論文の公開元へDriving the Photochemical Reaction Cycle of Proteorhodopsin and Bacteriorhodopsin Analogues by Photoisomerization of Azo Chromophores
概要
Retinal is a natural polyene chromophore that forms a highly efficient molecular energy transducer, rhodopsin, upon binding with the lysine residue in G helix of the protein opsin through a protonated Schiff base linkage.1 Photoisomerization of the retinal unit in rhodopsin catalyzes a series of photophysical and photochemical phenomena that results in the conversion of light to electrical signals in animals and to chemical energy or electrical signals in microorganisms.2 Furthermore, in microorganisms, rhodopsins bearing retinal in the all-trans configuration can act as molecular pumps, channels, and sensors.3 Proteorhodopsin (PR) and bacteriorhodopsin (BR) are well studied microbial rhodopsins that have essentially the same structures and functions.4,5 Photoexcitation of the retinal unit in PR or BR triggers a photocyclic reaction that drives the translocation of a proton across the membrane from the cytoplasmic (CP) side to the extracellular (EC) side, leading to a proton gradient that is coupled to adenosine triphosphate (ATP) synthesis.6
Azobenzene (azo), which undergoes reversible photoisomerization at its N=N double bond (Fig. 1.2), is a commonly used synthetic chromophore because of its high chemical stability, photofatigue resistance, and photosensitivity; its ready diversification; its strong UV–Vis–near-IR absorption band, which can be modified through ring substitution;7 and its readily induced and reversible photoisomerization.8 Furthermore, because of its high compatibility with biopolymers, especially proteins, azobenzene has been used to impart enzymes,9 ion channels,10 and motor proteins11 with photoswitching functions. Taking advantage of the photochromic properties of azo chromophores, the author suspected that azo derivatives could function an alternative to retinal and, thereby, modulate the photochromic properties of microbial rhodopsin and potentially lead to the development of artificial molecular machines. Moreover, unlike retinal, the molecular structure of an azo chromophore is readily modified, such that its photochromic properties could be tailored to modulate the photofunctional properties of microbial rhodopsin. The author is aware of only one previous attempt to develop azo chromophore–bound bacterioopsin derivatives, prepared from the azo chromophores (Fig. 1.3) 4-[[4´-(N,Ndimethylamino)phenyl-1´]azo]benzaldehyde (Az I) and 3-[4-[[4´-(N,Ndimethylamino)phenyl-1´]azo]phenyl-1]prop-2-enal (Az II). 12 Nevertheless, the photochemical reactions and photoinduced proton pumping functions of those azo analogues of BR were ambiguous.
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