Liposome binding constants and singlet oxygen quantum yields of hypericin, tetrahydroxy helianthrone and their derivatives: Studies in organic solutions and in liposomes

Mary Roslaniec, Hana Weitman, Dalia Freeman, Yehuda Mazur, Benjamin Ehrenberg

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88 Scopus citations

Abstract

The spectroscopy and photophysics of several hypericin and helianthrone derivatives were studied in methanol and when bound to liposomes. The singlet oxygen quantum yields (Φ(Δ)) were measured indirectly relative to Rose Bengal and hematoporphyrin IX, employing 9,10-dimethylanthracene as a singlet oxygen trap. Hypericin was found to have a Φ(Δ) of 0.39±0.01 in methanol, and 0.35±0.05 in lecithin vesicles, in agreement with literature values. A heavy atom effect was evident upon bromination, resulting in Φ(Δ) for tetrabromohypericin of 0.72±0.02, presumably due to enhanced intersystem crossing. Elimination of the anionic hydroxyls by methylation also enhanced Φ(Δ) to 0.81±0.01. Conversely, addition of anionic sulfate groups drastically reduced Φ(Δ) resulting in Φ(Δ)'s of 0.12±0.01, 0.052±0.003 and 0.40±0.01 for hypericin disulfonate, hypericin tetrasulfonate and hexamethyl hypericin tetrasulfonate, respectively. The non-sulfonated helianthrones exhibited low Φ(Δ)'s in solution. The liposome binding constants, K(b), were measured using a spectroscopic assay. Except for hexamethyl hypericin, all non-sulfonated compounds bound well with K(b)'s ranging from 15.5±0.1 to 48.7±3.9 (mg/ml)-1. None of the tetrasulfonated compounds bound, however the hypericin disulfonate had a K(b) of 4.1±0.2 (mg/ml)-1. The Φ(Δ)'s of the compounds capable of binding were measured and, in the case of the hypericin derivatives, were found not to vary dramatically from those in the free state. Liposome-bound helianthrone and dimethyl tetrahydroxy helianthrone both exhibited high Φ(Δ)'s, i.e. >0.5. The variations in binding constant and sensitization efficiencies are explained in conjunction with the molecular structure. The relevance of the above data to photodynamic therapy is briefly discussed. (C) 2000 Elsevier Science S.A. All rights reserved.

Original languageEnglish
Pages (from-to)149-158
Number of pages10
JournalJournal of Photochemistry and Photobiology B: Biology
Volume57
Issue number2-3
DOIs
StatePublished - Sep 2000

Bibliographical note

Funding Information:
This work was supported by the Falk Chair in Laser Therapy.

Funding

This work was supported by the Falk Chair in Laser Therapy.

FundersFunder number
Falk Foundation

    Keywords

    • Helianthrone
    • Hypericin
    • Liposomes
    • Membrane
    • Photosensitization
    • Singlet oxygen quantum yield

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