LIGHT PROMOTION OF SEED GERMINATION IN Datura ferox IS MEDIATED BY A HIGHLY STABLE POOL OF PHYTOCHROME

Abstract— The duration of the far-red light-absorbing form of phytochrome (Pfr) of the photoreceptor pool involved in the control of seed germination was investigated for Datura ferox seeds. These seeds require both Pfr and alternating temperatures (20/30°C) to germinate. After 24 h imbibition (25°C), the seeds received pretreatment-light pulses providing different phytochrome photoequilibria (Pfr/P), followed by a 24 h dark incubation (25°C), and test-light pulses providing different Pfr/P immediately prior to transfer to alternating temperatures. Germination increased with increasing Pfr/P provided by the test-light pulses, but was unaffected by the pretreatment-light pulses. This suggests that phytochrome synthesis, phytochrome degradation and phytochrome-mediated changes in response to phytochrome were negligible. In other experiments, red light-pretreatment pulses were followed by dark incubations (25°C) of different duration before transfer to alternating temperatures. The proportion of Pfr remaining after the 25°C incubation period was estimated by comparing germination rates with those of seeds that received test-light pulses of known calculated Pfr/P immediately prior to the start of the cycles of alternating temperatures. More than 80% of the Pfr established by a Pfr/P= 0.87 light pulse was present and active even after 48 h dark incubation at 25°C. Surprisingly, when a pretreatmentlight pulse providing a Pfr/P= 0.70 was given, the reduction in [Pfr] was significantly faster.
Germination of Datura ferox seeds is under the control of a highly stable (type II like) phytochrome pool. Apparently, this pool follows Pfr dark reversion to the red light-absorbing form, the times to reach half the original Pfr pool being > 96 h or <14 h after light pulses providing Pfr/P= 0.87 or 0.70, respectively.     

PHOTOPHYSIOLOGY OF TURION GERMINATION IN Spirodela polyrhiza (L.) SCHLEIDEN–V. DEMONSTRATION OF A CALCIUM-REQUIRING PHASE DURING PHYTOCHROME-MEDIATED GERMINATION

Abstract— Etiolated turions of Spirodela polyrhiza are positively photoblastic and show a phytochrome-mediated low fiuence germination response. The far-red light (FR) reversibility decreased with the delay of FR irradiation (lag phase 1.06 ± 0.03 days after red light irradiation; half-maximal response 1.9 days). The action of the far-red-absorbing form of phytochrome (Pfr) was only realized by a germination response if exogenously applied Ca²+ was present. Calcium step-down (from 1 mM to 0.9 μ M Ca²+) and Ca²+ step-up (from 0.9 μ M to 1 m M Ca²+) experiments were carried out to determine the Ca²+-sensitive phase. There was no time gap between the two phases determined by the step-down and step-up experiments but a clear coincidence of both curves. Pulse treatments (24 h) with Ca²+ (1 m M ) showed the upper part of this common curve to represent the most Ca²+-sensitive phase. The Ca²+-sensitive phase was within the Pfr-requiring phase. After reversion of Pfr by FR pulses there was only a negligible response to the high Ca²+-concentration, independent of the delay between the red light (R) and FR pulses. These results are compatible with the assumption of Ca²+ acting as a second messenger of Pfr. However, the Ca²+-insensitivity in the first 12 h after the R pulse points against this hypothesis.     

Phytochrome-mediated Phototropism in Adiantum Protonemata II. Participation of Phytochrome Dark Reversion

A microbeam irradiation technique was used to analyze phytochrome-mediated phototropism of the protonema of the fern Adiantum capillus-veneris. One side of the sub-apical zone of a dark-adapted protonema was irradiated with a red-light (R) microbeam to induce phototropic curvature toward the irradiated side. Except the cases where fluence-response relationships were examined, the protonema was stimulated with the microbeam for about 30 s to provide a fluence (5.3–5.5 mmol m^-2) optimal for curvature. When the whole protonema was pretreated with a high fluence of R (about 9 mmol m^-2), no significant curvature could be induced by immediately subsequent one-sided microbeam stimulation. It was found, however, that curvature became inducible progressively as the time of microbeam stimulation was delayed after the R pretreatment. The protonema gained nearly full responsiveness within 40 min after the pretreatment. Moreover, the inductive effect of microbeam R escaped rapidly from the reversing effect of far-red light. These results indicated that most of the far-red-absorbing form of phytochrome (Pfr), which mediates the phototropic response, undergoes relatively fast dark reversion to the red-absorbing form. The dark reversion kinetics was analyzed further by taking into account that the phototropic responsiveness after the R pretreatment was measured in the presence of background Pfr over the protonema. This analysis revealed a rate constant of about 0.002 s^-1 (t_1/2? 6 min) for the dark reversion. It is considered that the Pfr dark reversion plays a role in establishing a lateral Pfr gradient in the unilaterally irradiated protonema.     

Effect of the Cationic Form of a Fiban K-1 Fibrous Sulfo Cation Exchanger on the Degree of Reduction and Activity of Palladium in the Oxidation of Hydrogen

The effect of the nature of the exchanged cation M ^z+ (M ^z+ = Li⁺, Na⁺, Rb⁺, Cs⁺, Mg²⁺, Ca²⁺, and Ba²⁺) of a Fiban K-1 fibrous sulfo cation exchanger on the degree of reduction of the immobilized complex cations [Pd(NH₃)₄]²⁺ to Pd⁰ was studied. A linear correlation was found between the degree of palladium reduction and the difference of the relative electronegativities of atoms that participate in the O–M ^z+ bond. The activity of the catalysts in the oxidation of H₂ depends on the degree of palladium reduction.     

MECHANISM OF THE PHOTOSENSITIZED REDOX REACTIONS OF ACRIDINE ORANGE IN AQUEOUS SOLUTIONS-A SYSTEM OF INTEREST IN THE PHOTOCHEMICAL STORAGE OF SOLAR ENERGY†

-We have carried out a very detailed study, using fluorescence and optical flash photolysis techniques, of the photoreduction of methyl viologen (MV²⁺) by the electron donor ethylene diamine tetraacetic acid (EDTA) in aqueous solution sensitized by the dye acridine orange (AOH⁺). A complete mechanism has been proposed which accounts for virtually all of the known observations on this reaction. This reaction is novel in that both the triplet and the singlet state of AOH⁺ appear to be active photochemically. We have shown that mechanisms previously proposed for this reaction are probably incorrect due to an artifact. At pH 7 the fluorescence quantum yield φ_s of AOH⁺ is 0.26 ± 0.02 and the fluorescence lifetime is 1.8 ± 0.2 ns. φ_s is pH dependent and reaches a maximum of 0.56 at pH 4. The fluorescence of AOH⁺ is quenched by MV²⁺ at concentrations above 1 mM and the quenching obeys Stern-Volmer kinetics with a quenching rate constant of (1.0 ± 0.1) × 10¹⁰M^?1 s^?1. The quenching of the AOH⁺ excited singlet state by MV²⁺ almost certainly returns the AOH⁺ to its ground state with no photochemistry occurring. EDTA also quenches the fluorescence of AOH· with Stern-Volmer kinetics but with a smaller rate constant (6.4 ± 0.5) × 10⁸M^?1s^?1 at pH 7. In this case the quenching is reactive resulting in the formation of semireduced AOH. In the presence of MV²⁺, flash irradiation of AOH⁺ does result in the reversible formation of the semireduced MV? which absorbs at 603 nm. We attribute this to a photochemical reaction of the triplet state of AOH⁺ with MV²⁺. The initial quantum yield for formation of MV? (φ_MV:)₀ was found to be constant at 0.10 ± 0.05 for [MV²⁺] from 5 × 10^?5 to 1.0 × 10^?3 with [AOH⁺] = 8 × 10^?6M. Previous workers had found that (φ_MV:)₀ appears to decrease with decreasing [AOH⁺]; however, on careful investigation, we found this was most probably due to quenching of the triplet state of AOH⁺ by trace amounts of oxygen. When EDTA is added to a mixture of AOH ⁺ and MV²⁺ at pH 7, the photochemical formation of MV? becomes irreversible as the [EDTA] is increased. The quantum yield for the irreversible formation of MV? exceeds 0.10 becoming as large as 0.16 for [EDTA] = 0.014M. This fact requires that an alternative photochemical process must be operative and we present evidence that this is a reaction of EDTA with the excited singlet state of AOH⁺ to produce the semi-reduced AOH- which then reacts with MV²⁺ to produce MV?. The full kinetic scheme was tested by computer simulation and found to be totally consistent. This also enabled the processing of a full set of rate constants. When colloidal PtO₂ was added to the optimal mixture [EDTA] = 3.4 × 10^?2M; [MV²⁺] = 5 × 10^?4M; [AOH⁺] = 4 × 10^?5M; pH6 H₂ gas was produced at a rate of 0.2μmol H₂h^?1. Thus, acridine orange should serve as an effective sensitizer in reactions designed to use solar energy to photolyze water.     

Salt catalyzed heterogeneous dilute acid hydrolysis of the difficultly accessible portion of cellulose: Effect of nature of metal co-ion

The effect of nature of metal co-ion on the rate of hydrolysis, V_c, of the difficulty accessible portion of cellulose was investigated for Li⁺, Na⁺, K⁺, and Ca²⁺. For the univalent ions V_c decreases exponentially with an increase in ionic radius for a given concentration of co-ion. For Ca²⁺, V_c is lower than would be predicted on the basis of ionic radius. This is attributed to the increased charge and/or increased degree of hydration. For the univalent ions the data presented suggest V_c increases sharply with concentration up to 0.25M, while for Ca²⁺, V_c increases more gradually up to 2.25M, before V_c begins to drop.     

Conductometric and microcalorimetric analysis of the alkaline-earth/alkali-metal ion exchange onto polyacrylic acid

The specificity of the exchange between divalent (Di²⁺ = Ca²⁺ or Ba²⁺) and monovalent (M⁺ = Li⁺, Na⁺ or K⁺) ions onto a polyacrylic chain is examined using conductometric and microcalorimetric techniques. Assuming the formation of a bidentate complex between the Di²⁺ and the carboxylate groups, the conductometric data give the exchange ratio (Di²⁺/M⁺) and the speciation of the acrylic groups. No significant difference is observed between the three alkali-metal ions for a given Di²⁺ ion. Comparisons between Ca²⁺ and Ba²⁺ show a stronger hydrophobicity of the former as it precipitates at a complexation ratio r = 0.33 versus r = 0.45 for the Ba²⁺ salt. Microcalorimetric data show that all Di²⁺/M⁺ exchange energies are positive and depend significantly on the type of cations. The largest displacement energy (the more positive) is found for the binding of Ca²⁺ with sodium polyacrylate (8.13 kJ · mol⁻¹) and the smallest for Ba²⁺ with lithium polyacrylate (1.88 kJ · mol⁻¹). The interpretation of the data leads to the conclusion that specificity of the Di²⁺ binding originates in the dehydration phenomenon and specificity between the three alkali-metal ions is due to the decrease in the electrostatic bond strength with an increase in the ionic radii. The Di²⁺/M⁺ exchange is entropically driven. Received: 28 January 1999 Accepted in revised form: 7 April 1999     

Effect of ionic charge on energy contributions to the interaction of an ion with a polar molecule: ab initio calculations for MH2O (M = Mg, Mg, Mg, Ca, Ca, Ca)

For MH₂O (M = Mg, Mg⁺, Mg²⁺, Ca, Ca⁺, Ca²⁺) various energy contributions (first-order, induction and charge-transfer, dispersion) are compared. Near the minimum, stability due to the first-order energy decreases and that due to dispersion increases from M²⁺ to M⁰. For M²⁺, dispersion represents only 1–7% of the total energy; it may reach 25% with M⁺ and is largely responsible for stability of the neutral complex.     

Effect of ionic charge on energy contributions to the interaction of an ion with a polar molecule: ab initio calculations for M⋯H2O (M = Mg,Mg+, Mg2+, Ca,Ca+, Ca2+)

For M?H₂O (M = Mg, Mg⁺, Mg²⁺, Ca, Ca⁺, Ca²⁺) various energy contributions (first-order, induction and charge-transfer, dispersion) are compared. Near the minimum, stability due to the first-order energy decreases and that due to dispersion increases from M²⁺ to M⁰. For M²⁺, dispersion represents only 1–7% of the total energy; it may reach 25% with M⁺ and is largely responsible for stability of the neutral complex.     

On the role of interlayer atoms and molecular water in formation of crystal structure of salts of uranoarsenic acid HAsUO<Subscript>6</Subscript> · 4H<Subscript>2</Subscript>O

A number of uranoarsenates of alkali metals (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, NH₄⁺), alkaline-earth metals (Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺), and rare-earth metals (Y³⁺, La³⁺, Ln³⁺) of the composition M ^k (AsUO₆) _k · nH₂O were prepared by ion exchange in the M ^k -HAsUO₆ · 4H₂O system. On the basis of the data of X-ray diffraction, IR spectroscopy, thermal analysis, and elemental analysis, molecular water and the nature of the interlayer atom M ^k were found to play the key role in structure formation for the compounds.     

Synthesis and complexing properties of four imidazolyl acetamido p-tert-butylcalix[4]arenes

Four imidazolyl acetamido p-tert-butylcalix[4]arenes 5–8 have been prepared by reacting the corresponding methyl esters derivatives 1–4 with histamine in 1:1 mixture of methanol:toluene. The yields ranged from 56 to 68%. 5–8 have been shown to be in cone conformation. The complexation behaviour of 5–8 towards monovalent metal picrates M⁺Pic⁻ with M⁺ = Li⁺, Na⁺, K⁺, Rb⁺ and Cs⁺ and divalent metal picrates M²⁺(Pic⁻)₂ with M²⁺ = Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Pb²⁺, Cd²⁺, Zn²⁺ and Co²⁺ are given. Tentative localisation of the metal cations in the receptors is given. The binding properties towards these cations have been determined along with stoichiometries of the complexes.     

Chiral single-stranded metallohelix: metal-mediated folding of linear oligooxime ligand

A linear tetraoxime ligand H₄L bearing chiral (S)-2-hydroxylpropyl groups at both ends was synthesized. Complexation between H₄L with Zn²⁺ and Mⁿ⁺ (=Ca²⁺, Y³⁺, La³⁺) afforded helical trinuclear complex [LZn₂M]ⁿ⁺. The helical sense was the most effectively induced when Ca²⁺ was used as the central metal Mⁿ⁺.     

Modeling Ca-Polyanion Crosslinking in Secretory Networks. Assessment of Charge Density and Bond Affinity in Polyanionic Secretory Networks

Materials released by secretory cells are stored inside intracellular membrane-bound vesicles. These moieties are not freely diffusible in the vesicle but remain immobilized in a Ca²⁺-crosslinked condensed-phase polyanionic polymer matrix. During exocytosis a Na⁺/Ca²⁺ ion exchange process triggers a volume phase transition resulting in massive swelling and release of the materials to the extracellular space. Here we formulate a simple model to assess Ca²⁺-ion binding from the swelling kinetics of polymer networks. We found the diffusivity of the networks (D) exhibits a power-law dependency on the Ca²⁺ concentration where D ∝ [Ca²⁺]^−2/3. The model yields an estimate of charge density and ionic affinity of the polymer chains. Studies of post-exocytic swelling kinetics in airway mucin granules, mast cell granules and granules from the microalga (Phaeocystis globosa) were used to validate predictions from our model. These results suggest that independent of the cell type, from animal to plant cells, a single polyelectrolyte interaction mechanism appear to be responsible for product release in exocytosis.     

Effect of counterion condensation on the acid-base equilibrium of a pH-sensitive merocyanine dye covalently attached to polyelectrolytes

Effects of monovalent and divalent counterions on the acid-base equilibrium of a pH-sensitive merocyanine dye covalently attached to copolymers of acrylic acid and acrylamide with varying charge densities (0.28 < ξ < 2.8) were investigated. Added chloride salts of Li⁺, Na⁺, K⁺, and NH⁺₄ (< 0.2 mM) had essentially no effect on pK observed (pK^obs) for the equilibrium. By contrast, the salts of Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺ caused a significant decrease in pK^obs for the copolymers with larger ξ. With smaller ξ, most likely when ξ < 0.5, no decrease in pK^obs was observed upon addition of the salts of divalent cations. A competitive effect of Ca²⁺ and Na²⁺ ions on pK^obs in the presence of an excess of Na⁺ ions implied that Ca²⁺ ions at very low concentrations were preferentially, and therefore exhaustively, condensed on the polyanions with sufficiently large ξ probably until effective charge density was lowered to 0.5. The observed difference in the influence of the monovalent and divalent cations on pK^obs was discussed in terms of the difference in the microscopic behavior of the condensed monovalent and divalent cations. © 1993 John Wiley & Sons, Inc.     

Quantum chemical study of the interaction between selenium analog of methimazole as an anti-thyroid drug and metal cations

Energetic and structural properties of complexes formed from interaction between selenium analog of methimazole (MSeI) as an anti-thyroid drug and M^z+ (Li⁺, Na⁺, K⁺, Be²⁺, Mg²⁺ and Ca²⁺) cations have been investigated using B3LYP, M062X, PBE1PBE, and MP2 methods with 6-311++G(d,p) and 6-311++G(2d,2p) basis sets. Two planar and perpendicular complexes were predicted from interaction of MSeI and M^z+ cations. From the Gibbs free energy difference between the planar and perpendicular forms of MSeI–M^z+ complexes, it is found that the perpendicular forms are the predominant ones. In addition, the comparison of interaction energies shows that the order of energies increases in the following order: K⁺ < Na⁺ < Li⁺ < Ca²⁺ < Mg²⁺ < Be²⁺. The results of natural bond orbital analysis showed that the charge transfer occurs from MSeI to metal cations. The atom in molecule analysis shows that the charge density and its Laplacian at the Se–M^z+ bond critical point of the MSeI–M²⁺ complexes are greater than the MSeI–M¹⁺ ones. Also, it was revealed that the Se–M^z+ interactions in perpendicular complexes of alkali and alkaline metal cations are electrostatic and partially covalent in nature, respectively.     

Host‐Guest Complexes of Dodeka(ethylene)octamine: Prediction of Ion Selectivity by Quantum Chemical Calculations IX

In this contribution we investigated the ion complexation of Bühl's cryptand, dodeka(ethylene)octamine by quantum chemical methods (B3LYP/LANL2DZp). This cryptand is an isomer of a well‐known Lehn‐type cryptand [TriPip222]. The ion selectivity was determined based on the energetic criteria derived by model reactions starting from solvated metal ions and empty dodeka(ethylene)octamine, and by comparing the M–N bond length in [M ? dodeka(ethylene)octamine]^m+ and [M(NH₃)₆]^m+. We calculated that Bühl's cryptand will complex best Na⁺ followed by Li⁺ as alkaline cations and Ca²⁺ followed by Mg²⁺ as alkaline earth metal ions. Based on this data we conclude that Bühl's cryptand offers a smaller cavity to nest ions than the Lehn‐type [TriPip222].     

Enthalpy and Entropy of Formation of Alkali and Alkaline-Earth Macrobicyclic Cryptate Complexes [1]

The enthalpies and entropies of complexation of alkali and alkaline-earth metal cations by several macrobicyclic ligands have been obtained from calorimetric measurements and from the previously determined stability constants [2]. Both enthalpy and entropy changes play an important role in the stability and selectivity of the complexes. Particularly noteworthy are the large enthalpies and the negative entropies of complexation obtained for the alkali cation complexes (Na⁺, K⁺, Rb⁺ and Cs⁺ cryptates). The Sr²⁺ and Ba²⁺ as well as [Li⁺ ? 2.1.1]

1 For use of the symbols see [2].

and [Na⁺ ? 2.2.1] cryptates are of the enthalpy dominant type with also a favourable entropy change. The Ca²⁺ and [Li⁺ ? 2.2.1] cryptates are entirely entropy stabilized with about zero heat of reaction. The high stability of the macrobicyclic complexes as compared to the macromonocylcic ones, the cryptate effect, is of enthalpic origin. The enthalpies of complexation display selectivity peaks, as do the stabilities, whereas the entropy changes do not. The high M²⁺/M⁺ selectivities found in terms of free energy, may be reversed when enthalpy is considered in view of the very different role played by the entropy term for M²⁺ and M⁺ cations. The enthalpies and entropies of ligation show that whereas the cryptate anions are similar in terms of entropy irrespective of which cation is included, the ligands, despite being more rigid than the hydration shell, are nevertheless able to adjust to some extent to the cation. This conclusion agrees with published X-rays data. The origin of the enthalpies and entropies of complexation is discussed in terms of structural features of the ligands and of solvation effects.     

Hyperproduction of l-glutamate oxidase in submerged fermentation of Streptomyces sp. N1 with culture pH control and calcium addition

Production of l-glutamate oxidase (GluOx) by Streptomyces sp. N1 was investigated by controlling culture pH at 6.2, 6.7, 7.0, and 7.3 in a 5-l stirred fermentor. The corresponding GluOx activities obtained were 2.8, 4.2, 6.0, and 5.3 U/mL, respectively. Microbial growth was inhibited by increasing the medium pH from 6.2 to 7.0. The inhibitory effect was also observed in plate colony growth under incubation with a different initial pH value. The effect of calcium on GluOx production was also studied in the pH-controlled bioreactor. When the culture pH was controlled at 6.2 or 7.0, GluOx production could not be improved or was only improved slightly by initial addition of calcium to the medium. However, when the culture pH was kept at 6.7, initial Ca²⁺ addition (60 mM) conspicuously enhanced GluOx production up to 9.3 U/mL, which was about twofold of that without Ca²⁺ addition. The enzyme production level was the highest ever reported in the literature. During fermentation the inhibition of cell growth by Ca²⁺ addition was observed. For the morphological changes, the cells mostly existed as pellets in the medium without Ca²⁺ addition, whereas few pellets were found and almost all the cells were dispersed mycelia in the broth with Ca²⁺ addition.     

Positive/negative liquid secondary ion mass spectrometry ofLn-EDTA (1:1) complexes. Formation of molecular ion adducts with neutral species of the matrix orLn-EDTA

Summary The mass spectra of 1:1 complexes ofEDTA with lanthanide cations (Ln=Sm, Eu, Gd, Tb or Dy) upon positive/negative LSIMS are presented. In glycerol used as a matrix, adduct-ions such as [M+H]⁺, [M+H+nGly]⁺, [2M+H]⁺, [2M+H+Gly]⁺ (positive LSIMS) or [M-H]^–, [M-H+nGly]^–, [2M-H]^–, [2M-H+Gly]^– (negative LSIMS), wheren=1–3, are formed. Reactions leading to the formation of adduct-ions are suggested.

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Positive/negative Liquid-Sekundärionen-Massenspektrometrie vonLn-EDTA-(1:1)-Komplexen. Bildung von molekularen Ionenaddukten mit neutralen Spezies aus der Matrix oderLn-EDTA

Zusammenfassung Die Massenspektren von 1:1-Komplexen vonEDTA mit Lanthanidenkationen (Ln=Sm, Eu, Gd, Tb oder Dy) mittels positiver/negativer LSIMS werden präsentiert. In Glycerin als Matrix bilden sich Adduktionen wie [M+H]⁺, [M+H+nGly]⁺, [2M+H]⁺, [2M+H+Gly]⁺ (positive LSIMS) oder [M-H]^–, [M-H+nGly]^–, [2M-H]^–, [2M-H+Gly]^– (negative LSIMS), wobein=1–3. Es werden Reaktionen vorgeschlagen, die zur Bildung von Adduktionen führen.

     

Kinetics of cerium (IV) oxidation of mandelic acid. Ionic strength and specific cation effects

The kinetics of the redox reaction between mandelic acid (MA) and ceric sulfate have been studied in aqueous sulfuric acid solutions and in H₂SO₄? MClO₄ (M⁺ = H⁺, Li⁺, Na⁺) and H₂SO₄? MHSO₄ (M⁺ = Li⁺, Na⁺, K⁺) mixtures under various experimental conditions of total electrolyte concentration (that is, ionic strength) and temperature. The oxidation reaction has been found to occur via two paths according to the following rate law: rate = k[MA] [Ce(IV)], where k = k₁ + k₂/(1 + a)²[HSO₄^?]² = k₁ + k₂/(1 + 1/a)²[SO₄^2?]², a being a constant. The cations considered exhibit negative specific effects upon the overall oxidation rate following the order H⁺ ? Li⁺ < Na⁺ < K⁺. The observed negative cation effects on the rate constant k₁ are in the order Na⁺ < Li⁺ < H⁺, whereas the order is in reverse for k₂, namely, H⁺ ? Li⁺ < Na⁺. Lithium and hydrogen ions exhibit similar medium effects only when relatively small amounts of electrolytes are replaced. The type of the cation used does not affect significantly the activation parameters.