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Abstract. Measurements of the temperature dependence of the circular dichroism spectra of l-stercobilin and d-urobilin show that the conformations of these optically-active urobilinoids change with temperature between 163 and 2970K. These conformational changes depend critically on the hydrogen bonding characteristics of the solvent. Thus, in methanol-glycerol (9:1), the chiral sense of the helical conformation of the dipyrrylmethene chromophore is reversed on lowering the temperature, whereas in chloroform, reversal does not occur.The optical activity of certain urobilinoid members of the bile pigment family was recognized in l-stercobilin (1)1 and d-urobilin (2).2 Later, Gray, Jones, Klyne, and Nicholson3 measured the optical rotatory dispersion (ORD) curves for the hydrochlorides and noted the large amplitudes of rotation associated with the intense dipyrrylmethene absorption in the vicinity of 490 nm. The origin of the optical activity was more fully explained by Mioscowitz et al.,4 who reported more extensive ORD measurements on 1 and 2. Their interpretation of the data emphasized the importance of internal hydrogen (H) bonding, of the type depicted in Fig. 1, which can constrain the dipyrrylmethene chromophore to twisted (helical) conformations of a single chiral sense. The chiral dipyrrylmethene chromophore so generated falls into the class of inherently dissymmetric chromophores,5 and the extremely large rotations observed in the vicinity of the 490 nm absorption,3 4 are typical for this class of chromophores.5 Con-CH2CH2COOH CH2CH2COOH

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