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The equilibria and kinetics aspects of the binding of small gold nanoparticles, AuNPs, stabilized with tiopronin to DNA in B and C conformation (B-DNA and C-DNA), has been investigated in ethanol/water mixtures using different techniques. Two modes of binding are displayed: groove binding and partial intercalation, depending on the ethanol content, [EtOH], and the molar ratio, R = CAuNPs/CDNA. Two reaction mechanisms are proposed for AuNPs/DNA interaction in each polymer conformation, and the reaction parameters are evaluated. For lower ethanol levels, ([EtOH] up to 30%), when DNA is in the B form, the simplest mechanism according to the kinetic and thermodynamic results proved to be a three-step series mechanism reaction scheme which evolves in the formation of the groove complex. In this context, solvent hydration as well as the solvent effective viscosity are the main factors that influence kinetics. In contrast, for high ethanol levels, when DNA is in a C-like conformation, the mechanism is more complex involving three parallel reactions, in which AuNPs self-aggregation plays a key role in the switch from partial intercalation to groove binding. On the whole, it is evident that AuNPs aggregation and the DNA conformation are two key factors that must be taken into account in order to control the mechanism of AuNPs/DNA interaction.

We report herein results regarding reactivity and assembly of citrate-capped gold nanoparticles (AuNPs) mediated by captopril (cap) and S-nitrosocaptopril (NOcap), two angiotensin converting enzyme inhibitors and antihypertensive agents. The results were compared with that of cysteine (Cys), a thiol-containing amino acid found in plasma. The interparticle interactions were characterized by monitoring the evolution of the surface plasmon resonance band using the spectrophotometric method. The original gold nanoparticles were efficiently modified by small amounts of Mn(+2) ions, which are adsorbed onto the surface of 15.4 nm citrate-capped gold nanoparticles, giving rise to manganese-gold nanoparticles (Mn-AuNPs) that, in mild acid medium, have proved to be highly sensitive and a rapid colorimetric detection method for thiols. Depending on the concentration of the Mn(+2) ions the aggregation of AuNPs can be rapidly induced. The kinetics of the assembly process has been studied. Good first-order kinetics has been observed, with the exception of captopril-mediated nanoparticle aggregation at low concentration of either cap or acid. The rate of Cys-mediated assembly of gold nanoparticles in aqueous 10 mM acetic acid is more than 20-times faster than pure AuNPs and concentrations of Cys as low as 34 nM can be detected in less than 40 min under conditions of stable Mn-AuNPs. Similar effects were observed with cap or NOcap. The assembly-disassembly reversibility is shown with cap and NOcap and depends highly on pH.

It is well known that single-stranded DNA (ssDNA) is easily able to adsorb on citrate-capped, non-functionalized gold nanoparticles (AuNPs). However, the affinity of double-stranded DNA (dsDNA) for them is much more limited. The present work demonstrates that long dsDNA suffers from a bending conformational change when anionic nanoparticles are present in solution. A striking decrease in the persistence length of the double helix in the absence of salt is observed through dynamic light scattering (DLS), viscometric, and atomic force microscopy (AFM) methods. Long dsDNA is therefore shown to be able to interact with anionic gold nanoparticles. To date, only ssDNA detection has been described by making use of interparticle cross-linking aggregation mechanisms; however, the data shown in this work allow for the development of new methods for detecting dsDNA in solution by using aggregated AuNPs as a starting point. The aggregation state is induced by the controlled addition of an inert electrolyte. A deconvolution procedure of the experimental plasmon shows how individual bands corresponding to aggregated nanoclusters diminish as the DNA concentration increases in the presence of 0.075 M NaCl.

AlOOH nanoparticles have been used to study the strength and character of the binding of the persulphate ion (S2O82−), at pH = 5.4, to them. The strength of the binding has been studied using a kinetic approach which consists of the study of the oxidation kinetics of a metal cation complex ([Ru(NH3)5pz]2+ (pz = pyrazine)) by S2O82− at different NaCl concentrations. When the ionic strength increases, the strength of the binding decreases, as a consequence of the partial neutralization of the charge on the AlOOH surface which, at pH = 5.4, is positively charged. The increase of the ionic strength also produces a change in the character of the binding, which changes from anticooperative to noncooperative when the ionic strength increases. The nonelectrostatic and electrostatic components of the free energy of binding are determined. The nonelectrostatic component of the free energy of binding is almost zero. On the other hand, the values of the differences of electrostatic potential at the AlOOH/solutions in...

Gold nanoparticles (AuNPs) capped with N-(2-mercaptopropionyl)glycine have been used to study the strength and character of the binding of a cationic metal complex, [Ru(NH3)5pz]2+ (pz = pyrazine), at pH = 8, to these nanoparticles. The strength of the binding has been studied using a kinetic approach consisting of the study of the kinetics of the oxidation of this ruthenium complex by S2O82- at different NaCl concentrations. When the ionic strength increases, the strength of the binding decreases, as a consequence of the partial neutralization of the charge on the AuNPs which, at pH = 8, has the tiopronin residue negatively charged. The increase of the ionic strength also produces a change in the character of the binding, which changes from anticooperative to noncooperative when the ionic strength increases. The nonelectrostatic and electrostatic components of the free energy of binding are determined. From the latter, we have obtained the values of the electrostatic potential differences at the AuNPs/solutions interface.

The title reaction was studied in different reaction media: aqueous salt solutions (NaNO3) and water-cosolvent (methanol) mixtures. The observed rate constants, k(obs), show normal behavior in the solutions containing the electrolyte, that is, a negative salt effect. However, the solvent effect is abnormal, because a decrease of the rate constant is observed when the dielectric constant of the reaction medium decreases. These effects (the normal and the abnormal) can be explained using the Marcus-Hush treatment for electron transfer reactions. To apply this treatment, the true, unimolecular, electron-transfer rate constants, k(et), have been obtained from k(obs) after calculation of the rate constants corresponding to the formation of the encounter complex from the separate reactants, k(D), and the dissociation of this complex, k(-D). This calculation has been carried out using an exponential mean spherical approach (EMSA).

Salt and solvent effects on the kinetics of the reactions [Fe(CN)6]3- + [Ru(NH3)5pz](2+) right arrow over left arrow [Fe(CN)6]4- + [Ru(NH3)5pz]3+ (pz = pyrazine) have been studied through T-jump measurements. The forward and reverse reactions show different behaviors: "abnormal" salt and solvent effects in the first case and normal effects in the second one. These facts imply an asymmetric behavior of anion/cation reactions depending on the charge of the oxidant. The results can be rationalized by using the Marcus-Hush treatment for electron-transfer reactions.

The kinetics of the reaction between ruthenium pentaammine pyrazine and acetonitrile pentacyanoferrate (II) to obtain the binuclear anionic complex [Fe(CN) 5 pzRu(NH 3 ) 5 ] − , and the reverse (dissociation) process, have been studied in solutions containing DNA. The results corresponding to this reaction and those corresponding to the reverse (dissociation) process show a clear influence of DNA on their kinetics. The results can be interpreted using a modified Pseudophase Model. From the results obtained for the dissociation reaction one can conclude that the binuclear anionic complex [Fe(CN) 5 pzRu(NH 3 ) 5 ] − interacts with DNA.

The kinetics of the electron transfer reactions between [Co(C2O4)3]3− and different ruthenium complexes [Ru(NH3)5L]2+ (L = pyridine, pyrazine and 4-cyanopyridine) have been studied in water–methanol mixtures. A method for the evaluation of the reorganization energy of these reactions, based on comparison of their rate constants, is proposed. The results showed a decrease of the rate constants and a decrease of the reorganization energies when the dielectric constant of the medium decreases. This behaviour is according to that obtained from the use of classical continuum methods. The slight differences found between the distinct methods could be related to the existence of specific effects of the medium.

The kinetics of the electron transfer reactions between S(2)O(8)(2-) and the complexes (Ru(NH(3))(5)L)(2+) (L = pyridine, pyrazine, and 4-cyanopyridine) have been studied in micellar (SDS) solutions. A method for the evaluation of the reorganization energy of these reactions, based on the comparison of their rate constants, is proposed. From the results obtained, we concluded that the observed rate constants go through a minimum for the surfactant concentration in which the reorganization energy goes through a maximum. The method can be applied to any kind of restricted geometry conditions.

The kinetics of electron transfer reactions between [Fe(CN) 6 ] 4 - and [Co{NH 3 ) 5 pz] 3 + and between [Ru(NH 3 ) 5 pz] 2 + and [Co(C 2 O 4 ) 3 ] 3 - was studied in concentrated salt solutions (Na 2 SO 4 , LiNO 3 , and Ca(NO 3 ) 2 ). An analysis of the experimental kinetic data, k o b s , permits us to obtain the true (unimolecular) electron transfer rate constants corresponding to the true electron transfer process (precursor complex → successor complex), k e t . The variations of both, k o b s and k e t , with salt concentrations are opposite for these reactions These opposite tendencies can be rationalized by using the Marcus-Hush treatment for electron transfer reactions. The conclusion is that the negative salt effect found for the first reaction ([Fe(CN) 6 ] 4 - + [Co(NH 3 ) 5 pz] 3 + ) is due to the increase of the reaction and reorganization free energies when the concentration of salt increases. In the case of the second reaction ([Ru(NH 3 ) 5 pz] 2 + + [Co(C 2 O 4 ) 3 ] 3 - ), the positive salt effect observed is caused by the fact that the driving force becomes more favorable when the concentration of salt increases. Thus, it is shown that for anion/cation electron transfer reactions the kinetic salt effect depends on the charge sign of the oxidant (and the reductant).

The kinetics of the electron transfer reaction between [Ru(NH3)5pz]2+ (pz = pyrazine) and [Co(C2O4)3]3− ( C 2 O 4 2 - = oxalate anion ) was studied in solutions of the SB1.5G dendrimer at different concentrations of a supporting electrolyte (NaCl) and in solutions of the SB4.5G dendrimer. The results are interpreted on the basis of a modified pseudophase model, which takes into account the anti-cooperative character of the cationic reactant binding to the dendrimers. With regard to the binding characteristics of small cationic ligands, and their influences on the kinetics of reactions in which these small ions participate, the conclusion was that dendrimers and DNA behave similarly.

The kinetics of substitution of H2O by Ru(NH3)5pz2+ (pz = pyrazine) in Fe(CN)5H2O3− have been studied in micellar aqueous solutions of sodium dodecylsulfate (SDS). Experimental results are discussed by using an approach based on the transition-state theory. This approach is better than others based on the pseudophase model, which can also be used, because it is able to give a clear meaning to the parameters of the model. Trends in the observed reactivity are explained by a change in the degree of association of one of the reactants to the micelles (Ru(NH3)5pz2+ in the present work). This association is governed by an equilibrium constant that depends on the electrostatic potential at the surface of the micelles. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 627–633, 2004

The ionic strength effect on the binding of [Ru(NH 3 ) 5 pz] 2+ to DNA has been studied using a kinetic approach consisting in the study of the kinetics of the oxidation of this ruthenium complex by [Co(C 2 O 4 ) 3 ] 3− (C 2 O 4 =oxalate anion) at different ionic strength and in the presence and absence of buffers. When the ionic strength increases, the strength of the binding decreases, as a consequence of the partial neutralization of the charge on the DNA. The increase of the ionic strength also produces a change in the character of the binding, which changes from anti-cooperative to non-cooperative when the ionic strength increases.

The kinetics of the electron-transfer reaction between pentaamminepyridine ruthenium (II) perchlorate, [Ru(NH3)5py](ClO4)2, and sodium peroxodisulfate, Na2S2O8, was studied in DNA and SDS micellar ...

The kinetics of replacement of H2O by [Ru(NH3)pz]2+ (pz = pyrazine) in [Fe(CN)5H2O]3− have been studied in various concentrated electrolyte solutions and in various water–cosolvent mixtures, at 298 K. Salt and cosolvent effects can be rationalized taking into account specific medium effects on both the encounter complex formation process and the ligand-substitution process, once the encounter complex is formed. These effects in water–cosolvent mixtures depend on the solvation of the reactants by the components of the mixture, as well as on the solvent–solvent interactions in these mixtures. Salt effects seem to be related to a primary salt effect as well as to the effect of the cations on the electronic density on the iron complex. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 367–373, 2003

A variety of fluorescent imino and secondary amino chitosans were synthesized under very mild conditions by reaction of the biopolymer amino functions with aromatic aldehydes in an acidified methanolic suspension. Simultaneous reactions of several aldehydes with chitosan were successfully carried out, and kinetic studies showed that 1-pyrenecarboxaldehyde reacts the fastest among them. An unprecedented study on the evaluation of the degree of N-substitution (DS, ranging from 31.7% to 12.0%) for the chitosan Schiff bases by using solid state CPMAS (13)C NMR is performed. A linear correlation between the DS obtained for the secondary amino chitosans by (1)H NMR (55.3-10.2%) and those obtained by CPMAS (13)C NMR (34.4-13.8%) has allowed us to calculate an empirical correlation factor that could be applied on chitosan-based aromatic systems. The new chiral-labelled chitosan derivatives exhibit a stable fluorescent behaviour, which was used to explore solvent sensoring applications.

DAPI (4',6-diamidino-2-phenylindole) is a widely used fluorescent dye, whose complicated binding features to DNAs and RNAs have been the object of debates and are still not fully understood. In this study, different approaches were employed, including binding equilibrium measurements (spectrofluorometry), melting experiments (spectrophotometry), viscometric measurements, circular dichroism, and T-jump kinetic analyses; all data concur in shedding light on the complex mechanistic aspects of the binding mode of DAPI to natural DNA. Conditions are found that induce the mode of the DAPI/DNA interaction to change from groove binding to intercalation. Moreover, it is observed, for the first time, that DAPI is able to induce the formation of a rather compact polymer-dye adduct under particular conditions. The results suggest that this form is a folded or coiled DNA structure stabilized by DAPI dye bridges.

The kinetics of the interaction of a fluorescent probe, 1-pyrenecarboxaldehyde, with calf thymus DNA has been studied in different water/alcohol mixtures (ethanol, 2-propanol, and ter-butanol) at 25 degrees C, by using the stopped flow technique. The kinetic curves are biexponential and reveal the presence of two processes whose rates differ by about 1 order of magnitude on the time scale. The dependence of the reciprocal fast relaxation time on the DNA concentration is linear, whereas the concentration dependence of the reciprocal slow relaxation time tends to a plateau at high DNA concentrations. The simplest mechanism consistent with the kinetic results involves a simple two-step series mechanism reaction scheme. The first step corresponds to the formation of a precursor complex, (DNA/Py)(I), while the second one corresponds to full intercalation of the pyrene dye between the DNA base pairs. The values of the rate constants of both steps decrease as water activity decreases. The results have been discussed in terms of solvation of the species and changes in the viscosity of the solution.

The electron-transfer reaction between [Fe(CN) 6 ] 4- and [CoC1(NH 3 ) 5 ] 2+ was studied in the presence of 18-crown-6 ether (18C6) in different reaction media constituted by water and acetonitrile as organic cosolvent at 298.2 K. The results corresponding to this reaction show a clear influence of 18C6 on the kinetics: a positive catalytic effect. Trends in the observed reactivity are explained by a change in the degree of association of one of the reactants (the cobalt complex) with the 18C6. This association is governed by an equilibrium constant that depends on the dielectric constant of the medium. The results show an increase of the rate constants for the electron-transfer process as the 18-crown-ether concentration increases and an increase of the binding free energy of the cobalt complex to the 18C6 when the electrostatic field of the medium becomes weaker. An analysis of the experimental data allows not only the reactivity changes associated with adducts formation processes for an electron-transfer reaction but also information on the binding free energy of the cobalt complex to the 18C6 to be obtained, which can be quantified by using a two-state model. We have found a good correlation between the energy of binding and the Kosower's Z-value. The influence of the 18C6 in the intramolecular electron transfer in the binuclear complex [Fe(CN) 5 pzCo(NH 3 ) 5 ] has been also investigated.

The binding free energy of complex [Co(C(2)O(4))(3)](3-) to three peptides H-Lys-Gly-Lys-Gly-Lys-Gly-Lys-NH(2) (P-1), H-(Lys-Gly-Lys-Gly-Lys-Gly-Lys)(2)-NH(2) (P-2), H-(Lys-Gly-Lys-Gly-Lys-Gly-Lys)(3)-NH(2) (P-3) and to the monomers (amino acids) forming the peptides has been obtained using the kinetics of the electron-transfer reaction between [Ru(NH(3))(5)py](2+) and [Co(C(2)O(4))(3)](3-) as the probe. The polymerization of the monomers increases the negative free energy of binding and changes its character, noncooperative for the monomers and anticooperative for the peptides. This increase in the negative free energy represents a driving force for the polymerization process. The magnitude of the gain in negative free energy, as a consequence of the anticooperative character of the binding of the cobalt complex to the peptide, depends on the ratio of [complex]/[monomers].

The kinetics of the oxidation of dicyanobis(2,2′-bipyridine) iron(II) [Fe(CN)2(bpy)2] by peroxodisulfate (S2O82-) has been studied in different water–cosolvent mixtures. The cosolvents used were methanol, tert-butyl alcohol, ethylene glycol and glycerol. The results are explained assuming an additional component of the reorganization free energy of the solvent in the mixtures, caused by a translation of the solvent molecules, as a consequence of the changes in the composition of the (at least) innermost solvation shell. A quantitative estimation of this component is attempted.