Biphasic Absorbance Research Articles

The CO2 absorption and desorption performance of the triethylenetetramine + N,N-diethylethanolamine + H2O system

Abstract Post-combustion CO2 capture (PCC) process faces significant challenge of high regeneration energy consumption. Biphasic absorbent is a promising alternative candidate which could significantly reduce the regeneration energy consumption because only the CO2-concentrated phase should be regenerated. In this work, aqueous solutions of triethylenetetramine (TETA) and N,N-diethylethanolamine (DEEA) are found to be efficient biphasic absorbents of CO2. The effects of the solvent composition, total amine concentration, and temperature on the absorption behavior, as well as the effect of temperature on the desorption behavior of TETA–DEEA–H2O system were investigated. An aqueous solution of 1 mol·L− 1 TETA and 4 mol·L− 1 DEEA spontaneously separates into two liquid phases after a certain amount of CO2 is absorbed and it shows high CO2 absorption/desorption performance. About 99.4% of the absorbed CO2 is found in the lower phase, which corresponds to a CO2 absorption capacity of 3.44 mol·kg− 1. The appropriate absorption and desorption temperatures are found to be 30 °C and 90 °C, respectively. The thermal analysis indicates that the heat of absorption of the 1 mol·L− 1 TETA and 4 mol·L− 1 DEEA solution is − 84.38 kJ·(mol CO2)− 1 which is 6.92 kJ·(mol CO2)− 1 less than that of aqueous MEA. The reaction heat, sensible heat, and the vaporization heat of the TETA–DEEA–H2O system are lower than that of the aqueous MEA, while its CO2 capacity is higher. Thus the TETA–DEEA–H2O system is potentially a better absorbent for the post-combustion CO2 capture process.

CO2 capture by biphasic absorbent–absorption performance and VLE characteristics

Amine dissolved in alcohol is one of the possible biphasic absorbents for CO2 capture. Monoethanolamine (MEA) and methylaminoethanol (MAE) in isooctanol and 1-heptanol, respectively, were tested as absorbents by using a self-made batch mode chemisorption apparatus. CO2 absorption vapour-liquid equilibrium (VLE) curves were obtained. Phase separation of the absorbent MEA in alcohols occurred readily upon CO2 absorption at the absorption temperature. The CO2 bubbling absorption tests indicated that MAE in the alcohols did not experience phase separation in the whole possible operating temperature range from ambient to 90 °C. The principle for identifying potential CO2 phase separation absorbents is by a good match of the relative strength of the polarity of the amines with the solvent (alcohol in this study), and its change upon CO2 absorption and/or on further temperature increase of the CO2 loaded absorbent. The VLE curves are one of the most important properties of a CO2/absorbent system, based on which, some of the engineering design and operation conditions of a CO2 absorption process can be determined.

Synthesis, Acid-Base Properties, Kinetics and Mechanism of Cu2+ Incorporation into Water-Soluble Highly Substituted Porphyrins

The synthesis, acid-base properties, and kinetics of Cu[Formula: see text] incorporation into water-soluble highly substituted porphyrins were studied. The basicity increased and the stepwise acid-base equilibrium was clarified by increasing the number of phenyl groups at the [Formula: see text] position, and the basicity of a dodeca-substituted porphyrin increased with the ionic strength. The metalation reaction of the dodeca-substituted porphyrin with Cu[Formula: see text] in aqueous solution revealed a biphasic absorbance change at 453 nm. Plots of [Formula: see text] or[Formula: see text] vs. the Cu[Formula: see text] concentration and of log ([Formula: see text] or[Formula: see text]/[Formula: see text] 0) vs. the ionic strength show that [Formula: see text] is dependent on the Cu[Formula: see text] concentration and ionic strength, while [Formula: see text] is independent of these parameters. These results confirm the stepwise metalation mechanism and the existence of an intermediate in aqueous solution, which is indicated by the biphasic absorbance change at 453 nm.

Kinetic equivalence of the subunits of liver alcohol dehydrogenase.

1 The kinetics of two-substrate enzyme reactions proceeding by an ordered ternary-complex mechanism have been analyzed theoretically with respect to the precise magnitude of amplitudes of the two slowest transients governing such reactions under single-turnover conditions. Relationships derived have been applied to the liver alcohol dehydrogenase system for detailed interpretation of amplitudes of the biphasic absorbance changes associated with the enzymic reduction of aromatic aldehydes in catalytic-suicide experiments. 2 Amplitudes of the slow phase 328–330-nm absorbance changes occurring during the enzymic reduction of various para-substituted benzaldehydes by NADH or NAD2H correspond to 6–26% of the total absorbance change under conditions of substrate and coenzyme saturation. Saturation values of amplitudes determined in the present and previous investigations agree within 2 % of the total absorbance change with those calculated theoretically for an ordered mechanism of enzyme action at equivalent sites. In particular, amplitudes observed exhibit the expected sensitivity to electronic substituent and deuterium isotope effects. 3 Enzyme recycling, aldehyde binding, and alcohol desorption provide negligible contributions to the 328–330-nm absorbance changes observed during aldehyde reduction in the presence of 20 mM pyrazole. The maximum slow phase amplitude contribution provided by pyrazole binding corresponds to 5–6% of the total absorbance change. Additional slow phase absorbance changes persisting at saturating concentrations of coenzyme and substrate reflect incomplete accumulation of the enzymic intermediate formed through hydride transfer in the rapid reaction phase. This intermediate contains no detectable amounts (less than 2 %,) of NADH, but exhibits spectral properties indistinguishable from those of enzyme. NAD+. alcohol complexes examined. 4 It has been concluded in recent publications that an ordered ternary-complex mechanism involving equivalent sites is insufficient to account for the transient-state kinetics of liver alcohol dehydrogenase catalysis. These conclusions are shown to be based on an inadequate interpretation of the observations made. Results reported up to date provide no evidence whatsoever for the kinetic significance of subunit interactions leading to a non-equivalence of the enzymic sites. Subunit interactions resulting in a “half-site7rdquo; reactivity of the enzyme are definitely not at hand.

Hairpin formation of d(CGCG-TA-CGCG), d(CGCG-TG-CGCG) and their cytosine methylated analogs.

Hairpin formations of decamers d(CGCG-TA-CGCG), d(CGCG-TG-CGCG), and their m5dC analogs are evidenced by the existence of biphasic absorbance melting profiles in which the lower transition temperature increases with increasing oligomer concentration, whereas the higher melting temperature is concentration independent. The corresponding temperature dependent CD intensity at 285 nm exhibits a maximum around 55 degrees C. These observations are consistent with the interpretation that the lower temperature transition corresponds to the duplex to hairpin transformation while the melting of hairpins into single strands constitutes the higher temperature transition. The CD spectrum of the hairpin conformation appears to be characterized by a couplet with nearly equal positive and negative intensities at 285 and 255 nm, respectively, while a significantly smaller intensity at 285 nm is apparent for the duplex form. The hairpin conformation is suspected to contain a two-nucleotide loop. Titrations with NaCl further suggest that, in contrast to the TA sequence, the TG sequence with wobble base pairing favors Z formation under high salt conditions.

Kinetics and mechanism of the reaction between 4-t-butylphenolate anion and tetrahydroxoargentate(III) in aqueous alkaline media

The reaction of [Ag(OH)4]– with 4-t-butylphenolate (L–) follows a biphasic absorbance change, each phase exhibiting pseudo-first-order kinetics under limiting conditions of the silver(III). A subsequent, slow, and very small absorbance increase (t0.5 > 1 h) is accompanied by the precipitation of metallic silver. In the [L–] range 0.5 × 10–3– 10 × 10–3 and [OH–] range 0.12–1.2 mol dm–3 at 25 °C and in aqueous medium, the pseudo-first-order rate constant for the initial, faster phase, Kf(in s–1), corresponds to the first one-electron transfer from L– to the initial silver(III) species and obeys the relation (i). The pseudo-first-order rate constant for the second, kf=(0.330 ± 0.09)+(1.26 ± 0.09)× 103[L–]+(2.64 ± 0.28)× 102[L–]/[OH–](i), slower phase, Ks(in s–1), represents the second one-electron transfer from L– to an intermediate silver(II) species produced as a result of the first one-electron transfer and satisfies an [OH–]-independent expression (ii). The kf and ks values are not influenced by the ionic strength (0.3–1.2, ks=(1.46 ± 0.02)× 102[L–]/{1 +(56.0 ± 3.5)[L–]}(ii) mol dm–3) of the medium nor by the presence of added pyrophosphate or silver(I) ions. Kinetic and spectral results are interpreted as suggesting that each of the processes mentioned most probably proceeds by a substitution-controlled mechanism at the silver centre.

Interaction of the ruthenium red cation with nucleic acid double helices

The hexapositive complex cation ruthenium red very effectively stabilizes DNA and RNA double helices against thermal denaturation. In the presence of nucleic acid helices, this symmetric cation acquires an extrinsic CD spectrum near the wavelength of the dye's maximum absorbance. Competition experiments with single-stranded polyd(T) show this induced CD to be the result of selective binding to helical sites. The preferential affinity of ruthenium red for double helical binding sites is so great that it brings about biphasic absorbance- temperature profiles of polyd(A-T) at low [cation]: [polynucleotide phosphate]. The visible CD signal and fraction of helix melting at the upper transition increases with ruthenium red concentration until approximate charge neutrality is reached. These interactions, which have been studied in detail with the poly(U-U) helix as well as polyd(A-T), are likely largely electrostatic, since sufficient [NaCl] eliminates the bipliasic melting of polyd(A-T), renders the ultraviolet absorbance of poly (U) insensitive to ruthenium red, and abolishes the induced CD effects. The bipliasic melting of polyd(A-T) at intermediate [dye] is attributed to saturation of remaining double helical segments by cation migration from newly melted regions- Furthermore, virtually no change was observed in the induced CD upon melting through the first transition, whereas the effect is destroyed upon inciting through the second transition. A quantitative treatment of the data is used to obtain binding site size and association constant for the complex. The induced effect may prove useful in the exploration of exposed nucleic acid helical structure in such complex particles as nucleosomes or ribosomes.

Stopped‐Flow Spectrophotometric Characterization of Enzymic Reaction Intermediates in the Anaerobic Reduction of Pig‐Plasma Benzylamine Oxidase by Amine Substrates

Reduction of benzylamine oxidase by p-methoxybenzylamine under anaerobic conditions leads to biphasic absorbance changes at 470 nm. These reflect the intermediate formation of an enzyme substrate complex with spectral properties different from those of native enzyme and fully reduced enzyme. The spectrally modified enzyme-substrate complex exhibits a broad difference absorption band centered around 360 nm. The transient accumulation of this intermediate during reaction can be conveniently followed by stopped-flow techniques at wavelengths between 320 and 360 nm, where contributions from the subsequent reduction of the enzymic 470-nm chromophore are of minor significance. 2. Analogous intermediates exhibiting similar absorption spectra seem to be formed on reduction of the enzyme by benzylamine and other amine substrates which were tested. Substitution of benzylamine as the reducing substrate by [alpha, alpha-2H]benzylamine results in a decreased accumulation of the spectrally modified intermediate. This indicates that its formation is preceded by deprotonation of the alpha-carbon of the amine substrate. 3. Circular dichroism spectra of benzylamine oxidase exhibit a positive band at 360 nm, lending support to the previous conclusion that benzylamine oxidase is a pyridoxal enzyme. Formation of the spectrally modified enzyme-substrate complex then most likely reflects the prototropic shift converting an amine-pyridoxal Schiff-base obtained by rapid pre-equilibration between enzyme and substrate into an aldehyde-pyridoxamine Schiff-base.