Aquation Rate Research Articles

Cytotoxicity of Alizarine versus Tetrabromocathecol Cyclometalated Pt(II) Theranostic Agents: A Combined Experimental and Computational Investigation.

Platinum compounds cytotoxicity is strictly related to their ability to be converted into active mono- and di-aquated species and consequently to the replacement of labile ligands by water molecules. This activation process makes the platinum center prone to nucleophilic substitution by DNA purines. In the present work, quantum mechanical density functional theory (DFT) computations and experimental investigations were carried out in order to shed light on the relationship between the internalization, aquation, and DNA binding of two isostructural anionic theranostic complexes previously reported by our group, NBu4[(PhPy)Pt(Aliz)], 1 (IC50 1.9 ± 1.6 μM), and NBu4[(PhPy)Pt(BrCat)], 2 (IC50 52.8 ± 3.9 μM). Cisplatin and a neutral compound [(NH3)2Pt(Aliz)], 3, were also taken as reference compounds. The computed energy barriers and the endergonicity of the hydrolysis reactions showed that the aquation rates are comparable for 1 and 2, with a slightly higher reactivity of 1. The second hydrolysis process was proved to be the rate-determining step for both 1 and 2, unlike for compound 3. The nucleophilic attack by the N7 site of guanine to both mono- and di-aquated forms of the complexes was computationally investigated as well, allowing to rationalize the observed different cytotoxicity. Computational results were supported by photostability data and biological assays, demonstrating DNA as the main target for compound 1.

Cytotoxic properties of rhenium(I) tricarbonyl complexes of N-heterocyclic carbene ligands.

A family of eight rhenium(I) tricarbonyl complexes bearing pyridyl-imidazolylidene or bis-imidazolylidene ligands in combination with a series of N-acetyl amino acids ligands (glycine, isoleucine, and proline) and an acetate have been synthesised and characterised. These complexes are of interest as potential anticancer agents, where the oxygen bound carboxylate ligand can exchange with water giving rise to cytotoxic cationic complexes. The pseudo-first-order aquation rate constants for the complexes were evaluated using 1H NMR time-course experiments and for the complexes of the bis-imidazolylidene ligand the average k1 value was 6.22 × 10-5 s-1 while for the pyridyl-imidazolylidene ligand the aquation rate was slower with the average k1 value being 3.00 × 10-5 s-1. Cytotoxicity studies in three cancer cell lines (MDA-MB-231, PC3 and HEPG2) showed that the Re(I) complexes of the bis-imidazolylidene ligand were significantly more toxic than those of the pyridyl-imidazolylidene ligand.

1H NMR study of the effect of cucurbit[7]uril on the aquation of carboplatin in biologically relevant media

The aquation of carboplatin, a second-generation Pt(II)-based antitumor drug, in two biologically relevant media (PBS buffer solution and RPMI-1640 medium for cell growth) has been studied by means of 1H nuclear magnetic resonance spectroscopy. The effect of the macrocyclic cavitand cucurbit[7]uril on the carboplatin aquation rates in these two types of media has also been studied. Although, the cucurbit[7]uril does not form stable inclusion complex with carboplatin, it greatly affects the carboplatin aquation rates, presumably, through the two mechanisms: prevention of the carboplatin dimer formation and encapsulation of some components of the medium.

Hydrolysis of chlorido complexes of d8 metals: Old models, new facts

The discovery of the antitumor activity of cisplatin in 1969 has accelerated research in platinum chemistry. In particular, cisplatin hydrolysis, the rate-determining process activating the drug for reaction with DNA, was extensively studied and discussions of the mechanism of square-planar substitutions were rekindled. One intriguing aspect of cisplatin hydrolysis is the fact that the rates of the two aquation steps are almost identical, in spite of the charge-separation effect which would be expected to slow down the second step. This result is analyzed here in the context of the more general observation made by Martin et al. that aquation of chlorido-ammine complexes of Pt(II) proceeds at similar rates, independently of complex charge. Ancient hypotheses put forward to explain this observation are confronted with recent data. The discussion is extended to aquation of chlorido-am(m)ine complexes of Pd(II) and Au(III), and to chloride substitutions by thiourea and pyridine. It is shown that the charge independence is not limited to chloride substitution by water. This new correlation of published data invalidates our recent hypothesis according to which non-conventional hydrogen bonding from the attacking water molecule to the Pt(II) center as acceptor counterbalances the charge separation effect. Recent data also show that the frequently used kinetic model for square-planar substitutions featuring a metastable five-coordinate intermediate does not apply to complexes involving only simple σ-donating ligands such as amines, water, or chloride. By inference, the Martin-Basolo model, explaining the charge-independence of aquation rate constants with the mutual compensation between charge separation and charge neutralization in the transition state, remains the only model compatible with available data.

Organoruthenium Prodrugs as a New Class of Cholinesterase and Glutathione-S-Transferase Inhibitors.

A small library of 17 organoruthenium compounds with the general formula [RuII (fcl)(chel)(L)]n+ (in which fcl=face capping ligand, chel=chelating bidentate ligand, and L=monodentate ligand) were screened for inhibitory activity against cholinesterases and glutathione-S-transferases of human and animal origins. Compounds were selected to include different chelating ligands (i.e., N,N-, N,O-, O,O-, S,O-) and monodentate ligands that can modulate the aquation rate of the metal species. Compounds with a labile ruthenium chloride bond that provided rapid aquation were found to inhibit both sets of enzymes in reversible competitive modes and at pharmaceutically relevant concentrations. When applied at concentrations that completely abolish the activity of human acetylcholinesterase, the lead compound [(η6 -p-cymene)Ru(pyrithionato)Cl] (C1 a) showed no undesirable physiological responses on the neuromuscular system. Finally, C1 a was not cytotoxic against non-transformed cells at pharmaceutically relevant concentrations.

Mechanisms and Factors Controlling Photoisomerization Equilibria, Ligand Exchange, and Water Oxidation Catalysis Capabilities of Mononuclear Ruthen­ium(II) Complexes

AbstractThe photoisomerization equilibrium between distal‐ and proximal‐[Ru(tpy)(pyqu)OH2]2+ [d‐ and p‐RuH2O, tpy = 2,2′;6′,2″‐terpyridine, pyqu = 2‐(2′‐pyridyl)quinoline] is characterized. The kinetic analysis of the pD‐dependent photoisomerization reactions (monitored by 1H NMR) of d‐RuH2O and p‐RuH2O shows (1) that both hydroxo isomers, distal‐ and proximal‐[Ru(tpy)(pyqu)OH]+, are inert to photoisomerization, and (2) that the back reaction (distal to proximal) is 3.0 times faster than the forward reaction (proximal to distal). Isolation of distal‐ and proximal‐[Ru(tpy)(pyqu)Cl]+ (d‐ and p‐RuCl) as well as d‐ and p‐RuH2O isomers enabled comprehensive studies on geometric structures, ligand exchange and redox reactions, and water oxidation catalysis for these isomers. The observed aquation rate constant (9.2 × 10–2 s–1 at 40 μM) of p‐RuCl to form p‐RuH2O is 1700 times higher than that (5.4 × 10–5 s–1 at 63 μM) of d‐RuCl at 298 K owing to the steric repulsion between a chloro ligand and the 8‐proton of the quinoline moiety. The turnover frequency (TOF = 1.7 × 10–3 s–1) of p‐RuH2O for catalytic water oxidation is 1.7 times greater than that (1.0 × 10–3 s–1) for d‐RuH2O, in contrast to the [Ru(tpy)(pynp)OH2]2+ isomer system, in which the TOF of the distal isomer is higher than that of the proximal one by one order of magnitude. The mechanisms and factors controlling the photoisomerization equilibria and water oxidation catalysis of the d‐ and p‐RuH2O isomers are discussed on the basis of experimental and theoretical investigations.

Metal ion remediation by polyamidoamine dendrimers: a comparison of metal ion, oxidation state, and titania immobilization

The exceptional ability of dendrimers to coor- dinate metal ions yields the potential for many applications including wastewater remediation, which is the focus of this study. Here, the comparison of metal ion removal rate from simulated wastewater by generation 4 dendrimers with external hydroxyl functional groups (G4-OH) is evaluated for Ni 2? ,F e 2? , and Fe 3? ions. Ni 2? to amine complexation occurred more rapidly than Fe 3? , which was more rapid than Fe 2? complexation. These results indicate that both charge density and d-electron configuration are important toward the chelation rate. The impact of both factors is discussed in light of existing models in which precursor aquation rates have been proposed as a key intermediate step. Additionally, the application of the dendrimers as chelation agents is further advanced by immobilizing the dendrimer to titania and re-evaluating its chelation ability for Ni 2? removal. The dendrimer immobilization decreased the pseudo-first-order rate coefficient for Ni 2? —amine complexation at a pH of 7 by a factor of 7.5. This result is significant as it suggests that mass transfer becomes important following immobilization of the dendrimer to titania.

Kinetics of Ion-Pair Effect of Aquation of Bromopentaammine Cobalt(III) Complex in Different Dicarboxylate Media

The rate of aquation of bromopentaammine cobalt(III) ion in the presence of different types of dicarboxylate solutions containing tert-butanol (40% V/V) have been measured spectrophotometrically at different temperatures (30-600°C) in the light of the effects of ion-pairing on reaction rates and mechanism. The thermodynamic and extrathermodynamic parameters of activation have been calculated and discussed in terms of solvent effect on the ion-pair aquation reaction. The free energy of activation ∆Gip* is more or less linearly varied among the studied dicarboxylate ion-pairing ligands indicating the presence of compensation effect between ∆Hip* and ∆Sip*. Comparing the kip values with respect of different buffers at 40% of ter-butanol is introduced.

Electrochemical Biosensor Assay of the Interaction between [PtCln(NH3)4–n](2–n) (n = 0–4) Complexes and ds‐DNA

AbstractWe report on the results of electrochemical DNA biosensor assay of the interaction between double‐stranded DNA (ds‐DNA) and a series of six neutral, anionic or cationic Pt complexes of the general formula [PtCln(NH3)4–n](2–n) [n = 4, 1; n = 3, 2; n = 2, isomers cisplatin (3) and transplatin (4); n = 1, 5; n = 0, 6]. The ability of the electrophilic PtII agents generated in solution to interact with DNA, and hence to form Pt–DNA adducts, was measured as a function of the decrease in the guanine oxidation signal recorded on a screen‐printed electrode by using square wave voltammetry. Hydrolysis of the platinum complexes was studied by using time‐resolved RP‐HPLC and conductivity measurements to determine the aquation rate, which modulates the formation of the electrophilic agent prone to quickly interact with DNA. Our findings indicate that, if time is allowed for sufficient hydrolysis to occur, the interaction of these PtII complexes with ds‐DNA follows the order 2 > 1 > 3 ≈ 4 > 5 >> 6.

Synthesis, characterization, structure, molecular modeling studies and biological activity of sterically crowded Pt(II) complexes containing bis(imidazole) ligands

The syntheses, structures and biological evaluation of a series of cisplatin-like complexes containing bis(imidazole) derivatives – the so-called Joseph ligands – are described. Their cytotoxicity is discussed in terms of their polar surface area, rate of aquation, and lipophilicity. The X-ray crystal structure of the platinum diiodido derivative of dimethyl 2-(di(1 H-imidazol-2-yl)methyl)malonate) is reported and compared to those of related systems. Molecular modeling studies are focused on the hydrogen bonding properties of such systems, and their relevance to antitumor activity.

Different Features of the DNA Binding Mode of Antitumor cis-Amminedichlorido(cyclohexylamine)platinum(II) (JM118) and Cisplatin in Vitro

cis-Amminedichlorido(cyclohexylamine)platinum(II) (JM118) is an antitumor Pt(II) analogue of cisplatin exhibiting considerably higher activity than cisplatin in human tumor cells. JM118 is also the major metabolite of the first orally administered Pt(IV) drug satraplatin. In an effort to design improved platinum antitumor agents, it is important to elucidate the biochemical factors that affect the cytotoxic properties of existing platinum drugs. Since DNA is considered the major pharmacological target of platinum drugs, the objective in the present work was to understand more fully the DNA binding mode of antitumor JM118. We examined the rate of aquation of the first chloride of bifunctional JM118 and found that it was considerably lower than that of cisplatin; consequently, the rate of the reaction of JM118 with DNA was lower compared to cisplatin. The influence of global modification by JM118 and its major site-specific adducts on DNA conformation by biochemical methods was investigated as well. While examination of the global modification revealed in several cases no substantial differences in the lesions induced by JM118 and cisplatin, DNA bending due to the 1,2-GG intrastrand adduct of JM118 was lower than that of cisplatin. The bending angles afforded by the adducts of JM118 were only slightly affected by the orientation of the cyclohexylamine ligand toward the 3' or 5' direction of the duplex. We also used in vitro assays that make it possible to monitor DNA repair synthesis by cell-free extracts and DNA-protein cross-linking to probe properties of DNA adducts of JM118. These results showed a higher DNA-protein cross-linking efficiency of JM118 and a less efficient removal from DNA of the adducts of JM118 in comparison with cisplatin. Thus, the results of the present work provide additional evidence that DNA binding of JM118 is in several aspects different from that of conventional cisplatin.

1H, 15N] Heteronuclear Single Quantum Coherence NMR Study of the Mechanism of Aquation of Platinum(IV) Ammine Complexes

The aquation and hydrolysis of a series of platinum(IV) complexes of the general form cis, trans, cis-[PtCl 2(X) 2( (15)NH 3) 2] (X = Cl (-), O 2CCH 3 (-), OH (-)) have been followed by [ (1)H, (15)N] Heteronuclear Single Quantum Coherence NMR spectroscopy. Negligible aquation (<5%) is observed for the complexes where X = O 2CCH 3 (-) or OH (-) over 3-4 weeks. Aquation of cis-[PtCl 4( (15)NH 3) 2] ( 1) is observed, and the rate of aquation increases with increasing pH and upon the addition of 0.01 mol equiv of the platinum(II) complex cis-[PtCl 2( (15)NH 3) 2] (cisplatin). The first aquated species formed from cis-[PtCl 4(NH 3) 2] has one of the axial chloro groups (relative to the equatorial NH 3 ligands) replaced by an aqua/hydroxo ligand. The second observed substitution occurs in an equatorial position. Peaks that are consistent with five of the eight possible aquation species were observed in the NMR spectra.

Effect of Triton‐X‐100 Solvent in the Micellar Catalysis of the Aquation of Tris(2‐nitroso‐1‐naphtholate) Ferrate (II)

The effect of triton‐X‐100 micelles on the aquation of Fe(C10H6N2O)3 2+ has been investigated with triton‐X‐100 as solvent. In liquid triton‐X‐100, over a range of [H2O] T (0.0–3 M), significant rate enhancement factors of 50–150 are observe. Acid inhibits the rate of aquation at fixed [H2O] T . A mechanism based on effective solvent participation in a chemical environment similar to that in reversed micelles is proposed in liquid triton‐X‐100 with dispersed water pockets. This mechanism predicts direct H2O substitution into the coordination sphere of Fe(C10H6N2O)3 2+ in the highly polar water pockets or cavities where the Fe (II) complex molecules are solubilized. Changes in the tumbling rate, structure, and activity of water are suggested to account for the observed changes in the rate of aquation as a function OH [H2O] T . All k ψ–[H2O] T profiles are structured and exhibit maxima with k ψ(max) shifted to progressively higher [H2O] T as the fixed concentration [H+] T is increased.

Structure−Activity Relationships for NAMI-A-type Complexes (HL)[trans-RuCl4L(S-dmso)ruthenate(III)] (L = Imidazole, Indazole, 1,2,4-Triazole, 4-Amino-1,2,4-triazole, and 1-Methyl-1,2,4-triazole): Aquation, Redox Properties, Protein Binding, and Antiproliferative Activity

Imidazolium [trans-tetrachloro(1H-imidazole)(S-dimethylsulfoxide)ruthenate(III)] (NAMI-A) and indazolium [trans-tetrachlorobis(1H-indazole)ruthenate(III)] (KP1019) are the most promising ruthenium complexes for anticancer chemotherapy. In this study, the azole ligand of NAMI-A was systematically varied (from imidazole of NAMI-A to indazole, 1,2,4-triazole, 4-amino-1,2,4-triazole, and 1-methyl-1,2,4-triazole), and the respective complexes were evaluated with regard to the rate of aquation and protein binding, redox potentials, and cytotoxicity by means of capillary zone electrophoresis, electrospray ionization mass spectrometry, cyclic voltammetry, and colorimetric microculture assays. Stability studies demonstrated low stability of the complexes at pH 7.4 and 37 degrees C and a high reactivity toward proteins (binding rate constants in the ranges of 0.02-0.34 and 0.01-0.26 min-1 for albumin and transferrin, respectively). The redox potentials (between 0.25 and 0.35 V) were found to be biologically accessible for activation of the complexes in the tumor, and the indazole-containing compound shows the highest antiproliferative activity in vitro.

Anation kinetics of pentacyanoaquachromate(III) by thiocyanate and azide

The kinetics of the reaction of Cr(CN) 5(H 2O) 2− with NCS − and N 3 - were studied at pH 5.0 and at pH 6.3–7.0, respectively, as a function of the temperature between 25.0 and 55.0 °C, and at various ionic strengths. Anation occurs in competition with aquation of CN −, with rate constants that exhibit less-than-first-order dependence on the concentration of the entering anions. The results are interpreted in terms of ligand interchange in a context of association of the two reacting anions mediated by the Na + or Ca 2+ counterions. The degree of aggregation depends mainly on the total cationic charge rather than on the ionic strength, and is ca. 2-fold larger for N 3 - than for NCS −. Within the associated species, N 3 - is a better entering ligand than NCS − by a factor of 4.5. The Cr(CN) 5(NCS) 3− and Cr(CN) 5(N 3) 3− complexes were also synthesized, and the rates of aquation of NCS − and N 3 - were measured at pH 5.0 and between 55.0 and 80.0 °C, over the same range of ionic strengths. The ionic strength enhances the anation rates but has little effect on the aquation rates. The average activation enthalpies of the interchange step are 80 ± 3 and 76 ± 3 kJ mol −1 for entry of NCS − and N 3 - , respectively. Those of the corresponding aquation reactions are 94 ± 4 and 107 ± 4 kJ mol −1. Within error limits, all Δ H ‡ values are independent of the ionic strength. The results are consistent with an I d mechanism for substitution in Cr(CN) 5X z− complexes.

Effects of Steric Constraint on Chromium(III) Complexes of Tetraazamacrocycles, 2. Comparison of the Chemistry and Photobehavior of the trans-Dichloro- and trans-Dicyano- Complexes of Cyclam, 1,4-C2-Cyclam, and 1,11-C3-Cyclam

The synthesis and characterization of several Cr(III) complexes of the constrained macrocyclic ligand 1,11-C3-cyclam (1,4,8,11-tetraazabicyclo[9.3.3]heptadecane) is reported. Only trans complexes are formed, and the structure of trans-[Cr(1,11-C3-cyclam)Cl2]PF6 is presented. The chemical and photophysical behavior of the 1,11-C3-cyclam complexes are compared with those of the corresponding cyclam (1,4,8,11 tetraazacyclotetradecane) and 1,4-C2-cyclam (1,4,8,11-tetraazabicyclo[10.2.2]hexadecane) complexes. The aquation rate of trans-[Cr(1,11-C3-cyclam)Cl2]+ is similar to that of the corresponding 1,4-C2-cyclam complex and is more than 5 orders of magnitude faster than the cyclam counterpart. A monotonic increase in the extinction coefficient is observed on going from the cyclam complexes to the 1,11-C3-cyclam complexes to the 1,4-C2-cyclam complexes, and this is related to the degree of centrosymmetry in each complex. The trans-[Cr(1,11-C3-cyclam)(CN)2]+ complex is a weak emitter in aqueous solution with a room-temperature emission maximum at 724 nm (tau=23 micros). Like the corresponding 1,4-C2-cyclam complex (tau=0.24 micros), the 1,11-C3-cyclam complex shows no deuterium-isotope effect in room-temperature solution. This is in marked contrast to the corresponding cyclam complex which has an emission lifetime of 335 micros and a significant deuterium isotope effect in room-temperature solution. Low temperature (77K) data are also presented in an attempt to understand the differences in photophysical behavior.

Ligand Exchange in Pd(II)-NaCl-H2O and Pd(II)-HCl-H2O Systems: Quantum-Chemical Consideration

The behavior of potassium tetrachloropalladate(II) in media simulating biological liquids is studied. The rate of aquation in aqueous NaCl solutions is shown to be higher than the rate at which the Cl− ligand enters the inner coordination sphere of the Pd atom. In HCl solutions, the formation of the Pd chloro complexes predominates due to protonation of water molecules in the composition of aqua complexes. The reactions of replacement of the ligands (H2O molecules and H3O+ ion) in the planar Pd(II) complexes by the chloride ion are studied by the ZINDO/1 method. All the complexes containing H2O and H3O+ ligands, except for [Pd(H2O)4]2+, contain intramolecular hydrogen bonds. The ZINDO/1 and RHF/STO-6G(d) calculations revealed “nonclassic” symmetrical O⋯ H⋯O hydrogen bond in the [[Pd(H2O)3(H3O)]3+ and trans-[Pd(H2O)2(H3O)Cl]2+ complexes. The replacement of the H3O+ ion by the Cl− ion at the first three steps is thermodynamically more advantageous than the displacement of water molecules from the metal coordination sphere. The logarithms of stepwise stability constants of Pd(II) chloro complexes are found to correlate linearly with the enthalpies (ZINDO/1, PM3) of reactions of H2O replacement by Cl−.

Influence of dT20 and [d(AT)10]2 on Cisplatin Hydrolysis Studied by Two‐Dimensional [1H,15N] HMQC NMR Spectroscopy

The influence of the presence of DNA on the kinetics of cisplatin (cis-[PtCl2(NH3)2]) aquation (replacement of Cl- by H2O) and anation (replacement of H2O by Cl-) involved in the hydrolysis of cisplatin have been determined by two-dimensional [1H,15N] HMQC NMR spectroscopy. Single-stranded dT20 and double-stranded [d(AT)10]2 oligonucleotides were used as DNA models, avoiding guanines which are known to react rapidly with aquated cisplatin forms. Reactions starting from cis-[PtCl2(15NH3)2], or from a stoichiometric mixture of cis-[Pt(15NH3)2(H2O)2]2+ and Cl- (all 0.5 mM Pt(II); in ionic strength, adjusted to 0.095 M or 0.011 M with NaClO4, pH between 3.0 and 4.0) were followed in an NMR tube in both the absence and presence of 0.7 mM dT20 or [d(AT)10]2. In the presence of dT20, we observed a slight and ionic-strength-independent decrease (15-20 %) of the first aquation rate constant, and a more significant decrease of the second anation rate constant. The latter was more important at low ionic strength, and can be explained by efficient condensation of cis-[Pt(15NH3)2(H2O)2]2+ on the surface of single-stranded DNA, in a region depleted of chloride anions. At low ionic strength, we observed an additional set of [1H,15N] HMQC spectral signals indicative of an asymmetric species of PtN2O2 coordination, and we assigned them to phosphate-bound monoadducts of cis-[Pt(15NH3)2(H2O)2]2+. Double-stranded [d(AT)10]2 slowed down the first aquation step also by approximately 15 %; however, we could not determine the influence on the second hydrolysis step because of a significant background reaction with cis-[Pt(NH3)2(H2O)2]2+.

Aquation of α-cis-[CoCl2(edda)]- and of β-cis-[CoCl2(trien)]+ in cationic and anionic reverse micelles

The aquation of α-cis-[CoCl2(edda)]- and of β-cis-[CoCl2(trien)]+ has been studied in reverse micelles of water/AOT/n-heptane and water/CTAB/n-heptane+CHCl3 for various concentrations of water and AOT at 298.1 K. The reaction rate depends on the molar ratio W as well as on the concentration of AOT. These results demonstrate the important effect of micelle size as well as of structure and state of confined water in the aquation rate.

Use of chelating ligands to tune the reactive site of half-sandwich ruthenium(II)-arene anticancer complexes.

We show that the chelating ligand XY in Ru(II) anticancer complexes of the type [Ru(eta6-arene)(XY)Cl]n+ has a major influence on the rate and extent of aquation, the pKa of the aqua adduct, and the rate and selectivity of binding to nucleobases. Replacement of neutral ethylenediamine (en) by anionic acetylacetonate (acac) as the chelating ligand increases the rate and extent of hydrolysis, the pKa of the aqua complex (from 8.25 to 9.41 for arene=p-cymene), and changes the nucleobase specificity. For the complexes containing the hydrogen-bond donor en, there is exclusive binding to N7 of guanine in competitive nucleobase reactions, and in the absence of guanine, binding to cytosine or thymine but not to adenine. In contrast, when XY is the hydrogen-bond acceptor acac, the overall affinity for adenosine (N7 and N1 binding) is comparable to that for guanosine, but there is little binding to cytidine or thymidine.