J. Phys. Chem. B 2009, 113, 9373–9378
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Cooperative and Noncooperative Binding of *Ru(bpy)32+ to DNA and SB4.5G Dendrimers T. Lopes-Costa, F. Sanchez, and P. Lopez-Cornejo* Departamento de Quı´mica Fı´sica, Facultad de Quı´mica, UniVersidad de SeVilla; c/ Profesor Garcı´a Gonza´lez s/n, 41012 SeVilla (Spain) ReceiVed: March 9, 2009; ReVised Manuscript ReceiVed: May 25, 2009
The process *Ru(bpy)32+ + S2O82- in two different reaction media, the SB4.5G dendrimer and DNA solutions, was studied. In both media, the receptors have anionic characteristics. This fact will produce a binding of the ruthenium complex to the two receptors by attractive electrostatic interactions. On the contrary, the peroxodisulfate ions will be preferentially located in the aqueous solution due to electrostatic repulsions with the receptors. Despite the similarities of the receptors, some differences are observed in these two reaction media. These differences arise from the fact that the binding of the *Ru(bpy)32+ complex to DNA shows a negative cooperativity, whereas the binding to the dendrimer is noncooperative in character. The anticooperative character of the binding that happens in DNA solutions becomes noncooperative when an electrolyte, NaNO3, is added to the medium. This is related to a condensation of the salt’s counterions on the surface of the DNA which produces a decrease of the equilibrium constant corresponding to the binding of the complex to the receptor. Therefore, it is shown that the ionic strength of the reaction medium exerts a great influence on the cooperative nature of the ligand/receptor binding. This also explains the different behavior observed in DNA and dendrimer solutions. Introduction
K)
Noncovalent interactions between a ligand and a receptor have grown in interest during the past decade,1-3 the large number of systems in which such interactions play a significant role being the reason for this growth. For example, in biochemistry, noncovalent interactions are involved in the vast majority of molecular recognition processes,4 stabilization of membranes,5 drug delivery systems,6 etc. Along with that, this type of interaction is also important in the construction of molecular machines,7 sensors,8 on-off switches,9 etc. Noncovalent ligand/receptor interactions produce effects on chemical reactions.10 In this sense, the catalytic processes in general, and the enzymatic in particular, are caused by such interactions between the substrate and the catalyst. In previous papers,11 we have shown that the pseudophase model proposed by Menger and Portnoy12 to rationalize kinetic data in micellar systems can be used to explain kinetic behavior in other systems in which noncovalent interactions play a role. On the basis of this model, a reaction can take place in one (or both) of the two pseudophases that the media has: the bulk and a dispersed pseudophase. In the case of micellar systems, this dispersed pseudophase is called micellar pseudophase, and by analogy, we will name it receptor pseudophase throughout this paper to refer to DNA and dendrimer. The possibility that the reaction could take place in the two pseudophases of the system is due to the distribution of one or more reactants between the two pseudophases. This distribution can be expressed as
* Author to whom correspondence should be addressed. E-mail:
[email protected].
[SR] [Sw][R]
(1)
where [Sw] and [SR] are the concentrations of the reactant located in the bulk and in the receptor, respectively. [R] represents concentration of the receptor. For a unimolecular process, or a bimolecular process where only one of the reactants is partitioned between the two media pseudophases (see Discussion section), the observed rate constant can be written as
kobs )
kw + kRK[R] 1 + K[R]
(2)
Here kw and kR represent the rate constants corresponding to the processes that take place in the bulk and in the receptor, respectively. Equation 1 is true only if one assumes that structural parameters of the receptor (size, shape, and structure) do not change when the receptor concentration is changed. Besides, the receptor concentration used must be much higher than the reactant concentration that binds to the receptor, to avoid saturation of the latter. When this does not happen, the K equilibrium constant must be defined by using other equations.13 It must also be taken into account that K is only a true constant when the interaction of the reactant with the receptor is noncooperative in character. In another case, eq 1, and therefore eq 2, must be modified. The cooperativity of a binding can be defined as follows: when a receptor has various binding sites, the interaction of a first ligand to a receptor can produce some influence on the interactions of other ligands.14 Thus, the interaction is cooperative when the binding of a first ligand to the receptor makes the binding of a second ligand to the same receptor stronger
10.1021/jp902110x CCC: $40.75 2009 American Chemical Society Published on Web 06/18/2009
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and so on. On the contrary, the interaction is anticooperative when the binding of a first ligand to the receptor prevents the binding of a second one. Finally, the interaction is noncooperative when the binding of a ligand with a receptor shows no influence on the following bindings. The variations on the equilibrium constant, K, corresponding to a ligand/receptor interaction give information about the cooperativity of the process. So, when the interaction is noncooperative, this equilibrium constant is a true constant. On the contrary, when the interaction is cooperative in character, this equilibrium constant shows a dependence on the receptor concentration. In the present work, the electron transfer reaction *Ru(bpy)32+ + S2O82- in two different systems, dendrimer and DNA, was studied. In both systems, the receptors have anionic character. This fact will produce a binding of the ruthenium complex to the two receptors by attractive electrostatic interactions. On the contrary, the peroxodisulfate ions will mainly be located in the aqueous pseudophase due to electrostatic repulsions. Taking into account the characteristics of the two receptors used here, the behavior of the process studied could expect to be similar in the two media. However, different behavior is observed. This reveals the importance that working conditions, more concretely the ionic strength of the medium, have when the cooperative character of a binding is studied.
Lopes-Costa et al. TABLE 1: Stern-Volmer Constant (KSV) Lifetime Values in the Absence of Quencher (τo) and Observed Rate Constants (kobs) of the Reaction *Ru(NH3)5pz2+ + S2O82- in DNA Solutions at Different NaNO3 Concentrations Added 104[DNA]
KSV
τo
mol dm-3
mol-1 dm3
ns
0 0.25 0.40 0.50 1.0 2.5 5.0 10.0 17.5 25.0
mol-1 dm3 s-1 -3
6.15 4.46 3.71 2.30 0.91 0.50 0.31 0.19 0.14 0.09
0 0.25 0.40 0.50 1.0 2.5 5.0 10.0 17.5 25.0
[NaNO3] ) 0.05 mol dm-3 3215 599 1315 600 910 619 805 632 683 647 680 655 641 664 600 687 573 709 536 725
5.36 2.19 1.47 1.27 1.06 1.04 0.97 0.87 0.81 0.74
0 0.25 0.40 0.50 1.0 2.5 5.0 10.0 17.5 25.0
[NaNO3] ) 0.10 2791 1390 1157 1104 724 569 557 554 548 527
mol dm-3 605 606 607 628 638 644 649 658 672 682
4.62 2.29 1.90 1.76 1.13 0.88 0.86 0.84 0.81 0.77
0 0.25 0.40 0.50 1.0 2.5 5.0 10.0 17.5 25.0
[NaNO3] ) 0.15 mol dm-3 2547 604 1302 606 1125 612 983 614 735 618 552 621 535 622 522 628 518 631 508 635
5.36 2.21 1.47 1.27 1.06 1.04 0.97 0.87 0.81 0.74
0 0.25 0.40 0.50 1.0 2.5 5.0 10.0 17.5 25.0
[NaNO3] ) 0.30 mol dm-3 1321 604 1055 606 988 607 976 604 877 608 602 603 504 603 492 602 471 604 457 604
2.19 1.74 1.63 1.61 1.44 0.99 0.83 0.82 0.78 0.76
Experimental Section Materials. Ru(bpy)3(ClO4)2 and Na2S2O8 were obtained from Aldrich and Fluka, respectively (bpy ) 2,2′-bipyridine). NaNO3 came from Merck and was used as received. Calf thymus DNA was purchased from Fluka and used without further purification. Polynucleotide concentrations were determined spectrophotometrically from molar absorptivity (6600 mol-1 dm3 at 258 nm).15 The commercial starburst dendrimer (SB4.5G) was from Aldrich and also used as received. The water used for the preparation of solutions had a conductivity of ≈ 10-6 S m-1 and was deoxygenated before used. Fluorescence Measurements. (a) Intensity measurements were carried out in a spectrofluorimeter (Hitachi F-2500) interfaced to a PC for the reading and handling of the spectra. The concentration of the ruthenium complex used was 5 × 10-6 mol dm-3, and the concentration of the quencher (S2O82-) ranged from 0 to 3 × 10-3 mol dm-3. In the case of measurements in the presence of the quencher, a constant Na+ concentration was maintained by adding an inert salt (NaNO3). (b) Fluorescence lifetimes of the ruthenium complex excited state were measured with a FL920 Fluorescence Lifetime Spectrometer from Edinburgh Instruments using the time correlated single photon counting technique. Fluorescence decays were analyzed by an iterative deconvolution procedure based on the Marquardt algorithm.16 The goodness of the fit was measured by the magnitude χ2 (χ2 ) ∑i|Fi - fi|2, where Fi is the value of the ith data point and fi is the value obtained from the fit) and the shape functions of the weighed residuals. The solutions were deoxygenated by bubbling argon through them for at least 30 min before lifetime and intensity measurements. Emission and excitation wavelengths were 597 and 452 nm, respectively, for the two receptors studied. The excitation and emission wavelengths were those that corresponded to the maxima of the absorption and emission spectra, respectively, in the different reaction media. The temperature was always maintained at 298.2 K.
[NaNO3] ) 0 mol dm 3680 598 2739 614 2311 622 1460 634 612 672 371 743 239 766 147 791 116 808 75 819
10-9 kobs
Results Stern-Volmer constants, KSV, obtained by using eq 3 are collected in Tables 1 and 2.
I0 ) 1 + KSV[S2O82-] I
(3)
Binding of *Ru(bpy)32+ to DNA and SB4.5G Dendrimer
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TABLE 2: Stern-Volmer Constant (KSV) Lifetime Values in the Absence of Quencher (τo) and Observed Rate Constants (kobs) of the Reaction *Ru(NH3)5pz2+ + S2O82- in Dendrimer Solutions 105[dendrimer]
KSV
τo
10-9 kobs
mol dm-3
mol-1 dm3
ns
mol-1 dm3 s-1
0 0.018 0.036 0.045 0.060 0.090 0.30 0.80 2.0 3.6 7.0 9.5 15 20
3680 3038 2660 2524 1865 1434 946 468 203 141 139 125 121 119
598 604 605 606 610 612 631 640 650 657 682 692 699 702
6.15 5.03 4.39 4.16 3.06 2.34 1.50 0.73 0.31 0.21 0.20 0.18 0.17 0.17
Figure 2. Decay of the excited state of [Ru(bpy)3]2+ in dendrimer solutions ([dendrimer] ) 3.6 × 10-7 mol dm-3) at 298.2 K (the blue line represents experimental data, and the pink line is the best fit obtained).
These tables also contain the observed rate constant, kobs, calculated as KSV/τo. Good linear plots were obtained for the plots of the Io/I ratio versus the [S2O82-] in all the media used and in the range of quencher concentrations studied here. An example is shown in Figure 1. Lifetime values measured in the absence of quencher, τo, also appear in Tables 1 and 2. For all the media studied, monoexponential decays were observed at the different receptor concentrations. Figure 2 shows an example of the decay of the excited ruthenium complex as well as the residuals obtained. Discussion (a) DNA Solutions. According to Table 1, a decrease of kobs is observed by increasing the DNA concentration. Taking into account the characteristics of the receptor and of the reactants that participate in the process, it is clear that only one of the reactants interacts significantly with DNA. This reactant must be the ruthenium complex which probably binds to the phosphate groups of the polymer by electrostatic interactions. This is according to the variation that lifetime values of the [Ru(bpy)3]2+ shows with the DNA concentration (see Table 1), as well as with the slight red shift obtained for the maximum wavelength in the emission spectra of the same complex by increasing the receptor concentration. The latter, as well as the fact that no variation in the emission intensity when the DNA concentration is changed, seems to indicate that the interaction of the metal complex with this receptor is slight; that is,
2-
Figure 1. Plot of Io/I versus the quencher concentration, [S2O8 ], in DNA solutions ([DNA] ) 1 × 10-4 mol dm-3) at 298.2 K.
interactions such as intercalation or groove binding do not occur between this ruthenium complex and the DNA molecules. Besides, this is in agreement with previous results of Kumar et al. for the same metal complex.17 Results confirm that the interaction between [Ru(bpy)3]2+ and DNA is of an electrostatic type. At first, eq 2 could be applied to describe the kinetic behavior in the solutions containing DNA. This equation, strictly speaking, is only valid for unimolecular reactions. However, as mentioned in the Introduction section, it can be applied to bimolecular processes, provided that, as here, only one of the reactants is concentrated in the close vicinity of the receptor and the concentration of the second reactant is depleted. This situation was considered by us in a previous paper,18 where the meaning of parameters appearing in eq 2 was considered. However, eq 2 fails if one tries to fit our results to it. This means that some, or all, of the parameters appearing in this equation depend on the receptor concentration. Since kw is fixed and kR is small (see below) and, thus, does not contribute appreciably, one must assume that the binding constant K is dependent on the DNA concentration. One could think that variations of K could be a consequence of the condensation of counterions (Na+) of DNA, and that, as a consequence, could be explained from the pseudophase ion-exchange (PPIE) model.19 Notice, however, that the condensation of counterions would produce a diminution of K when the concentration of the DNA increases because the condensation, which reduces the charge on the polymer, increases when the concentration of DNA does so.20 Here, we observed the opposite effect: an increase of K when the DNA concentration increases. Consequently, Variations in K are not due to sodium ion condensation, and thus, the PPIE model cannot be applied. Another possibility for the change in K, when the [DNA] changes, could be the anticooperative character of the [Ru(bpy)3]2+/DNA interaction. Thus, for an anticooperative binding, a sigmoidal dependence of K on the receptor concentration was used in a previous paper.21 This dependence can be expressed as
K)
Kmaxet 1 + et
(4)
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Lopes-Costa et al. TABLE 3: Equilibrium Constants, K, and Surface Potential Values, Ψ, in DNA Solutions at Different NaNO3 Concentrations [NaNO3] mol dm 0.05 0.10 0.15 0.30
Figure 3. Plot of kobs/mol-1 dm3 s-1 versus kcalc/mol-1 dm3 s-1 (calculated from eqs 2, 4, and 5) for the process *Ru(NH3)5pz2+ + S2O82- in DNA solutions.
Figure 4. Plot of K/mol-1 dm3 (experimental) versus Kcalc/mol-1 dm3 (calculated from eqs 4 and 5) for the process *Ru(NH3)5pz2+ + S2O82in DNA solutions.
where
t)
[DNA] - h j
-3
-Ψ
K mol
-1
dm
3
114621 56488 49447 13397
mV 32.3 23.6 21.5 4.65
To get a deeper insight into the cause of the anticooperative character of the binding between the ligand and the receptor, the reaction was studied in DNA solutions in the presence of various NaNO3 concentrations. The observed rate constants are also collected in Table 1. As happens in the absence of the electrolyte, a decrease of the observed rate constant is obtained by increasing the polymer concentration at the different salt concentrations. However, now the behavior is different in the sense that, in the presence of NaNO3, the kobs values can be rationalized by using the pseudophase model in its original version (see eqs 1 and 2). That is, the equilibrium constant is a true constant in the DNA solutions when salt is added, and therefore, the binding between the ruthenium complex and the polymer is noncooperative in character. The K values obtained by using eq 2 are collected in Table 3. The cooperative character of a binding ligand/DNA depends on the ionic strength of the medium. When the ionic strength increases, a condensation of the counterions of the electrolyte on the phosphate groups of the DNA happens. So, a partial neutralization of the DNA charge and, therefore, a decrease of the surface potential of the receptor take place. This will produce a decreasing of the free energy corresponding to the ligand/ receptor binding and, thus, a decrease in the equilibrium constant K. In this case, the anticooperative character of this binding would be less important, and as is observed in the presence of NaNO3, the anticooperative binding becomes noncooperative in character. As was mentioned above, a decrease of K when increasing salt concentration represents a change of the free energy corresponding to the distribution process of the metal complex between the aqueous and the receptor pseudophases
(5)
Kmax being the maximum (limiting) value of K; j an adjustable parameter which measures the degree of anticooperativity; and h the value of [DNA] for which K ) (1/2)Kmax. In fact, the experimental data collected in Table 1 are wellfitted by using eq 2 if one takes into account a sigmoidal dependence of K on the [DNA] with the following adjustable parameters: Kmax ) 8.0 × 105 mol-1 dm3, h ) 1.2 × 10-2 mol dm-3, j ) 3.4 × 10-3 mol dm-3, kw ) 6.2 × 109 mol-1 dm3 s-1, and kR ) 2.8 × 107 mol-1 dm3 s-1. Figures 3 and 4 show the quality of the fit. It is interesting to note that there are two possible causes of the anticooperative character of the binding of the ruthenium complex to DNA: first, the repulsion of a second ruthenium complex caused by the binding of the first complex (according to the definition of cooperativity given in the Introduction section) and, second, a change in the DNA structure caused by a folding induced by the presence of the ruthenium complex.22 However, we cannot say which of them is the main factor determining the anticooperative character from data obtained here.
K ) e-∆G/RT
(6)
This variation free energy can be defined as the sum of two contributions: a nonelectrostatic contribution, ∆Gnel, and a electrostatic one, ∆Gel
∆G ) ∆Gnel + ∆Gel
(7)
The electrostatic contribution takes into account the surface potential of the DNA as follows
∆Gel ) zRFΨ
(8)
where F is the Faraday constant, z the charge of the ligand which interacts with the receptor (+2 in the case of the ruthenium complex used here), and Ψ the surface potential of the polymer. R is a parameter that is related to the location of the ruthenium complex in the DNA, that is, gives information about the fraction of surface potential that exerts some influence on ∆Gel. By using eqs 6, 7, and 8, one can write
Binding of *Ru(bpy)32+ to DNA and SB4.5G Dendrimer
ln K ) ln Knel -
zRFΨ RT
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(9)
Knel being the nonelectrostatic contribution of the equilibrium constant. By using eq 9 and the Lippard equation23
ln K ) ln Knel + a ln[Na+]
(10)
one can obtain the surface potential Ψ of the DNA at different salt concentrations. Table 3 collects the values obtained. Of course, these are only estimated values because they depend on the location of the species which binds to the receptor, that is, on the R value. In any case, the values obtained assuming R ) 1 are of the same order as those obtained previously by us in DNA24 and other charged receptors using other methods.21,25 (b) Dendrimer Solutions. The SB4.5G dendrimer used here bears a negative surface as a consequence of its carboxylate groups. Therefore, this is a negatively charged system like the DNA solutions. This means that similar behavior of the reaction studied, *Ru(bpy)32+ + S2O82-, could be expected in the two receptors: there is a binding of the ruthenium complex with the receptors due to electrostatic interactions, while the peroxodisulfate ions will be principally located in the bulk.18 The emission spectra of the ruthenium complex show no dependence on the dendrimer concentration; that is, no movement of the band or variation on the intensity value is observed when the dendrimer concentration is changed. On the contrary, the lifetime of the ruthenium complex increases with the dendrimer concentration (see Table 2). This behavior seems to indicate that there is an interaction between the ruthenium complex and the dendrimer. A decrease of the observed rate constants is obtained by increasing the [dendrimer] as happens with the DNA solutions (see Tables 1 and 2). However, this decrease can be rationalized by using the pseudophase model in its original version. Figure 5 shows the good fit obtained with eq 2 and the following adjustable parameters: kw ) 6.2 × 109 mol-1 dm3 s-1, kR ) 1.5 × 108 mol-1 dm3 s-1, and K ) 1.5 × 106 mol-1 dm3. That is, the binding of the ruthenium complex with the dendrimer is noncooperative in character, while an anticooperative binding takes place for the same ruthenium complex in the case of DNA solutions. This result is also different from that obtained for the same receptor, SB4.5G dendrimer, and a similar process, the Ru(NH3)5pz2+ + S2O82- reaction, in which an anticooperative binding of the ruthenium complex to the dendrimer was observed.21 The range of dendrimer concentration used here and in the previous paper is the same, the only difference being the concentration of the reactants, ruthenium, and peroxodisulfate ions (here [Ru(bpy)32+] ) 5 × 10-6 mol dm-3 and [S2O82-] ) 3 × 10-3 mol dm-3, and in the previous work [Ru(NH3)5pz2+] ) 2 × 10-5 mol dm-3 and [S2O82-] ) 2 × 10-4 mol dm-3).26 Fluorescence measurements are known to need the use of small fluorescence species concentrations to avoid reabsorption phenomena that produce erroneous results. This explains the use of different concentrations for the ruthenium complexes. With respect to the S2O82- concentration, we have used a higher Na+ concentration here than that in the previous work to obtain good experimental data. Thus, the ionic strength of the medium will be higher in this work. A higher ionic strength produces a greater condensation of sodium ions on the negative surface of the dendrimer and, so, a higher partial neutralization
Figure 5. Plot of kobs/mol-1 dm3 s-1 versus the dendrimer concentration. Points represent experimental values, and the line corresponds to the best fit obtained by using eq 2.
of the dendrimer charge, as happens in DNA solutions when salt is added. This explains why the anticooperative binding of the [Ru(NH3)5pz]2+ to SB4.5G dendrimers previously obtained becomes cooperative for the binding of [Ru(bpy)3]2+ to the same dendrimer type. It is interesting to note that the ionic strength required to neutralize part of the dendrimer’s surface and, therefore, to change the cooperative character of the ligand/receptor binding is lower in dendrimer than in DNA solutions (in the case of DNA a [salt] ) 0.05 mol dm-3 is necessary).27 This is related to the fact that the size of the dendrimers is smaller than that for DNA and, thus, the charge. In any case, it must be mentioned that the location of the two ruthenium complexes compared is different given the nature of the ligands. This fact can also exert some influence on the cooperative character in the ligand/receptor binding. In conclusion, the pseudophase model is a broad spectra model that can be applied to different systems. However, its use not only is determined by the characteristics of the reaction media but also shows an impressive connection to the experimental working conditions used, in the sense that the cooperative effect on the equilibrium constant does not only depend on the receptor concentration, or better on the [receptor]/[ligand] molar ratio, but also on the ionic strength of the reaction media. Acknowledgment. This work was financed by the D.G.I.CYT (CTQ2008-00008/BQU) and the Consejerı´a de Educacio´n y Ciencia de la Junta de Andalucı´a. T. Lopes-Costa thanks the Direccio´n General de Investigacio´n for a grant of the “Programa de Becas Predoctorales de Formacio´n de Investigadores”. References and Notes (1) (a) Gutsche, C. D.; Nam, K. C. J. Am. Chem. Soc. 1988, 110, 6153. (b) Mohammed-Ziegler, I.; Hamdi, A.; Abidi, R.; Vicens, J. Supramol. Chem. 2006, 18, 219. (2) Gutsche, C. D. Host Guest Complex Chemistry Macrocycles: Synthesis, Structures, Aplications; Weber, E., Eds.; Springer: Berlin, 1985. (3) (a) Mustafina, A. R.; Skripacheva, V. V.; Gubaidullin, A. T.; Latipov, S. K.; Toropchina, A. V.; Yanilkin, V. V.; Solovieva, S. E.; Antipin, I. S.; Konovalov, A. I. Inorg. Chem. 2005, 44, 4017. (b) Miyagawa, T.; Yamamoto, M.; Muraki, R.; Onouchi, H.; Yashima, E. J. Am. Chem. Soc. 2007, 129, 3676. (c) Galia, A.; Navarre, E. C.; Scialdone, O.; Ferreira, M.; Filardo, G.; Tilloy, S.; Monflier, E. J. Phys. Chem. B 2007, 111, 2573. (4) (a) Hamada, F.; Kondo, Y.; Ito, R.; Suzuki, I.; Osa, T.; Ueno, A. J. Inclusion Phenom. 1993, 15, 273. (b) Reczek, J. J.; Kennedy, A. A.; Halbert, B. T.; Urbach, A. R. J. Am. Chem. Soc. 2009, 131, 2408. (5) Howarth, G.; Sovak, M. Biochim. Biophys. Acta 1973, 298, 850.
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