胞外高钾对大鼠背根神经元超极化电流的影响

应用全细胞膜片钳技术研究了胞外高钾对大鼠背根神经元（DRG）超极化电流（Ih）的影响．结果表明,Ih随着胞外钾离子浓度的增大而增大,同时随着外加刺激电压的增高而增强．当胞外钾离子浓度[Kext]为4,8和16mmol／L时,半数激活电压V1/2分别为-98±1．9,-106±1．3（p〈0．05）和-110±1．0mV（p〈0．05）,其对应的最大电流的峰值分别为1085±340,1576±409和2124±614pA,与4mmol／L比较,后者分别增长了45％和92％．胞外高钾使,Ih激活曲线显著左移,说明高Kext改变了超极化电流的激活过程．提高细胞外钾离子浓度,可以使早期钾通道（又称快通道）的激活时间常数增大,使晚期钾通道（又称慢通道）激活时间常数减小．当胞外钾离子浓度由4mmol／L升高到8和16mmol／L时,翻转电位右移,但不具有显著性差异．结果提示,细胞外高浓度钾可增强大鼠背根神经元超极化电流,Ih,改变Ih的激活过程,从而提高了神经元的兴奋性,产生不正常的动作电位,对神经细胞产生损伤．     

Gold(III) amine complexes in aqueous alkali solutions

The gold(III) 1,3-diaminopropane complex [Au(1,3-pn)(1,3-pn-H)]Cl₂ has been synthesized. Its dissociation constant has been determined: Au(1,3-pn)₂³⁺ = Au(1,3-pn-H)²⁺ + H⁺, logK _a1 = −7.03 ± 0.05 (I = 0.1 mol/L NaClO₄). Considerable spectral changes are observed for strong alkali solutions (pH 11–14) containing the monoamido forms of the gold(III) ethylenediamine, 1,3-diaminopropane, and diethylenetriamine complexes (Au(en)(en-H)²⁺, Au(1,3-pn)(1,3-pn-H)²⁺, Au(dien-H)OH⁺). These changes are attributed to the formation of the diamido species Au(en-H)₂⁺, Au(1,3-pn-H)₂⁺, and Au(dien-2H)OH⁰. The dissociation constants of the monoamido complexes have been determined: Au(en)(en-H)²⁺ (logK _a2 = −10.9 ± 0.1 at I = 0.001–0.01 mol/L NaCl); Au(1,3-pn)(1,3-pn-H)²⁺ (logK _a2 = −11.3 ± 0.1 at I = 0.1 mol/L NaCl); Au(dien-H)OH⁺ (logK _a2 = −12.4 ± 0.1 at I = 0.1 mol/L NaCl).     

The rheological and colloidal properties of bentonite dispersions in the presence of organic compounds III. The effect of alcohols on the coagulation of sodium montmorillonite

The critical coagulation concentration,c _K, of sodium chloride for sodium montmorillonite dispersed in water (solid content 0.025 percent) is 8 mmol/L. It remains virtually constant (7.5–8.5 mmol/L) in water-rich alcohol mixtures (below 50% (v/v) methanol and 40% ethanol and propanol). At higher alcohol contents thec _K decreases to 3.6 mmol/L (70 percent methanol), 1.2 mmol/L (70 percent ethanol), and 0.8 mmol/L (60 percent propanol). In the presence of 10^–4 M sodium diphosphate thec _K in water rises to 195 mmol/L. In contrast to the behavior in the absence of diphosphate, even small amounts of alcohol reduce the critical coagulation concentration. In 70% methanol thec _K is 7.5 mmol/L, in 70 percent ethanol 2.5 mmol/L and in 60% propanol 5 mmol/L. The main mechanism is coagulation by contacts between negatively charged edges and faces.At high alcohol contents montmorillonite-alcohol complexes (interlayer solvates) are formed and the colloidal dispersions become unstable even in the absence of salt. Transition from the state with diffuse ionic layers into the quasi-crystalline structure of the interlayer solvates is also evident from the sediment volume which changes with the alcohol content. Maxima are observed which are indicative of band-type structures as intermediate states between the colloidally dispersed particles with repulsive interaction and the discrete particles of the montmorillonite-alcohol complexes.     

Protolytic properties and complexation with copper(II) ions of some azo derivatives of β-arylaminopropionic acids

The azo coupling reaction of N-(2-carboxyethyl)anthranilic acid and N,N,N′,N′-tetrabis(2-carboxyethyl)-1,3-phenylenediamine with diazosulfanilic acid yielded the complexones sodium 4-N-(2-carboxyethyl)amino-5-carboxyazobenzene-4′-sulfonate (I) and 2,4-N,N,N′,N′-tetrabis(2-carboxyethyl)diaminoazobenzene-4′-sulfonic acid (II), respectively. The acidity constants of I and II (20°C, μ = 0.1M KCl) were determined to be as follows: for I, pK ₀₀ = 1.29 ± 0.13, pK ₀ = 2.92 ± 0.07, pK ₁ = 3.92 ± 0.05, pK ₂ = 5.16 ± 0.03; for II, pK ₀₀ = 2.35 ± 0.06, pK ₀ = 2.81 ± 0.09, pK ₁ = 3.21 ± 0.11, pK ₂ = 3.81 ± 0.09, pK ₃ = 4.34 ± 0.04, pK ₄ = 5.03 ± 0.06, pK ₅ = 6.67 ± 0.07. The electronic absorption spectra of I and II were measured, and acid-base equilibrium scheme for I and II in aqueous solutions were suggested. The complexation constants of I and II with copper(II) ions were determined to be logK _CuQ^I= 5.47 ± 0.06 and logK _CuQ^II= 5.72 ± 0.13 (20°C, μ = 0.1 M KCl).     

Interaction of Cucurbit[n = 6∼8]urils and Benzimidazole Derivatives

The structures and optical properties of host–guest complexes produced from cucurbit[n = 6–8]urils and some benzimidazole derivatives have been investigated by ¹H NMR spectroscopy, electronic absorption spectroscopy and fluorescence spectroscopy. The experimental results reveal that calculations of A∼N_Q[n]/N_guest and I_f∼N_Q[n]/N_guest for the same association complex both support a good fit to an identical binding model. In particular, the A∼N_Q[n]/N_guest, I_f∼N_Q[n]/N_guest calculations and the ¹H NMR determinations for three Q[6]–ge(1∼3) complexes and three Q[8]–ge(1∼3) complexes all support a binding model of 1:1 and 1:2 respectively.     

Effect of pH and ionic strength on the interaction of humic acid with aluminium oxide

The effect of pH and neutral electrolyte on the interaction between humic acid/humate and γ-AlOOH (boehmite) was investigated. The quantitative characterization of surface charging for both partners was performed by means of potentiometric acid–base titration. The intrinsic equilibrium constants for surface charge formation were logK _a,1 ^int=6.7±0.2 and logK _a,2 ^int = 10.6±0.2 and the point of zero charge was 8.7±0.1 for aluminium oxide. The pH-dependent solubility and the speciation of dissolved aluminium was calculated (MINTEQA2). The fitted (FITEQL) pK values for dissociation of acidic groups of humic acid were pK ₁ = 3.7±0.1 and pK ₂ = 6.6±0.1 and the total acidity was 4.56 mmol g⁻¹. The pH range for the adsorption study was limited to between pH 5 and 10, where the amount of the aluminium species in the aqueous phase is negligible (less than 10⁻⁵ mol dm⁻³) and the complicating side equilibria can be neglected. Adsorption isotherms were determined at pH ∼ 5.5, ∼8.5 and ∼9.5, where the surface of adsorbent is positive, neutral and negative, respectively, and at 0.001, 0.1, 0.25 and 0.50 mol dm⁻³ NaNO₃. The isotherms are of the Langmuir type, except that measured at pH ∼ 5.5 in the presence of 0.25 and 0.5 mol dm⁻³ salt. The interaction between humic acid/humate and aluminium oxide is mainly a ligand-exchange reaction with humic macroions with changing conformation under the influence of the charged interface. With increasing ionic strength the surface complexation takes place with more and more compressed humic macroions. The contribution of Coulombic interaction of oppositely charged partners is significant at acidic pH. We suppose heterocoagulation of humic acid and aluminium oxide particles at pH ∼ 5.5 and higher salt content to explain the unusual increase in the apparent amount of humic acid adsorbed. Received: 20 July 1999 /Accepted in revised form: 20 October 1999     

Protonation acidity constants for benzotoluidides in sulfuric acid solutions

The protonation of o-, m-and p-benzotoluidide in sulfuric acid solutions is studied by UV spectroscopy in the 190–350 nm region. Principal component analysis is applied to estimate the contributions of the effect of protonation and the medium effect. For the substances studied in this work, the first principal component (PC) captures about 98 % of the variance and the second PC ∼100 % of the cumulative percentage variance in the 210–350 nm region. The same spectral region is used for calculation of the ionization ratio from the coefficients of the first PC and mole fractions of the base and its conjugate acid. Using these data and Hammett's equation (pK _{BH +} = H _X + log I), the pK _{BH +} values for the protonation reaction are obtained. The dissociation constants as well as the solvent parameters m* (∼0.43) and ϕ (∼0.60) are calculated using the Excess Acidity Method (-pK _{BH +}= 2.28–2.30) and the Bunnett-Olsen Method (-pK _{BH +}= 2.24–2.28). The probable sites of protonation are discussed.     

Crystal and molecular structure of the [Y(Phen){( i -C4H9)2PS2}2NO3] mixed-ligand compound

The [Y(Phen){(i-C₄H₉)₂PS₂}₂NO₃] mixed-ligand compound (I) was synthesized. Its structure was determined by X-ray diffraction analysis (X8 APEX diffractometer, λMoK _α , 3788 F _hkl , R = 0.0677). Crystals I are orthorhombic: a = 10.6495(8) Å, b = 20.006(2) Å, c = 35.317(4) Å, V = 7524.4(13) ų, Z = 8, ρ_calc = 1.324 g/cm³, space group Pbca. The structure is built of mononuclear discrete molecules. The N₂O₂S₄ coordination polyhedron of the Y atom is a distorted dodecahedron. Original Russian Text Copyright ? 2008 by V. L. Varand, L. A. Glinskaya, R. F. Klevtsova, and S. V. Larionov __________ Translated from Zhurnal Strukturnoi Khimii, Vol. 49, No. 1, pp. 182–186, January–February, 2008.     

Study of equilibria and kinetics of the acid catalysed interaction of iron(III) with salicylaldehyde ando-hydroxyacetophenone

The equilibria and kinetics of the reaction of Fe^III with salicylaldehyde ando-hydroxyacetophenone, leading to 1∶1 chelate formation, have been studied at different temperatures (25–35°C) and ionic strength, I = 1.0 mol dm⁻³ (NaClO₄+HClO₄). A dual path mechanism involving both Fe _aq ³⁺ and Fe(OH) _aq ²⁺ species and undissociated free ligand (LH) is consistent with the experimental observations where [H⁺]≫[Fe]_T≫[L]_T (where [Fe]_T and [L]_T stand for total concentrations of iron and ligand respectively). The results conform to k_obs/B = k₁[H⁺]+k₂K_h where B = [Fe]_T/(K_h+[H⁺])+1/Q; K_h = hydrolysis constant of Fe _aq ³⁺ ; k₁, k₂ are the forward second order rate constants of Fe _aq ³⁺ and Fe(OH) _aq ²⁺ , respectively, and Q is the equilibrium constant of the reaction, Fe³⁺+LH⇋FeL²⁺+H⁺. Thermodynamic parameters for each of the steps have been determined. Fe(OH) _aq ²⁺ appears to react in a dissociative fashion (Eigen-Tamm mechanism), whilst Fe _aq ³⁺ appears to react through the associative inter-change (I_a) mechanism. The equilibrium constants (Q) obtained spectrophotometrically are compared with those obtained from kinetic studies. TMC 2638     

Water solubility and octanol/water-partitioning of hydrophobic chlorinated organic substances determined by using SPME/GC

The critical step in the determination of water solubilitiy (S _w) and octanol-water partition coefficient (K _ow) of hydrophobic organic chemicals by using the generator-column technique and the slow-stirring procedure, respectively, is the exact quantification of the low water-phase concentrations of the substances under investigation. We have tested the applicability of solid-phase microextraction (SPME) and gas chromatography with seven chlorinated organic compounds. The substances cover a S _w range from 500 mg/L to 7 ng/L and a log K _ow range from 3 to 8. The results show that SPME can be a valuable alternative to common preconcentration techniques in the quantification of hydrophobic organics in pure and octanol-saturated water. The apparent SPME distribution constants K _SPME (obtained with the 100 μm-PDMS fiber for analyte’s partitioning between fiber coating and aqueous sample) do not correlate directly with octanol/water partition coefficients and thus cannot be recommended as a surrogate parameter for K _ow. Received: 15 January 1997 / Revised: 2 May 1997 / Accepted: 8 May 1997     

Acoustic Study of Solvent Coordination in the Hydration Shells of Potassium Iodide

The isentropic compressibilities of aqueous solutions of potassium iodide, from dilute to almost saturated, were determined at 288 to 308 K based on precise measurements of the speed of ultrasound. Using proper correlations, the hydration numbers (h) were calculated as well as the molar volume and compressibility parameters of the hydrated complexes (V _h , β _h V _h ) of water in the hydration shell (V _1h , β_1h V _1h), and of the cavity containing stochiometric mixtures of K⁺ and I⁻ ions (V _2h, β_2h V _2h). It is revealed that under the studied conditions, the obtained values of h and β _h V _h are independent of temperature whereas the molar compressibility of the hydration shell β _h V _h) is independent of concentration. The electrostatic field of the ions is shown to influence the temperature dependence of the molar volume of water in the hydration shell more substantially than a change of pressure alone influences the temperature dependence of the molar volume of pure water.     

Influence of electrostatic effect on the stability of mixed-ligand complexes: Uranyl-complexone-phenol/phenolic acid ternary systems

The formation constants (logK _MAL ^MA ) of the complexes of the type (UO₂.A.L] (whereA = IMDA, NTA or EDTA;L = catechol, resorcinol, phloroglucinol, pyrogallol, β-resorcylic acid or protocatechuic acid) have been determined potentiometrically at 25°C and ionic strength,I = 0·2 (mol dm⁻³, NaClO₄) using the Irving-Rossotti approach. The formation constants of the binary complexes (logK _KMA ^KM ) have been found to lie in the sequence IMDA < NTA < EDTA, whereas those for the mixed ligand complexes (logK _MAL ^MA ) follow the reverse sequence, IMDA > NTA > EDTA. due to the electrostatic effect.     

Determination of ion-pair formation constants of univalent metal–crown ether complex ions with anions in water using ion-selective electrodes: application of modified determination methods to several salts

Ion-pair formation constants (mol^–1 dm³ unit), K_MX for a univalent metal salt (MX) and K_MLX for its ion-pair complex (ML⁺X^–) with a crown ether (L) in water, were determined at various ionic strengths (I) and 25°C by potentiometry with ion-selective electrodes for MX=NaPic, NaMnO₄, NaBPh₄, KPic, and KMnO₄; and MLX=Na(18C6)Pic, K(18C6)Pic, and Na(18C6)BPh₄, where Pic^– and 18C6 denote a picrate ion and 18-crown-6 ether, respectively. Equations for analyzing I-dependence of logK_MLX and logK_MX were derived and fitted well to the I-dependence using a non-linear regression analysis. The equilibrium constants at I=0 mol dm^–3, K_MLX° and K_MX°, were simultaneously obtained from the analysis. The experimental values of K_MLX and K_MX were only in agreement with the values calculated from K_MLX° and K_MX°, respectively, in the ranges of higher I.     

Simulaaneous ethanol and cellobiose inhibition of cellulose hydrolysis studied with integrated equations assuming constant or variable substrate concentration

The integrated forms of the Michaelis-Menten equation assuming variable substrate (depletion) or constant substrate concentration were used to study the effect of the simultaneous presence of two exoglucanase Cel7A inhibitors (cellobiose and ethanol) on the kinetics of cellulose hydrolysis. The kinetic parameters obtained, assuming constant substrate (K _m =21 mM, K _ic =0.035 mM; K _icl =1.5×10¹⁵mM; k _cat=12 h⁻¹) or assuming variable substrate (K _m =16 mM, K _ic =0.037 mM; K _icl =5.8×10¹⁴ mM; k _cat=9 h⁻¹), showed a good similarity between these two alternative methodologies and pointed out that bothethanol and cellobiose are competitive inhibitors. Nevertheless, ethanol is a very weak inhibitor, as shown by the large value estimated for the kinetic constant K _icl . In addition, assuming different concentrations of initial accessible substrate present in the reaction, both inhibition and velocity constants are at the same order of magnitude, which is consistent with the obtained values. The possibility of using this kind of methodology to determine kinetic constants in general kinetic studies is discussed, and several integrated equations of different Michaelis-Menten kinetic models are presented. Also examined is the possibility of determining inhibition constants without knowledge of the true accessible substrate concentration.     

全细胞膜片钳法研究铜对急性分离大鼠海马细胞钾电流的影响

Using the whole cell patch clamp technique, the effect of Cu^2＋on transient outward K^＋current （/to） and delayed rectifier K^＋ current （Idr） was studied in acutely isolated rat hippocampal neurons.Ito and Idr were increased when the concentration of Cu^2＋ was lower than 2 × 10^-5 and 10^-5 tool/L, respectively, and increased ratio was decreased with increasing Cu^2＋concentration in the bath solutions. When the concentration continued to increase to 5× 10^-5 and 2 × 10^- 5 mol/L, the currents were hardly changed, while the concentration was more than 10^-4 and 5 × 10^-5 mol/L, the currents were inhibited remarkably. Cu^2＋ （10^-5 mol/L） did not affect the activation and inactivation process of Ito. The activation curve of Idr was shifted toward positive potential, but 10^-5 mol/L Cu^2＋did not affect slope factor. According to these results, it was considered that Cu^2＋at low concentration in the bath solution could promote Ito and Idr while at high concentration could inhibit them, and change of amplitude was different with different membrane voltage. Conclusion was drawn： Cu^2＋may be involved in the pathophysiologic mechanism of diseases with neuropathological components.     

Pulse radiolysis study of the oxidation of the I<Superscript>−</Superscript> ions with the radical anions Br<Subscript>2</Subscript><Superscript>·−</Superscript> in an aqueous solution: formation and properties of the radical anion BrI<Superscript>·−</Superscript>

The radiation chemical redox transformations in solutions of bromides in the presence of minor additives of iodides were studied by pulse radiolysis. The change in the concentrations of the Br⁻ and I⁻ ions changes the ratio of the formed short-lived radical anions Br₂ ^·−, BrI^·−, and I₂ ^·−. The spectrum of the mixed radical anion BrI^·− contains a broad optical band at 370 nm with ɛ₃₇₀ = 9650 L mol⁻¹ cm⁻¹. The reduction potential of the BrI^·−/Br⁻, I⁻ pair is 1.25 V. The rate constants for the forward and backward reactions Br₂ ^·− + I⁻ ⇌ BrI^·− + Br⁻ are k _f = 4.3·10⁹ and k _r = 1.0·10⁵ L mol⁻¹ s⁻¹, respectively; for the reactions BrI^·− ⇌ Br⁻ + I^·, k _f = 5.7·10⁸ s⁻¹ and k _r = 1.0·10¹⁰ L mol⁻¹ s⁻¹. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1787–1792, September, 2008.     

d-xylose transport by Candida succiphila and Kluyveromyces marxianus

The kinetics and regulation of d-xylose uptake were investigated in the efficient pentose fermentor Candida succiphila, and in Kluyveromyces marxianus, which assimilate but do not ferment pentose sugars. Active high-affinity (K _m ∼ 3.8 mM; V _max ∼ 15 nmol/[mg·min]) and putative facilitated diffusion low-affinity (K _m ∼ 140 mM; V _max ∼ 130 nmol/[mg·min]) transport activities were found in C. succiphila grown, respectively, on xylose or glucose. K. marxianus showed facilitated diffusion low-affinity (K _m ∼ 103 mM; V _max ∼ 190 nmol/[mg·min]) transport activity when grown on xylose under microaerobic conditions, and both a low-affinity and an active high-affinity (K _m ∼ 0.2 mM; V _max ∼ 10 nmol/[mg·min]) transport activity when grown on xylose under fully aerobic conditions.     

Colloidal magnesium aluminum hydroxide and heterocoagulation with a clay mineral. I. Properties of colloidal magnesium aluminum hydroxide

Magnesium aluminum hydroxide, the most important member of layered double hydroxides, was peptized by intense washing. The particle diameter, 70–130 nm, depended on the temperature of aging the parent material. The electrophoretic mobility of the particles decreased with increasing pH, from 3.7 × 10⁻⁸ m²/Vs at pH 5 to 0.5 × 10⁻⁸ m²/Vs at pH 12.3. An isoelectric point at pH∼7 was reached with the addition of 87 mmol/l NaSCN, 3 mmol/l Na₂SO₄ and Na₂CO₃, and 0.7 mmol/l Na₂HPO₄. The critical coagulation concentration for the 2% (w/w) dispersion was 88 mmol/l NaCl, 1.8 mmol/l Na₂CO₃, 1.4 mmol/l Na₂SO₄, and 1.2 mmol/l Na₂HPO₄ at pH∼7. The 2% dispersion at pH∼7 showed an almost Newtonian flow behavior. Yield values were developed after salt addition. The 2% dispersion reached a yield value of 2 Pa at 100 mmol/l NaCl, 3 Pa at 100 mmol/l Na₂SO₄, and 5 Pa at 100 mmol/l Na₂CO₃. Sodium phosphate in comparison with the other salts showed a liquefying effect. The yield value increased to 3 Pa at 1–10 mmol/l Na₂HPO₄ and decreased to 0.5 Pa at 100 mmol/l Na₂HPO₄. Received: 28 February 2001 Accepted: 8 March 2001     

Crystal and molecular structure and properties of [Ni(4-NH2Py)2(iso-Bu2PS2)2] and [Co(4-NH2Py)(iso-Bu2PS2)2]

The interaction of the Co(iso-Bu₂PS₂)₂ chelate with 4-NH₂Py afforded a paramagnetic complex [Co(4-NH₂Py)(iso-Bu₂PS₂)₂] (μ_eff = 4.53 BM). Single crystals of [Ni(4-NH₂Py)₂(iso-Bu₂PS₂)₂] (I) and [Co(4-NH₂Py)(iso-Bu₂PS₂)₂] (II) were grown and used for X-ray diffraction investigation (X8 APEX diffractometer, MoK _α radiation). Crystals I are monoclinic with unit cell parameters a = 12.5336(5) Å, b = 9.4356(4) Å, c = 16.4095(6) Å; β = 111.351(1)°; V = 1807.4(1) ų; Z = 2, ρ = 1.223 g/cm³, space group P2₁/n. Crystals II are triclinic with unit cell parameters a = 8.7572(4) Å, b = 9.6934(6) Å, c = 18.665(1) Å; α = 79.374(2)°, β = 87.049(2)°, γ = 75.640(2)°; V = 1508.6(1) ų; Z = 2, ρ = 1.259 g/cm³; space group . The structures of I and II are formed by isolated mononuclear molecules. The coordination unit is NiN₂S₄ (octahedron) in I and CoNS₄ (tetragonal pyramid) in II. The 4-NH₂Py molecule is coordinated through the N atom of the heterocycle. Electronic spectroscopy data for II agree with the symmetry of the NS₄ polyhedron found by X-ray diffraction (XRD) analysis. The noncoordinated amine groups link the complex molecules via N-H...S hydrogen bonds. __________ Translated from Zhurnal Strukturnoi Khimii, Vol. 46, No. 6, pp.1072–1080, November–December, 2005. Original Russian Text Copyright ? 2005 by T. E. Kokina, L. A. Glinskaya, E. A. Sankova, R. F. Klevtsova, and S. V. Larionov     

Synthesis and properties of M2V8O21 (M = K, Tl) octavanadates and K2?xTlxV8O21 (0 ≤ x ≤ 2) solid solutions

Formation parameters and properties of M₂V₈O₂₁ octavanadates where M = K and Tl were studied using X-ray powder diffraction, microscopy, thermogravimetry, vibrational spectroscopy, EPR, and voltammetry. Original synthesis processes for these compounds were developed; their thermal stability parameters were determined. Potassium and thallium(I) octavanadates were shown to form complete solid solutions with each other. Potassium octavanadate in air is stable to 450°C; above this temperature, it transforms, on account of partial reduction of vanadium, to vanadium bronze by the reaction K₂V₈O₂₁ ↔ K₂V₈O_20.8 + 0.1O₂. The K₂V₈O_20.8 percentage in the sample increases with rising temperature. The substitution of small thallium amounts for potassium (x ≥ 0.025) enhances the stability of the phase until it melts at ∼525°C.