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Circular dichroism studies at variable temperature: urobilinoid conformation.

D A Lightner, E L Docks, J Horwitz

    Proceedings of the National Academy of Sciences of the United States of America
    |November 1, 1970
    PubMed
    Summary

    The study reveals that the molecular conformations of optically-active urobilinoids, l-stercobilin and d-urobilin, change with temperature. Solvent hydrogen bonding critically influences these temperature-dependent conformational shifts.

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    Area of Science:

    • Biochemistry
    • Spectroscopy
    • Organic Chemistry

    Background:

    • Urobilinoids are optically active compounds derived from heme degradation.
    • Their conformations are crucial for their biological functions and spectral properties.
    • Understanding their behavior in different environments is key to their study.

    Purpose of the Study:

    • To investigate the temperature dependence of circular dichroism (CD) spectra of l-stercobilin and d-urobilin.
    • To determine how solvent properties affect the conformational changes of these urobilinoids.
    • To elucidate the role of hydrogen bonding in modulating urobilinoid conformations.

    Main Methods:

    • Circular dichroism (CD) spectroscopy was employed to measure spectral changes.
    • Temperature variations were applied between 163 K and 297 K.

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  • Experiments were conducted in different solvent systems, specifically methanol-glycerol and chloroform.

    • Main Results:

    • Significant temperature-dependent conformational changes were observed for l-stercobilin and d-urobilin.
    • These conformational changes were critically dependent on the solvent's hydrogen bonding characteristics.
    • A reversal of the chiral sense of the dipyrrylmethene chromophore's helical conformation occurred in methanol-glycerol upon cooling, but not in chloroform.

    Conclusions:

    • The study demonstrates that urobilinoid conformations are sensitive to temperature and solvent environment.
    • Hydrogen bonding plays a critical role in dictating the temperature-induced conformational transitions.
    • Solvent choice can control the chiral behavior of the dipyrrylmethene chromophore in urobilinoids.