Indian Journal of Biochemistry & Biophysics Vol. 36, August 1999, pp. 233-239 Solubilization and binding ofDNA-CTAB complex with SDS in aqueous media SA Gani, D K Chattoraj* and D C Mukherjee t Department of Food Technology and Biochemical Engineering, Jadavpur University, Calcutta 700 032 Received 5 August /998; revised /9 March /999 Extent of binding of sodium dodecyl sulphate (SDS) to the binary complex fonned between calf- thymus DNA. and cetyltrimethylammonium bromide (CTAB) has been measured in mole per mole of nucleotide in the complex as function of concentration of SDS by using equilibrium dialysis technique at different temperatures and pH. Binding of SDS to thennally denatured DNA-CT AB complex has also been studied. The most interesting aspect to be noted in this experiment is that the water insoluble DNA-CTAB binary complex gets solubilized in the ternary mixture in presence of SDS but when DNA is thennally denatured, the ternary system DNA-CTAB-SDS remains insoluble. Significant change in the extent of binding has been noted with the variation of the relative composition of DNA and CT AB in their binary mixture. The data of binding of SDS to DNA-CTAB complex are compared more precisely in terms of the standard Gibbs' free energy decrease (-MJ") for the saturation of the binding sites in the complex with the change of SDS activity from zero to unity in the rational mole fraction scale. It well-known that anionic surfactant sodium dodecylsulphate (SDS) has been frequently used for the lysis of living cell whereby intact high molecular DNA on separation from cell becomes solubilized in SDS solution along with some basic protein histone1.2. It has also been reported that the dissociation of chromatin to form histone and DNA is enhanced in the presence of non-ionic surfactane Triton-x I 00. Chatterjee and Chattora/ have earlier shown from equilibrium dialysis experiments that in the aqueous media DNA polyanion is totally unable to bind SDS anion in aqueous solvent because of high electrostatic repulsion effect which apparently indicates that there is no direct relation between high solubility of cellular DNA to its gross binding with SDS dissolved in aqueous media. Chatterjee and Chattora/, however, observed from equilibrium dialysis experiments that cationic surfactant cetyltrimethylammonium bromide (CTAB) in aqueous solvent bind to dissolved DNA exteQsively forming saturated insoluble DNA-CTAB complex at neutral pH. In the saturated state, it has *To whom all correspondence regarding this paper be made. lUniversity College of Science and Technology, Department of Pure Chemistry, Calcutta University, 92, A.P.C. Road, Calcutta 700 009. been noted that one negatively charged nucleotide ion is in ion-pairing interaction with one surfactant cation under suitable physicochemical condition. Further hydrophobic groups of bound CTAB are associated in the complex as a result of strong hydrophobic effect. From a recent studl of the kinetics of binding of CTAB to DNA, it has been observed that electrostatic, hydrophobic and conforrnatiomil change effects of biopolymers have significant role in the interaction of CTAB to DNA. Earlier from spectro- scopic study7.8, hyp 'chromic shift of DNA 7 ,8 and DNA-CTAB comple.. 9 and the melting temperatures were observed to be close to each other which indicated the presence of large fraction of double- helical DNA in DNA-CTAB complex, Recently, Kunjappu and Nair lO from the analysis of optical data reported that DNA-CTAB complex solubilizes in SDS micelles by a mechanism of some complex interactions, In the light of solubilization of cellular DNA in presence of SDS and in the absence of any interaction of pure DNA with SDS earlier reported 4 by us, further analysis of inertness of SDS against binding to pure DNA in aqueous solution'and strong affinity of SDS for solubilization of DNA- CT AB complex have been made in the present investigation.