Interactions of chlorophyll-derived photosensitizers with human serum albumin are determined by the central metal ion

Abstract

Two structurally similar derivatives of chlorophyll a, chlorophyllide a (Chlide) and zinc-pheophorbide a (Zn-Pheide), differing only in central metal ion (Mg²⁺ or Zn²⁺, respectively) substituting the tetrapyrrole ring, were investigated with regard to their binding to human serum albumin (HSA). Chlide and Zn-Pheide are very promising photosensitizers with potential application in photodynamic therapy, therefore it is desirable to investigate their interactions with serum proteins. The studies included absorption and steady-state fluorescence spectroscopy, as well as molecular docking. It was found that both investigated compounds form complexes with HSA. Experimental data revealed two classes of binding sites for each compound. The affinities (K_a) for the first class were in the range of 10⁵ and 10⁶ M⁻¹ for Chlide and Zn-Pheide, respectively, while the second class was characterized by the affinities of the order of 10⁴ M⁻¹ for both derivatives. Molecular docking simulations together with displacement studies revealed that the primary binding site of the studied compounds is the heme site, localized in the subdomain IB, however the best characterized binding sites of HSA, namely the Sudlow’s sites I and II are also involved. The interactions between the derivatives of chlorophyll and HSA were found to be predominantly hydrophobic and to a lesser extent hydrogen bonding. Our results demonstrate that the centrally bound metal ion determines both the affinity and mode of binding to HSA, which may be a feature differentiating these compounds in terms of their pharmacokinetics.

Communicated by Ramaswamy H. Sarma

Keywords:

• Chlorophyll derivatives
• photosensitizers
• human serum albumin
• molecular docking
• spectroscopy

Acknowledgements

The author is grateful to Prof. Leszek Fiedor and Dr. Małgorzata Szczygieł (Jagiellonian University, Kraków, Poland) for kindly providing chlorophyllide a. Many thanks to Prof. Joanna Grzyb (University of Wrocław, Poland) for sharing the spectrofluorometer and providing valuable advice. Prof. Andrzej Gamian (Hirszfeld Institute of Immunology and Experimental Therapy, Polish Academy of Sciences, Wrocław, Poland) is gratefully acknowledged for critical reading of the manuscript. We also thank Tomasz Ziółkowski for kind help with the software installation.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by the grant from the National Science Centre (2015/19/N/NZ1/00578).