In 1971, the first halobacterial retinal protein was discovered and named bacteriorhodopsin. Bacteriorhodopsin (BR) is a protein of 248 amino acids that is integrated into the bacterial membrane. It is a light driven pump, moving protons from the inside to the outside of the cell. The resulting gradient drives the production of ATP (adenosine triphosphate) for the organism. In the 1970s, rapidly growing research activities focused on the biochemical and biophysical properties of BR and resulted in a linearly increasing number of publications per year. The constant level of about 200 publications/year in the last decade signifies the steadily growing knowledge about the structure and bioenergetic function of BR and related molecules. Nowadays, BR is one of the best-investigated membrane proteins (e.g., References 3–10).
Only five years after the unusual chemical and photophysical properties of BR were reported, several articles appeared suggesting technical applications of BR for sunlight energy conversion. Although no commercial applications of BR have resulted from these studies, new ideas for application have arisen, covering about 10% of the total publications on BR during the last two years. Is this an indicator of real potential for BR as a biological material in technology?