Some Physicochemical Peculiarities of Poplar Plastocyanins a and b

2009 ◽  
Vol 64 (5-6) ◽  
pp. 399-404 ◽  
Author(s):  
Petya K. Christova ◽  
Anthony A. Donchev ◽  
Alexandra C. Shosheva ◽  
Vladimir I. Getov ◽  
Mitko I. Dimitrov
Keyword(s):  
Plant Species ◽  
Ph Dependence ◽  
Equilibrium Constants ◽  
The Other ◽  
Redox Potentials ◽  
Extinction Coefficients ◽  
Structural Differences ◽  
Ph Dependent ◽  
Ph Range ◽  
Reduced Forms

The redox potentials of poplar plastocyanins a and b (PCa, PCb) were determined by spectro photometric titrations of their reduced forms with [Fe(CN)6]3-. It was found that the two isoforms have the following millimolar extinction coefficients ε597, equilibrium constants Keq of one-electron exchange with [Fe(CN)6]4-/[Fe(CN)6]3-, and standard electron potentials E0′: PCa: ε597 = (4.72 ± 0.08) mM-1 cm-1, Keq = 0.133 ± 0.009, E0′ = (354 ± 11) mV; PCb: ε597 = (5.23 ± 0.16) mM-1 cm-1, Keq = 0.175 ± 0.010, E0′ = (363 ± 12) mV. The pH dependence of the redox potential of PCb was studied too. It was found, that the value of E0′ for PCb is constant in the pH range 6.5 - 9.5, but decreases in the range 4.8 - 6.5. On the whole, the dependence resembles that of PC from some well-known plant species, including poplar PCa. The changes of E0′ in the pH-dependent region for poplar PCb, however, are smaller and are 13 mV per pH unit, whereas in the other well-known plant species the changes are about 50 - 60 mV per pH unit. It has been assumed that the weaker pH dependence of E0′ of PCb accounts for some structural differences between PCa and PCb

Humate Complexation of Neptunium(V) and its Modeling

1994 ◽  
Vol 353 ◽  
Author(s):  
Hirotake Moriyama ◽  
Yasuhiro Nakata ◽  
Kunio Higashi
Keyword(s):  
Functional Groups ◽  
Ph Dependence ◽  
Complexation Constants ◽  
Complexation Constant ◽  
Ph Dependent ◽  
Ph Range

AbstractThe complexation behavior of Np(V) with humate was investigated by spectrophotometry in the pH range from 5.7 to 10 in 0.01–0.5M NaClO4. The absorption peaks of NpO2+ and Np(V) humate were observed at 979.6nm and 989.9nm, respectively, and the apparent complexation constant of the Np(V) humate was found to be pH-dependent. A two-site complexation model was applied to the interpretation of the observed pH-dependence by considering the participation of the two functional groups of carboxylate and phenolate. The dissociation and complexation constants of both functional groups were determined.

An Infrared Spectroscopic Study of the Hydrogen-Deuterium Exchange of Bovine Submaxillary Mucin

1969 ◽  
Vol 23 (3) ◽  
pp. 245-248 ◽  
Author(s):  
Frank S. Parked ◽  
Martin H. Stryker
Keyword(s):  
Ph Dependence ◽  
Spectroscopic Method ◽  
Deuterium Exchange ◽  
Infrared Spectroscopic Study ◽  
Hydrogen Deuterium Exchange ◽  
Infrared Spectroscopic ◽  
Hydrogen Deuterium ◽  
Bovine Submaxillary Mucin ◽  
Ph Dependent ◽  
Ph Range

An infrared spectroscopic method was used to study the hydrogen-deuterium exchange of a glycoprotein, bovine submaxillary mucin (BSM), dissolved in D2O. The pH-dependence of the rate and extent of the H—D exchange of BSM was determined. The rate constant of the exchange decreased as pH increased from 3.7 to 5.3 and remained constant at a minimum value, (0.82 ± 0.09) X 10−2 min−1, from pH 5.3 to 7.2. The extent of the exchange decreased with increasing pH in the pH range 3.7 to 5.3 and levelled off from pH 5.3 to 7.2. It is suggested that these results are due to a pH-dependent conformational change.

scholarly journals Metal Ions and Hydrogen Peroxide. XXV

1972 ◽  
Vol 27 (2) ◽  
pp. 95-100 ◽  
Author(s):  
Peter Waldmeier ◽  
Bernhard Prijs ◽  
Helmut Sigel
Keyword(s):  
Hydrogen Peroxide ◽  
Ionic Strength ◽  
Metal Ions ◽  
Experimental Evidence ◽  
Initial Rate ◽  
Reaction Order ◽  
Ph Dependence ◽  
Initial Concentration ◽  
Ph Dependent ◽  
Ph Range

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

scholarly journals The origin of multiply sigmoid curves of pH-dependence. The partitioning of groups among titration pK values

1991 ◽  
Vol 278 (1) ◽  
pp. 279-284 ◽  
Author(s):  
H B F Dixon ◽  
S D Clarke ◽  
G A Smith ◽  
T K Carne
Keyword(s):  
Titration Curve ◽  
Dissociation Constants ◽  
Ph Dependence ◽  
The Other ◽  
Equimolar Mixture ◽  
Ionization State ◽  
Degree Of Dissociation ◽  
Titration Curves ◽  
Ph Range ◽  
The One

An acid, HnA, with n ionizing groups is known to have the same titration curve as an equimolar mixture of n hypothetical monobasic acids, whose dissociation constants are known as the ‘titration constants’ of the real acid. We show that the pH-dependence of any property of HnA is also represented by the sum of one-site titration curves, characterized by these same titration constants. Since one such property is the degree of dissociation of one of the dissociating groups, a fraction of each group shows each of the various titration pK values, so that the group partitions among them. The n groups therefore share the same n titration pK values but differ in the fractions belonging to each. The one H+ ion per molecule that titrates with each pK is thus made up of the fractions, one from each group, that share this pK value. A group may possess a single pK value, in that it contributes virtually all of this pK and almost nothing to the others, only if either (1) in titrates in a different pH range from the other groups or (2) its affinity for H+ is unaffected by their ionization state.

scholarly journals Species-Specific, pH-Independent, Standard Redox Potential of Selenocysteine and Selenocysteamine

Antioxidants ◽  
2020 ◽  
Vol 9 (6) ◽  
pp. 465
Author(s):  
Tamás Pálla ◽  
Arash Mirzahosseini ◽  
Béla Noszál
Keyword(s):  
Redox Potential ◽  
Equilibrium Constants ◽  
Biological Significance ◽  
Redox Potentials ◽  
Acid Base ◽  
Redox Equilibrium ◽  
Standard Redox Potential ◽  
Ph Dependent ◽  
Species Specific ◽  
Redox Equilibria

Microscopic redox equilibrium constants and standard redox potential values were determined to quantify selenolate-diselenide equilibria of biological significance. The highly composite, codependent acid-base and redox equilibria of selenolates could so far be converted into pH-dependent, apparent parameters (equilibrium constants, redox potentials) only. In this work, the selenolate-diselenide redox equilibria of selenocysteamine and selenocysteine against dithiothreitol were analyzed by quantitative nuclear magnetic resonance (NMR) methods to characterize the interfering acid-base and redox equilibria. The directly obtained, pH-dependent, conditional redox equilibrium constants were then decomposed by our method into pH-independent, microscopic constants, which characterize the two-electron redox transitions of selenocysteamine and selenocysteine. The 12 different, species-specific parameter values show close correlation with the respective selenolate basicities, providing a tool to estimate otherwise inaccessible site-specific selenolate-diselenide redox potentials of related moieties in large peptides and proteins.

scholarly journals Local pH-dependent conformational changes leading to proteolytic susceptibility of cystatin C

1994 ◽  
Vol 302 (2) ◽  
pp. 411-416 ◽  
Author(s):  
P J Berti ◽  
A C Storer
Keyword(s):  
Ionic Strength ◽  
Conformational Change ◽  
Cystatin C ◽  
Conformational Changes ◽  
Ph Dependence ◽  
Significant Proportion ◽  
Cysteine Protease Inhibitor ◽  
Ionic Strength Dependence ◽  
Ph Dependent ◽  
Ph Range

Cystatin C, a cysteine protease inhibitor, was subject to hydrolysis at two sites when complexed with papain and in the presence of excess papain. A pH-dependent cleavage at His-86 increases Asp-87 was observed, as well as a pH-independent one at Gly-4 increases Lys-5. His-86 increases Asp-87 hydrolysis increased with decreasing pH and was characterized kinetically. It could be described by a single ionization with pKa = 3.4 +/- 0.2 and (kcat./Km)max. = 1.4 (+/- 0.4) x 10(4) M-1.s-1 at I = 0.3 M. C.d. spectroscopy, also at I = 0.3 M, demonstrated a conformational change with pKa = 3.2 +/- 0.2, indicating that the pH-dependence of hydrolysis was due to a conformational change in cystatin C. At I = 0.15 M, the pKa of the conformational change observed by c.d. shifted to 4.1 +/- 0.1. This indicates that at physiological ionic strength of 0.15 M, a significant proportion of cystatin C complexed with protease would be in a proteolytically labile conformation over the pH range 4.5 to 5, which is encountered in lysosomes. This may constitute a mechanism for clearing inappropriately localized cystatins. A pH-dependent conformational variability in this region of the inhibitor could explain the differences in the X-ray crystallographic and n.m.r. structures of the homologous chicken cystatin. The ionic-strength dependence of ionization indicates a hydrophobic stabilization of the ionizable group. The lack of pH-dependence of hydrolysis at Gly-4 increases Lys-5, with kcat./Km = 220 +/- 41 M-1.s-1 in the pH range 3.89 to 7.96 was unexpected in light of the normal, bell-shaped pH-dependence of papain-catalysed hydrolyses. This may reflect a different rate-limiting step of cystatin C hydrolysis.

scholarly journals Studies on the stabilized ubisemiquinone species in the succinate-cytochrome c reductase segment of the intact mitochondrial membrane system

1980 ◽  
Vol 192 (3) ◽  
pp. 769-781 ◽  
Author(s):  
J C Salerno ◽  
T Ohnishi
Keyword(s):  
Redox State ◽  
Ph Dependence ◽  
Specific Inhibitor ◽  
Membrane System ◽  
Dipolar Interaction ◽  
Energy Transduction ◽  
Redox Potentials ◽  
Succinate Cytochrome C Reductase ◽  
Ph Range ◽  
Reduced State

1. Evidence is presented for the presence of a stable ubisemiquinone pair in the vicinity of iron-sulphur centre S-3, based on its thermodynamic and spin relaxation properties. 2. These semiquinones are coupled by dipolar interaction; quantitative analysis of the signals of the spin-coupled semiquinones (at pH 7.4) gives midpoint redox potentials E1 (oxidized to semiquinone state) and E2 (semiquinone to fully reduced state) of 140 and 80mV, respectively, for individual ubiquinones. 3. Values of pKS (pK of the semiquinone form) below 6.5 and pKR (pK of the fully reduced ubiquinone) of about 8.0 or above were estimated from the pH-dependence of the midpoint potentials of the spin coupled signals. Thus the ubisemiquinone associated with succinate dehydrogenase (designated as SQS) functions mostly in the anionic form of the physiological pH range. 4. Theonyltrifluoroacetone, a specific inhibitor of the succinate-ubiquinone reductase segment of the respiratory chain, destabilized the intermediate redox state; thus it quenches both the g = 2.00 signal and ubisemiquinone (SQS) and split signals from the spin coupled pair. This inhibitor has no significant effect on another bound ubisemiquinone species present in the cytochrome bc1 region (designated as SQC). 5. The possible function and location of these stabilized ubisemiquinone species were discussed in connection with Site-II energy transduction.

scholarly journals pH dependent octanol–water partitioning coefficients of microcystin congeners

2018 ◽  
Vol 16 (3) ◽  
pp. 340-345 ◽  
Author(s):  
James McCord ◽  
Johnsie R. Lang ◽  
Donna Hill ◽  
Mark Strynar ◽  
Neil Chernoff
Keyword(s):  
Algal Blooms ◽  
Environmental Fate ◽  
Ph Dependence ◽  
Significant Heterogeneity ◽  
Algal Toxins ◽  
Amino Acid Region ◽  
Log Kow ◽  
Partitioning Coefficients ◽  
Ph Dependent ◽  
Ph Range

Abstract Hazardous algal blooms can generate toxic compounds with significant health impacts for exposed communities. The ubiquitous class of algal toxins known as microcystins exhibits significant heterogeneity in its peptide structure, which has been minimally studied, given the significant impact this has on hydrophobicity, acid/base character and related environmental fate and health effects. Octanol–water partition coefficients for the microcystin congeners MCLR, MCRR, MCLY, MCLF, and MCLA were calculated over an environmentally and physiologically relevant pH range. Microcystin-LR log(Kow) partition coefficient values were found to be consistent with previously established literature values, 1.67 to −1.41 between pH 1 and 8. Microcystin RR was found to be pH insensitive with a log(Kow) of −0.7. The remaining congeners exhibit similar pH dependence as MCLR, with systematic increases in hydrophobicity driven by the introduction of more hydrophobic residues to their variable amino acid region. The variation in pH dependent hydrophobicity suggests increased propensity for bioaccumulation and alternate environmental fates for differing microcystin forms, requiring further investigation.

Rationalizing the pH-Activity Response for Escherichia Coli’s Glycinamide Ribonucleotidetransformylase Through Computational Methods

2019 ◽  
Author(s):  
Adrian Roitberg ◽  
Pancham Lal Gupta
Keyword(s):  
Transfer Reaction ◽  
Catalytic Mechanism ◽  
Ph Dependence ◽  
Ph Effects ◽  
Dependent Manner ◽  
E Coli ◽  
Gar Tfase ◽  
Activity Curve ◽  
Ph Dependent ◽  
Formyl Transfer

<div>Human Glycinamide ribonucleotide transformylase (GAR Tfase), a regulatory enzyme in the de novo purine biosynthesis pathway, has been established as an anti-cancer target. GAR Tfase catalyzes the formyl transfer reaction from the folate cofactor to the GAR ligand. In the present work, we study E. coli GAR Tfase, which has high sequence similarity with the human GAR Tfase with most functional residues conserved. E. coli GAR Tfase exhibits structural changes and the binding of ligands that varies with pH which leads to change the rate of the formyl transfer reaction in a pH-dependent manner. Thus, the inclusion of pH becomes essential for the study of its catalytic mechanism. Experimentally, the pH-dependence of the kinetic parameter kcat is measured to evaluate the pH-range of enzymatic activity. However, insufficient information about residues governing the pH-effects on the catalytic activity leads to ambiguous assignments of the general acid and base catalysts and consequently its catalytic mechanism. In the present work, we use pH-replica exchange molecular dynamics (pH-REMD) simulations to study the effects of pH on E. coli GAR Tfase enzyme. We identify the titratable residues governing the pH-dependent conformational changes in the system. Furthermore, we filter out the protonation states which are essential in maintaining the structural integrity, keeping the ligands bound and assisting the catalysis. We reproduce the experimental pH-activity curve by computing the population of key protonation states. Moreover, we provide a detailed description of residues governing the acidic and basic limbs of the pH-activity curve.</div>

Removal of Trace Metals From Wastewater by Activated Carbon

1973 ◽  
Vol 8 (1) ◽  
pp. 110-121
Author(s):  
A. Netzer ◽  
J.D. Norman
Keyword(s):  
Activated Carbon ◽  
Trace Metals ◽  
Organic Compounds ◽  
The Other ◽  
Published Data ◽  
Ph Adjustment ◽  
Nickel Silver ◽  
Ph Range ◽  
Carbon Treatment ◽  
Cobalt Copper

Abstract The merits of activated carbon for removal of organic compounds from wastewater have been well documented in the literature. On the other hand there is a lack of published data on the use of activated carbon for the removal of trace metals from wastewater. Experiments were designed to assess the possibility that activated carbon treatment would remove aluminum, cadmium, chromium, cobalt, copper, iron, lead, manganese, mercury, nickel, silver and zinc from wastewater. All metals studied were tested over the pH range 3-11. Greater than 99.5% removal was achieved by pH adjustment and activated carbon treatment for most of the metals tested.

Sign in / Sign up

Export Citation Format

Share Document