Synthesis, micellar properties, DNA binding and antimicrobial studies of some surfactant–cobalt(III) complexes
Introduction
DNA plays a fundamental role in the storage and expression of genetic information in a cell. DNA is not only an important biological material with a unique double helical rodlike structure, but also an interesting anionic polyelectrolyte. Studies on the interaction of transition metal complexes with DNA have been pursued in recent years [1], [2], [3], [4]. These complexes are stabilized in binding to DNA through a series of weak interactions, such as the π-stacking interactions associated with intercalation of a planar aromatic group between the base pairs, hydrogen-bonding and van der Waals interactions of functionalities bound along the groove of the DNA helix, and the electrostatic interaction of the cation with phosphate group of DNA. Studies directed toward the design of site- and conformation-specific reagents provide rationales for new drug design as well as a means of developing sensitive chemical probes of nucleic acid structure.
Surfactants, sometimes called suface-active agents, are among the most versatile chemicals available. They are amphiphilic molecules consisting of a hydrophilic head group and a hydrophobic (lipophilic) tail and are, thus, able to interact with both polar and non-polar compounds. Accordingly, surfactants are often classified as non-ionic or ionic (cationic, anionic or zwitterionic). Surfactants are major building blocks of many physical, chemical and biological systems. They have been introduced into several commercial products such as antiseptic agents in cosmetics and as germicides [5], and also have found a wide range of applications because of their unique solution properties such as detergency, solubilization and surface wetting capabilities, in diverse areas such as mining, petroleum and pharmaceutical industries [6]. Cationic surfactants offer some additional advantages over other classes of surfactants. These substances, besides their surface activity, do show antitumor properties [7]. Cationic surfactant–DNA interactions have been the subject of many studies over the past few decades because they are of interest both in fundamental science and in biotechnological applications [8], [9], [10]. Studies have shown that the binding of surfactant to DNA is cooperative based on the binding isotherm, and is similar to the interaction of surfactant with a synthetic polymer [11], [12], [13].
Surfactant–metal complexes are a special type of surfactants, where a coordination complex (containing a central ion with surrounded ligands coordinated to the metal) acts as the surfactant (Scheme 1). In these surfactants, the metal complex part containing the central metal ion with its primary coordination sphere acts as the head group and the hydrophobic part of one or more ligands acts as tail part. Like any other well-known surfactants, these metallosurfactant complexes also form micelles at a specific concentration called critical micelle concentration (CMC) in aqueous solution. There are but a few reports [14], [15], [16], [17] on the synthesis, isolation and characterization of surfactant transition metal complexes, in contrast to numerous reports of the formation and study of such surfactants in solution without isolation. We have been interested in the synthesis and micelle forming properties of cobalt(III)/chromium(III) complexes containing lipophilic ligands for a long time [18], [19], [20], [21]. As in biology, such compounds may exhibit novel physical and chemical properties with interesting and useful associated applications.
A characteristic feature of transition metals is their ability to form complexes with a variety of neutral molecules such as bipyridine (bpy) and phenanthroline (phen). These are widely used as a classical N,N΄-bidentate ligand to prepare mixed-ligand complexes in coordination chemistry. Metal complexes of bipyridine and phenanthroline chelators are of great interest since they exhimit numerous biological properties such as antitumor, anticandida and antibacterial activity [22], [23], [24]. At the same time, metal complex bearing ethylenediamine have also been interest because in the classical antitumor agent cis-platinum, one of the ligands must be a N-donor and posses at least one hydrogen atom attached to the nitrogen [25].
In spite of the greatest effort and success in the study of metallosurfactants of cobalt(III) complexes, such complexes still attract much attention due to their interesting properties and the relative simplicity of their synthesis. To the best of our knowledge no previous studies are available to find the interaction of DNA with metallosurfactants. From this point of view the results presented here are of interest . In the present paper, we report the synthesis, CMC determination and DNA binding properties of various surfactant–cobalt(III) complexes using different physico-chemical methods. Also we have reported the antibacterial and antifungal activities of these surfactant–cobalt(III) complexes against certain human pathogenic microorganisms.
Section snippets
Materials and methods
All the reagents were of analytical grade (Aldrich and Merck). Calf thymus DNA obtained from Sigma-Aldrich, Germany, was used as such. The spectroscopic titration was carried out in the buffer (50 mM NaCl-5 mM Tris–HCl, pH 7.1) at room temperature. A solution of calf thymus DNA in the buffer gave a ratio of UV absorbance at 260 and 280 nm of ~ 1.8–1.9:1, indicating that the DNA was sufficiently free of protein [26]. Milli-Q water was used to prepare the solutions.
Absorption spectra were recorded
Spectroscopic characterization
Infrared spectroscopy is used to distinguish the mode of coordination of the ligand with the central metal ion. Various workers have employed the NH2 deformation mode (1700–1500 cm− 1 region), the CH2 rocking mode (950–850 cm− 1 region) and Co–N stretching mode in the 600–500 cm− 1 region to distinguish between cis and trans isomers [34], [35]. The cis-isomers always show two peaks, whereas the trans-isomers usually have only one peak in the CH2 rocking region. In the present study, the NH2
Conclusion
As mentioned in our previous reports [20], [21], the critical micelle concentration values of surfactant–cobalt(III) complexes in the present study are also very low compared to that of the simple organic surfactant, dodecylammonium chloride (CMC = 1.5 × 10− 2 mol dm− 3). Thus it is concluded that these metal surfactant complexes have more capacity to associate themselves, forming aggregates, compared to those of ordinary synthetic organic surfactants. The binding behavior of these
Acknowledgements
We are grateful to the UGC-SAP & COSIST and DST-FIST programmes. Council of Scientific and Industrial Research (CSIR), New Delhi is gratefully acknowledged for financial support (Grant No. 01(2075)/06/EMR-II) and a Senior Research Fellowship to RSK. We also thank UGC for sanction of a research scheme (F. 32-274/2006) to SA.
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