The key function of both natural and artificial photoreceptors is the generation, upon the absorption of a photon, of a signal which can be transduced and elaborated by the biological system or the man-made device [1,2]. Photocycling chromophores incorporated into a macromolecular matrix. can constitute efficient synthetic photo-detectors/-transducers, able to mimic the performances of natural photoreceptors. The photochromic moiety undergoes reversible stereochemical rearrangements between two or more isomeric forms, the direction being determined by the wavelength of the incident light. Such an isomerization can induce conformational changes in the macromolecular matrix thus giving rise to the "photosignalling state", which yields, directly or indirectly, the functional response to light [1, 3, 4].
We have shown that spiropyran-modified poly(L-glutamate)s dissolved in hexafluoro-2-propanol (HFP) undergo reversible variations of the macromolecular structure upon exposure to light or dark conditions. The isomerization reaction of the photochromic side chains is the very primary event of the photoresponse but the actual driving force responsible for the conformational variation -helix random-coil are intermolecular interactions between the photochromic side chains producing merocyanine dimers [3]. In other words, light, bringing the chromophores either into interacting or into non-interacting isomers, controls the shape and the size of the molecular nanostructure.
In the presence of trifluoroacetic acid (TFA), however, this system can not be photomodulated, the polypeptide chain being in a random coil configuration when the chromophores are in both the merocyanine and the spiropyran form. This molecular behavior is due to the protonation of the side chains that makes the chromophores mutually repulsive, compelling the macromolecule to adopt a disordered structure. When cosolvents such as methanol (MeOH) or trifluoroethanol (TFE) are added to the HFP/TFA solutions, the repulsive interactions among the chromophores can be removed and the macromolecules can adopt the -helical structure The amount of cosolvent needed to disrupt the repulsive forces was found to be different for the the dark-adapted and the irradiated samples. So, at appropriate amounts of MeOH or TFE, the shape and the size of the nanostructure can be controlled by light, the extent of the structural variation ("Photoresponse") depending on solvent composition.
The properties of this relatively simple system, whose frame and
dimensions can be modulated by the combined action of light and of the
chemical environment, could be exploited in designing molecular nanomachines
acting as efficient photo-chemosensors/photo-chemoswitches.
References
[1] Hellingwerf, K. J., W. D. Hoff and W. Crielaard. 1996. Photobiology of microorganisms: how photosensors catch a photon to initialize signalling. Mol. Microbiol. 21: 683-693.
[2] Willner, I. 1997. Photoswitchable biomaterials: en route to optobioelectronic systems. Acc. Chem. Res. 30: 347-356.
[3] Angelini, N., B. Corrias, A. Fissi, O. Pieroni and F. Lenci. 1998. Photochromic polypeptides as synthetic models of biological photoreceptors: a spectroscopic study. Biophys. J. 74: 2601-2610.
[4] Pieroni, O. and F. Ciardelli. 1995. Photoresponsive polymeric materials. Trends Polym. Sci. 3: 282-287.