Synthesis of New Five Coordinated Copper(II) and Nickel(II) Complexes of L-Valine and Kinetic Study of Copper(II) with Calf Thymus DNA

Five coordinated novel complexes of Cu II and Ni II have been synthesized from benzil and 1,3- diaminopropane- Cu II / Ni II complex and characterized by elemental analysis, i.r., n.m.r., e.p.r, molar conductance and u.v-vis, spectroscopy. The complexes are ionic in nature and exhibit pentaeoordinated geometry around the metal ion. The reaction kinetics of C 25 H 36 N 5 O 2 CuCl with calf thymus DNA was studied by u.v-vis, spectroscopy in aqueous medium. The complex after interaction with calf thymus DNA shows shift in the absorption spectrum and hypochromicity indicating an intercalative binding mode. The K obs values have been calculated under pseudo-first order conditions. The redox behaviour of complex C 25 H 36 N 5 O 2 CuCl in the presence and in the absence of calf thymus DNA in the aqueous solution has been investigated by cyclic voltammetry. The cyclic voitammogram exhibits one quasi-reversible redox wave corresponding to Cu II / Cu I redox couple with E 1 / 2 values of -0.377 and -0.237 V respectively at a scan rate of 0.1V s - 1 .On interaction with calf thymus DNA, the complex C 25 H 36 N 5 O 2 CuCl exhibits shifts in both E p as well as in E 1 / 2 values, indicating strong binding of the complex to the calf thymus DNA.


INTRODUCTION
Copper is widely distributed in the biological systems. Complexes of Cu are capable of interaction with nucleic acids and have been intensively studied/1-3/. Sigman et al. have proved Cu(Phen)2 + to be an efficient nuclease which can be employed for understanding the DNA binding mechanism in DNA complexes/4-6/. Artificial nucleases have been proved to be an efficient tool for the footprinting and sequence specific targeting of nucleic acids/7,8/. Despite the intense study, the exact mode of binding remains unknown/9-1 l/. Current researchers are developing new applications and finding new ways to design more efficient and Author for correspondence t'ol. 9, Nos, I-2, 2002 ,S),nthesis of New Five Coordinated Copper(ll) and Nickel(ll) Complexes of L-lZaline and Kinetic useful catalysts for DNA cleavage. CuU-L-histidine is one of the well defined complexes which has been demonstrated to cleave plasmid DNA at physiological pH and temperature /12/. Metal ion-L-histidine interaction (with Cu", Nin, Zn n etc) has also been well demonstrated and can be probed for other applications as well, viz, protein purification/13-15/. In continuation to our earlier report on Co" complexes of five coordinated amino acid porphyrin interaction with calf thymus DNA, we have designed new Cu"/Nin-L valine complexes to understand the mechanistic pathway of DNA binding at physiological pH.
Amino acids are the basic structural units of protein and reports on different amino acid residues reveal that they are capable of controlling the binding and reaction of the Cu" complexes on DNA/16,17/. These amino acids play an important role at and around the active sites of various metalloproteins including regulating the redox potential of the metal ion, arranging the specific geometry of the metal coordination site and restricting the conformation of the peptide chain/18/. Thus amino acids interaction promotes specificity of the molecule to the active site including regulating the redox properties, which we have demonstrated through cyclic voltammetric data. In this paper we report the synthesis of new valine complexes of type C25H36N502CuCI/C_sH36NO2NiCl derived from benzil and 1,3-diaminopropaneCu"/Ni" complex and the interaction of Cu u complex with calf thymus DNA.

Synthesis of Complex C6H20N4CuCI2
To the solution of the copper chloride (1.71 g, 10mmol) in MeOH (50 cm"3) was added 1,3diaminopropane (1.70 g, 10mmol) in 1:2 molar ratio. A blue precipitate was obtained, washed with ether and dried in vacuo. Similarly Ni complex was also synthesized A.Ahmad et al.

Synthesis of Complex C20H26 N4CuCI2
To a solution ofbenzil (0.210 g, lmmol) in MeOH (50 cm"3) was added C6H20N4CuCI2 complex (0.282 g, mmol) in 1"1 molar ratio. The resulting solution was boiled to reflux for ca. 20 h, while dark green precipitate was obtained, which was washed thoroughly with ether and dried in vacuo.

Synthesis of Complex C2sH3NOzCuC!
To a hot solution of L-valine (0.117 g, mmol) in MeOH (50 cm"3) was added the complex C20H26NaCCI2 (0.456g, lmmol) in DMF. The solution was boiled to reflux for ca. 36 h and further concentrated. The solution was then allowed to cool and left overnight in the refrigerator. Blue coloured complex was obtained, filtered, washed with ether and dried in vacuo. Ni" complex was also prepared in a similar way.

I.R. Spectra
The most prominent bands in the spectra of the complexes are listed in Table 2 The e.p.r, spectrum of the complex C25H36NsO2CuCl, recorded at room temperature, shows signals for gl and g+/-at 2.25 and 2.07 respectively, which is consistent with the square pyramidal geometry around Cu u ion.
The Cu m exhibits dxE.y ground state with g> g+/-> 2.0 which is the most common ground state of these compounds/24/.

Electronic Spectra
The electronic spectrum of C25H36NsO2NiCI recorded in MeOH shows bands at 610 nm and 360 nm which could be assigned to the 3B1. 3E (F) and 3BI 3A2, 3E(p) transitions respectively/25/. These values are consistent with pentacoordinated environment around Ni /26,27/. The electronic spectrum of C25H36NsO2CuCI complex recorded in MeOH shows a broad band at 645nm, which is due to the dd transition assigned to the pentacoordinate geometry around Cu ion/28/.

N.M.R Spectra
In order to arrive at the correct structure of the complexes C20H26N4NiCI2 and C25H36NOaNiCI, IH and 3C n.m.r, spectra have been carried out. The H n.m.r, spectrum of the C20H26N4NiC12 recorded in DMF shows a multiplet at 7.51-7.93 ppm, due to the phenyl protons. The multiplet due to CH2 protons and CH3 protons was observed at 3.70-3.80 ppm. and 0.62-1.80 ppm respectively. The primary amine proton signal was observed at 5.0 ppm. In the complex C:sH36NsOaNiCl, the absence of carboxylic proton signal of the amino acid confirms coordination to the metal through oxygen atom of carboxylic group. There was no major shiR in the 5 values after coordination of amino acid to the metal. The primary amine signal appears at 4.90 ppm /29/ and CH2 proton signal at 3.62-3.74 ppm respectively. The mutliplet due to the phenyl protons appears at 7.17-7.43 ppm.   Synthesis of New Five Coordinated Copper(ll) and Nickel(ll) Complexes of L-Valine and Kinetic,

Redox Behaviour
The cyclic voltammetric studies provide an insight to the calf thymus DNA binding. The cyclic voltammogram recorded for the complex C2Ha6NsO2CuC1 in H20/MeOH (95:5) at a scan rate of 0.1Vs reveals one electron quasireversible CuII/I wave with Ea values as-0.377V and 0.237V respectively (Fig.l).
The ratio of anodic to cathodic peak currents is---1. At different scan rates the voltammogram do not show any major change (Fig. 2). For a reversible wave Ep is independent of scan rate and Ip (as well as the current at any point of the wave) is proportional to the v/2/30/. The AEp value is larger than the Nernstian value observed for the one-electron transfer couple. Large peak width for one electron couple Cu" Cu in these complexes is not an uncommon observation/31/. This is due to the reorganization of the coordination sphere during the electron transfer and has been observed in a number of the copper complexes as well/32/.  16 18 [DNA] 10 ' (tool dm") Metal-Based Drugs with calf thymus DNA, there is a red shift of 9 nm and large hypochromicity. Small changes in ,r,ax and hypochromicities have been observed in some copper complexes upon interaction with calf thymus DNA [34,35]. Strong hypochromism and red shifts are usually attributed for intercalation /36/. The rate constant kobs s'l values were calculated at varying concentration of calf thymus DNA by plotting logA versus time (Fig. 3) and the plot of the kobs versus [DNA] gave a straight line, suggesting a pseudo-first order dependence (Fig. 4). On the basis of the kinetic data, the following mechanism is proposed.