Synthesis and characterization of photoswitchable lipids containing hemithioindigo chromophores.

The synthesis of four lipids containing the hemithioindigo chromophore as part of the fatty acid is described. Heck reaction of bromophenyl thioacetate esters with acrylonitrile, followed by reduction, ester hydrolysis, and Friedel--Craft acylation--cyclization gave a substituted thioindoxyl that condensed with an alkoxy benzaldehyde to produce the hemithioindigo. "Solventless" nitrile hydrolysis followed by mixed anhydride coupling of the acid with glycerophosphocholine produced lipids bearing two hemithioindigo chromophores. The photochemistry of various hemithioindigo derivatives was studied to confirm the expected photoisomerization in both homogeneous organic solution, and in vesicle bilayer membranes. Characteristic changes in the UV--visible spectra are consistent with fully reversible Z--E photoisomerization. Chromatographic separation of the Z and E isomers of a compound containing a single hemithioindigo chromophore confirmed the spectroscopic analysis and provided a quantitative analysis of the compositions of Z--E isomer mixtures.