Bernhard Prijs

Macrochelate formation in monomeric metal ion complexes of nucleoside 5'-triphosphates and the promotion of stacking by metal ions. Comparison of the self-association of purine and pyrimidine 5'-triphosphates using proton nuclear magnetic resonance

The concentration dependence of the chemical shifts of the protons H-2, H-8, and H-I’ or H-5, H-6, and H-1’ of A T P , I T P , and G T P or C T P and U T F ( = N T P ) , respectively, and of the corresponding nucleosides has been measured. The results are consistent with the isodesmic model of indefinite noncooperative stacking; the association constants for NTP4are between 1.3 (ATP4-) and about 0.4 M-’ (UTP4-) and for the nucleosides between 15 (adenosine) and 1.2 M-’ (uridine). The self-stacking tendency decreases within the series adenosine > guanosine > inosine > cytidine uridine. Due to the repulsion of the negatively charged phosphate moieties, this trend is much less pronounced for the corresponding N T F series. The charge effect also governs the series adenosine >> AMP2> ADP3ATP4-. Likewise the self-association tendency of ATP4-, ITP, and GTP‘is promoted by a factor of about 3-5 by the coordination of Mg2+ to the phosphate moiety, which neutralizes part of the negative charge a t this residue. However, the self-association tendency of Zn(ATP)2and Cd(ATP)2is much larger than of Mg(ATP)’-; this is explained by an increased tendency to form an intermolecular metal ion bridge in the dimeric stacks in which Zn2+ or Cd2+ is coordinated to the phosphate moiety of one ATP4and to N-7 of the adenine residue of the other A T P . The shifts of H-8 for complete stacking (6,) agree with this interpretation. There is no significant increase in stability in Zn(ITP)2and Zn(GTP)2-: Le., the stability of these stacks is governed only by the charge neutralization-the effect of Zn2+ is the same as that of Mg2+. Comparison of the shifts of H-8 at infinite dilution (6,) for several systems reveals that an M2+/N-7 interaction exists in the monomeric Zn2+ and CdZt complexes of the purine 5’-triphosphates; Le., a macrochelate is formed through an inrramolecular coordination of the metal ion to the phosphate moiety and to N-7. The position of this concentration-independent equilibrium between the open isomer (with phosphate coordination only) and the macrochelated isomer is estimated by comparing 6o of M(NTP)2with the shifts of H-8 for complete complex formation of the corresponding metal ion-nucleoside complexes, which were also determined. The N M R study gives no hint for such an N-7 interaction either for the corresponding Mg(NTP)2complexes or for a base interaction in any of the pyrimidine 5’-triphosphate complexes. These N M R results prompted an evaluation of stability data (obtained earlier under conditions where no self-association occurs), which give further evidence that macrochelate formation also occurs in the M(NTP)’complexes of purine nucleotides with Mn2+ Co2+, Ni2+, Cu2+, and Zn2+; no evidence for such an interaction is observed in the pyrimidine nucleotide complexes. Ionizkon of the base moiety in I T P , G T F , UTF, and TTP favors, however, the base-metal ion interaction and therefore also the formation of macrochelates in the M(NTP-H)3complexes. The percentage of the macrochelated isomer is estimated for all these systems: the whole range from nearly 100% ring back-bonding to only insignificant traces is observed. The ambivalent coordinating properties of nucleotides and their structural versatility are discussed. I t is now well-known tha t metal ions a r e essential in a large variety of biological processes, including those with nucleic acids and their der ivat ives4 For example, D N A polymerase contains tightly bound Zn2+, and there is evidence that this metal ion binds the enzyme to DNA.5s6 To fulfill its function the enzyme must also be activated by a divalent cation such as Mg2+ or Mn2+, and these metal ions bind the nucleoside triphosphate substrates to the enzyme.’,* Thus the interplay between metal ions and nucleotides, or their derivatives, is receiving much attention at

Stability and structure of binary and ternary complexes of α-lipoate and lipoate derivatives with Mn2+, Cu2+, and Zn2+ in solution☆

Abstract The stabilities of the 1:1 complexes of Mn 2+ , Cu 2+ , and Zn 2+ with lipoate and its chainshortened catabolites, viz., bisnorlipoate and tetranorlipoate, were studied by potentiometric titrations in water containing 50% dioxane ( I = 0.1, NaClO 4 ; 25 °C). A comparison of the stabilities of these complexes with those of simple carboxylates reveals that the catabolite complexes formed with Cu 2+ and Zn 2+ are more stable than expected from only the basicity of the carboxylate groups. This is evidence that chelates involving the disulfide group are formed. The stability of all Mn 2+ complexes is determined by the basicity of the carboxylate groups. The same pattern of stability holds for the mixed-ligand complexes formed by Cu 2+ or Zn 2+ , 2,2′-bipyridyl, and lipoate or one of its derivatives. It is evident that the disulfide group of the 1,2-dithiolane moiety can participate in the formation of binary and ternary complexes. The somewhat less-pronounced coordinating properties of the 1,2-dithiolane moiety compared with the tetrahydrothiophene moiety are discussed. It is apparent that the electron density at S(1) and S(2) in the dithiolane moiety of lipoate is not equivalent: S(1) is favored over S(2) in electrophilic reactions; possible biological implications are indicated.

Metal ion/buffer interactions. Stability of alkali and alkaline earth ion complexes with triethanolamine (tea), 2-amino-2(hydroxymethyl)-1,3-propanediol (tris)and 2-[bis(2-hydroxyethyl)-amino] 2(hydroxymethyl)-1,3-propanediol (Bistris) in aqueous and mixed solvents☆

Abstract The acidity constants of the protonated buffers given in the title, i.e. of H(Tea) + , H(tris) + and H(Bistris) + , have been measured at 25 °C in water, 50% aqueous dioxane or methanol, and in 75 or 90% dimethylsulfoxide (Dmso) with tetramethylammonium nitrate as background electrolyte. The interaction of Tea, Tris, or Bistris (L) with the alkali or alkaline earth ions (M n+ was studied by potentiometric pH titrations in the same solvents and the stability constants of the ML n+ complexes were determined. The stability constants, log K M ML , of the Na + complexes with the several buffer-ligands in the given solvents vary from −1.05 [Na(Tea) + in H 2 O; I = 1.0] to 0.54 log units [Na(Bistris) + in 90% Dmso; I = 0.25]; the corresponding values for the Mg 2+ complexes range from 0.24 [Mg(Tea) 2+ in H 2 O; I = 1.0] to 0.91 log units [Mg(Bistris) 2+ in 90% Dmso; I = 0.25]. Unexpectedly, Ca(Bistris) 2+ is the most stable among the alkaline earth ion complexes in aqueous solution (log K Ca Ca(Bistris) = 2.25; the corresponding values for the Mg 2+ , Sr 2+ and Ba 2+ complexes are 0.34, 1.44 and 0.85, respectively; I = 1.0), while in 90% Dmso Sr(Bistris) 2+ is most stable (log K Sr Sr(Bistris) = 1.87; the corresponding values for the Mg 2+ , Ca 2+ and Ba 2+ complexes are 0.91, 1.64 and 1.14, respectively; I = 0.25). A similar, but less pronounced pattern is observed for the M(Tea) n+ complexes. obviously, the stabilities of the alkaline earth ion complexes with Bistris and Tea follow neither the order of the ionic radii nor that of the hydrated radii of the cations. In contrast, in all solvents the stability of the alkali ion complexes increases with decreasing ionic radii; this being also true for the alkaline earth iron complexes of Tris in aqueous solution. The possible reasons for these observations, the structures of the complexes in solution, and some biological implications are discussed. Calculations of the extent of complex formation show that in the physiological pH range the concentration of certain complexes may be quite pronounced; hence reservations should be exercised in employing these buffers in systems which also contain metal ions.

The dimerization, polymerization, and hydrolysis of FeIII-4,4′,4″,4″′-tetrasulfophthalocyanine

Abstract The Fe III complex of 4,4′,4″,4″′-tetrasulfophthalocyanine (H 2 PTS) dimerizes and in concentrations > 10 −5 M polymerizes (additionally favored by NaClO 4 ). At natural ionic strength ([Na + ] ≲ 5·10 −5 M) log K D = 7.11 at 25°C and 5.90 at 60°C; in 0.1 M NaClO 4 the values were estimated. — The log K D values are compared with those of other MePTS dimers. — Fe III PTS hydrolyzes in the alkaline pH region. Due to the di- and polymerization, the tendency for hydrolysis is dependent upon the concentration of the complex.

“Hard and soft” behavior of Mn2+, Cu2+, and Zn2+ with respect-to carboxylic acids and α-oxy- or α-thio-substituted carboxylic acids of biochemical significance

Abstract An investigation has been made on the coordination of Mn2+, Cu2+, and Zn2+, by several simple carboxylic acids and certain of α-oxy- and α-thiocarboxylic acids which have biochemical significance. The stability constants of the 1:1 complexes have been measured in water containing 50% dioxane (I = 0.1; t = 25 °). With all three metal ions, the expected correlations between the basicity of the monodentate ligands and the stability of the complexes were found. The following potential bidentate α-oxyor α-thiocarboxylic acids were included in the measurements: Tetrahydrofuran 2-carboxylic acid (XI), hydroxyacetic acid (XII), tetrahydrothiophene 2-carboxylic acid (XIII), and S-carboxymethyl ethyl mercaptan (XIV). The complexes of the carboxylic acids with an oxygen in the α-position, (XI, XII), are more stable with all three metal ions than is expected on the basis of participation of the carboxyl function alone, i.e., chelates are formed. For the carboxylic acids with sulfur in the α-position, (XIII, XIV), such chelation is definite only for the Cu2+ complexes. The values of the Zn2+ and especially the Mn2+ complexes are in the order expected from basicity of the carboxylate groups, i.e., there is no or only a weak coordination of the sulfur atom. NMR spectra of S-carboxymethyl ethyl mercaptan (XIV) with increasing amounts of the paramagnetic ions, Cu2+ and Mn2+, confirm these results: In the case of Cu2+, the sulfur is strongly coordinated and a bidental chelate ligand results, while with Mn2+ the spectra suggest that interaction with sulfur is weak and the simple monodental carboxylate complex probably dominates. These results are discussed with respect to Pearsons “hard-soft” rule and the selection of the coordinating groups in biological complexes, i.e., the question about the “selection rules” which determine the structure and reactivity in such complexes is considered.

Mn2+, Cu2+, and Zn2+ 1:1 Complexes with biochemically significant thioether carboxylic acids and some of the sulfoxide and sulfone derivatives

Abstract The 1:1 complexes of Mn 2+ , Cu 2+ , and Zn 2+ with S -carboxymethyl alkyl and S -carboxymethyl aryl mercaptans were studied in water containing 50% dioxane (I = 0.1; t = 25 °). The determination of the stability constants and a comparison with simple carboxylate complexes reveals that the complexes of Cu 2+ (and slightly also of Zn 2+ ) with the S -carboxymethyl alkyl mercaptans are more stable than expected from only basicity of the carboxylate groups. This suggests that the thioether group participates in complex formation, i.e., chelates are formed. The Mn 2+ complexes of both kinds of ligands, and the Cu 2+ or Zn 2+ complexes with S -carboxymethyl aryl mercaptans have the stability expected according to the basicity of the carboxylate groups. NMR experiments with S -carboxymethyl ethyl mercaptan confirm the formation of chelates with Cu 2+ and suggest simple carboxylate complexes with Mn 2+ . Analogous experiments with ( S -carboxymethyl phenyl mercaptan do not allow an unequivocal statement about the distribution between simple carboxylate complexes and chelates for both metal ions. Also, as the thioether acids are biologically oxidized, the complex stabilities of several of such oxidized derivatives were measured.

On the kinetics and mechanism of the catalase-like activity of diaquocobinamide: Evidence for the formation of a μ-peroxo-bis(cobinamide) species☆

Abstract The disproportionation of H2O2, catalyzed by diaquocobinamide (CoIIICob), was investigated in the pH range 2–10 by measuring the initial rate, νin0 = d[ O 2 ] d t (Ms−1), of the increase in the concentration of O2 (25°C; [Na+] = 0.1). In contrast to the usual inertness of CoIII-complexes the fifth and sixth coordination positions in CoIIICob are sufficiently labile toward substitution to make CoIIICob a usable catalyst. There is some indication of a pH independent reaction at pH about 2, while within the remaining pH range ν0 increases with increasing pH and at pH about 10 saturation seems to occur. At low ratios of [ H 2 O 2 ] [ Co III Cob ] ν 0 is proportional to [H2O2]2, indicating is proportional to [CoIIICob]; this result is explained by the monomer-dimer equilibrium, 2 CoIIICob + H2O2↔CobCoIII(O22−)CoIIICob + 2H+ and the assumption that only that only the monomer is the active catalyst. Indeed, the high stability of the μ-peroxo-bis(cobinamide) species could be confirmed with experiments where [CoIIICob]≳[H2O2]tot. [H2O2]tot. The main difference between aquocobalamin and CoIIICob is the presence of an additional coordinating group, the 5,6-dimethylbenzimidazolyl moiety, in the former; indeed the catalase-like activity of aquocobalamin is approx. 10 times smaller than that of CoIIICob. This suggests that the catalysis of the decomposition is only efficient if both apical positions are available for the coordination of H2O2 or derivatives thereof. This corresponds to observations made earlier for CoIII-hematoporphyrin IX. Indeed, the catalase-like activity of this latter complex and of CoIIICob are of comparable order. However the activity of CoIII (dimethylglyoximate)2, which lacks an extended π-system, is considerably smaller.

Ternary Complexes in Solution, XII. Models for Biological Mixed-Ligand Complexes: 2,2′-Bipyridyl-Cu2+-Oligoglycine Systems

The stability constants of the binary Cu2+ complexes of glycine amide, diglycine, diglycine amide, triglycine, and tetraglycine were determined, as were those of the mixed-ligand Cu2+ systems containing 2,2′-bipyridyl and one of the mentioned oligoglycines. The results evidence that all these complexes have the same structure and, therefore, the binding sites of the ligands have to be the terminal amino group and the oxygen of the neighbored amide group. The stability differences between the ternary and the binary complexes are in agreement with this interpretation. It is of interest to note that these ternary complexes are significantly more stable than expected on statistical reasons. With increasing pH, the amide groups in the binary complexes are successively deprotonated. Thus, with tetraglycine finally all three amide protons are displaced, and the amide nitrogens are bound to the square-planar coordination sphere of Cu2+. As in the Cu2+-2,2′-bipyridyl 1 : 1 complex, only two coordination positions are left for the binding of the oligoglycine, in the tenary complexes, only one amide group can be deprotonated. An increase in pH with deprotonation of other amide groups leads to a displacement of 2,2′-bipyridyl, i. e. the simple binary complexes result. No evidence could be observed for the coordination of a deprotonated amide group to an apical position of the coordination sphere of Cu2+. Additionally, while the displacement of the first amide proton in the several binary Cu2+ oligoglycine complexes occurs over a large pH range (4 to 7), the deprotonation in all the mixed-ligand complexes takes place at pH approximately 8.

Metal Ions and Hydrogen Peroxide. XXV

The decomposition of H2O2, catalyzed by the Co2® complex of 4,4′,4″,4″′-tetrasulfophthalocyanine (CoIIPTS), was investigated in the pH range 3.8 through 10 by measuring the initial rate, v0=d(O2)/dt, of the increasing formation of O2 (25°; I=0.1). In this pH range v0 is proportional to the initial concentration of H2O2 (determined at pH 5.0 and 9.2). Due to the dimerization (log KD=5.47 ±0.09 at natural ionic strength and about 7.63 ±0.16 in 0.1 M NaClO4; 25°) and polymerization of CoIIPTS the catalyst and its reaction order are difficult to establish: Based on the experimental evidence it is suggested that v0 is proportional to the concentration of monomer CoIIPTS. Additionally, there is evidence that the experimentally determined v0 contains the contributions of a pH-independent and a pH-dependent reaction course. These results are analog to those obtained earlier with FeIIIPTS as catalyst. A mechanism for the catalyzed disproportionation of H2O2 by CoIIPTS is proposed. The catalase-like activity of CoIIIPTS (OH) is smaller than that of CoIIPTS and the pH-dependence is different.