Calcium hydroxyapatite solubilisation in the hydrochloric and perchloric acids

Summary Enthalpy of solution of calcium hydroxyapatite Ca₁₀(PO₄)₆(OH)₂ (CaHap) in hydrochloric and perchloric acid solutions is measured by an isoperibol calorimeter and a C80 microcalorimeter. The former device is adapted to reactions occurring in concentrated acid solutions, whereas the microcalorimeter is suitable for slow processes happening in diluted acid solutions. Some solution mechanisms are suggested for different pH ranges. They are confirmed by complementary solution in the same solvent of the entities produced by the reaction between CaHap and the acid. These entities are CaCl₂; Ca(ClO₄)₂·nH₂O; Ca(H₂PO₄)₂ and H₃PO₄. Extrapolation of solution enthalpies to pH=7 leads to the solution enthalpy of (CaHap) in pure water, which is -406.2 kJ mol^-1 from HCl and -437.3 kJ mol^-1 from HClO₄.     

One‐Electron Oxidation and Sensitivity of Uric Acid in On‐Line Electrochemistry and in Electrospray Ionization Mass Spectrometry

The sensitivity of detection of uric acid (H₂U) in positive ion mode electrospray ionization mass spectrometry (ESI MS) was enhanced by uric acid oxidation during electrospray ionization. With a carrier solution of pH 6.3>pK_a1=5.4 of H₂U, protonated unoxidized uric acid [H₂U+H]⁺ (m/z 169) was detected together with the protonated uric acid dimer [2H₂U+H]⁺ (m/z 337). The dimer likely forms by 1e^? oxidation of urate (HU^?) followed by rapid radical dimerization. A covalent structure of the dimer was verified by H/D exchange experiments. Efficiency of 2e^?, 2H⁺ oxidation of uric acid is low during ESI in pH 6.3 carrier solution and improves when a low on‐line electrochemical cell voltage is floated on the high voltage of the ES in on‐line electrochemistry ESI MS (EC/ESI MS). The intensity of the uric acid dimer decreases with an increase in the low applied voltage. In a carrier solution with 0.1 M KOH, pH 12.7>pK_a2=9.8 of H₂U, allantoin (Allnt) (MW 158.04), the final 2e^?, 2H⁺ oxidation product of uric acid, was detected as a potassium complex [K(Allnt)+K]⁺ (m/z 235) and the [2H₂U+H]⁺ dimer was not detected. In direct ESI MS analysis of 1000‐fold diluted urine [NaHU+H]⁺ (pK_sp NaHU=4.6) was detected in 40/60 (vol%) water/methanol, 1 mM NH₄Ac, pH ca. 6.3 carrier solution. A new configuration of the ESI MS instrument with a cone‐shaped capillary inlet significantly enhanced sensitivity in ESI and EC/ESI MS measurements of uric acid.     

Electrochemical Characterization of Pyrocatechol Sulfonephthalein-Modified Glassy Carbon Electrode and Separation of Electrocatalytic Responses for Ascorbic Acid and Dopamine Oxidation

A pyrocatechol sulfonephthalein- (PS-) modified glassy carbon (PS/GC) electrode has been prepared by adsorption of PS on a glassy carbon electrode surface. Cyclic voltammograms of the PS/GC electrode indicate the presence of a couple of well-defined redox peaks, and the formal potential shifts in the negative direction with increasing solution pH. The relation between formal potential,E^0′, and solution pH can be fit to the equationE^0′(mV) = −51.4 pH + 538.7. The PS/GC electrode shows high electrocatalytic activity toward ascorbic acid oxidation, with an overpotential ca. 380 mV less than that of the bare electrode and a drastic enhancement of the anodic currents. The electrocatalytic reaction rate constant (k), which was decreased with increasing concentration of H₂A, was determined using rotating disk electrode measurements. The values ofkwas also affected by the solution pH. The electrode can also separate the electrochemical responses of ascorbic acid and dopamine. The separation between the anodic peak potentials of ascorbic acid and dopamine is more than 50 mV by the differential pulse voltammetry.     

Efficient Molecular Catalysis of ATP-Hydrolysis by Protonated Macrocyclic Polyamines

Molecular catalysis of ATP-hydrolysis by a number of protonated macrocyclic polyamines 1–9 has been investigated by ³¹P-NMR spectroscopy, and marked rate enhancements have been obtained. The largest acceleration is produced by the [24]-N₆O₂ macrocycle 1 , and the process displays the following properties: 1. protonated 1 forms very stable complexes with ATP, as well as with ADP and AMP; 2. it enhances the rate of ATP-hydrolysis by a factor of 10³ at pH = 8.5; the rate of hydrolysis is constant over a wide pH-range, from pH = 2.5 to 8.5; 3. 1 is more efficient than acyclic analogues; 4. the products of the reaction are orthophosphate (OP) and ADP, which is subsequently hydrolyzed to OP and AMP at a slower rate; 5. at pH > 6.5, a transient species is detected, which is tentatively identified as a phosphoramidate intermediate, resulting from phosphorylation of the macrocycle 1 ; 6. the reaction presents first-order kinetics and is catalytic. The mechanism of the process is discussed in terms of initial formation of a complex between ATP and protonated 1 , followed by an intracomplex reaction which may involve a combination of nucleophilic or acid catalysis with electrostatic catalysis.     

NMR Study of CO2 Capture by Butylamine and Oligopeptide KDDE in Aqueous Solution: Capture Efficiency and Gibbs Free Energy of the Capture Reaction as a Function of pH**

We have been interested in the development of rubisco-based biomimetic systems for reversible CO₂ capture from air. Our design of the chemical CO₂ capture and release (CCR) system is informed by the understanding of the binding of the activator CO₂ (^ACO₂) in rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase). The active site consists of the tetrapeptide sequence Lys-Asp-Asp-Glu (or KDDE) and the Lys sidechain amine is responsible for the CO₂ capture reaction. We are studying the structural chemistry and the thermodynamics of CO₂ capture based on the tetrapeptide CH₃CO−KDDE−NH₂ (“KDDE”) in aqueous solution to develop rubisco mimetic CCR systems. Here, we report the results of ¹H NMR and ¹³C NMR analyses of CO₂ capture by butylamine and by KDDE. The carbamylation of butylamine was studied to develop the NMR method and with the protocol established, we were able to quantify the oligopeptide carbamylation at much lower concentration. We performed a pH profile in the multi equilibrium system and measured amine species and carbamic acid/carbamate species by the integration of ¹H NMR signals as a function of pH in the range 8≤pH≤11. The determination of ΔG₁(R) for the reaction R−NH₂+CO₂ R−NH−COOH requires the solution of a multi-equilibrium equation system, which accounts for the dissociation constants K₂ and K₃ controlling carbonate and bicarbonate concentrations, the acid dissociation constant K₄ of the conjugated acid of the amine, and the acid dissociation constant K₅ of the alkylcarbamic acid. We show how the multi-equilibrium equation system can be solved with the measurements of the daughter/parent ratio X, the knowledge of the pH values, and the initial concentrations [HCO₃⁻]₀ and [R-NH₂]₀. For the reaction energies of the carbamylations of butylamine and KDDE, our best values are ΔG₁(Bu)=−1.57 kcal/mol and ΔG₁(KDDE)=−1.17 kcal/mol. Both CO₂ capture reactions are modestly exergonic and thereby ensure reversibility in an energy-efficient manner. These results validate the hypothesis that KDDE-type oligopeptides may serve as reversible CCR systems in aqueous solution and guide designs for their improvement.     

KINETICS AND PHASE TRANSFORMATIONS DURING THE REACTION OF NaCl-SODALITE IN AMMONIUM CHLORIDE SOLUTIONS

The reaction behavior of sodium chloride sodalite Na₈[AlSiO₄]₆Cl₂ in ammonium chloride solution has been investigated under mild hydrothermal conditions (T = 473 K) for reaction times up to 72 h. Reactions under weak acid conditions led to an amorphous aluminosilicate phase comparable to a leaching process. This material forms an amorphous layer around the sodalite grains, preventing the framework from further decomposition. About 52% of sodalite was damaged by acid leaching after 11 hours and this amount remains nearly constant even at longer reaction periods up to 72 hours. Cation exchange was observed in sodalite only on a very low level (< 10%). Beside these reactions under acid conditions (pH » 5) some additional experiments in alkaline solutions were done to improve ion exchange of sodalite. Thus an ammonium/ammonia buffer solution was used (pH » 9) at various temperatures in a range of 353 – 473 K. Neither cation exchange nor decomposition of the sodalite was obtained at 353 and 393 K after 72 hours. Formation of amorphous material started at 433 K. In contrast to the acid conditions a total transformation of sodalite into a crystalline ammonium aluminosilicate phase was observed at 473 K.     

SYNTHESIS,ELECTROCHEMICAL CHARACTERIZATION AND POTENTIOMETRIC TITRATION of trans-[RuCl2(L)4] COMPLEXES (L ˭ PYRIDINE DERIVED LIGANDS: 3-PYRIDINECARBOXYLIC ACID AND 4-PYRIDINECARBOXYLIC ACID)

Abstract

In this work we report the synthesis, spectroscopic characterization, potentiometric titration and electrochemical studies performed on trans-[RuCl₂(nic)₄] (1) and trans-[RuCl₂(i-nic)₄] (II) complexes, where nic = 3-pyridinecarboxylic acid and i-nic = 4-pyridinecarboxylic acid. The complexes were synthesized using a ruthenium blue solution as a precursor in the synthetic route, and characterized by electronic spectroscopy, vibrational FT-IR spectroscopy, and ¹H and ¹³C NMR. The results indicated a trans geometry. Cyclic voltammetry performed in water/ acetone (1:1) solution revealed a quasi-reversible process centered on the Ru(II) species and a dependence of the redox potential, E _1/2, on the pH. The electronic spectra showed that the MLCT bands were also affected by the pH, undergoing a hypsochromic shift (blue shift) as the pH increased. The spectroelectrochemical analysis indicated that the bands in the visible region progressively faded as new UV bands emerged during the oxidation process. The equilibrium constants for trans-[RuCl₂(nic)₄] and trans-[RuCl₂(i-nic)₄] were determined by potentiometric titration, indicating that protonic species dominated at pH values lower than 2.6, whereas non-protonic species dominated at pH values higher than 5.0.     

An imidazole functionalized copper(II)-organic framework for highly selective sensing of picric acid and metal ions in water

An imidazole functionalized metal–organic framework (MOF), [Cu(HL)(H₂O)]·(H₂O)·(DMA) ( HBU-166 , H₃L = 4,4′,4″-(1H-imidazole-2,4,5-triyl)tribenzoic acid, DMA = N,N-dimethylacetamide) was synthesized and characterized by single-crystal X-ray diffraction. HBU-166 was observed as a two-dimensional MOF and showed good stability in water, an acidic solution (pH = 4), and an alkaline solution (pH = 9). HBU-166 exhibited ligand-based luminescence with a blue shift, which could be attributed to the coordination effect. Moreover, HBU-166 could be applied to detect nitroaromatic compounds (NACs) and metal ions in water with preferable selectivity and sensitivity. In particular, HBU-166 could be used as a promising luminescent sensor for picric acid (PA) with enhancement of emission intensity. The mechanism for PA detection likely involved electron transfer and weak interaction between ligand and electron-deficient of NACs at the beginning to increase the emission intensity. Additionally, HBU-166 exhibited excellent selectivity in the sensing of 4-nitrophenol and Fe³⁺ through fluorescence quenching.     

含吡啶的抗肿瘤转移NAMI-A衍生物的制备和水解机理动力学

目的研究配体结构对NAMI-A衍生物水解机理、电化学性质的影响。方法制备了trans-[RuCl4(DMSO)(3-MePy)][(3-MePy)H](3-MePy=3-甲基吡啶,化合物1)和trans-[RuCl4(DMSO)(4-MePy)][(4-MePy)H](4-MePy=4-甲基吡啶,化合物2)。用UV、NMR、CV法研究化合物1、化合物2的水解机理-动力学、溶液稳定性及电化学性质。结果化合物1和化合物2与NAMI-A相似,在pH7.40的缓冲液中发生脱氯水解反应(Ⅰ氯水解及Ⅱ氯水解)(分步反应);在pH 5.00缓冲液中DMSO(二甲亚砜)及少量吡啶水解。测定各水解反应表观速率常数及半衰期、溶液稳定性及氧化还原电位。结论化合物1、化合物2的Ⅰ氯、Ⅱ氯及DMSO水解反应机理与NAMI-A相似,而且各水解速率与NAMI-A相差不大,即用甲基吡啶取代咪唑环,对NAMI-A衍生物的Ⅰ氯、Ⅱ氯及DMSO水解反应速率影响较小。化合物在酸性溶液中的稳定性明显高于中性溶液。     

High-temperature degradation of polyacrylic acid in aqueous solution

Poly(acrylic acid) (PAA) was decarboxylated in aqueous solution as a function of pH and ionic strength in the temperature range 100–350°C. Degradation kinetics were first order with respect to acid functionality. The rate of decarboxylation at high pH (E_α = 51.1 kcal/mol) was much slower than that at low pH (E_α = 20.3 kcal/mol). At intermediate pH, the reactivity was found to depend on the degree of dissociation of PAA as a functions of ionic strength, pH, and temperature. No monomer was observed in the reaction product. © 1996 John Wiley & Sons, Inc.     

A kinetic study of the oxidation of L-ascorbic acid by chromium(VI)

The reaction between chromium(VI) and L-ascorbic acid has been studied by spectrophotometry in the presence of aqueous citrate buffers in the pH range 5.69–7.21. The reaction is slowed down by an increase of the ionic strength. At constant ionic strength, manganese(II) ion does not exert any appreciable inhibition effect on the reaction rate. The rate law found is where K_p is the equilibrium constant for protonation of chromate ion and k_r is the rate constant for the redox reaction between the active forms of the oxidant (hydrogenchromate ion) and the reductant (L-hydrogenascorbate ion). The activation parameters associated with rate constant k_r are E_a = 20.4 ± 0.9 kJ mol^?1, ΔH^≠ = 17.9 ± 0.9 kJ mol^?1, and ΔS^≠=?152 ± 3 J K^?1 mol^?1. The reaction thermodynamic magnitudes associated with equilibrium constant K_p are ΔH⁰ = 16.5 ± 1.1 kJ mol^?1 and ΔS⁰ = 167 ± 4 J K^?1 mol^?1. A mechanism in accordance with the experimental data is proposed for the reaction. © 1993 John Wiley & Sons, Inc.     

Reactions of p‐Substituted Phenols with Nitrous Acid in Aqueous Solution

The reaction of phenols with nitrite (nitrous acid HONO, or its conjugated base, NO₂^?) is of importance in stomach fluids (low pH) and in atmospheric hydrometeors (mild acid and basic pH). The initial reaction associated with the oxidation/nitration of 4‐substitued phenols promoted by HONO/NO₂ depends on the pH of the solution. At low pH, the initial step involves the reaction between HONO and phenol, whereas at basic conditions this involves an electron transfer from the phenoxy anion to nitrogen dioxide (NO₂) producing the nitrite anion. The rate of both processes is determined by the donor capacity of the substituent at the 4‐position of the phenol, and the data obtained at pH 2.3 follow a linear Hammett‐type correlation with a slope equal to –1.23. The partition of the gaseous intermediates (NO and NO₂) makes the rate of HONO‐mediated oxidation dependent on their gas–liquid distribution. At low pH, the main process is phenol oxidation, even in oxygen‐free conditions, and the presence of any 4‐substituted phenol decreases the rate of HONO auto‐oxidation.     

Electrocatalytic Nitrite Reduction to Nitrogen Oxide by a Synthetic Analogue of the Active Site of Cu‐Containing Nitrite Reductase Incorporated in Nafion Film

The electrocatalytic reduction of nitrite to NO by [CuMe₂bpa(H₂O)(ClO₄)]⁺ ( 1 ), which is a model for the active site of copper‐containing nitrite reductase, incorporated in Nafion film was investigated. The Cu complex in the Nafion matrix exhibits an intense band at 267 nm and a broad band around 680 nm, assigned to d–d and ligand field transitions, respectively. The 77‐K EPR spectrum of 1 in the Nafion matrix reveals the typical axial signals (g_//=2.28, g =2.08, A_//=13.3 mT) of a tetragonal Cu²⁺ chromophore. The redox potential, which is related to the Cu⁺/Cu²⁺ couple, was ?146 mV (ΔE=72 mV) at pH 5.5. The redox reaction of 1 in Nafion was not dependent on pH and was a diffusion‐controlled process. The electronic structure and redox properties of 1 in the negatively charged polymer matrix were almost the same as those in aqueous solution. In the presence of nitrite, an increase in the cathodic current was observed in the cyclic voltammogram of 1 in the Nafion matrix. The current increase was dependent on the nitrite concentration and pH in solution. Upon reaching ?400 mV, a linear generation of NO was observed for the 1 /Nafion film coated electrode. The relationship between the rate of NO generation and the nitrite concentration in solution was analyzed with the Michaelis–Menten equation, where V_max=45.1 nM s^?1 and K_m=15.8 mM at pH 5.5. The Cu complex serves the function of both the catalyst and electron transport in the Nafion matrix. The sensitivity of the electrode was estimated to be 3.23 μA mM^?1 in the range of 0.1–0.4 mM nitrite.     

Effect of pH on radical polymerization of butadiene 1-carboxylic acid in aqueous solutions

Radical polymerization of butadiene 1-carboxylic acid (Bu-1-Acid) was carried out in aqueous solutions at 50°C with ammonium persulfate (APS) as an initiator. Kinetic studies led to the rate equation, R_p = k[APS]^1/2 [Bu-1-Acid]¹ at pH 6.8. The overall activation energy for the polymerization was 16.0 kcal/mole. The polymerization rate R_p of Bu-1-Acid decreased with an increase of pH in the range 2.4–6.8 and increased with an increase of pH in the range 6.8–8.4. Moreover, in the pH range 8.4–13, the rate of polymerization was not affected by the pH of the system. In copolymerization with acrylonitrile, the trends of changes in the monomer reactivity ratios r₁, r₂ and Q-e values caused by changes in pH were similar to trends found in homopolymerization described above. In addition, it was observed that the resultant polymer was extended in alkaline solution and contracted in acidic solution.     

Kinetic studies of the reactions of pentacyanoferrate(III) complexes with L‐ascorbic acid

The reduction of Fe(CN)₅L^2? (L = pyridine, isonicotinamide, 4,4′‐bipyridine) complexes by ascorbic acid has been subjected to a detailed kinetic study in the range of pH 1–7.5. The rate law of the reaction is interpreted as a rate determining reaction between Fe(III) complexes and the ascorbic acid in the form of H₂A(k₀), HA^?(k₁), and A^2? (k₂), depending on the pH of the solution, followed by a rapid scavenge of the ascorbic acid radicals by Fe(III) complex. With given Ka₁ and Ka₂, the rate constants are k₀ = 1.8, 7.0, and 4.4 M^?1 s^?1; k₁ = 2.4 × 10³, 5.8 × 10³, and 5.3 × 10³ M^?1 s^?1; k₂ = 6.5 × 10⁸, 8.8 × 10⁸, and 7.9 × 10⁸ M^?1 s^?1 for L = py, isn, and bpy, respectively, at μ = 0.10 M HClO₄/LiClO₄, T = 25°C. The kinetic results are compatible with the Marcus prediction. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 126–133, 2005     

Synthesis and ferromagnetic properties of composites of a water-soluble polyaniline copolymer containing iron oxide

Composites of water-soluble conducting polyaniline copolymers, poly(aniline-co-aminobenzenesulfonic acid) (PAOABSA), containing γ-Fe₂O₃magnetic particles with nanometer size, were synthesized by a chemical method. The ferromagnetic properties of the resulting PAOABSA composites were measured as a function of the pH value of the reaction solution, the sulfonated degree of the copolymer, and the concentration of FeCl₂. The structure of the composites was characterized by means of elemental analysis, FTIR, XPS, and X-ray diffraction. It was found that increasing the pH value of the reaction solution and the concentration of FeCl₂ is favorable for an enhancement of the saturated magnetization. As high as 33.2 emu/g of saturated magnetization for the PAOABSA composites was observed. No hysteresis loop (i.e. H_c = 0) was observed, which is independent of the preparation conditions. Structure characterizations show that iron oxide existing in the composite is mainly γ- Fe₂O₃, which is responsible for the ferromagnetic properties of the PAOABSA composites, whereas γ- Fe₂O₃ magnetic particles nanometer in size (∼85 nm) may be attributed to a lower coercive force (i.e. H_c = 0) of the composites. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 2749–2755, 1998     

Specific assay for homoserine and its lactone in Pisum sativum. Preparation of homoserine hydroxamic acid

The existence of homoserine lactone in Pisum sativum seedlings is demonstrated. L-Homoserine lactone reacts with hydroxylamine, at neutral or alkaline pH, to form homoserine hydroxamic acid. Procedures are described for preparing L-homoserine lactone and L-homoserine hydroxamic acid. The hydroxamic acid yields a color with maximum absorbance at 492 nm with Fe³⁺ in 0.25 N HCl. This reaction permitted assay for total homoserine and homoserine lactone. Six-day old Pisum sativum seedlings, with cotyledons removed, were extracted with 90% ethanol. Evaporation of the ethanol and addition of Na₂SO₄ solution and toluene and centrifugation removed protein lipids and esters. After clarification with activated charcoal, homoserine lactone content was estimated by reaction with NH₂OH and Fe³⁺ reagents. For total homoserine, protein precipitation was with 2 N HCl and toluene. Evaporation to dryness at 60 °C under vacuum converted all homoserine to the lactone. The values found for total homoserine (μmols/g, wet weight) and preformed lactone (%) with the various growth media used were as follows: nitrate 87.4 (14.7%), NH₂OH 75.2 (6.3%), water 70.5 (7.9%), urea 56.4 (18.9%). Acetic anhydride added to homoserine hydroxamic acid forms acetohydroxamic acid, which yields a color with maximum absorbance at 505 nm with Fe³⁺. This color reaction is seven times as sensitive as the reaction of Fe³⁺ with homoserine hydroxamic acid itself.     

Universal Reaction Mechanism of Boronic Acids with Diols in Aqueous Solution: Kinetics and the Basic Concept of a Conditional Formation Constant

To establish a detailed reaction mechanism for the condensation between a boronic acid, RB(OH)₂, and a diol, H₂L, in aqueous solution, the acid dissociation constants (${K{{{\rm BL}\hfill \atop {\rm a}\hfill}}}$ ) of boronic acid diol esters (HBLs) were determined based on the well‐established concept of conditional formation constants of metal complexes. The pK_a values of HBLs were 2.30, 2.77, and 2.00 for the reaction systems, 2,4‐difluorophenylboronic acid and chromotropic acid, 3‐nitrophenylboronic acid and alizarin red S, and phenylboronic acid and alizarin red S, respectively. A general and precise reaction mechanism of RB(OH)₂ with H₂L in aqueous solution, which can serve as a universal reaction mechanism for RB(OH)₂ and H₂L, was proposed on the basis of (a) the relative kinetic reactivities of the RB(OH)₂ and its conjugate base, that is, the boronate ion, toward H₂L, and (b) the determined pK_a values of HBLs. The use of the conditional formation constant, K′, based on the main reaction: RB(OH)₂+H₂L ${{\mathop \leftrightarrow \limits ^{K{_{1}}}_{}}}$ RB(L)(OH)^?+H₃O⁺ instead of the binding constant has been proposed for the general reaction of uncomplexed boronic acid species (B′) with uncomplexed diol species (L′) to form boronic acid diol complex species (esters, BL′) in aqueous solution at pH 5–11: B′+L′ ${{\mathop \leftrightarrow \limits ^{K{^\prime}}_{}}}$ BL′. The proposed reaction mechanism explains perfectly the formation of boronic acid diol ester in aqueous solution.     

Confromational Studies on Peptides Containing Enantiometric α-Methyl α-Amino Acids. Part I. Differential conformational properties of (R)- and (S)-2-methylaspartic acid

The conformational properties of four model peptides of the general formula Ac-Tyr-Xaa-Yaa-Zaa-Ala-Lys-Glu-ala-Ala-Glu-Lys-Ala-Zaa-Yaa-Xaa-Lys-NH₂ (Xaa-Yaa-Zaa = Ala-Ala-(R)-Asp(2-Me), 1 ; Ala-Ala-(S)-Asp(2-Me), 2 ; Ala-Aib-Asp, 3 ; Ala-Ala-Asp, 4 ; Asp(2-Me) = 2-methylaspartic acid; Aib = 2-aminoisobutyric acid) were studied by CD spectroscopy in solution, to evaluate the helix-inducing potential of enantiomerically pure 2-methylaspartic acid as a function of its chirality at C(2). At neutral pH and 1°, all peptides exhibit significant helix formation in aqueous solution, the degree of helicity increasing in the order 4 3 ≈ 1 . Lowering the pH to 2 results in a dramatic increase in helicity for peptide 1 , while the diastereoisomeric peptide 2 now exists in a predominantly unordered conformation. Helix induction by protonated (R)-Asp(2-Me) exceeds Aib-induced helix formation in peptide 3 , and the helix content of 1 in aqueous solution at pH 2 is comparable to the degree of helicity in the strongly helix-inducing solvent 2,2,2-trifluoroethanol.     

Free-radical-mediated oxidation of ascorbic acid by peroxomonosulphate

The kinetics of the oxidation of ascorbic acid (AH₂) by peroxomonosulphate (PMS) were determined in aqueous medium in acidic (pH 4.4), neutral (pH 7.0) and alkaline (pH 9.0) conditions over the temperature range 13-28°C. The reactions were found to obey total second-order kinetics, first-order each with respect to peroxomonosulphate and ascorbic acid concentration, obeying-d[AH₂]/dt = k ₂[PMS][AH₂]. Dehydroascorbic acid was detected as the product of the reaction. The stoichiometry of the reaction, [peroxomonosulphate]/[ascorbicacid] = 1 : 1, indicated the absence of self-decomposition of peroxomonosulphate. The influence of neutral salt (NaClO₄) was found to increase the reaction rate. Evidence for the formation of radical intermediates was obtained. A mechanism involving the formation of hydroxyl, sulphate and ascorbate free radicals as intermediates is proposed. The rate and activation parameters were evaluated to substantiate the mechanism proposed. A comparison is made with the corresponding reactions of the similar peroxides, S₂O₈ ^2- and P₂O₈ ^4-.