Photomodulation of Polypeptide Conformation

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. 

On the other hand, in 1989 Ciardelli et al. at the Universtia di Pisa reported the first photomodulation of the conformation of spiropyran-containing poly- 

Phn4Qchmniic helm-tor of spiro pv lan-modlficd pcriytL-gluiainic acid) I (in hc afluoro-2-prupanol (HKP). 

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. 

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F. Ciardelli, D. Fabbri, O. Pieroni, and A. Fissi, Photomodulation of polypeptide conformation by sunlight in spiropyran-containing poly(L-glutamic acid), J. Am. Chem. Soc. Ill, 3470-3472... 

Following up the previous work, the poly[Asp(OBzl)] 181 modified with different proportions of a diazobenzene moiety linked via an ester group was prepared by the co-polymerization of the respective NCAs of 179 (180) and L-Asp(OBzl) (Scheme 50)J92-94 Again, the idea was to use the well-known photoinduced cis-trans isomerization of the diazobenzene group to control the helical sense of the polypeptide framework. Impressive results obtained by CD showed that indeed the sense of the helix could be modulated by irradiation of the diazo group at 320-390 nm.[ l However, the changes observed are sensitive to the nature of the solvent. The photomodulation of the conformation was also observed with the para analogue of 181. 

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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