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Solutions of biliverdin dimethyl ester (I) and its monoprotonated form (IH+) exhibit a solvatochromic and thermochromic behaviour. Based on absorption, fluorescence and fluorescence excitation, and solvent-induced circular dichroism (SICD) data, these properties are interpreted in terms of variations of the relative populations of conformers (including readily interconvertible configurational isomers). Thus, folded helical forms of I and IH+ are more abundant at room temperature in, e.g., ethanol than in ethyl lactate, and stretched conformations other than the already known stable (E, Z, Z) and (Z, Z, E) photoisomers are quite generally favoured with increasing temperature. Irradiation of I in ethanol at 173 K increases the population with stretched, thermally equilibrating conformers. IH+ in ethanol undergoes an irreversible photochemical change at room temperature or below. Its nature has not yet been identified.

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