *Department of Aerospace Engineering, Indian Institute of Science, Bangalore 560 012, India The phenomenon of reversible photobleaching in bacte- riorhodopsin (BR) molecules exposed to 570 nm laser is presented. Creation of non-absorbing molecules in the bleached state is analysed theoretically using a simple theoretical model based on the excited-state absorption process. The analysis helps in ascertaining the percentage of molecules in the bleached state in the BR-based memory devices at any given time. The theory provides a bleaching parameter quantifying the number of photons the BR molecules can absorb before they reach a bleached state. THE biological molecule bacteriorhodopsin (BR), which is found in the purple membrane of Halobacterium halo- bium, is finding many applications in the field of photon- ics due to its unique advantages 1–3 . The advantages of the BR molecule include high quantum efficiency of convert- ing light into a state change, large absorption cross- section and nonlinearities, robustness to degeneration by environmental perturbations, capability to form thin films in polymers and gels, and existence of genetic variants with enhanced spectral properties for specific device appli- cations 4–7 . The photochromic property of the BR mole- cule has resulted in applications like pattern recognition systems 8 , three-dimensional memories 5 , holography 9 , second-harmonic generation 10 , mode locking 11 , spatial light modulation 12 and logic gates 3 . It is commonly known that one of the distinct differen- ces between the widely used photochromic dyes and the BR molecules is the photobleaching effect, which seve- rely limits the application of dyes in many optical devices 13 . In this communication we show that the BR molecule also suffers an equivalent bleaching mechanism due to the existence of the branched-photocycle. However, photo- bleaching in the BR molecule is reversible. Our main aim is to present the mechanism of reversible photobleaching in BR molecules in solid polymer matrix and to determine the bleaching parameter, defined as the average number of photons that the BR molecule absorbs before reaching the bleached state. The bleaching phenomenon described here is different from the photobleaching of purple mem- brane due to hydroxylamine presented recently, which occurs due to the disassembly of the purple membrane crystal into smaller crystals 14 . When exposed to light at 570 nm, the BR molecule undergoes a complex photocycle that generates interme- diates shown in Figure 1. These intermediate states start- ing from the short-lived J state through Q state have absorption maxima spanning the entire visible region of the spectrum. The molecule can be switched to the origi- nal BR state, designated as B state, from any of the inter- mediate states using appropriate laser signal of peak wavelength indicated as indices in the photocycle. Except from the P and Q states, the BR molecule from all other intermediate states relaxes back into the original B state by thermal decay, as shown in the photocycle. However, the molecule in the O state can follow a branching path by absorbing appropriate photons and reach the P state instead of decaying to the B state. The molecule in the P state ultimately decays into the Q state, and hence we designate these states as a single P(Q) state. The mole- cule in the Q state has an absorption peak at 380 nm and has negligible absorption at 570 nm and hence remains in this state indefinitely, until exposed to 380 nm light. Thus the molecules which reach the P(Q) state are not avail- able for the photochromic activity of the BR molecules excited by the 570 nm laser signal. This is equivalent to dye molecules reaching a bleached state which has negli- gible absorption at the excitation wavelength 15,16 . Hence, we define photobleaching of the BR molecule as the mechanism of exciting the BR molecule to the P state, which eventually decays into the Q state. Unlike the dye molecules, regeneration of the photobleached BR mole- cule into the original B state can be achieved by exposing the sample to 380 nm laser signal. Thus, photobleaching of the BR molecule exposed to 570 nm wavelength is completely reversible. Detailed analysis of the BR pho- tocycle has been reported previously in a series of pa- pers 12,17,18 . Figure 1. Schematic representation of the photochemical cycle indi- cating various intermediate states the BR molecule undergoes when exposed to 570 nm laser light. In the analysis, only O and P(Q) inter- mediate states are considered.