Solvent-induced conformational changes of arylazo-substituted polyaspartates.

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Copolymers of β-benzyl L-aspartate and β-[p-(phenylazo)phenethyl] L-aspartate were prepared and their conformations studied by circular dichroism (CD) spectroscopy. The copolymers with less than 40% azo amino acid residues were found to be left-handed (LH) helical, those with more azo amino acid residues right-handed (RH) helical in 1,2-dichloroethane (DCE) solution. The former undergo LH helix→random coil transition on addition of hexafluoro-2-propanol (HFIP) to their DCE solutions, the latter RH helix→LH helix→random coil transition on addition of HFIP. All polymers in DCE exhibit a negative CD band in the side-chain n-π* transition region. The side-chain π-π* CD band was observed only for the copolymers with the highest azo amino acid residues (67%). Its absolute ellipticity decreases with increasing HFIP content, finally disappearing at 10% HFIP. The sign of the n-π* CD band of the copolymer remains negative irrespective of the helix reversion, its absolute ellipticity being unchanged below 10% HFIP and decreasing on further increase in HFIP content. The results suggest solvent-induced changes in orientation or mobility of the side-chain phenylazo moieties.

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