Phytochrome Models: Part 10. Concentration, Sonication and Temperature Affecting the Population of the Ground‐state Conformers of Biliverdin Dimethyl Ester in Solution*

UV‐vis absorption and fluorescence spectroscopy were used to show that in alcoholic solutions (methanol, ethanol, 2‐methylbutan‐1‐ol, and 2‐octanol) concentration, temperature, and sonication affect the relative populations of the more rigid stretched and the more flexible helically coiled conformers of biliverdin dimethyl ester. In aromatic solvents, e.g. toluene, no such effects could be detected. At concentrations larger than 3 10‐5M in the alcoholic solvents aggregates and monomeric coiled species prevail. Upon dilution, the fraction of stretched monomeric conformers increases. Heating and sonication at concentrations smaller than 3 10‐5M further and irreversibly increase the fraction of the stretched conformers. The activation parameters for this change are compatible with the assessment that no equilibrium is attained between the two families of conformers even after several hours at room temperature, since the barriers arc higher than kT at this temperature. The present results and those previously reported on the excited‐state processes of biliverdin dimethyl esters are explained on the basis of a scheme in which the eight possible ring B/C isomers of biliverdin dimethyl ester are interconverted through intra‐and intermolecular proton transfers, rotations around C‐C single bonds, and E‐Z photoisomerizations around C = C double bonds.