Effect of phosphate buffer on photodegradation reactions of riboflavin in aqueous solution

The effect of phosphate buffer on aerobic photodegradation reactions of riboflavin (RF) at pH 7.0 has been studied. The photoproducts of the two major reactions, viz., intramolecular photoreduction and intramolecular photoaddition, have been determined by a specific multicomponent spectrophotometric method. The overall photodegradation of riboflavin in the presence of phosphate buffer involves the participation of both H2PO4-and HPO4(2-) species. The second-order rate constants for the H2PO4(-)-catalysed photodegradation of riboflavin (normal photolysis) to lumichrome (LC) and HPO4(2-)-catalysed photodegradation of riboflavin (photoaddition) to cyclodehydroriboflavin (CDRF) are 0.93 x 10(-4) and 4.0 x 10(-4) M(-1) s(-1), respectively. The addition of 0.25-2.00 M phosphate to RF solutions at pH 7.0 gives rise to RF-HPO4(2-) complex and hence the quenching of 4-36% fluorescence, respectively. This results in the suppression of normal photolysis leading to the formation of LC in favour of photoaddition to yield CDRF. The present study shows the involvement of H2PO4- anions in the base-catalysed degradation of riboflavin by normal photolysis vis-a-vis the involvement of HPO42- anions in photoaddition reactions of riboflavin suggested earlier [M. Schuman Jorns, G. Schollnhammer, P. Hemmerich, Intramolecular addition of the riboflavin side chain. Anion-catalysed neutral photochemistry, Eur. J. Biochem. 57 (1975) 35-48].     

Effect of light intensity and wavelengths on photodegradation reactions of riboflavin in aqueous solution

A study of the effect of light intensity and wavelengths on photodegradation reactions of riboflavin (RF) solutions in the presence of phosphate buffer using three UV and visible radiation sources has been made. The rates and magnitude of the two major photodegradation reactions of riboflavin in phosphate buffer (i.e., photoaddition and photoreduction) depend on light intensity as well as the wavelengths of irradiation. Photoaddition is facilitated by UV radiation and yields cyclodehydroriboflavin (CDRF) whereas photoreduction results from normal photolysis yielding lumichrome (LC) and lumiflavin (LF). The ratios of the photoproducts of the two reactions at 2.0 M phosphate concentration, CDRF/RF (0.09-0.22) and CDRF/LC (0.54-1.75), vary with the radiation source and are higher with UV radiation than those of the visible radiation. On the contrary, the ratios of LF/LC (0.15-0.25) increase on changing the radiation source from UV to visible. The rate is much faster with UV radiation causing 25% degradation of a 10(-5) M riboflavin solution in 7.5 min compared to that of visible radiations in 150-330 min.     

Effect of borate buffer on the photolysis of riboflavin in aqueous solution

The photolysis of riboflavin (RF) in the presence of borate buffer (0.1-0.5M) at pH 8.0-10.5 has been studied using a specific multicomponent spectrophotometric method for the determination of RF and photoproducts, formylmethylflavin (FMF), lumichrome (LC) and lumiflavin (LF). The overall first-order rate constants for the photolysis of RF (1.55-4.36 x 10(-2)min(-1)) and the rate constants for the formation of FMF (1.16-3.52 x 10(-2)min(-1)) and LC (0.24-0.84 x 10(-2)min(-1)) have been determined. The values of all these rate constants decrease with an increase in buffer concentration suggesting the inhibition of photolysis reaction by borate species. The kinetic data support the formation of a RF-borate complex involving the ribityl side chain to cause the inhibition of photolysis. The second-order rate constants for the borate inhibited reaction range from 1.17-3.94 x 10(-2)M(-1)min(-1). The log k-pH profiles for the reaction at various buffer concentrations indicate a gradual increase in rate, with pH, up to 10 followed by a decrease in rate at pH 10.5 probably due to ionization of RF and quenching of fluorescence by borate species. A graph of second-order rate constants against pH is a sigmoid curve showing that the rate of photolysis increases with an increase in pH. The results suggest the involvement of excited singlet state, in addition to excited triplet state, in the formation of LC.     

Selective recognition of monohydrogen phosphate by fluorescence enhancement of a new cerium complex

Bis(8-hydroxy quinoline-5-solphonate) cerium(III) chloride (Ce(QS)(2)Cl) (L) was synthesized and then used as a novel fluorescent sensor for anion recognition. Preliminarily study showed that fluorescence of L enhanced selectively in the presence of HPO(4)(2-) ion. This enhancement is attributed to a 1:1 complex formation between L and HPO(4)(2-) anion. The association constant of 1:1 complex of L-HPO(4)(2-) was calculated as 3.0×10(6). Thus, L was utilized as a basis for a selective detection of HPO(4)(2-) anion in solution. The linear response range of the proposed fluorescent chemo-sensor covers a concentration range of HPO(4)(2-) from 3.3×10(-7) to 5.0×10(-6) mol L(-1) with a detection limit of 2.5×10(-8) mol L(-1). L showed selective and sensitive fluorescence enhancement response toward HPO(4)(2-) ion in comparison with I(3)(-), NO(3)(-), CN(-), CO(3)(2-), Br(-), Cl(-), F(-), H(2)PO(4)(-) and SO(4)(2-) ions. It was probably attributed to the higher stability of the inorganic complex between HPO(4)(2-) ion and L. The method was successfully applied for analysis of phosphate ions in some fertilizers samples.     

Photodegradation and photosensitization of mycosporine-like amino acids

The photodegradation and photosensitization of several mycosporine-like amino acids (MAAs) were investigated. The photodegradation of the MAA, palythine, was tested with three photosensitizers: riboflavin, rose bengal and natural seawater. For comparison of degradation rates, the riboflavin-mediated photosensitization of six other MAAs was also examined. When riboflavin was used as a photosensitizer in distilled water, MAAs were undetectable after 1.5h. Palythine showed little photodegradation when rose bengal was added as the photosensitizer (k=0.12x10(-3)m(2)kJ(-1)). Palythine dissolved in natural seawater containing high nitrate concentrations also showed slow photodegradation rate constants (k=0.26x10(-3)m(2)kJ(-1)) over a 24-h period of constant irradiation. Similar experiments in deep seawater with porphyra-334 and shinorine resulted in 75% of the initial MAA remaining after 4h of irradiation and rates of 0.018 and 0.026x10(-3) m(2) kJ(-1), respectively. Experiments conducted in deep seawater with riboflavin additions resulted in photodegradation rate constants between 0.77x10(-3) and 1.19x10(-3)m(2)kJ(-1) for shinorine and porphyra-334, respectively. Photoproduct formation appeared to be minimal with the presence of a dehydration product of the cycloheximine ring structure indicated as well as the presence of amino acids. Evidence continues to build for the role of MAAs as potent and stable UV absorbers. This study further highlights the photostability of several MAAs in both distilled and seawater in the presence of photosensitizers.     

Kinetics and mechanism of iron(III)-nitrilotriacetate complex reactions with phosphate and acetohydroxamic acid

The kinetics and mechanism of the substitution of coordinated water in nitrilotriacetate complexes of iron(III) (Fe(NTA)(OH(2))(2) and Fe(NTA)(OH(2))(OH)(-)) by phosphate (H(2)PO(4)(-) and HPO(4)(2)(-)) and acetohydroxamic acid (CH(3)C(O)N(OH)H) were investigated. The phosphate reactions were found to be pH dependent in the range of 4-8. Phosphate substitution rates are independent of the degree of phosphate protonation, and pH dependence is due to the difference in reactivity of Fe(NTA)(OH(2))(2) (k = 3.6 x 10(5) M(-)(1) s(-)(1)) and Fe(NTA)(OH(2))(OH)(-) (k = 2.4 x 10(4) M(-)(1) s(-)(1)). Substitution by acetohydroxamic acid is insensitive to pH in the range of 4-5.2, and Fe(NTA)(OH(2))(2) and Fe(NTA)(OH(2))(OH)(-) react at equivalent rates (k = 4.2 x 10(4) and 3.8 x 10(4) M(-)(1) s(-)(1), respectively). Evidence for acid-dependent and acid-independent back-reactions was obtained for both the phosphate and acetohydroxamate complexes. Reactivity patterns were analyzed in the context of NTA labilization of coordinated water, and outer-sphere electrostatic and H-bonding influences were analyzed in the precursor complex (K(os)).     

Cation mobility and kinetics of ion exchange in zirconium hydrogen monothiophosphate hydrate, Zr(HPO(3)S)(2)x1.5H(2)O

The ion conductivity of zirconium hydrogen monothiophosphate (Zr(HPO(3)S)(2)x1.5H(2)O) has been measured by impedance spectroscopy. The measured value of proton conductivity is 3 x 10(-5) S/cm at 298 K. Conductivity was shown to decrease with increasing temperature due to a dehydration process. Above 450 K, the conductivity is likely governed by proton transport in the anhydrous phase Zr(HPO(3)S)(2). The activation energies of proton conductivity were measured to be 18 +/- 2 kJ/mol for Zr(HPO(3)S)(2)x1.5H(2)O and 60 +/- 3 kJ/mol for the anhydrous compound. The kinetics of ion exchange was studied with the use of potentiometric titration for several ion pairs, H(+)/Na(+), H(+)/Zn(2+), and Na(+)/Zn(2+) in Zr(HPO(3)S)(2)x1.5H(2)O. The diffusion coefficient values for H(+)/Na(+) ion exchange in Zr(HPO(3)S)(2)x1.5H(2)O are lower than those reported in alpha-zirconium phosphate. At the same time, the mobility of zinc ions in Zr(HPO(3)S)(2)x1.5H(2)O is higher than sodium ion mobility. The ion exchange H(+)/Zn(2+) is accompanied by the slow hydrolysis of the initial compound. In all cases, the powdered solids were evaluated by powder X-ray diffraction, and particle sizes were controlled by grinding and sieving the powders.     

Synthesis and phosphate uptake behavior of Zr4+ incorporated MgAl-layered double hydroxides

We synthesized Zr(4+) incorporated MgAl-layered double hydroxides, Mg(AlZr)-LDH(A) (where A denotes a counteranion in the interlayer space and is expressed as CO(3) for carbonate and Cl for chloride ions), with different molar ratios of Mg/(Al+Zr). Then we characterized their uptake behavior toward phosphate ions. CO(3)-type tertiary LDH materials synthesized at room temperature show low crystallinity, whereas the highly crystalline Cl-type tertiary LDH, [Mg(0.68)Al(0.17)Zr(0.14)(OH)(2)][Cl(0.26)(CO(3))(0.04)1.24H(2)O], was synthesized for the first time using a hydrothermal treatment at 120 degrees C. The distribution coefficients (K(d)) of oxo-anions were measured with a mixed solution containing trace amounts of the anions. The selectivity sequences were Cl(-), NO(-)(3)相似文献   

Kinetics and mechanisms of bromine chloride reactions with bromite and chlorite ions

Chloride ion catalyzes the reactions of HOBr with bromite and chlorite ions in phosphate buffer (p[H(+)] 5 to 7). Bromine chloride is generated in situ in small equilibrium concentrations by the addition of excess Cl(-) to HOBr. In the BrCl/ClO(2)(-) reaction, where ClO(2)(-) is in excess, a first-order rate of formation of ClO(2) is observed that depends on the HOBr concentration. The rate dependencies on ClO(2)(-), Cl(-), H(+), and buffer concentrations are determined. In the BrCl/BrO(2)(-) reaction where BrCl is in pre-equilibrium with the excess species, HOBr, the loss of absorbance due to BrO(2)(-) is followed. The dependencies on Cl(-), HOBr, H(+), and HPO(4)(2)(-) concentrations are determined for the BrCl/BrO(2)(-) reaction. In the proposed mechanisms, the BrCl/ClO(2)(-) and BrCl/BrO(2)(-) reactions proceed by Br(+) transfer to form steady-state levels of BrOClO and BrOBrO, respectively. The rate constant for the BrCl/ClO(2)(-) reaction [k(Cl)(2)]is 5.2 x 10(6) M(-1) s(-1) and for the BrCl/BrO(2)(-) reaction [k(Br)(2)]is 1.9 x 10(5) M(-1) s(-1). In the BrCl/ClO(2)(-) case, BrOClO reacts with ClO(2)(-) to form two ClO(2) radicals and Br(-). However, the hydrolysis of BrOBrO in the BrCl/BrO(2)(-) reaction leads to the formation of BrO(3)(-) and Br(-).     

Synthesis of a single four-ring (S4R) molecular zinc phosphate and its assembly to an extended polymeric structure: a single-crystal and in-situ MAS NMR investigation

A reaction of ZnO, HCl, H(3)PO(4), and 2-pyridylpiperazine in THF/H(2)O mixture at 75 degrees C for 72 h produces a new zinc phosphate, [(C(5)NH(5))(C(4)N(2)H(10))][Zn(H(2)PO(4))(2)(HPO(4))], I. Zinc phosphate I consists of single four-ring (S4R) units with terminal phosphoryl groups hanging from the Zn center. On reaction with zinc acetate dihydrate in the presence of water at 100 degrees C, I gave another new zinc phosphate, [(C(5)NH(5))(C(4)N(2)H(10))][Zn(2)(H(2)PO(4))(HPO(4))(PO(4))] x 2H(2)O, II. II has a layer structure with apertures formed by 4- and 8-T atoms (T = Zn, P). An examination of the two structures reveals that I and II are related, II being formed by the direct addition of Zn(2+) ions to I. Room-temperature (31)P MAS NMR studies show the presence of different phosphorus species in both compounds. An in-situ (31)P MAS NMR investigation on the formation of II from I in the presence of Zn(2+) ions and water reveals the transformation to be facile. What is noteworthy in this study is that the structural integrity of the S4Rs has been maintained during the formation of II. Donor-acceptor hydrogen bond interactions and pi-pi interactions involving the pyridyl groups also appear to play subtle roles in both phosphates. This study, the first attempt of its kind, combines the principles of supramolecular organic chemistry with inorganic building units and contributes to our understanding of the formation of framework solids.     

Electrospray ionization tandem mass spectrometric study of salt cluster ions: part 2--salts of polyatomic acid groups and of multivalent metals

Salt cluster ions formed from 0.05 M solutions of CaCl(2), CuCl(2) and Na(A)B (where A = 1 or 2 and B = CO(3)(2-), HCO(3)(-), H(2)PO(4)(-) and HPO(4)(2-)) were studied by electrospray ionization tandem mass spectrometry. The effects on salt cluster ions of droplet pH and of redox reactions induced by electrospray provide information on the electrospray process. CaCl(2) solution yielded salt cluster ions of the form (CaCl(2))(n)(CaCl)(x)(x+) and (CaCl(2))(n)(Cl)(y)(y-), where x, y = 1-3, in positive- and negative-ion modes, respectively. Upon collision induced dissociation (CID), singly charged CaCl(2) cluster ions fragmented, doubly charged cluster ions generated either singly or both singly and doubly charged fragment ions, depending on the cluster mass, and triply charged clusters fragmented predominantly by the loss of charged species. CuCl(2) solution yielded nine series of cluster ions of the form (CuCl(2))(n)(CuCl)(m) plus Cu(+), CuCl(+), or Cl(-). CuCl, the reductive product of CuCl(2), was observed as a neutral component of positively and negatively charged cluster ions. Free electrons were formed in a visible discharge that bridged the gap between the electrospray capillary and the sampling cone brought about the reduction of Cu(2+) to Cu(+). Upon CID, these cluster ions fragmented to lose CuCl(2), CuCl, Cl, and Cl(2). Na(2)CO(3) and NaHCO(3) solutions yielded cluster ions of the form (Na(2)CO(3))(n) plus Na(+) or NaCO(3)(-). Small numbers of NaHCO(3) molecules were found in some cluster ions obtained with the NaHCO(3) solution. For both Na(2)HPO(4) and NaH(2)PO(4) solutions, ions of the form (Na(2)HPO(4))(h), (NaH(2)PO(4))(i), (Na(3)PO(4))(j), (NaPO(3))(k) plus Na(+), PO(3)(-) or H(2)PO(4)(-) were observed. In addition, ions having one or two phosphoric acid (H(3)PO(4)) molecules were observed from the NaH(2)PO(4) solution while ions containing one sodium hydroxide (NaOH) molecule were observed from the Na(2)HPO(4) solution. The cluster ions observed from these four salts of polyatomic acid groups indicate that changes in pH occur in both directions during the electrospray process principally by solvent evaporation; the pH value of the acidic solution became lower and that of the basic solution higher.     

Protective effect of Boldo and tea infusions on the visible light-mediated pro-oxidant effects of vitamin B2, riboflavin

The effect of Boldo and black tea infusions on the pro-oxidant effects of vitamin B2, riboflavin (RF), when exposed to the action of visible light was studied. The amounts of antioxidants present in Boldo and tea infusions were evaluated by a procedure based on the bleaching of preformed 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) radical cations and were expressed as 6-hydroxy-2,5,7,8-tetramethyl-chroman-2-carboxylic acid equivalent concentrations. The quenching rate constants of singlet oxygen (1O2; [kq]Boldo = 6.0 x 10(7) M(-1) s(-1) and [kq]Tea = 3.2 x 10(7) M(-1) s(-1)) and triplet RF (3RF; [3RFkq]Boldo = 10 x 10(8) M(-1) s(-1) and [3RFkq]TEA = 3.2 x 10(8) M(-1) s(-1)) with Boldo and tea were determined by flash photolysis. These data allow a quantitative interpretation of the results obtained. Our data suggest that most of the oxygen consumption observed in the photolysis of RF in the presence of tea and Boldo infusions is caused by 1O2 reactions. The oxygen consumption quantum yield is considerably smaller than the fraction of RF triplets trapped by the additives (AH) present in the infusion, indicating that their interaction with 3RF does not lead to chemical reactions or that the AH*+ radical ions initially formed participate in secondary processes that do not consume oxygen. Boldo and tea infusions have a significant protective effect when a system containing RF and tryptophan (Trp) is exposed to visible light, not only by quenching the 1O2 and interfering with the Type-I mechanism but also by repairing the damage to Trp molecules associated with the latter mechanism.     

Reactivity and selectivity of the N-acetyl-Glu-P-1, N-acetyl-Glu-P-2, N-acetyl-MeIQx,and N-acetyl-IQx nitrenium ions: comparison to carbocyclic N-arylnitrenium ions

The model ultimate carcinogens 1a-d, related to the metabolites of the food-derived carcinogenic heterocyclic amines Glu-P-1, Glu-P-2, MeIQx, and IQx, spontaneously decompose in neutral aqueous solution to generate the heterocyclic nitrenium ions, 2a-d. The less reactive esters 1a and 1b also undergo acid-catalyzed ester hydrolysis to generate the corresponding hydroxamic acids at pH <2, while the more reactive 2c and 2d are prone to rearrangement in nonaqueous solvents. The reactions of the nitrenium ions with AcO(-), HPO(4)(2-), N(3)(-), and 2'-deoxyguanosine (d-G) were characterized in aqueous solution by using a combination of competitive trapping methods and product isolation and identification. The reactions with N(3)(-) and d-G generally follow patterns previously established for carbocyclic nitrenium ions, but the reactions with AcO(-) and HPO(4)(2-) are unusual. Similar reactions have previously only been reported for heterocyclic 1-alkyl-2-imidazolium ions. The N(3)(-)/solvent selectivities of these ions (5.1 x 10(6) M(-1) for 2a, 2.3 x 10(6) M(-1) for 2b, 1.2 x 10(5) M(-1) for 2c, and 5.2 x 10(4) M(-1) for 2d) are comparable to those of highly selective carbocyclic nitrenium ions. If k(az) for these ions is diffusion limited at ca. 5 x 10(9) M(-1) s(-1) the aqueous solution lifetimes of these ions range from 10 micros (2d) to 1 ms (2a). These ions are also highly selective for trapping by d-G, but comparisons to other nitrenium ions show that they are 10- to 50-fold less selective for trapping by d-G than they would be if both the N(3)(-) and d-G reactions were diffusion limited. This is not a consequence of their heterocyclic structures. Several carbocyclic ions show similar behavior. The relatively inefficient trapping of 2c and 2d by d-G may account for the observation of the unusual minor N-2 d-G adduct that is isolated for both of these nitrenium ions, but has not previously been observed for the reactions of other nitrenium ions with monomeric d-G.     

Water/silica nanoparticle interfacial kinetics of sulfate, hydrogen phosphate, and dithiocyanate radicals

The values of the rate constants for the reactions of the sulfate (2.5 x 10(9) M(-1) s(-1)) and hydrogen phosphate (2.2 x 10(8) M(-1) s(-1)) radicals with silica nanoparticles are obtained by flash photolysis experiments with silica suspensions containing S(2)O(8)(2-) or P(2)O(8)(4-), respectively. The interaction of these radicals with the silica nanoparticles leads to formation of transients, probably adsorbed sulfate and hydrogen phosphate radicals, with absorption maxima at around 320 and 350 nm, respectively. A different mechanism takes place for the interaction of the less oxidizing dithiocyanate radicals with the silica nanoparticles. These radicals selectively react with the dissociated silanol groups of the nanoparticles with a rate constant at 298.2K of 7 x 10(7) M(-1) s(-1) (per mol of SiO(-) groups), and there is no evidence for their adsorption at the surface. All the results are discussed in terms of the Smoluchowski equation and redox potential of the inorganic radicals.     

Oxidation of di- and tripeptides of tyrosine and valine mediated by singlet molecular oxygen, phosphate radicals and sulfate radicals.

Kinetics and mechanism of the oxidation of tyrosine (Tyr) and valine (Val) di- and tripeptides (Tyr-Val, Val-Tyr and Val-Tyr-Val) mediated by singlet molecular oxygen [O(2)((1)Delta(g))], phosphate (HPO(4)(*-) and PO(4)(*2-)) and sulfate (SO(4)(*-)) radicals was studied, employing time-resolved O(2)((1)Delta(g)) phosphorescence detection, polarographic determination of dissolved oxygen and flash photolysis. All the substrates were highly photooxidizable through a O(2)((1)Delta(g))-mediated mechanism. Calculated quotients between the overall and reactive rate constants for the quenching of O(2)((1)Delta(g)) by Tyr-derivatives (k(t)/k(r) values, accounting for the efficiency of the effective photooxidation) were 1.3 for Tyr, 1 for Tyr-Val, 2.8 for Val-Tyr and 1.5 for Val-Tyr-Val. The effect of pH on the kinetics of the photooxidative process confirms that the presence of the dissociated phenolate group of Tyr clearly dominates the O(2)((1)Delta(g)) quenching process. Products analysis by LC-MS indicates that the photooxidation of Tyr di- and tripeptides proceeds with the breakage of peptide bonds. The information obtained from the evolution of primary amino groups upon photosensitized irradiation is in concordance with these results. Absolute rate constants for the reactions of phosphate radicals (HPO(4)(*-) and PO(4)(*2-), generated by photolysis of the P(2)O(8)(4-) at different pH) and sulfate radicals (SO(4)(*-), produced by photolysis of the S(2)O(8)(2-)) with Tyr peptides indicate that for all the substrates, the observed tendency in the rate constants is: SO(4)(*-) > or = HPO(4)(*-) > or = PO(4)(*2-). Formation of the phenoxyl radical of tyrosine was detected as an intermediate involved in the oxidation of tyrosine by HPO(4)(*-).     

Photophysical and photochemical processes of riboflavin (vitamin B_2) by means of the transient absorption spectra in aqueous solution

Using time-resolved techniques of 337 and 248 nm laser flash photolysis, the photo-physical and photochemical processes of riboflavin (RF, vitamin B2) were studied in detail in aqueous solution. The excited triplet state of riboflavin (3RF*) was produced with 337 nm laser, while under 248 nm irradiation, both 3RF* and hydrated electron (eaq) formed from photoionizationcould be detected. Photobiological implications have been inferred on the basis of reactivity of 3RF* including energy transfer, electron transfer and hydrogen abstraction. The RF.+ was generated by oxidation of SO4.- radical with the aim of confirming the results of photolysis.     

The mechanism of riboflavin fluorescence quenching by nucleophiles

It is shown that the quenching of riboflavin (RF) fluorescence by nucleophiles, i.e., sulfite-, nitrite-, thiocyanate-, iodide-, and bromide ions, occurs by both a dynamic and static mechanism. The constants of quenching excited RF by anions and the constants of equilibrium of RF in the ground state and anions are determined. The formation of RF radicals in the presence of low concentrations of nucleophiles is revealed using flash photolysis.     

Solid-state NMR study of the transformation of octacalcium phosphate to hydroxyapatite: a mechanistic model for central dark line formation

For many years, octacalcium phosphate (OCP) has been postulated as the precursor phase of biological apatite in bones and teeth. In this work, we study the molecular mechanism of OCP to hydroxyapatite (HAp) transformation in vitro by several physical techniques, with particular emphasis on solid-state (31)P homonuclear double-quantum (DQ) NMR spectroscopy. The in vitro system is prepared by mixing urea, sodium phosphate monobasic dehydrate, and calcium nitrate tetrahydrate at 100 degrees C. The images obtained by scanning electron microscopy and transmission electron microscopy show that the bladelike OCP crystals will transform into hexagonal rod-shaped HAp crystals as the pH of the reaction mixture increases slowly from 4.35 to 6.69 in 12 h. Powder X-ray diffraction patterns indicate that a trace amount of monetite was also precipitated when the pH was around 5. Together with computer-assisted lattice matching, our DQ NMR data reveal that OCP crystals transform to HAp topotaxially, where the [000](HAp) and [20](HAp) axes are along the same directions as the [001](OCP) and [010](OCP) axes, respectively. On the basis of our in vitro results, the formation of the central dark line commonly found in biological hard tissues could be explained by the inherent lattice mismatch between OCP and HAp. Furthermore, the data of the (31)P{(1)H} cross-polarization NMR suggest that water molecules enter the hydration layers of OCP crystals via the hydrolysis reaction HPO(4)(2)(-) + OH(-) = PO(4)(3)(-) + H(2)O, which also accounts for the deprotonation of the HPO(4)(2)(-) ions during the transformation.     

Rapid three-step cleavage of RNA and DNA model systems promoted by a dinuclear Cu(II) complex in methanol. energetic origins of the catalytic efficacy

A dinuclear Cu(II) complex of 1,3-bis-N(1)-(1,5,9-triazacyclododecyl)propane with an associated methoxide (2-Cu(II)(2):(-OCH(3))) was prepared, and its kinetics of reaction with an RNA model (2-hydroxypropyl-p-nitrophenyl phosphate (1, HPNPP)) and two DNA models (methyl p-nitrophenyl phosphate (3) and iso-butyl p-chlorophenyl phosphate (4)) were studied in methanol solution at (s)(s)pH 7.2 +/- 0.2. X-ray diffraction structures of 2-Cu(II)(2):(-OH)(H(2)O)(CF(3)SO(3)-)(3):0.5CH(3)CH(2)OCH(2)CH(3) and 2-Cu(II)(2):(-OH)((C(6)H(5)CH(2)O)(2)PO(2)-)(CF(3)SO(3)-)2 show the mode of coordination of the bridging -OH and H(2)O between the two Cu(II) ions in the first complex and bridging -OH and phosphate groups in the second. The kinetic studies with 1 and 3 reveal some common preliminary steps prior to the chemical one of the catalyzed formation of p-nitrophenol. With 3, and also with the far less reactive substrate (4), two relatively fast events are cleanly observed via stopped-flow kinetics. The first of these is interpreted as a binding step which is linearly dependent on [catalyst] while the second is a unimolecular step independent of [catalyst] proposed to be a rearrangement that forms a doubly Cu(II)-coordinated phosphate. The catalysis of the cleavage of 1 and 3 is very strong, the first-order rate constants for formation of p-nitrophenol from the complex being approximately 0.7 s(-1) and 2.4 x 10(-3) s(-1), respectively. With substrate 3, 2-Cu(II)(2):(-OCH(3)) exhibits Michaelis-Mentin kinetics with a k(cat)/K(M) value of 30 M(-1) s(-1) which is 3.8 x 10(7)-fold greater than the methoxide promoted reaction of 3 (7.9 x 10(-7) M(-1) s(-1)). A free energy calculation indicates that the binding of 2-Cu(II)(2):(-OCH(3)) to the transition states for 1 and 3 cleavage stabilizes them by -21 and -24 kcal/mol, respectively, relative to that of the methoxide promoted reactions. The results are compared with a literature example where the cleavage of 1 in water is promoted by a dinuclear Zn(II) catalyst, and the energetic origins of the exalted catalysis of the 2-Cu(II)(2) and 2-Zn(II)(2) methanol systems are discussed.     

A metallic cobalt electrode for the indirect potentiometric determination of calcium and magnesium in natural waters using flow injection analysis

A flow injection analysis of Ca(2+) and Mg(2+) using indirect potentiometric detection in natural waters is proposed, where Ca(2+) or Mg(2+) are injected into a buffer carrier containing phosphate, resulting in the formation of Ca(3)(PO(4))(2) or Mg(3)(PO(4))(2). The consequent reduction in free phosphate in the carrier solution is detected using a metallic cobalt wire electrode. Indirect electrode response was used and the experimental conditions affecting electrode response were optimized. Responses were linear in the concentration range 5x10(-4) to 5x10(-3) M with a detection limit of 1x10(-5) M in 20 mM phosphate buffer at pH 8.0. The relative standard derivation at 1 mM of Ca(2+) and Mg(2+) were 3.9 and 3.7% (n=10), respectively. EGTA and 8-hydroxyquinoline were used as the masking agents for Ca(2+) and Mg(2+), respectively. Concentrations of Ca(2+) and Mg(2+) in natural waters were successfully determined by the proposed method.