Probing the binding of tetraplatinum(pyridyl)porphyrin complexes to DNA by means of surface plasmon resonance

Abstract

Tetrapyridylporphyrins containing four chloro(2,2′-bipyridine)platinum(II) complexes attached at the meta (3-H₂TPtPyP) and para (4-H₂TPtPyP) positions of the peripheral pyridine ligands were synthesized and their interaction with DNA investigated. The compounds were isolated in the solid state and characterized by means of spectroscopic and analytical techniques. According to molecular simulations, the two isomers exhibit contrasting structural characteristics, consistent with a saddle shape configuration for 3-H₂TPtPyP and a planar geometry for 4-H₂TPtPyP. Surface plasmon resonance studies were carried out on the interaction of the complexes with calf thymus DNA, revealing a preferential binding of 3-H₂TPtPyP, presumably at the DNA major grooves.

Introduction

The biological activity of platinum(II) complexes was discovered in 1969 by Rosenberg et al. [1] and is mainly related to their ability to coordinate nucleic acids or to interact with DNA via supramolecular interactions. Since then, research concerning the interaction of platinum(II) and related metal complexes with biomolecules has expanded considerably [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], pursuing not only the discovery of new antitumor drugs, but also the understanding of structure–function aspects of metal containing proteins and nucleic acids. In this sense, we have already shown that porphyrins containing four ruthenium–bipyridine complexes at the peripheral positions are able to bind DNA and exhibit pronounced photochemical activity in inducing DNA cleavage [15], [16], [17].

A great variety of interesting supermolecules has already been obtained by attaching transition metal complexes such as ruthenium–polypyridines, ruthenium–edta, pentacyanoferrate and trinuclear μ₃-ORu₃(OAc)₆ clusters to the peripheral pyridyl-groups of metallated meso-tetra(pyridyl)porphyrins [18], [19], [20], [21], [22]. In such systems, the peripheral complexes play a wide variety of roles. For instance, they can modify the local environment around the porphyrin ring and exert protection effects against radicals and other highly reactive species [21], [22]. In addition they can improve the solvation and the solubility properties, or act in parallel, as cofactors in redox processes. They also provide tunable electron donor–acceptor groups and relays, inducing electronic effects which can change the intrinsic activity of the metalloporphyrin center [20]. Finally, they can add important sites for intra- and/or intermolecular interactions, favoring the assembly of supramolecular structures and materials.

Therefore, the association of porphyrin with platinum(II) complexes can provide an interesting route to be explored, specially for the generation of compounds exhibiting useful cytotoxic and photodynamic therapy features. In this work we report on two new, symmetric supermolecules containing four platinum(II)–bipyridine complexes directly bound to the peripheral 4-pyridine and 3-pyridine groups of meso-tetrapyridylporphyrins, i.e. 4-H₂TPtPyP and 3-H₂TPtPyP, as shown in Fig. 1.

The important aspect to be mentioned is the contrasting molecular geometries exhibited by the 4-pyridine (4-TPtPyP) and 3-pyridine (3-TPtPyP) isomers. The tetraplatinum complex derived from 4-TPyP exhibit an overall planar geometry with the four metal centers in the plane of the porphyrin ring. In the case of 3-H₂TPtPyP, because of the substitution position at the pyridine group, the metal complexes are displaced above and below the porphyrin ring, acquiring a saddle shape configuration. This particular aspect is expected to influence their interactions with DNA, justifying a detailed investigation on the subject. The attempts to monitor the TPtPyP/DNA system spectrophotometrically were precluded by the occurrence of precipitation reactions. For this reason, we have employed the surface plasmon resonance (SPR) technique.

SPR has become a powerful optical technique for a variety of applications, from the measurement of adsorbed mass, to chemical and biological sensing [23], [24], [25], [26], [27]. It exploits the fact that, at certain conditions, the surface plasmons on thin metallic films can be excited by photons. Such conditions depend on the refractive index of the adsorbate. In this way, SPR has been successfully used in affinity biosensors, allowing real-time analysis of biospecific interactions without the use of labeled molecules [28], [29], [30]. However, to the best of our knowledge, the application of SPR in the investigation of the interaction of metal porphyrins and DNA has never been reported before.