Optoelectronic Applications of Bacteriorhodopsin and its Mutants as Interfacial Layers of the Solid-Liquid Junction Cell

Tsutomu Miyasaka and Koichi Koyama

Ashigara Research Laboratories, Fuji Photo Film Co. Ltd., Minami-ashigara, Kanagawa 250-0193, Japan

e-mail: miyasak-t@ashiken.fujifilm.co.jp
 

Photoelectric conversion and its potential applications to optical sensors and artificial vision systems have been studied in our laboratories by using the interfacial layer of bacteriorhodopsin (bR) immobilized at the electrochemical junction of oxide electrode and aqueous electrolyte. The bR layer emits a transient d.c. response with unique time-differential responsivity to a change in light intensity, holding linear current-intensity characteristics [1]. Although charge displacement mechanisms within the bR molecule can also be involved, the response mainly reflects a capacitic current induced by a rapid potential change (Nernst response to pH change) on the oxide electrode surface, developed by proton release and uptake events at the photo-excited bR. The response profile is thus distinctive between mutated bRís with altered proton pumping mechanisms. In the differential profile, symmetric cathodic and anodic transients indicate the moments of proton release and uptake, respectively, which are pH-dependent, with their directions reversed in acidic media [2], and are significantly influenced by amino-acid displacements in bR. Pulse excitation (YAG, 532 nm) shows a rise time of the cathodic response to be 10-5 s, which matches the step of L to M conversion in the bR photocycle. In the mutant D96N, the response lacks the anodic counterpart, while in the mutants E204Q and E194Q, the profile is reversed, indicating that proton uptake precedes the release. The response corresponds to the pH-dependent B2 [3] or B3 [4] component of the bR photoelectric signals that are conventionally assigned, with other electrochemical cells, to proton pumping step after the trans-cis photo-isomerization.

Aritificial vision is one of the fittest applications of the bR-based photocell. The differential responsivity is the essential function in mobile target detection and edge pattern perception in human retina. A model array sensor (256 pixels) with SnO2-aqueous gel junction showed that visual information processings for mobile image extraction are performed within 100µs, far faster than computer-assisted image processing, at the material level without the need of differential circuitry and external d.c. driving power for the sensor. The use of chitin gel as an electrolyte proved to protect bR from biological decay, allowing the sensor to function for years. For ultimate end of the application, however, molecular wiring to the molecule of bR to elicit its charge displacement and proton transfer events is the key subject, common to the implementation of molecular electronic devices that surpass the silicon-based ones. One for this end may be implantation of the photoelectric bR layer on the retinal membrane for the purpose of visual prosthesis for those who lack the sensory retinal but retain the neural network (sensitivity to electric stimuli).

Literature

[1] T. Miyasaka, K. Koyama, and I. Itoh, Science 255, 342 (1992).

[2] Y. Saga, T. Watanabe, K. Koyama, and T. Miyasaka, J. Phys. Chem. B 103, 234 (1999).

[3] F. H. Hong and F. T. Hong, Bioelectrochem. Bioenerg. 37, 91 (1995).

[4] J. Wang, L. Song, S. Yoo, and M. A. El-Sayed, J. Phys. Chem. B 101, 10599 (1997).