Poly spiropyran-modifie

Sunlight-induced Conformational Transitions in Spiropyran-containing Polypeptides 13.2.2.1 Spiropyran-modified Poly(L-glutamate)s... 

The structures of spiropyran-modified poly(L-glutamate)s are strongly affected by light or dark conditions, as demonstrated by the CD spectra in Figure 10. Before irradiation, the colored solutions show the CD spectrum of a random coil conformation. After exposure to sunlight, the colorless solutions display the typical CD pattern of the a-helix, thus indicating that the isomerization of the side chains causes a transition from coil to helix in the polypeptide chains. The photoinduced conforma-... 

Quite interesting photoresponsive behavior was observed when spiropyran-modified poly(L-glutamate) was dissolved in HFP and a small amount of trifluoroacetic acid... 

Fig. 13 Reverse photochromic reactions of spiropyran-modified poly(L-lysine) (XVI) in hexafluoro-2-propanol.

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... 

The spiropyran-modified poly(L-glutamic acid) PGA-2 undergoes a coil helix transition upon exposure to visible light in hexafluoro-2-propanol solution. In the dark, the polypeptide, containing 30-80 mol% chromophore units in the open charged form, adopts a random coil conformation. Irradiation causes isomerization in the side chains, as indicated by complete bleaching of the colored solution (see Scheme 5.4). The formation of the colorless and uncharged spiropyran form induces spiralization of the polypeptide chain. The coil helix transition can be followed with the aid of CD spectra, as shown in Fig. 5.4. 

Spiropyran-modified poly(L-glutamate)s in hexafluoroisopropanol (HFP) exhibit reverse photochromism , that is, a photochromic behavior which is... 

The analogy of these reversible processes with those observed in spiropyran-modified poly(Glu) suggests the occurrence of similar photochromic reactions. Accordin y, HFP stabilizes the colored ionic merocyanine structure, while irradiation gives the colorless spiro structure. 

Poly(L-glutamate)s containing various molar percentages of spiropyran units in the side chains (XV) have been prepared by treating poly(L-glutamic acid) with N-(2-hy-droxyethyl) -spiropyran in the presence of dicyclohexylcarbodiimide and 4-pyrrolidino-pyridine.[60 62] The structure and photochromic behavior of the modified polymers are shown in Figure 8. 

Photomechanical effects have been also observed in monolayers obtained from poly(L-glutamic acid) modified with carbocyanine1831 and spiropyran dyes.184 In the latter case, irradiation at 254 nm produced changes in the molecular conformation, which in turn caused photomodulation of the surface pressure and surface area of the films. From all these examples, it appears that photoresponsive monolayers are quite fascinating systems, which may eventually come to be regarded as a machine to transform light into mechanical energy . 81 ... 

Synthetic polymers containing photochromic units can undergo reversible changes of their physical and chemical characteristics. (For recent reviews, see Refs. 24-26). Recent examples involving photochromic compounds other than spiropyrans are reported in Refs. 27-31. Spiropyran-containing polymers such as polyacrylates have also been prepared and were found to show photoinduced variations of their viscosity.18- 0 The change in the viscosity of the polymers partly reflects the polymer conformation. Thus, spiropyran-attached poly(L-tyro-sine) and poly(L-lysine) were synthesized by Vandewyer and Smets at the University of Louvain in 1970.32 33 No photoresponsiveness was observed however, for these modified peptides. 

Light-induced transformations from one structure to another have been studied with many modified polypeptides [13, 14] bearing pendant photochromic groups such as azobenzene or spiropyran groups. Typical examples are the modified poly(L-glutamic acids) PGA-1 and PGA-2 presented in Chart 5.5. 

In addition to spiropyrans, azobenzenes have also been incorporated into polymer Aims to modulate cell adhesion [273]. For instance, polyelectrolyte multilayers of poly(acrylic acid)-poly(allylamine hydrochloride) (PAA-PAH) functionalized with RGD-modified azobenzene side chains were used to produce tunable substrates with different intrinsic capacities to support cell adhesion and survival [273]. Light-driven isomerization from trans to cis of the azobenzene on the PAA-PAH-RGD polymer film resulted in an enhanced cell adhesion. 

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