Chinese Journal of Chemistry, 2005, 23, 1157—1164 Full paper * E-mail: jdpandey@rediffmail.com; jyotsna_chhabra@rediffmail.com; drranjan@hotmail.com. Received November 11, 2004; revised February 6, 2005; accepted April 19, 2005. Project supported by UGC, New Delhi, India, and Council of Scientific and Industrial Research, New Delhi, India. © 2005 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Effect of Na ＋ , K ＋ and Ca 2＋ Ions on Physico-chemical Properties of Thymine, Cytosine, Thymidine and Cytidine in Aqueous Urea Solution PANDEY, Jata Dhari * HAROON, SHahla CHHABRA, Jyotsna DEY, Ranjan MISRA, Krishna Department of Chemistry, University of Allahabad, Allahabad, 211-002, India Experimental determination of density, ultrasonic velocity and viscosity of two pyrimidine bases thymine and cytosine along with their respective nucleosides, thymidine and cytidine has been carried out in aqueous urea solu- tions in the presence of different concentrations of three salts, viz. NaCl, KCl and CaCl 2 . The experimental data have been used for the computation of various thermodynamic parameters, viz. apparent molar volume, apparent molar compressibility, coefficients A and B of the Jones-Dole equation, internal pressure, acoustic impedance, etc. Structural studies of solutions under investigation have also been carried out by ultraviolet spectroscopy, and an at- tempt has been made to collaborate the findings of ultraviolet spectroscopy with results obtained thermodynami- cally. Keywords interaction, bivalent ion, thermodynamic property, ultraviolet spectroscopy, ionic size Introduction Biomolecules are a special class of compounds found in living systems. Each individual component in the cell of a living organism, e.g. lipid, protein or nu- cleic acid has its own specific functions. There has been a lot of active interest in the study of solute-solute and solute-solvent interactions during recent years. Various workers 1,2 have discussed nucleic acid-water interac- tions. Aqueous urea and its derivatives are important solvents and have a wide range of applica- tions. Aque- ous urea solutions have been widely reported to be nu- cleic acid denaturants. It has been seen that urea in high concentration of 3—6 mol•kg －1 exhibited strong hy- drophilic-hydrophilic ionic group interactions with nu- cleic acid bases, nucleosides and nucleotides, thereby supporting its denaturing characteristics. In an earlier work, 3 thermodynamic studies of urea and its deriva- tives in water and water-dioxane solutions at different temperatures had been carried out. The biochemistry of nucleic acids is frequently asso- ciated with metal ion interactions. 4 Metal ions act in several different roles in biological processes, viz. the transport of charges, the stabilization of structure, etc. Ions, such as Na ＋ , K ＋ and Ca 2＋ , present in the body and nucleic acids generally occur as complexes coor- dinated with metal ions. The significance of this binding involved in processes related to DNA replication and transcription has long been recognized. 5 The concentra- tion of ions significantly affects the conformational changes in both DNA and RNA and proteins. Recently interactions of adenine in water and aqueous urea solu- tions of different concentrations have been studied in the presence or absence of metal ions. 6 Thermochemical and sonochemical studies of adenosine in DMSO in the presence of Na ＋ or Ca 2＋ have been also carried out. 7 A wide variety of techniques have been applied 8-10 singly or jointly to understand the interaction behaviour of nucleic acids with metal ions. The most common methods that have so far been used in the study of in- teractions of metal ions with nucleic acids are conduc- tometry, spectroscopy, etc. For the quantitative evalua- tion of thermodynamic information on aqueous nucleic acid solutions, the thermodynamic data on the constitu- ents of nucleic acid have provided significant contribu- tion to understanding of the conformation of the nucleic acids in solutions. 11 Volumetric and ultrasonic data on the constituents of nucleic acids in the presence of salts are scarce, and therefore in the present investigation, two bases thymine and cytosine along with their respec- tive nucleosides, thymidine and cytidine, have been put under investigation. An attempt has been made to study the effect of three salts NaCl, KCl and CaCl 2 on phys- ico-chemical properties of nucleic acid bases and nu- cleosides under investigation in very dilute solution of urea. For this purpose, ultrasonic, volumetric and vis- cometric techniques in addition to spectroscopic UV technique have been used. Density, ultrasonic velocity and viscosity of solutions have been measured experi- mentally, and employing the experimental data, a num- ber of thermodynamic parameters like apparent molar