Spiropyran-modified polypeptides

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... 

In both cases (1 and 2) although the reason for the observed transitions on the basis of the photoisomerization of the spiropyran unit remains to be clarified, the spiropyran-modified polypeptides can be considered as macromolecular models resembling the behavior of naturally occurring photoreceptors. 

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-... 

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. 

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. 

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