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Spectrophotometric studies of molecular complex formation between water-soluble cobalt(II) Schiff base complex and nucleotides in mixed solvent systems
Spectrochimica Acta Part A 61 (2005) 3061–3065
Spectrophotometric studies of molecular complex formation between water-soluble cobalt(II) Schiff base complex and nucleotides in mixed solvent systems Davar M. Boghaei∗ , Mehrnaz Gharagozlou Department of Chemistry, Sharif University of Technology, P.O. Box 11365-9516 Tehran, Iran Received 15 September 2004; accepted 17 November 2004
Abstract The formation constants for 1:1 molecular complex formation between water-soluble cobalt(II) tetradentate Schiff base complex, disodium[{bis(4-methoxy-5-sulfo-salicylaldehyde)-4,5-dimethyl-o-phenylenediiminato}cobalt(II)], Na2 [Co(SO3 -4-meosal-4,5-dmophen)], and nucleotides, adenosine-5 -triphosphate (ATP) and cytidine-5 -triphosphate (CTP), in mixed solvent systems of ethanol and water with different volume fractions of ethanol and water have been determined spectrophotometrically at constant ionic strength (I = 0.2 mol dm−3 NaClO4 ) and temperature 278 K. Trends in the values of formation constants according to the volume fractions of ethanol and water in ethanol and water mixed solvent systems, suggest that the trend of molecular complex formation increases with increasing the volume fraction of ethanol in mixed solvent systems. © 2004 Elsevier B.V. All rights reserved. Keywords: Water-soluble cobalt(II) Schiff base complex; Nucleotides; Molecular complexes; Formation constant; Mixed solvent systems
1. Introduction Molecular complexes [1] are being regarded as important materials for use as organic superconductors [2] and photocatalysts [3]. Recent applications of the electron donor–acceptor (EDA) complexes include non-linear optical activity [4], surface chemistry [5] and molecular recognition [6,7]. Particular attention has recently been paid to the synthesis and study of the diimino tetradentate Schiff bases and their complexes [8,9]. This is due to their uses as biological models in understanding the structure of biomolecules and biological processes [10,11]. The crucial role of Schiff bases in the biological function of bacteriorhodospin is proven. The retinal chromophore is bound covalently to the protein via a protonated Schiff base [12,13]. Adenosine-5 -triphosphate (ATP), cytidine-5 -triphosphate (CTP), guanosine-5 -triphosphate (GTP) and other nucleoside triphosphates are the principal biological energy store ∗
Corresponding author. Tel.: +98 21 464 0649; fax: +98 21 601 2983. E-mail address: [email protected] (D.M. Boghaei).
1386-1425/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2004.11.027
and of great importance in the life science. The nucleotide ATP is one of the four main components of DNA and about one-sixth of all enzyme systems need ATP or a related adenine cofactor. Their biological importance rests on the fact that they function as major carriers of chemical energy in the cells. With this fact in mind, it is not surprising that the properties of nucleotides are receiving much attention. For a better understanding of the interactions between receptors and nucleotides, it is important to obtain information about their formation constants. Although studies have been performed to determine the equilibrium constants for the interaction of cobalt(II) ion [14] and cobalt(II) ferrocyanides [15] with nucleotides, there is a lack of information about determining formation constants for molecular complex formation between water-soluble cobalt(II) Schiff base complexes and nucleotides in mixed solvent systems. In earlier works we have synthesized and characterized some Schiff base complexes in non-aqueous media [16,17]. Here we report spectrophotometric studies of molecular complex formation between water-soluble cobalt(II) tetradentate Schiff base complex derived from 4,5-dimethyl-o-phenylenediamine, as
3062
D.M. Boghaei, M. Gharagozlou / Spectrochimica Acta Part A 61 (2005) 3061–3065
Fig. 1. The structure of the investigated water-soluble cobalt(II) Schiff base complex.
shown in Fig. 1, with ATP and CTP in mixed solvent systems of ethanol and water under physiological condition (pH 7.0) that the acceptor and product are stable at this pH. Since formation constant (K) is an important parameter which measures the extent of binding, the object of the present report is to determine K for molecular complex formation of two important biological molecules, ATP and CTP, with watersoluble cobalt complexes in ethanol and water mixed solvent systems.
2. Experimental Adenosine-5 -triphosphate disodium salt and cytidine-5 triphosphate disodium salt dihydrate were purchased from Biochemika, Fluka. The buffer solution was adjusted to 7.0 with 0.1 mol dm−3 Na2 HPO4 and NaH2 PO4 . Doubly distilled, deionized water was used throughout. Ethanol and sodiumperchlorate were purchased from Merck. All other chemicals were of analytical reagent grade (if not otherwise stated) and were used as received. The Na2 [Co(SO3 -4meosal-4,5-dmophen)] was prepared according to [18]. The purity of the prepared complex was checked by elemental analysis, for C24 H20 N2 S2 O10 Na2 Co (665.46): found (calculated): C 43.6 (43.3), H 3.3 (3.0), N 4.1 (4.2), Co 8.7 (8.8). UV–vis spectra were recorded by CARY 100 Bio VARIAN UV–vis spectrophotometer equipped with a PCB150 water circulator and thermostated multicell holder. METROHM AG Herisau, 620 pH-meter was employed for pH measurements.
Fig. 2. Variation of absorption spectrum of mixtures containing Na2 [Co(SO3 -4-meosal-4,5-dmophen)] and ATP in aqueous solution (x = 0, I = 0.2 mol dm−3 NaClO4 ) and T = 278 K, the concentration of the components are as in Table 1.
480 nm shifted to lower wavelengths with the increase in its intensity and a new charge transfer (CT) peak appeared at about 360 nm that its intensity increases with the increase in the concentration of nucleotides with very clear isosbestic points. Similar spectral features are observed for all systems. The molecular complexes formed show an absorption different from the acceptor, while the nucleotides do not absorb at those wavelengths. 3.1. Determination of formation constants The formation constants of the molecular complexes are determined by using Ketelaar’s [19] equation for cells with 1 cm optical path length: 0 C0 CA 1 1 0 0 D (1) = + C A + CD ε K A − A0A − A0D 0 and C 0 are the initial concentrations of the acHere CA D ceptor and donor, respectively, A the absorbance of the donor–acceptor mixture at λCT , A0A and A0D are the absorbances of the acceptor and donor solutions with same molar concentrations as in the mixture at λCT . The effective molar absorptivity (ε ) is given by
3. Results and discussion
ε = εC − εA − εD
Absorption spectra of mixtures containing a fixed concentration of Na2 [Co(SO3 -4-meosal-4,5-dmophen)] (10−5 mol dm−3 ) and varying concentrations of ATP in aqueous solution (x = 0) are shown in Fig. 2. The isosbestic points suggest that there are only two species in equilibrium. The same is valid for other systems. The increase in the concentration of the nucleotides causes that one of the original peaks of the cobalt Schiff base complexes at about 380 nm gradually vanished, the other original peak around
where εC , εA and εD are the molar absorptivities of the molecular complex, the acceptor and the donor, respectively, K the formation constant of the formed complex. A plot of 0 C 0 /(A − A0 − A0 ) versus C 0 + C 0 should produce a CA D D D A A straight line if only a 1:1 donor–acceptor complex is formed; while a mixture of 1:1 and 1:2 or only 1:2 complex in a system would lead to a curve [19]. The formation constants for molecular complex formation between studied water-soluble cobalt(II) Schiff base complex and nucleotides in mixed
(2)
D.M. Boghaei, M. Gharagozlou / Spectrochimica Acta Part A 61 (2005) 3061–3065
3063
Table 1 Formation constants and molar absorptivities of molecular complexes from absorbance data of mixtures containing Na2 [Co(SO3 -4-meosal-4,5-dmophen)] (10−5 mol dm−3 ) and ATP at 360 nm (temperature 278 K) Ethanol volume fraction (x)
Water volume fraction (1 − x)
104 [ATP]0 (mol dm−3 )
Absorbances of mixture
Formation constant (dm3 mol−1 )
Molar absorptivity, ε (dm3 mol−1 cm−1 )
0
1
2.5 3.5 5.5 14.5 31.0 65.5
0.0046 0.0056 0.0086 0.0170 0.0290 0.0500
166.7 ± 0.3
10000 ± 0.1
0.05
0.95
2.5 3.5 5.5 14.5 31.0 65.5
0.0045 0.0059 0.0092 0.0218 0.0348 0.0546
185.2 ± 1.2
10000 ± 0.7
0.08
0.92
2.5 3.5 5.5 14.5 31.0 65.5
0.0058 0.0076 0.0118 0.0268 0.0419 0.0631
238.1 ± 0.5
10000 ± 0.4
0.11
0.89
2.5 3.5 5.5 14.5 31.0 65.5
0.0081 0.0097 0.0149 0.0302 0.0515 0.0648
333.3 ± 0.9
10000 ± 0.3
0.15
0.85
2.5 3.5 5.5 14.5 31.0 65.5
0.0104 0.0130 0.0196 0.0426 0.0633 0.0885
347.8 ± 0.6
12500 ± 1.4
solvent systems of ethanol + water were calculated from the ratio of the slope to the intercept by least-square method. The K measurements were repeated at least twice and were reproducible. Experimental data are shown in Tables 1 and 2. In 0 C 0 /(A − A0 − A0 ) versus C 0 + C 0 all cases the plots of CA D D D A A according to Eq. (1) are fairly linear, which signify that only a 1:1 donor–acceptor complex is formed, one typical case being shown in Fig. 3. Values of K and ε of the molecular complexes for x ethanol and (1 − x) water mixed solvent systems where x and (1 − x) are volume fractions of ethanol and water, respectively, obtained from such plots are shown in Tables 1 and 2.
be related to the great differences between Gutmann acceptor number for water and ethanol, which has an effect on the formation constants of the molecular complexes through the hydrogen bonding formation with nucleotides [20]. The Gutmann acceptor number for water is 54.8 and for ethanol is 37.1 [20]. Hydrogen bonding for mixed solvent systems with
3.2. Effects of mixed solvent systems To investigate the effects of mixed solvent systems of ethanol and water, studies on the molecular complex formation between water-soluble cobalt(II) Schiff base complex and nucleotides were carried out in different volume fractions of ethanol (x) and water (1 − x) in ethanol and water mixed solvent systems. It was shown that mixed solvent systems, x ethanol and (1 − x) water where x is volume fraction of ethanol, affect the formation constants. These effects can
Fig. 3. Ketelaar’s plot for molecular complexes of Na2 [Co(SO3 -4-meosal4,5-dmophen)] with CTP in ethanol and water mixed solvent system 0 0 0 0 (x = 0.08) and T = 278 K, P = CA CD /(A − A0A − A0D ) and C = CA + CD , the concentrations and absorbances being as in Table 2.
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D.M. Boghaei, M. Gharagozlou / Spectrochimica Acta Part A 61 (2005) 3061–3065
Table 2 Formation constants and molar absorptivities of molecular complexes from absorbance data of mixtures containing Na2 [Co(SO3 -4-meosal-4,5-dmophen)] (10−5 mol dm−3 ) and CTP at 360 nm (temperature 278 K) Ethanol volume fraction (x)
Water volume fraction (1 − x)
104 [CTP]0 (mol dm−3 )
Absorbances of mixture
Formation constant (dm3 mol−1 )
Molar absorptivity, ε (dm3 mol−1 cm−1 )
0
1
2.5 3.5 5.5 14.5 31.0 65.5
0.0050 0.0065 0.0095 0.0170 0.0270 0.0312
454.5 ± 0.5
5000 ± 0.2
0.05
0.95
2.5 3.5 5.5 14.5 31.0 65.5
0.0040 0.0054 0.0078 0.0188 0.0194 0.0252
612.2 ± 1.0
3333.3 ± 1.4
0.08
0.92
2.5 3.5 5.5 14.5 31.0 65.5
0.0051 0.0058 0.0090 0.0179 0.0221 0.0242
769.2 ± 0.5
3333.3 ± 0.8
0.11
0.89
2.5 3.5 5.5 14.5 31.0 65.5
0.0054 0.0057 0.0092 0.0196 0.0238 0.0242
833.3 ± 0.3
3333.3 ± 1.2
0.15
0.85
2.5 3.5 5.5 14.5 31.0 65.5
0.0208 0.0250 0.0275 0.0558 0.0517 0.0728
909.1 ± 0.9
10000 ± 0.1
higher ethanol:water ratios is lower due to the lower Gutmann acceptor number for ethanol. Therefore, nucleotides are better solvated through hydrogen bonding in mixed solvent systems with lower ethanol:water ratios, resulting in lower formation constants (Tables 1 and 2). Water has a very high dielectric constant, 81.7, and for ethanol is 24.3 [20]. Therefore, adding ethanol decreases the dielectric constant of the solution resulting in a greater attraction forces and hence higher formation constants. The donor property of the nucleotides is dependent on the steric factors and solvation effects through the hydrogen bonding. CTP has less steric effects than ATP. In addition ATP is solvated more than CTP due to more sites for hydrogen bonding [21], therefore in all mixed solvent systems with different volume fractions of ethanol and water, the trend of molecular complex formation according to the nucleotides that is affected by both steric and solvation effects is as follow: CTP > ATP (Tables 1 and 2). The 1:1 molecular complex formation between watersoluble cobalt(II) Schiff base complex and nucleotides is of great importance with regard to modulate the selectivity of the phosphate recognition of the biomolecules in mixed solvent systems.
Acknowledgement We are grateful to the Research Council of Sharif University of Technology for their financial support.
References [1] R.S. Mulliken, J. Am. Chem. Soc. 74 (1952) 811–824. [2] A. Eychmuller, A.L. Rogach, Pure Appl. Chem. 72 (2000) 179–188. [3] R. Dabestani, K.J. Reszka, M.E. Sigman, J. Photochem. Photobiol. A 117 (1998) 223–233. [4] S.D. Bella, I.L. Fragala, M. Ratner, T.J. Marks, J. Am. Chem. Soc. 115 (1993) 682–686. [5] S.M. Andrade, S.M.B. Costa, R. Pansu, J. Colloid Interface Sci. 226 (2000) 260–268. [6] J.L. Atwood, G.A. Koutsoantonis, C.L. Raston, Nature 368 (1994) 229–231. [7] F.C. Tucci, D.M. Rudkevich, J. Rebek Jr., J. Org. Chem. 64 (1999) 4555–4559. [8] J.P. Costes, G. Cros, M.H. Darbieu, Inorg. Chim. Acta 60 (1982) 111–114. [9] D.F. Averill, R.F. Broman, Inorg. Chem. 17 (1978) 3389–3394. [10] J.E. Kovacic, Spectrochim. Acta A 23 (1987) 183–191. [11] R. Atkins, G. Brfwer, E. Kokto, G.M. Mockler, E. Sinn, Inorg. Chem. 24 (1985) 128–134.
D.M. Boghaei, M. Gharagozlou / Spectrochimica Acta Part A 61 (2005) 3061–3065 [12] L. Rousso, N. Friedman, M. Sheves, M. Ottolenghi, Biochemistry 34 (1995) 12059–12065. [13] T. Bassov, M. Sheves, Biochemistry 25 (1986) 5249–5258. [14] L.A. Herrero, A. Terron, Polyhedron 17 (1998) 3825–3833. [15] S.R. Ali, J. Ahmad, Kamaluddin, Colloid Surf. A: Physicochem. Eng. Aspects 236 (2004) 165–169. [16] D.M. Boghaei, M. Lashanizadegan, Synth. React. Inorg. Met. Org. Chem. 30 (2000) 1393–1404.
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[17] D.M. Boghaei, M. Lashanizadegan, Synth. React. Inorg. Met. Org. Chem. 30 (2000) 1535–1545. [18] D. Chen, A.E. Martell, Y. Sun, Inorg. Chem. 28 (1989) 2647–2652. [19] J.A.A. Ketelaar, C. Van De Stolpe, A. Coulsmit, W. Dzcubes, Rec. Trav. Chim. 71 (1952) 1104–1114. [20] V. Gutmann, Coordination Chemistry in Nonaqueous Solutions, Springer-Verlag, Berlin, 1968. [21] L. Mao, Y. Wang, Y. Liu, X. Hu, J. Mol. Biol. 336 (2004) 787–807.
Spectrophotometric studies of molecular complex formation between water-soluble cobalt(II) Schiff base complex and nucleotides in mixed solvent systems Davar M. Boghaei∗ , Mehrnaz Gharagozlou Department of Chemistry, Sharif University of Technology, P.O. Box 11365-9516 Tehran, Iran Received 15 September 2004; accepted 17 November 2004
Abstract The formation constants for 1:1 molecular complex formation between water-soluble cobalt(II) tetradentate Schiff base complex, disodium[{bis(4-methoxy-5-sulfo-salicylaldehyde)-4,5-dimethyl-o-phenylenediiminato}cobalt(II)], Na2 [Co(SO3 -4-meosal-4,5-dmophen)], and nucleotides, adenosine-5 -triphosphate (ATP) and cytidine-5 -triphosphate (CTP), in mixed solvent systems of ethanol and water with different volume fractions of ethanol and water have been determined spectrophotometrically at constant ionic strength (I = 0.2 mol dm−3 NaClO4 ) and temperature 278 K. Trends in the values of formation constants according to the volume fractions of ethanol and water in ethanol and water mixed solvent systems, suggest that the trend of molecular complex formation increases with increasing the volume fraction of ethanol in mixed solvent systems. © 2004 Elsevier B.V. All rights reserved. Keywords: Water-soluble cobalt(II) Schiff base complex; Nucleotides; Molecular complexes; Formation constant; Mixed solvent systems
1. Introduction Molecular complexes [1] are being regarded as important materials for use as organic superconductors [2] and photocatalysts [3]. Recent applications of the electron donor–acceptor (EDA) complexes include non-linear optical activity [4], surface chemistry [5] and molecular recognition [6,7]. Particular attention has recently been paid to the synthesis and study of the diimino tetradentate Schiff bases and their complexes [8,9]. This is due to their uses as biological models in understanding the structure of biomolecules and biological processes [10,11]. The crucial role of Schiff bases in the biological function of bacteriorhodospin is proven. The retinal chromophore is bound covalently to the protein via a protonated Schiff base [12,13]. Adenosine-5 -triphosphate (ATP), cytidine-5 -triphosphate (CTP), guanosine-5 -triphosphate (GTP) and other nucleoside triphosphates are the principal biological energy store ∗
Corresponding author. Tel.: +98 21 464 0649; fax: +98 21 601 2983. E-mail address: [email protected] (D.M. Boghaei).
1386-1425/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2004.11.027
and of great importance in the life science. The nucleotide ATP is one of the four main components of DNA and about one-sixth of all enzyme systems need ATP or a related adenine cofactor. Their biological importance rests on the fact that they function as major carriers of chemical energy in the cells. With this fact in mind, it is not surprising that the properties of nucleotides are receiving much attention. For a better understanding of the interactions between receptors and nucleotides, it is important to obtain information about their formation constants. Although studies have been performed to determine the equilibrium constants for the interaction of cobalt(II) ion [14] and cobalt(II) ferrocyanides [15] with nucleotides, there is a lack of information about determining formation constants for molecular complex formation between water-soluble cobalt(II) Schiff base complexes and nucleotides in mixed solvent systems. In earlier works we have synthesized and characterized some Schiff base complexes in non-aqueous media [16,17]. Here we report spectrophotometric studies of molecular complex formation between water-soluble cobalt(II) tetradentate Schiff base complex derived from 4,5-dimethyl-o-phenylenediamine, as
3062
D.M. Boghaei, M. Gharagozlou / Spectrochimica Acta Part A 61 (2005) 3061–3065
Fig. 1. The structure of the investigated water-soluble cobalt(II) Schiff base complex.
shown in Fig. 1, with ATP and CTP in mixed solvent systems of ethanol and water under physiological condition (pH 7.0) that the acceptor and product are stable at this pH. Since formation constant (K) is an important parameter which measures the extent of binding, the object of the present report is to determine K for molecular complex formation of two important biological molecules, ATP and CTP, with watersoluble cobalt complexes in ethanol and water mixed solvent systems.
2. Experimental Adenosine-5 -triphosphate disodium salt and cytidine-5 triphosphate disodium salt dihydrate were purchased from Biochemika, Fluka. The buffer solution was adjusted to 7.0 with 0.1 mol dm−3 Na2 HPO4 and NaH2 PO4 . Doubly distilled, deionized water was used throughout. Ethanol and sodiumperchlorate were purchased from Merck. All other chemicals were of analytical reagent grade (if not otherwise stated) and were used as received. The Na2 [Co(SO3 -4meosal-4,5-dmophen)] was prepared according to [18]. The purity of the prepared complex was checked by elemental analysis, for C24 H20 N2 S2 O10 Na2 Co (665.46): found (calculated): C 43.6 (43.3), H 3.3 (3.0), N 4.1 (4.2), Co 8.7 (8.8). UV–vis spectra were recorded by CARY 100 Bio VARIAN UV–vis spectrophotometer equipped with a PCB150 water circulator and thermostated multicell holder. METROHM AG Herisau, 620 pH-meter was employed for pH measurements.
Fig. 2. Variation of absorption spectrum of mixtures containing Na2 [Co(SO3 -4-meosal-4,5-dmophen)] and ATP in aqueous solution (x = 0, I = 0.2 mol dm−3 NaClO4 ) and T = 278 K, the concentration of the components are as in Table 1.
480 nm shifted to lower wavelengths with the increase in its intensity and a new charge transfer (CT) peak appeared at about 360 nm that its intensity increases with the increase in the concentration of nucleotides with very clear isosbestic points. Similar spectral features are observed for all systems. The molecular complexes formed show an absorption different from the acceptor, while the nucleotides do not absorb at those wavelengths. 3.1. Determination of formation constants The formation constants of the molecular complexes are determined by using Ketelaar’s [19] equation for cells with 1 cm optical path length: 0 C0 CA 1 1 0 0 D (1) = + C A + CD ε K A − A0A − A0D 0 and C 0 are the initial concentrations of the acHere CA D ceptor and donor, respectively, A the absorbance of the donor–acceptor mixture at λCT , A0A and A0D are the absorbances of the acceptor and donor solutions with same molar concentrations as in the mixture at λCT . The effective molar absorptivity (ε ) is given by
3. Results and discussion
ε = εC − εA − εD
Absorption spectra of mixtures containing a fixed concentration of Na2 [Co(SO3 -4-meosal-4,5-dmophen)] (10−5 mol dm−3 ) and varying concentrations of ATP in aqueous solution (x = 0) are shown in Fig. 2. The isosbestic points suggest that there are only two species in equilibrium. The same is valid for other systems. The increase in the concentration of the nucleotides causes that one of the original peaks of the cobalt Schiff base complexes at about 380 nm gradually vanished, the other original peak around
where εC , εA and εD are the molar absorptivities of the molecular complex, the acceptor and the donor, respectively, K the formation constant of the formed complex. A plot of 0 C 0 /(A − A0 − A0 ) versus C 0 + C 0 should produce a CA D D D A A straight line if only a 1:1 donor–acceptor complex is formed; while a mixture of 1:1 and 1:2 or only 1:2 complex in a system would lead to a curve [19]. The formation constants for molecular complex formation between studied water-soluble cobalt(II) Schiff base complex and nucleotides in mixed
(2)
D.M. Boghaei, M. Gharagozlou / Spectrochimica Acta Part A 61 (2005) 3061–3065
3063
Table 1 Formation constants and molar absorptivities of molecular complexes from absorbance data of mixtures containing Na2 [Co(SO3 -4-meosal-4,5-dmophen)] (10−5 mol dm−3 ) and ATP at 360 nm (temperature 278 K) Ethanol volume fraction (x)
Water volume fraction (1 − x)
104 [ATP]0 (mol dm−3 )
Absorbances of mixture
Formation constant (dm3 mol−1 )
Molar absorptivity, ε (dm3 mol−1 cm−1 )
0
1
2.5 3.5 5.5 14.5 31.0 65.5
0.0046 0.0056 0.0086 0.0170 0.0290 0.0500
166.7 ± 0.3
10000 ± 0.1
0.05
0.95
2.5 3.5 5.5 14.5 31.0 65.5
0.0045 0.0059 0.0092 0.0218 0.0348 0.0546
185.2 ± 1.2
10000 ± 0.7
0.08
0.92
2.5 3.5 5.5 14.5 31.0 65.5
0.0058 0.0076 0.0118 0.0268 0.0419 0.0631
238.1 ± 0.5
10000 ± 0.4
0.11
0.89
2.5 3.5 5.5 14.5 31.0 65.5
0.0081 0.0097 0.0149 0.0302 0.0515 0.0648
333.3 ± 0.9
10000 ± 0.3
0.15
0.85
2.5 3.5 5.5 14.5 31.0 65.5
0.0104 0.0130 0.0196 0.0426 0.0633 0.0885
347.8 ± 0.6
12500 ± 1.4
solvent systems of ethanol + water were calculated from the ratio of the slope to the intercept by least-square method. The K measurements were repeated at least twice and were reproducible. Experimental data are shown in Tables 1 and 2. In 0 C 0 /(A − A0 − A0 ) versus C 0 + C 0 all cases the plots of CA D D D A A according to Eq. (1) are fairly linear, which signify that only a 1:1 donor–acceptor complex is formed, one typical case being shown in Fig. 3. Values of K and ε of the molecular complexes for x ethanol and (1 − x) water mixed solvent systems where x and (1 − x) are volume fractions of ethanol and water, respectively, obtained from such plots are shown in Tables 1 and 2.
be related to the great differences between Gutmann acceptor number for water and ethanol, which has an effect on the formation constants of the molecular complexes through the hydrogen bonding formation with nucleotides [20]. The Gutmann acceptor number for water is 54.8 and for ethanol is 37.1 [20]. Hydrogen bonding for mixed solvent systems with
3.2. Effects of mixed solvent systems To investigate the effects of mixed solvent systems of ethanol and water, studies on the molecular complex formation between water-soluble cobalt(II) Schiff base complex and nucleotides were carried out in different volume fractions of ethanol (x) and water (1 − x) in ethanol and water mixed solvent systems. It was shown that mixed solvent systems, x ethanol and (1 − x) water where x is volume fraction of ethanol, affect the formation constants. These effects can
Fig. 3. Ketelaar’s plot for molecular complexes of Na2 [Co(SO3 -4-meosal4,5-dmophen)] with CTP in ethanol and water mixed solvent system 0 0 0 0 (x = 0.08) and T = 278 K, P = CA CD /(A − A0A − A0D ) and C = CA + CD , the concentrations and absorbances being as in Table 2.
3064
D.M. Boghaei, M. Gharagozlou / Spectrochimica Acta Part A 61 (2005) 3061–3065
Table 2 Formation constants and molar absorptivities of molecular complexes from absorbance data of mixtures containing Na2 [Co(SO3 -4-meosal-4,5-dmophen)] (10−5 mol dm−3 ) and CTP at 360 nm (temperature 278 K) Ethanol volume fraction (x)
Water volume fraction (1 − x)
104 [CTP]0 (mol dm−3 )
Absorbances of mixture
Formation constant (dm3 mol−1 )
Molar absorptivity, ε (dm3 mol−1 cm−1 )
0
1
2.5 3.5 5.5 14.5 31.0 65.5
0.0050 0.0065 0.0095 0.0170 0.0270 0.0312
454.5 ± 0.5
5000 ± 0.2
0.05
0.95
2.5 3.5 5.5 14.5 31.0 65.5
0.0040 0.0054 0.0078 0.0188 0.0194 0.0252
612.2 ± 1.0
3333.3 ± 1.4
0.08
0.92
2.5 3.5 5.5 14.5 31.0 65.5
0.0051 0.0058 0.0090 0.0179 0.0221 0.0242
769.2 ± 0.5
3333.3 ± 0.8
0.11
0.89
2.5 3.5 5.5 14.5 31.0 65.5
0.0054 0.0057 0.0092 0.0196 0.0238 0.0242
833.3 ± 0.3
3333.3 ± 1.2
0.15
0.85
2.5 3.5 5.5 14.5 31.0 65.5
0.0208 0.0250 0.0275 0.0558 0.0517 0.0728
909.1 ± 0.9
10000 ± 0.1
higher ethanol:water ratios is lower due to the lower Gutmann acceptor number for ethanol. Therefore, nucleotides are better solvated through hydrogen bonding in mixed solvent systems with lower ethanol:water ratios, resulting in lower formation constants (Tables 1 and 2). Water has a very high dielectric constant, 81.7, and for ethanol is 24.3 [20]. Therefore, adding ethanol decreases the dielectric constant of the solution resulting in a greater attraction forces and hence higher formation constants. The donor property of the nucleotides is dependent on the steric factors and solvation effects through the hydrogen bonding. CTP has less steric effects than ATP. In addition ATP is solvated more than CTP due to more sites for hydrogen bonding [21], therefore in all mixed solvent systems with different volume fractions of ethanol and water, the trend of molecular complex formation according to the nucleotides that is affected by both steric and solvation effects is as follow: CTP > ATP (Tables 1 and 2). The 1:1 molecular complex formation between watersoluble cobalt(II) Schiff base complex and nucleotides is of great importance with regard to modulate the selectivity of the phosphate recognition of the biomolecules in mixed solvent systems.
Acknowledgement We are grateful to the Research Council of Sharif University of Technology for their financial support.
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