Polypeptides photochromic

Other photochromic polypeptide systems have been described, in which the ability to photocontrol the specific conformation of polypeptides is an essential feature in the design of biomaterials for devices that can be photo switched. 

Photochromic polymers have been used in order to develop artificial membranes with particular physical properties and functions, such as permeability, conductivity, and membrane potential, that can be switched on and off or otherwise controlled in response to light.17,105,106 More specifically, photochromic polypeptides have been selected as useful materials due to their ability to undergo photoinduced structural change. 

The investigation was then extended to a monolayer formed from dipalmitoyl phosphatidyl choline and the same amphiphilic photochromic polypeptide XXIII.11211 When the monolayer was kept in the dark, the polypeptide molecules arranged themselves perpendicularly to the membrane (the water/air interface) and formed a bundle of helices which could be observed by atomic force microscopy as a transmembranous particle of about 4 nm in diameter. Irradiation with UV light and the consequent trans—>tis isomerization of the azobenzene moiety caused a bending of the molecular main chain, which in turn produced a destabilization and dena-turation of the bundle of helices in the monolayer. After removal of the light, the polypeptide molecules reverted to their original bundle structure. 1211... 

From this point of view, polypeptides containing photochromic units in the side chains are quite special polymers. They can exist in ordered or disordered conformations, and photoisomerization of their photochromic side chains can produce order = disorder conformational changes. These photostimulated structural variations, such as random coil a-helix, take place as highly cooperative transitions therefore photochromic polypeptides actually work as amplifiers and transducers of the primary photochemical events occurring in the photosensitive side chains. 

Topics reviewed during the year include the photochemistry of indoles, sulfoxides, pyrazoles and isothiazoles, (S-hetero)cyclic unsaturated carbonyl compounds, photoinduced single electron transfer (SET) reactions of amines and of azo compounds, SET reactions of organosilanes and organostannanes with Qo and ketones, photochromic polypeptides and di(hetero)arylethenes, processes in chromophore sequences on a-helical polypeptides,aryl-aryl coupling in furans, thiophenes and pyrroles," [3+2]cycloaddition of aromatic nitriles (and esters) with alkenes, and reactions of benzylsilane derivatives. ... 

Provided that an optically active molecular aggregate is photochemically perturbed to change the state of molecular alignment, the effect of a chiral environment on an achiral chromophore incorporated in the molecular aggregate will be also altered. It has been known that polypeptides bearing photochromic side groups change their optically active properties as a result of photochromic reaction(10-12). This phenomenon is likely to be related to non-linear photoresponsiveness. 

These polypeptides exhibited photochromic behavior associated with the photoisomerization of their azobenzene moieties, as discussed in Section 13.1 (Figure 1). At room temperature, in the dark, azo groups exist in the more stable trans config-... 

Poly(L-lysine) containing azobenzene units linked to the side chains by means of a sulfonamide function (Scheme 4, Structure VI), was obtained by treating poly(L-lysine) with p-phenylazobenzenesulfonyl chloride. The poly(a-amino acid) was modified quantitatively conversion to the azo-lysine units of VI was effectively 100%. The azo-modified polypeptide was soluble in HFP, in which it exhibited an intense photochromism attributed to the trans-cis photoisomerization of the azobenzene units. Like other sulfonated azobenzene compounds, 33 azosulfonyl-modified polymers of L-lysine were found to be very stable in their tis form, and no thermal decay was observed at room temperature over periods of times as long as several weeks. Interconversion between the two forms at room temperature could only be effected by irradiation at appropriate wavelengths. This behavior allowed the authors to purify the trans and cis forms of the model compound NE-azobenzenesulfonyl-L-lysine (VII) by chromatography, and to measure the absorption spectra of the two pure photoisomers. 

A photochromic polymer containing azobenzene units has also been prepared by modification of a naturally occurring microbial poly(E-L-lysine) (Scheme 5, Structure IX), and investigated by means of absorption and circular dichroism spectroscopy.1431 The structure of this polymer, however, does not correspond to those of polypeptides, which are poly(amide)s of a-amino acids, and therefore the results cannot be discussed in terms of the typical polypeptide structures (a-helix, P-structure, random coil) and their standard CD spectra. 

In nature, polypeptides with amphiphilic structures are known to form transmembrane channels formed by an assembly of several helices, so as to present their polar faces inward and their apolar faces outward. In view of such behavior, the photochromic amphiphilic polypeptide was incorporated into a cationic bilayer membrane composed of dipalmitoyl phosphatidyl choline.11201 Fluorescence and microscopic measurements provided evidence that the polypeptide was able to form bundles of helical molecules analogous to their natural counterparts, which acted as transmembrane channels for K+ ions. Irradiation, and the consequent transacts isomerization of the azobenzene link, caused a bending of the molecular structure and a destabilization of the transmembrane bundles. Therefore, formation of ion permeable channels would be favored or inhibited depending on whether the azo moiety... 

Like azobenzenes (III.A), polypeptides carrying spiropyrans have been studied to control the conformations of polypeptides. In polar solvents such as hexafluoropropanol, the colored merocyanine form is thermally more stable than the spiro form. Therefore it shows reverse (or negative) photochromism irradiation by visible light on the colored species bleaches it, while the spiro form formed... 

When azobenzenes are attached to polypeptides, photochromic reactions of azobenzenes can induce the change in helical properties of the polypeptides, which may be detected by CD spectrum as well as optical rotation. For 4-phenylazophen-ylamine-condensed poly(y-glutamic acid) A-9 containing up to 80 mol% of 4-phenylazophenylamide side chain, UV irradiation in organic solvents, such as... 

In contrast to the spiropyran-modified polypeptides derived from poly(L-glutamic acid), the poly(L-lysine) analogs have been reported to show no photomodulation of the side-chain conformation in pure HFP, although they exhibited similar photochromic behavior (Figure 2).32,33 However, when appropriate amounts of triethylamine were added to the HFP solution, exposure to visible light resulted in the reversible formation of a helix by the poly(L-lysine) chains.35 Thus, addition of triethylamine to the HFP solution induced the coil - helix transition, but the amount of base necessary was different for the dark-adapted sample as compared with the irradiated one. The authors claimed... 

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