A Study of Prolongation of the M-State Lifetime in Chemically Modified Bacteriorhodopsin Films

Baofang Li*, Bing Liang, Meiling Li, and Long Jiang

Laboratory of Colloid and Interface Science, Center for Molecular Science, Institute of Chemistry, The Chinese Academy of Sciences, and *De Wai Bei Sha Tan, Institute of Photographic Chemistry, The Chinese Academy of Sciences, Beijing 100101, China

e-mail: bfli@ipc.ac.cn

Bacteriorhodopsin (bR) is a photochromic protein present in the purple membrane (PM) isolated from Halobacterium salinarum. Upon absorption of light energy, bR undergoes a complex photocycle involving several intermediates. Of all these intermediates, the M-state is the most stable. In the past two decades, bR has long been recognized as a candidate material for optical applications, such as spatial light modulation and real-time holographic image processing. However, the lifetime of 10 ms of the M-state is too short for these applications. In this paper, we used 1,4,7,10-tetraoxa-13-azacyclopentadecane (aza-15-crown-5) as chemical additive to prolong the lifetime of the M-state. A series of bR-gelatin films with bR/aza-15-crown-5 weight ratio of 1:1.3,1:2,1:3 were prepared at neutral pH, and the photodynamics of these films were investigated by UV-vis spectrophotometry. It has been shown that the film with bR/aza-15-crown-5 weight ratio of 1:3 has the longest M-state lifetime of 360s (T1/e), which is about four orders magnitude over 10 ms of native bR. In addition, the thermal relaxation process of this film shows that the M-state is not yet empty after an hour. The present study suggests that aza-15-crown-5 has strong complexing property to H+, hence reducing the number of protons available for the M-state bR molecules to recapture to relax back to the B-state.