Radiation imagery chemistry: process – composition – or product th – Holographic process – composition – or product
Reexamination Certificate
1995-02-03
2001-08-14
Angebranndt, Martin (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Holographic process, composition, or product
C430S002000, C430S945000, C359S003000, C365S125000, C365S121000, C369S110040, C369S288000, C530S350000
Reexamination Certificate
active
06274279
ABSTRACT:
The invention relates to preparations of bacteriorhodopsin variants having increased memory time, and their use for reversible optical information recording.
The retinal protein bacteriorhodopsin is a biological photochrome which is distinguished by its high thermal, chemical and photochemical stability and its high photosensitivity ‘P. Kouyama, K. Kinositu and A. Ikegami: Structure and Function of Bacteriorhodopsin, Adv. Biophys. 24 (1988), pp. 123-175’. In applications, this retinal protein is preferably employed in the form of a purple membrane. Purple membranes are built up from a dense, two-dimensional, usually crystalline arrangement of the bacteriorhodopsin included in a lipid double layer. This form has particularly high stability; bacteriorhodopsin which has been dissolved out is much more unstable.
Owing to its photochromic property, this retinal protein can be used for optical information recording. When exposed to light, a configuration change is induced in the retinal residue. This change in configuration is reversible both thermally and photochemically. In wild-type BR, it proceeds from all-trans in the initial state to 13-cis in the longest-lived intermediate. Associated with this is a reversible change in the protonation state of the Schiff base bond by means of which the retinal residue is coupled to the protein.
In optical information recording, a distinction is generally made between short-term memory, which is required in data processing, and long-term memory, as is necessary for information storage.
The advantages of purple membranes for these applications lie in the high resolution that can be achieved, since the structure size of the active elements, i.e. the bacteriorhodopsin molecules, is in the order of a few nm, in the high light fastness of the material and in the excellent shelf life.
A review of possible industrial applications of purple membranes is given in ‘D. Oesterhelt, C. Bräuchle, N. Hampp: Bacteriorhodopsin: A Biological Material for Information Processing. Quarterly Reviews of Biophysics, 24 (1991), pp. 425-478’. This article also describes preparations, in particular films for holography, in which transient recordings of optically written information is in the forefront.
In addition to these applications as short-term memories in the area of information processing, there is also interest in storing optical information for a longer period. There is particular interest in media which can be written by means of light of a first wavelength &lgr;1 and erased with light of a second wavelength &lgr;2. These reactions can be described formally by a reaction scheme E-(&lgr;1)-P-(&lgr;2)-E, where E is the starting or initial state of the material and P is a long-lived photoproduct of E, known as the memory state, which is itself likewise photochemically active.
The memory time is defined as the period within which 36.8% (1/e) of the photoproduct relaxes without exposure to light, i.e. for example, thermally, into another state, for example the starting state. The memory time of a medium is proportional to the life of the state serving for information storage.
The memory time can be determined by measuring the time-dependent change in absorption at the maximum of the absorption band of the initial state or of the memory state.
If the absorption state of a storage medium is used for coding information, the achieveable contrast ratio, which is in turn wavelength-dependent, is the determining factor for the signal
oise ratio. The contrast ratio is defined as the quotient of the absorption of a photochromic material before and after exposure. The maximum contrast ratio is achieveable at the wavelength at which the difference spectrum between the unexposed and exposed material has a maximum. At this point, the contrast ratio is limited only by the maximum achievable conversion of the starting material into the photoproduct.
Of considerable interest for industrial applications is the number of possible write/erase cycles, i.e. how often the material can be switched to and fro between the states E and P using light. This number should be as large as possible.
A general problem in the use of photochromic materials for reversible information recording is that reading or scanning of the stored information with light wavelengths in the region of the absorption of initial or memory state results in a change in the information and thus a reduction in the signal
oise ratio. In the case of digital information (black/white), this can be compensated by refreshing the information after reading. In the is case of analog signal recording, this is impossible in practice. For this reason, photochromic materials are virtually never used for reversible recording of analog signals (grey value recording).
The photocycle of wild-type bacteriorhodopsin (BR
WT
) is known. The longest-lived intermediate in this photocycle is known as the M-state. The absorption maximum of the M-state, which is at 410 nm, differs by 160 nm from the initial state of BR
WT
, which is known as the B-state and has an absorption maximum of about 570 nm. Irradiation of the B-state with light having a wavelength in the region of its absorption band, for example having the wavelength 570 nm±60 nm, initiates a photochemical reaction of high quantum yield (on average≧64%) which results in population of the M-state within 50 &mgr;s. In an aqueous suspension of BR
WT
purple membrane, the M-state has a life of about 10 ms. The M-state can be switched back into the initial state by means of light having a wavelength in the region of its absorption band, for example the wavelength 410 nm. In this form, the B
WT
purple membrane is unsuitable as an information memory. In order to be able to employ BR
WT
as an information memory, the M-state must have the longest possible life.
Two possibilities are known for increasing the life of the M-state:
1. Low temperatures: At temperatures of <−50° C., the M-state can be “frozen”, i.e. its thermal relaxation is suppressed ‘S. P. Balashov, F. Litvin, Photochemical Conversions of Bacteriorhodopsin, Biophys. J. 26 (1981), pp. 566-581’. The possibility of returning BR
WT
into the ground state at these temperatures by photochemical means using blue light, for example 410 nm, is retained.
2
. Dehydration: By removing water from BR
WT
preparations, the life of the M-state can be extended to about 150 sec. ‘R. Korenstein, B. Hess, Hydration Effects of cis-trans Isomerization of Bacteriorhodopsin, FEBS Lett. 82 (1977), pp. 7-11’, ‘Z. Chen, A. Lewis, H. Takei, I. Nebenzahl, Application of Bacteriorhodopsin Oriented in Polyvinyl Alcohol Films as an Erasable Optical Storage Medium, Appl. Opt. 30 (1991), pp. 188-196’, ‘T. V. Dyukova, N. N. Vsevolodov, L. N. Chekulaeva, Change in the Photochemical Activity of Bacteriorhodopsin in Polymer Matrices on its Dehydration, Biophysics 30 (1985), 668-672’.
The use of purple membranes containing BR
WT
at low temperatures has the following disadvantages, inter alia:
Permanent cooling is necessary during use. Special equipment must be used to exclude condensation of atmospheric moisture on the storage medium. Loss of data is possible due to failure of the electric supply for cooling or due to other technical defects. These disadvantages are unacceptable for industrial use, the use of BR
WT
is therefore of no industrial importance.
The same applies to the extension of the M-life by dehydration. This has the disadvantage, inter alia, that the achievable memory times in the region of minutes are orders of magnitude below the memory times required industrially in the region of from one day to 10 years.
It is furthermore known that purple membranes containing BR
WT
are converted into a blue form at pH values of less than 3. This form is known as ‘blue membrane’ (K. Kimura, A. Ikegami, W. Stoeckenius, Salt and pH-dependent Changes of the Purple Membrane Absorption Spectrum, Photochem. Photobiol. 40 (1984), pp. 461-464). This form is also obtained if ions, in particular divalent cations (for example Ca
2+
, Mg
2+
)
Brauchle Christoph
Hampp Norbert
Oesterhelt Dieter
Popp Andreas
Angebranndt Martin
Collard & Roe P.C.
MIB Munich Innovative Biomaterials GmbH
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