Thermochromism of metal ion complexes of semiquinone radical anions. Control of equilibria between diamagnetic and paramagnetic species by Lewis acids

Metal ion complexes of semiquinone radical anions exhibit different types of thermochromism depending on metal ions and quinones. Metal ion complexes of 1,10-phenanthroline-5,6-dione radical anion (PTQ(.-)) produced by the electron-transfer reduction of PTQ by 1,1'-dimethylferrocene (Me(2)Fc) in the presence of metal ions (Mg(2+) and Sc(3+)) exhibit the color change depending on temperature, accompanied by the concomitant change in the ESR signal intensity. In the case of Mg(2+), electron transfer from Me(2)Fc to PTQ is in equilibrium, when the concentration of the PTQ(.-)-Mg(2+) complex (lambda(max) = 486 nm) increases with increasing temperature because of the positive enthalpy for the electron-transfer equilibrium. In contrast to the case of Mg(2+), electron transfer from Me(2)Fc to PTQ is complete in the presence of Sc(3+), which is a much stronger Lewis acid than Mg(2+), to produce the PTQ(.-)-Sc(3+) complex (lambda(max) = 631 nm). This complex is in disproportionation equilibrium and the concentration of the PTQ(.-)-Sc(3+) complex increases with decreasing temperature because of the negative enthalpy for the proportionation direction, resulting in the remarkable color change in the visible region. On the other hand, the p-benzosemiquinone radical anion (Q(.-)) forms a 2:2 pi-dimer radical anion complex [Q(.-)-(Sc(3+))(2)-Q] with Q and Sc(3+) ions at 298 K (yellow color), which is converted to a 2:3 pi-dimer radical anion complex [Q(.-)-(Sc(3+))(3)-Q] with a strong absorption band at lambda(max) = 604 nm (blue color) when the temperature is lowered to 203 K. The change in the number of binding Sc(3+) ions depending on temperature also results in the remarkable color change, associated with the change in the ESR spectra.     

Binding modes in metal ion complexes of quinones and semiquinone radical anions: electron-transfer reactivity.

9,10-Phenanthrenequinone (PQ) and 1,10-phenanthroline-5,6-dione (PTQ) form 1:1 and 2:1 complexes with metal ions (M (n+)=Sc (3+), Y (3+), Mg (2+), and Ca (2+)) in acetonitrile (MeCN), respectively. The binding constants of PQ--M (n+) complexes vary depending on either the Lewis acidity or ion radius of metal ions. The one-electron reduced species (PTQ(-)) forms 1:1 complexes with M (n+), and PQ(-) also forms 1:1 complexes with Sc(3+), Mg(2+), and Ca(2+), whereas PQ(-) forms 1:2 complexes with Y(3+) and La(3+), as indicated by electron spin resonance (ESR) measurements. On the other hand, semiquinone radical anions (Q(-) and NQ(-)) derived from p-benzoquinone (Q) and 1,4-naphthoquinone (NQ) form Sc(3+)-bridged pi-dimer radical anion complexes, Q(-)--(Sc(3+))(n)--Q and NQ(-)--(Sc(3+))(n)-NQ (n=2 and 3), respectively. The one-electron reduction potentials of quinones (PQ, PTQ, and Q) are largely positively shifted in the presence of M (n+). The rate constant of electron transfer from CoTPP (TPP(2-)=dianion of tetraphenylporphyrin) to PQ increases with increasing the concentration of Sc(3+) to reach a constant value, when all PQ molecules form the 1:1 complex with Sc(3+). Rates of electron transfer from 10,10'-dimethyl-9,9'-biacridine [(AcrH)(2)] to PTQ are also accelerated significantly by the presence of Sc(3+), Y(3+), and Mg(2+), exhibiting a first-order dependence with respect to concentrations of metal ions. In contrast to the case of o-quinones, unusually high kinetic orders are observed for rates of Sc(3+)-promoted electron transfer from tris(2-phenylpyridine)iridium(III) [Ir(ppy)(3)] to p-quinones (Q): second-order dependence on concentration of Q, and second- and third-order dependence on concentration of Sc(3+) due to formation of highly ordered radical anion complexes, Q()--(Sc(3+))(n)--Q (n=2 and 3).     

OFF-OFF-ON switching of fluorescence and electron transfer depending on stepwise complex formation of a host ligand with guest metal ions

Stepwise complex formation is observed between 2,3,5,6-tetrakis(2-pyridyl)pyrazine (TPPZ) and a series of metal ions (M(n+) = Sc3+, Y3+, Ho3+, Eu3+, Lu3+, Nd3+, Zn2+, Mg2+, Ca2+, Ba2+, Sr2+, Li+), where TPPZ forms a 2:1 complex [(TPPZ)2-M(n+)] and a 1:1 complex [TPPZ-M(n+)] with Mn+ at low and high concentrations of metal ions, respectively. The fluorescence intensity of TPPZ begins to increase at high concentrations of metal ions, when the 2:1 (TPPZ)2-M(n+) complex is converted to the fluorescent 1:1 TPPZ-M(n+) complex. This is regarded as an "OFF-OFF-ON" fluorescence sensor for metal ions depending on the stepwise complex formation between TPPZ and metal ions. The fluorescence quantum yields of the TPPZ-M(n+) complex vary depending on the metal valence state, in which the fluorescence quantum yields of the divalent metal complexes (TPPZ-M2+) are much larger than those of the trivalent metal complexes (TPPZ-M3+). On the other hand, the binding constants of (TPPZ)2-M(n+) (K1) and TPPZ-M(n+) (K2) vary depending on the Lewis acidity of metal ions (i.e., both K1 and K2 values increase with increasing Lewis acidity of metal ions). Sc3+, which acts as the strongest Lewis acid, forms the (TPPZ)2-Sc3+ and TPPZ-Sc3+ complexes stoichiometrically with TPPZ. In such a case, "OFF-OFF-ON" switching of electron transfer from cobalt(II) tetraphenylporphyrin (CoTPP) to O2 is observed in the presence of Sc3+ and TPPZ depending on the ratio of Sc3+ to TPPZ. Electron transfer from CoTPP to O2 occurs at Sc3+ concentrations above the 1:2 ratio ([Sc3+]/[TPPZ]0 > 0.5), when the (TPPZ)2-Sc3+ complex is converted to the TPPZ-Sc3+ complex and TPPZ-(Sc3+)2, which act as promoters of electron transfer (ON) by the strong binding of O2*- with Sc3+. In sharp contrast, no electron transfer occurs without metal ion (OFF) or in the presence at Sc3+ concentrations below the 1:2 ratio (OFF), when the (TPPZ)2-Sc3+ complex has no binding site available for O2*-.     

Effects of metal ions on photoinduced electron transfer in zinc porphyrin-naphthalenediimide linked systems

Zinc porphyrin-naphthalenediimide (ZnP-NIm) dyads and zinc porphyrin-pyromellitdiimide-naphthalenediimide (ZnP-Im-NIm) triad have been employed to examine the effects of metal ions on photoinduced charge-separation (CS) and charge-recombination (CR) processes in the presence of metal ions (scandium triflate (Sc(OTf)(3)) or lutetium triflate (Lu(OTf)(3)), both of which can bind with the radical anion of NIm). Formation of the charge-separated states in the absence and in the presence of Sc(3+) was confirmed by the appearance of absorption bands due to ZnP(.) (+) and NIm(.) (-) in the absence of metal ions and of those due to ZnP(.) (+) and the NIm(.) (-)/Sc(3+) complex in the presence of Sc(3+) in the time-resolved transient absorption spectra of dyads and triad. The lifetimes of the charge-separated states in the presence of 1.0 x 10(-3) M Sc(3+) (14 micros for ZnP-NIm, 8.3 micros for ZnP-Im-NIm) are more than ten times longer than those in the absence of metal ions (1.3 micros for ZnP-NIm, 0.33 micros for ZnP-Im-NIm). In contrast, the rate constants of the CS step determined by the fluorescence lifetime measurements are the same, irrespective of the presence or absence of metal ions. This indicates that photoinduced electron transfer from (1)ZnP(*) to NIm in the presence of Sc(3+) occurs without involvement of the metal ion to produce ZnP(.) (+)-NIm(.) (-), followed by complexation with Sc(3+) to afford the ZnP(.) (+)-NIm(.) (-)/Sc(3+) complex. The one-electron reduction potential (E(red)) of the NIm moiety in the presence of a metal ion is shifted in a positive direction with increasing metal ion concentration, obeying the Nernst equation, whereas the one-electron oxidation potential of the ZnP moiety remains the same. The driving force dependence of the observed rate constants (k(ET)) of CS and CR processes in the absence and in the presence of metal ions is well evaluated in terms of the Marcus theory of electron transfer. In the presence of metal ions, the driving force of the CS process is the same as that in the absence of metal ions, whereas the driving force of the CR process decreases with increasing metal ion concentration. The reorganization energy of the CR process also decreases with increasing metal ion concentration, when the CR rate constant becomes independent of the metal ion concentration.     

Scandium ion-promoted reduction of heterocyclic N=N double bond. Hydride transfer vs electron transfer

Hydride transfer from 10-methyl-9,10-dihydroacridine (AcrH(2)) to 3,6-diphenyl-1,2,4,5-tetrazine (Ph(2)Tz), which contains a N=N double bond, occurs efficiently in the presence of Sc(OTf)(3) (OTf = OSO(2)CF(3)) in deaerated acetonitrile (MeCN) at 298 K, whereas no reaction occurs in the absence of Sc(3+). The observed second-order rate constant (k(obs)) increases with increasing Sc(3+) concentration to approach a limited value. When AcrH(2) is replaced by the dideuterated compound (AcrD(2)), the rate of Sc(3+)-promoted hydride transfer exhibits the same primary kinetic isotope effect (k(H)/k(D) = 5.2+/-0.2), irrespective of Sc(3+) concentration. Scandium ion also promotes an electron transfer from CoTPP (TPP(2)(-) = tetraphenylporphyrin dianion) and 10,10'-dimethyl-9,9'-biacridine [(AcrH)(2)] to Ph(2)Tz, whereas no electron transfer from CoTPP or (AcrH)(2) to Ph(2)Tz occurs in the absence of Sc(3+). In each case, the observed second-order rate constant of electron transfer (k(et)) shows a first-order dependence on [Sc(3+)] at low concentrations and a second-order dependence at higher concentrations. Such dependence of k(et) on [Sc(3+)] is ascribed to formation of 1:1 and 1:2 complexes between Ph(2)Tz(*)(-) and Sc(3+) at the low and high concentrations of Sc(3+), respectively, which results in acceleration of the rate of electron transfer. The formation of 1:2 complex has been confirmed by the ESR spectrum in which the hyperfine structure is different from that of free Ph(2)Tz(*)(-). The 1:2 complex formation results in the saturated kinetic dependence of k(obs) on [Sc(3+)] for the Sc(3+)-promoted hydride transfer, which proceeds via Sc(3+)-promoted electron transfer from AcrH(2) to Ph(2)Tz, followed by proton transfer from AcrH(2)(*)(+) to the 1:1 Ph(2)Tz(*)(-)-Sc(3+) complex and the subsequent facile electron transfer from AcrH(*) to Ph(2)TzH(*). The effects of counteranions on the Sc(3+)-promoted electron transfer and hydride transfer reactions are also reported.     

Metal ion-catalyzed cycloaddition vs hydride transfer reactions of NADH analogues with p-benzoquinones

1-Benzyl-4-tert-butyl-1,4-dihydronicotinamide (t-BuBNAH) reacts efficiently with p-benzoquinone (Q) to yield a [2+3] cycloadduct (1) in the presence of Sc(OTf)(3) (OTf = OSO(2)CF(3)) in deaerated acetonitrile (MeCN) at room temperature, while no reaction occurs in the absence of Sc(3+). The crystal structure of 1 has been determined by the X-ray crystal analysis. When t-BuBNAH is replaced by 1-benzyl-1,4-dihydronicotinamide (BNAH), the Sc(3+)-catalyzed cycloaddition reaction of BNAH with Q also occurs to yield the [2+3] cycloadduct. Sc(3+) forms 1:4 complexes with t-BuBNAH and BNAH in MeCN, whereas there is no interaction between Sc(3+) and Q. The observed second-order rate constant (k(obs)) shows a first-order dependence on [Sc(3+)] at low concentrations and a second-order dependence at higher concentrations. The first-order and the second-order dependence of the rate constant (k(et)) on [Sc(3+)] was also observed for the Sc(3+)-promoted electron transfer from CoTPP (TPP = tetraphenylporphyrin dianion) to Q. Such dependence of k(et) on [Sc(3+)] is ascribed to formation of 1:1 and 1:2 complexes between Q(*)(-) and Sc(3+) at the low and high concentrations of Sc(3+), respectively, which results in acceleration of the rate of electron transfer. The formation constants for the 1:2 complex (K(2)) between the radical anions of a series of p-benzoquinone derivatives (X-Q(*)(-)) and Sc(3+) are determined from the dependence of k(et) on [Sc(3+)]. The K(2) values agree well with those determined from the dependence of k(obs) on [Sc(3+)] for the Sc(3+)-catalyzed addition reaction of t-BuBNAH and BNAH with X-Q. Such an agreement together with the absence of the deuterium kinetic isotope effects indicates that the addition proceeds via the Sc(3+)-promoted electron transfer from t-BuBNAH and BNAH to Q. When Sc(OTf)(3) is replaced by weaker Lewis acids such as Lu(OTf)(3), Y(OTf)(3), and Mg(ClO(4))(2), the hydride transfer reaction from BNAH to Q also occurs besides the cycloaddition reaction and the k(obs) value decreases with decreasing the Lewis acidity of the metal ion. Such a change in the type of reaction from a cycloaddition to a hydride transfer depending on the Lewis acidity of metal ions employed as a catalyst is well accommodated by the common reaction mechanism featuring the metal-ion promoted electron transfer from BNAH to Q.     

A thiazole-containing tripodal ligand: synthesis, characterization, and interactions with metal ions and matrix metalloproteinases

A new tripodal ligand, tris[2-(((2-thiazolyl)methylidene)amino)ethyl]amine (Tatren), has been synthesized and characterized by NMR, IR, and UV-visible absorbance spectroscopy and elemental analysis. Tatren forms stable complexes with transition metal ions (Zn(2+), 1; Mn(2+), 2; Co(2+), 3) and the alkaline earth metal ions (Ca(2+), 4; Mg(2+), 5). Single-crystal X-ray structures of 1, 2, and 5 revealed six-coordinate chelate complexes with formula [M(Tatren)](ClO(4))(2) in which the metal centers are coordinated by three thiazolyl N atoms and three acyclic imine N atoms. Crystals of 1, 2, and 5 are monoclinic, P2(1)/c space group. Crystals of 4 are triclinic, P space group. The Ca(2+) complex is eight-coordinate with all N atoms of Tatren and one water molecule coordinated to the metal ion. Spectrophotometric titrations show that formation constants for the chelates of metal ions are >1 in methanol. Free Tatren inhibits the catalytic domain of matrix metalloproteinase-13 (MMP-13, collagenase-3) with K(i) = 3.5 +/- 0.6 microM. Molecular mechanics-based docking calculations suggest that one leg of Tatren coordinates to the catalytic Zn(2+) in MMPs-2, -9, and -13 with significant hydrogen bonding to backbone amide groups. High-level DFT calculations suggest that, in the absence of nonbonded interactions between Tatren and the enzyme, the most stable first coordination sphere of the catalytic Zn(2+) is achieved with three imidazolyl groups from His residues and two imine N atoms from one leg of Tatren. While complexes (1-3) do not inhibit MMP-13 to a significant extent, 4 does (K(i) = 30 +/- 10 microM). Hence, this study shows that tripodal chelating ligands of this class and their Ca(2+) complexes have potential as active-site inhibitors for MMPs.     

A mechanistic dichotomy in scandium ion-promoted hydride transfer of an NADH analogue: delicate balance between one-step hydride-transfer and electron-transfer pathways

The rate constant (kH) of hydride transfer from an NADH analogue, 9,10-dihydro-10-methylacridine (AcrH2), to 1-(p-tolylsulfinyl)-2,5-benzoquinone (TolSQ) increases with increasing Sc(3+) concentration ([Sc(3+)]) to reach a constant value, when all TolSQ molecules form the TolSQ-Sc(3+) complex. When AcrH2 is replaced by the dideuterated compound (AcrD2), however, the rate constant (kD) increases linearly with an increase in ([Sc(3+)]) without exhibiting a saturation behavior. In such a case, the primary kinetic deuterium isotope effect (kH/kD) decreases with increasing ([Sc(3+)]). On the other hand, the rate constant of Sc(3+)-promoted electron transfer from tris(2-phenylpyridine)iridium [Ir(ppy)3]to TolSQ also increases linearly with increasing ([Sc(3+)]) at high concentrations of Sc(3+) due to formation of a 1:2 complex between TolSQ*- and Sc(3+), [TolSQ*--(Sc(3+)2], which was detected by ESR. The significant difference with regard to dependence of the rate constant of hydride transfer on ([Sc(3+)]) between AcrH2 and AcrD2 in comparison with that of Sc3+-promoted electron transfer indicates that the reaction pathway is changed from one-step hydride transfer from AcrH2 to the TolSQ-Sc3+ complex to Sc3+-promoted electron transfer from AcrD2 to the TolSQ-Sc3+ complex, followed by proton and electron transfer. Such a change between two reaction pathways, which are employed simultaneously, is also observed by simple changes of temperature and concentration of Sc3+.     

Formation and dissociation kinetics of triaza-crown-alkanoic acid complexes of transition metal(II) and lanthanide (III)

The formation and dissociation rates of some transition metal(II) and lanthanide(III) complexes of the 1,7,13-triaza-4,10,16-trioxacyclooctadecane N',N',N'-triacetic acid (1) and 1,7,13-triaza-4,10,16-trioxacyclooctadecane-N',N',N'- trimethylacetic acid (2) have been measured by the use of stopped-flow and conventional spectrophotometry. Experimental observations were made at 25.0 +/- 0.1 degrees C and at an ionic strength of 0.10 M KCl. The complexation of Zn(2+) and Cu(2+) ions with 1 and 2 proceeds through the formation of an intermediate complex (MH(3)L(+) *) in which the metal ion is incompletely coordinated. This may then lead to a final product in the rate-determining step. Between pH 4.68 and 5.55, the diprotonated (H(2)L(-)) form is revealed to be a kinetically active species despite its low concentration. The stability constants (log K (MH (3)L (+) *)) and specific base-catalyzed rate constants (k(OH)) of intermediate complexes have been determined from the kinetic data. The dissociation reactions of 1 and 2 complexes of Co(2+), Ni(2+), Zn(2+), Ce(3+), Eu(3+) and Yb(3+) were investigated with Cu(2+) ions as a scavenger in acetate buffer. All complexes exhibit acid-independent and acid-catalyzed contributions. The buffer and Cu(2+) concentration dependence on the dissociation rate has also been investigated. The metal and ligand effects on the dissociation rate of some transition metal(II) and lanthanide(III) complexes are discussed in terms of the ionic radius of the metal ions, the side-pendant arms and the rigidity of the ligands.     

Interactions of metal ions with monoaza crown ethers A15C5 and A18C6 carrying dansyl fluorophore as pendant in acetonitrile solution

The influence of metal cations Li(+), Na(+), K(+), Cs(+), Mg(2+), Ca(2+), Sr(2+), Ba(2+), Co(2+), Ni(2+), Cu(2+), Zn(2+), Pb(2+) and Al(3+) on the spectroscopic properties of the dansyl (1-dimethylaminonaphthalene-5-sulfonyl) group covalently linked to monoaza crown ethers 1-aza-15-crown-5 (1,4,7,10,-tetraoxa-13-azacyclopentadecane) (A15C5) and 1-aza-crown-6 (1,4,7,10,13-pentaoxa-16-azacyclooctadecane) (A18C6) was investigated by means of absorption and emission spectrophotometry. Interaction of the alkali metal ions with both fluoroionophores is weak, while alkaline earth metal ions interact strongly causing 50 and 85% quenching of dansyl fluorescence of N-(5-dimethylamine-1-naphthalenesulfonylo)-1,4,7,10,-tetraoxa-13-azacyclopentadecane (A15C5-Dns) and N-(5-dimethylamine-1-naphthalenesulfonylo)-1,4,7,10,13-pentaoxa-16-azacyclooctadecane (A18C6-Dns), respectively. The Cu(2+), Pb(2+) and Al(3+) cations interact very strongly with dansyl chromophore, causing a major change in absorption spectrum of the chromophore and forming non-fluorescent complexes. The Co(2+), Ni(2+), Zn(2+), Mg(2+) cations interact moderately with both fluoroionophores causing quenching of dansyl fluorescence by several percent only.     

Catalysis of the methanolysis of activated amides by divalent and trivalent metal ions. The effect of Zn(2+), Co(2+), and La(3+) on the methanolysis of acetylimidazole and its (NH(3))(5)Co(III) complex.

The metal ions Zn(2+), Co(2+), and La(3+) strongly catalyze the methanolysis of the activated amides acetylimidazole (1) and its ligand-exchange-inert Co(III) complex, (NH(3))(5)Co(III)-AcIm (2). Studies of the kinetics of methanolysis are performed with pH measurement and control, and the metal ions are soluble in the medium throughout the pH regions where ionization of the M(x+)(CH(3)OH)(y) occurs. Zn(2+) and Co(2+) act as Lewis acids toward 1, catalyzing attack of external methoxide on a 1:M(2+) complex at values only 100-fold lower than the diffusion limit, the k(OR) values being 5.6 x 10(7) M(-1) s(-1) and 2.5 x 10(7) M(-1) s(-1), while that for CH(3)O(-) attack on 2 is 4.69 x 10(7) M(-1) s(-1). Since neither Zn(2+) nor Co(2+) promotes the methanolysis of 2, these metals appear to be acting through transient binding to the distal N of 1, which activates the C=O of the complex to external CH(3)O(-) attack. La(3+) catalyzes the methanolysis of both 1 and 2, which occurs by a mechanism that is fundamentally different from that exhibited by Zn(2+) and Co(2+) in that the active species appears to be a bis-methoxy-bridged dimer (La(3+))(2)(CH(3)O(-))(2)(CH(3)OH)(x)() that interacts directly with the C=O unit of the substrate.     

Binding of the two-valence-electron metal ions Sc(+), Ti(2+), and V(3+) to the second-row hydrides BeH(2), BH(3), NH(3), OH(2), and FH: a computational study

We report results from a computational study of the binding in complexes formed from one of the transition-metal ions Sc(+), Ti(2+), or V(3+), each of which has two valence electrons outside an argon core, and one of the second-row hydrides FH, OH(2), NH(3), BH(3), or BeH(2). The complexes that involve the electron-rich ligands FH, OH(2), and NH(3) have strong ion-dipole components to their binding. There are large stabilization energies for sigma-interactions that transfer charge from occupied lone-pair natural bond orbitals on the F, O, or N atom of the (idealized) Lewis structure into empty non-Lewis orbitals on the metal ions; these interactions effectively increase electron density in the bonding region between the metal ion and liganded atom, and the metal ions in these complexes act in the capacity of Lewis acids. The complexes formed from the electron-poor hydrides BH(3) and BeH(2) consistently incorporate bridging hydrogen atoms to support binding, and there are large stabilization energies for interactions that transfer charge from the Be-H or B-H bonds into the region between the metal ion and liganded atom. The metal ions in Sc(+)-BeH(2), Ti(2+)-BeH(2), Ti(2+)-BH(3), and V(3+)-BH(3) act in the capacity of Lewis acids, whereas the scandium ion in Sc(+)-BH(3) acts as a Lewis base.     

Complex formation of p-nitrocalix[6]arene with rare earth metal ions.

p-Nitrocalix[6]arene (CALX-N6, L) formed a 1:1 metal complex, ML, with light rare earth metal ions (M3+), such as La3+, Pr3+ and Nd3+ except Ce3+, but formed a 1:2 (M(3+):L) complex, ML2 (the charge of the complex is omitted) with heavy rare earth metal ions, such as Sm(3+)-Lu3+ including Y3+. The conditional stability constants of these 1:1 and 1:2 complexes, KML and KML2, were measured by a ligand displacement method using absorption spectrophotometry in 4% (v/v) acetone aqueous solution at pH 9.65 +/- 0.15 and 25 degrees C.     

Effect of metal ions (Li+, Na+, K+, Mg2+, Ca2+, Ni2+, Cu2+, and Zn2+) and water coordination on the structure and properties of L-arginine and zwitterionic L-arginine

Interactions between metal ions and amino acids are common both in solution and in the gas phase. The effect of metal ions and water on the structure of L-arginine is examined. The effects of metal ions (Li(+), Na(+), K(+), Mg(2+), Ca(2+), Ni(2+), Cu(2+), and Zn(2+)) and water on structures of Arg x M(H2O)m , m = 0, 1 complexes have been determined theoretically by employing the density functional theories (DFT) and using extended basis sets. Of the three stable complexes investigated, the relative stability of the gas-phase complexes computed with DFT methods (with the exception of K(+) systems) suggests metallic complexes of the neutral L-arginine to be the most stable species. The calculations of monohydrated systems show that even one water molecule has a profound effect on the relative stability of individual complexes. Proton dissociation enthalpies and Gibbs energies of arginine in the presence of the metal cations Li(+), Na(+), K(+), Mg(2+), Ca(2+), Ni(2+), Cu(2+), and Zn(2+) were also computed. Its gas-phase acidity considerably increases upon chelation. Of the Lewis acids investigated, the strongest affinity to arginine is exhibited by the Cu(2+) cation. The computed Gibbs energies DeltaG(o) are negative, span a rather broad energy interval (from -150 to -1500 kJ/mol), and are appreciably lowered upon hydration.     

Capillary isotachophoretic determinations of metal ions by use of complexation equilibria in acetone-water medium

The determination of metal ions by capillary isotachophoresis and the complexation equilibria between metal ions and polyaminopolycarboxylic acids has been investigated. A seven-component mixture of metal ions can be separated in 45% v/v acetone-water medium when EDTA or DCTA is used as the terminating ion. Linear calibration graphs are obtained for a standard mixture of Mn(+), Cu(2+), Zn(2+), Cd(2+), Pb(2+) and Fe(3+) in the range 0.5-5.0 nmole, with relative standard deviations of 1.0% or better. The effective mobilities of the Mn(II), Co(II), Ni(II), Cu(II) and Zn(II) complexes increase in parallel with the stability constants, except for the Cu(II) complexes. It is concluded that the abnormal behaviour of the Cu(II) complexes may be attributed to a difference in steric configuration.     

Chemometric analysis of voltammetric data on metal ion binding by selenocystine

The behavior of selenocystine (SeCyst) alone or in the presence of various metal ions (Bi(3+), Cd(2+), Co(2+), Cu(2+), Cr(3+), Ni(2+), Pb(2+), and Zn(2+)) was studied using differential pulse voltammetry (DPV) over a wide pH range. Voltammetric data matrices were analyzed using chemometric tools recently developed for nonlinear data: pHfit and Gaussian Peak Adjustment (GPA). Under the experimental conditions tested, no evidence was found for the formation of metal complexes with Bi(3+), Cu(2+), Cr(3+), and Pb(2+). In contrast, SeCyst formed electroinactive complexes with Co(2+) and Ni(2+) and kinetically inert but electroactive complexes with Cd(2+) and Zn(2+). Titrations with Cd(2+), Co(2+), Ni(2+), and Zn(2+) produced data that were reasonably consistent with the formation of stable 1:1 M(SeCyst) complexes.     

The -Cys-Cys- motif in Helicobacter pylori's Hpn and HspA proteins is an essential anchoring site for metal ions

The Hpn and HspA proteins from H. pylori are significant for nickel homeostasis and protect the cells from higher concentrations of external metal ions. Both proteins have a unique histidine- and cysteine-rich domain at the C terminus. The interactions of Ni(2+), Bi(3+), Zn(2+) and Cd(2+) ions with C-terminal Ac-CCSTSDSHHQ-NH(2) and Ac-EEGCCHGHHE-NH(2) fragments from Hpn and the Ac-GSCCHTGNHD-NH(2) sequence from HspA were studied by potentiometry, mass spectrometry, circular dichroism and UV-Vis spectroscopy. Ac-CC-NH(2) was used as a reference peptide. The studies have shown that nickel ions form planar complexes with a {2S(-),N(-)} binding mode. The thiol sulfurs of the -Cys-Cys- motif are also the anchoring sites for Bi(3+), Zn(2+) and Cd(2+) ions. The studied protein fragments have the highest affinity for Bi(3+) ions. The thermodynamic stability of Ni(2+) is much higher then that of Zn(2+).     

Electron capture dissociation of complexes of diacylglycerophosphocholine and divalent metal ions: Competition between charge reduction and radical induced phospholipid fragmentation

Divalent metal complexes of phosphocholines, [Metal(II)(L)(n)](2+) (where Metal=Cu(2+), Co(2+), Mg(2+), and Ca(2+), L=1,2-dihexanoyl-sn-glycero-3-phosphocholine [6:0/6:0GPCho] and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine [16:0/18:1GPCho] and n=2-5), were formed upon electrospray ionization mass spectrometry (ESI/MS) of 8 mM solution of phosphocholine (L) with 4 mM metal salt (Metal). The electron capture dissociation (ECD) reactions of these [Metal(II)(L)(n)](2+) complexes were examined via Fourier-transform ion-cyclotron resonance mass spectrometry. A rich and complex chemistry was observed, including charge reduction and fragmentation involving losses of a methyl radical, trimethylamine, and the acyl chains. The predominant reaction channel was dependent on the size (n) of the complex, the metal and ligand used, and the size of the acyl chain. Thus charge reduction dominates the ECD spectra of the larger phosphocholine, 16:0/18:1GPCho, but is largely absent in the smaller 6:0/6:0GPCho. For complexes of 16:0/18:1GPCho, n=4-5, fragmentation from the head group mainly occurs via loss of the methyl radical and trimethylamine. At n=3, the relative abundance of fragments due to loss of acyl chain radicals increases. The abundances of ions arising from these radical losses increase further for the n=2 complexes, thereby providing information on the composition and position of the 16:0 and 18:1 acyl groups. Thus ECD of metal complexes provides structurally useful information on the phosphocholine, including the nature of the head group, the acyl chains, and the positions of the acyl chains.     

Metal ion induced allosteric transition in the catalytic activity of an artificial phosphodiesterase

An artificial phosphodiesterase () bearing two types of metal binding sites, a catalytic site and a regulatory bipyridine site showed a unique allosteric transition in the catalytic activity against the metal concentration. The rate constants for the hydrolysis reaction of 2-hydroxypropyl-p-nitrophenyl phosphate (HPNP) and RNA dimer (ApA) with and without an effector metal ion were evaluated; the k(obs) value of HPNP hydrolysis for .(Zn(2+))(3) (2.0 x 10(-4) s(-1)) is 3.3 times larger than that for .(Zn(2+))(2). In the case of and Cu(2+), a 19.4 times larger k(obs) value was obtained for .(Cu(2+))(3) (1.2 x 10(-3) s(-1)) against .(Cu(2+))(2). The increase in the catalytic activity is ascribed to the allosteric conformational transition of induced by the coordination of effector metal ion to the Bpy moiety. A detailed investigation revealed that a conformational change of induced by the third M(2+) complexation enhances the rate of hydrolysis rather than a change in the substrate affinity.     

Kinetics of metal ion exchange between citric acid and serum transferrin

The exchange of Fe(3+), Tb(3+), In(3+), Ga(3+), and Al(3+) between the C-terminal metal-binding site of the serum iron transport protein transferrin and the low-molecular-mass serum chelating agent citrate has been studied at pH 7.4 and 25 degrees C. The removal of Ga(3+), In(3+), and Al(3+) follows simple saturation kinetics with respect to the citrate concentration. In contrast, removal of both Fe(3+) and Tb(3+) shows a combination of saturation and first-order kinetic behavior with respect to the citrate concentration. The saturation component is consistent with a mechanism for metal release in which access to the bound metal is controlled by a rate-limiting conformational change in the protein. The first-order kinetic pathway is very rapid for Tb(3+), and this is attributed to a direct attack of the citrate on the Tb(3+) ion within the closed protein conformation. It is suggested that this pathway is more readily available for Tb(3+) because of the larger coordination number for this cation and the presence of an aquated coordination site in the Tb(3+)-CO(3)-Tf ternary complex. There is relatively little variation in the k(max) values for the saturation pathway for Tb(3+), Ga(3+), Al(3+), and In(3+), but the k(max) value for Fe(3+) is significantly smaller. It is suggested that protein interactions across the interdomain cleft of transferrin largely control the release of the first group of metal ions, while the breaking of stronger metal-protein bonds slows the rate of iron release. The rates of metal binding to apotransferrin are clearly controlled in large part by the hydrolytic tendencies of the free metal ions. For the more amphoteric metal ions Al(3+) and Ga(3+), there is rapid protein binding, and the addition of citrate actually retards this reaction. In contrast, the nonamphoteric In(3+) ion binds very slowly in the absence of citrate, presumably due to the rapid formation of polymeric In-hydroxo complexes upon addition of the unchelated metal ion to the pH 7.4 protein solution. The addition of citrate to the reaction accelerates the binding of In(3+) to apoTf, presumably by forming soluble, mononuclear In-citrate complexes.