Azobenzene-containing Poly L-lysine

Polymers with various contents (20-90 mol%) of azobenzoyl-L-lysine residues (V) are soluble in hexafluoro-2-propanol (HFP), in which they exhibit very similar CD spectra, independent of their azo content. The dark-adapted samples (azo groups in trans configuration) display the a-helix CD pattern below 250 nm. Weak bands are also present in the range of wavelengths between 250 and 500 nm, arising from 

Since the absorbance values of the pure trans and the pure cis isomers are known, the trans-cis isomeric composition at the photostationary state can be determined on the basis of the equations  

Poly(L-lysine) containing azobenzenesulfonyl groups in the side chains (VI) was random coil in pure HFP, and the disordered conformation was not affected by 

Of course, stability and formation of HFP-azosulfonyl-Lys complexes is less favored on going from pure HFP to HFP/MeOH or HFP/DCE solvent mixtures. At appropriate and critical solvent compositions, the formation of the electrostatic complexes described above might be favored or inhibited by the electronic situation of the azo moieties, which differs depending on whether they are in the apolar, conjugated trans form, or in the more polar, unconjugated cis form. In other words, the trans-cis photoisomerization of the azo units, which is the primary photochemical event, does not seem to be sufficient to induce appreciable variations of the back- 

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Fissi, A., Pieroni, O., and Giardelli, F. Photoreponsive Polymers Azobenzene Containing Poly(L-Lysine). Biopolymers 26, 1993 (1987). 

Malcolm, B. R., and O. Pieroni, O. The Photoresponse of an Azobenzene-Containing Poly(L-Lysine) in the Monolayer State. Biopolymers 29, 1121 (1990). 

Tedeschi, C., Fontana, M. P., Pieroni, O., Dei, L., Wilde, J., Pearson, C., Petty, M. C., and Tanner, B. K., Deposition and Characterisation of Langmuir-Blodgett Films of an Azobenzene-containing Poly-L-lysine. Thin Solid Films 335,197 (1998). 

A. Fissi, O. Pieroni, and F. Ciardelli, Photoresponsive polymers Azobenzene-containing poly(L-lysine), Biopolymers 26, 1993-2007 (1987). 

Malcolm BR, Pieroni 0.1990. The photoresponse of an azobenzene containing poly(L lysine) in the monolayer state. Biopolymers 29(6 7) 1121 1123. 

The photochromic behaviour of azobenzene-containing poly(L-lysine) has also been reported in the monolayer state [19]. When the polypeptide monolayer is kept at constant area, alternate irradiation with visible and ultraviolet light produces reversible changes ( 25%) of the surface pressure of the monolayer. 

Menzel1811 more recently described the properties and behavior of monolayers prepared from azobenzene-containing poly(L-glutamate)s possessing the structures XIX and XX (n= 2) shown in Scheme 11. These monolayers showed photomechanical effects opposite to those described above for azo-modified poly(L-lysine)s. In fact, they expanded when exposed to UV light (trans—>tis isomerization), and shrank when exposed to visible light (ds— trans isomerization). The expansion was found to be smaller than expected from comparison of the monolayer isotherms obtained from irradiated and nonirradiated solutions. This was attributed to the trans-cis photoconversion of the azo units, which occurs with lower yields in monolayers... 

The introduction of azobenzene units into the side chains of poly(L-lysine) has been achieved be means of various procedures and different azo reagents. The polymers described initially contained azobenzene units linked to the lysine side chains by means of an amide moiety 29-311 (Scheme 4, Structure V). More recently, Fissi et al. 321 have described azo-modified poly(L-lysine) in which the azobenzene units are linked to the Lys side chains by means of a sulfonamide function (Scheme 4, Structure VI). The two families of azo-modified poly(L-lysine) have been found to exhibit completely different conformational and photoresponsive behavior. 

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. 

Gallot B, Fafiotte M, Fissi A, Pieroni O. 1996. Liquid crystalline structure of poly(L lysine) containing azobenzene units in the side chain. Macromol Rapid Commun 17(8) 493 501. 

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. 

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