With the growth of Information over last few decades, demands for its efficient storage and faster processing has reached new dimensions. The need of the hour seems to be development of high capacity secondary storage devices as well as faster processors. The RAM used in most computers is the same type of memory used several years ago. The limit of increasing the density of RAM has already been reached.
These limitations are more economical than physical. Presently RAM modules vary in capacity from a few hundred kilobytes to about 64 Megabytes. Anything greater than this range is both expensive and rare. In comparison a 5 cubic centimeter block of Bacteriorhodopsin,( a light transducing protein in purple membrane of salt marsh archeon Halobacterium Salinarum ) studded in a polymer matrix could theoretically store 512 gigabytes of information.
Moreover Bacteriorhodopsin modules could be run 1000 times faster meeting the demands of both speed and power of future digital computers. Also the present secondary storage devices store one dimensional (serial) information in a 2D plane which to a greater extent limits the memory bandwidth. However 3D memory devices using Bacteriorhodopsin modules store 2D information (Bit planes) throughout a volume with information partitioned in binary planes that are stacked in third dimension. One memory operation on entire bit plane, gives rise to tremendous memory bandwidth.
This has opened up gates for what are the major areas of improvements in purely electronic computers:1) Ultra dense multi tetrabit memories (of the order of 1 Tb/cm3 ).2) Highly parallel processing.3) Nearly real time write and retrieval rates.4) Data retention for longer time.
Russian scientists, under the leadership of the late Yuri Ovchinnikov, were the first to recognize and explore the potential of bacteriorhodopsin through their projects, termed ‘Project Rhodopsin’, which were intended only for military applications. Soviet military was able to make microfiche films out of Bacteriorhodopsin, known as “Biochrome”.? About the moleculeBacteriorhodopsin (bR) is a light transducing protein dye found in purple membrane of salt marsh archeon Halobacterium Salinarum . This membrane serves a a protection against harsh environment of salt marshes where the salt concentration is six times the ordinary sea water and temperature ranges around 150 degree Fahrenheit . bR allows Halobacterium Salinarum the ability to convert light energy to a metabolically useful form when conditions are unfavorable for aerobic respiration.
It does so by switching to photosynthesis by acting as a proton pump, sufficient to sustain respirative oxidative phosphorylation.Structurally, it is ???26 KD (400nm*400nm*500nm) composed by a sequence of 248 residues arranged in 7 trans-membrane ?-helices and 1 ?-sheet .It also contains a retinal chromophore linked through a protonated Schiff base to Lysine-216.This cromophore gives the protein photoactive properties that on light excitation of certain wavelengths generates different isomerization states. The photocycle of bR consists of these various isomeric states.
? Photocycle of bR and data recording principlebR consists of two different paths of isomerization reactions. The main photocycle consists of the left hand side of the figure. In course of studying the material a branching reaction was identified marked as P and Q states. The first path starts at the state called bR which is the most stable state of the protein.
Green light induction with a wavelength of approx... 600nm triggers the first chain reaction that causes the protein to change its isomery in six different states.
. From bR the following states are K, L, M, N and O. If only blue light is applied after O state the protein returns to the initial stable state bR.Inorder to reach another stable state Q another light excitation is necessary during O state.
Red light of wavelength around 690 nm has to be induced before the reaction reaches bR state.The intermediate is converted from O state to P which subsequently decays to Q.Q state is the one used for recording data. Any two long lasting states Q or bR can be assigned value 0 or 1 ,making it possible to store information in a series of bR molecules in one state or the other. The entire photocycle takes 6-10 milliseconds to complete depending upon temperature.
The green light can be termed as the paging, the blue light as erasing pulse while the red light here acts as the writing pulse. In real devices the most important part is the precise control of the timing of the reaction The data beam and the addressing beam not only must intersect at a given data plane but also must follow a timed sequence with the paging pulse activating the media followed by the writing pulse 2ms later changing O state to P state and subsequently to Q.? Data writing techniqueA cube of bR in Acrylamide polymer gel is surrounded by two arrays of laser beams placed at 90 degree angles to each other. One of the array of lasers, all set to green (paging beam), activates the photo cycle of the protein in a square plane.2 milliseconds after a second array is programmed to strike only the region of activated square planes where the data bits are to be written, switching the molecules there to P state. The P intermediate then quickly relaxes to highly stable Q state.
The rest of the molecules in O state thermally revert to most stable bR state. The stable state bR is assigned a binary value 0, and P and Q states assigned a binary value of 1. As, the laser array can activate molecules at various places throughout the selected plane, multiple data locations can be written simultaneously i.e.
parallel.? Data Reading TechniqueThis process relies on selective absorption of red light by O intermediate state of bR.First the green paging beam is fired at the square of protein to be read. After 2 milliseconds, the entire red laser array is turned on at very low intensity of red light.
The molecules in binary state 1(P or Q) do not absorb red light or change their states as they have already been excited by intense red light during the data writing stage. However, the molecules which are in the binary state 0(bR state) do absorb low intensity red beams. A detector then images the light passing through the cube of memory and reads the binary codes 0 and 1.? ConclusionAs a future perspective in biomolecular computing Bacteriorhodopsin can easily be marked as having promising utility. With high density data storage capability, milliseconds of data read and write time, femtoseconds of switching time this could well be projected as a primary, secondary or tertiary storage device.
Keeping in view its two photon excitation property it could replace many electronic components associated with computers efficiently such as ultra-fast random access memory, neural logic gates , optical switches , picosecond photo detectors. Thus, bR is a possible material for hybrid computers of superior performance.
