INVESTIGATION OF LIGHT-INDUCED CONFORMATION CHANGES IN SPIROPYRAN-MODIFIED SUCCINYLATED POLY(L-LYSINE)

To determine the maximum range of coupling between side-chain photochr omism and polypeptide conformation change, we modified the carboxylate side chains of succinylated poly(L-lysine) with a spiropyran to form polypeptide I. The extent of modification was determined to be 35.5%. The spacer group length between the polypeptide alpha-carbon and the d ye was 12 atoms, providing minimum polypeptide-dye interaction. Confor mation changes were monitored by circular dichroism as a function of l ight adaptation and solvent composition (hexafluoroisopropanol [HFIP] vs trifluoroethanol [TFE]), Under all solvent compositions, the dark-a dapted dye was in the merocyanine form. Light adaptation by visible li ght converted the dye to the spiropyran form. When dissolved in TFE, I adopted a helical conformation insensitive to light adaptation. With increasing percentage HFIP, a solvent-induced helix-to-coil transition was observed around 80% (vol/vol) HFIP. At 100% HFIP, both light- and dark-adapted forms of I were in the coil state. Near the midpoint of the solvent-induced helix-to-coil transition, light adaptation caused conformation changes. Applying helix-to-coil transition theory, we mea sured a statistically significant difference in coil segment-HFIP bind ing constant for light- vs dark-adapted solutions (6.38 +/- 0.03 M(-1) vs 6.56 +/- 0.03 M(-1)), but not for the nucleation parameter sigma ( 1.2 +/- 0.4 10(-3) vs 1.3 +/- 0.3 x 10(-3)). The small binding constan t difference translated to a light-induced binding energy difference o f 17 cal/mol/monomer. Near the midpoint of the helix-to-coil transitio n, collective interactions between monomer units made possible the tra nslation of a small energy difference (less than RT) into large macrom olecular conformation changes. This work parallels similar behavior ob served in poly(isocyanate) (Green, M. M. et al. J. Am. Chem. Soc. 115, 4941-4942, 1993). The subtle differences in dye-backbone interaction in I suggested a maximum coupling distance (12 atoms) beyond which pol ypeptide conformation and dye state are uncoupled.