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1.
Nine new polynuclear 2-(5-tetrazolyl)ethyl podands have been obtained by the azidation of the corresponding nitriles. Using Bjerrum distribution functions, the values of pK a 1, pK a 2, pK a 3, and pK a 4 have been determined by a potentiometric method for 14 polynuclear tetrazoles in aqueous and aqueous methanolic solution. The found values lie in the range from 3.5 to 7.5 pH units. The overall rules and the sequence of the ionization of the spatially separated tetrazole fragments in these podand systems are discussed.  相似文献   

2.
The intrinsic acid‐base properties of the hexa‐2′‐deoxynucleoside pentaphosphate, d(ApGpGpCpCpT) [=(A1?G2?G3?C4?C5?T6)=(HNPP)5?] have been determined by 1H NMR shift experiments. The pKa values of the individual sites of the adenosine (A), guanosine (G), cytidine (C), and thymidine (T) residues were measured in water under single‐strand conditions (i.e., 10 % D2O, 47 °C, I=0.1 M , NaClO4). These results quantify the release of H+ from the two (N7)H+ (G?G), the two (N3)H+ (C?C), and the (N1)H+ (A) units, as well as from the two (N1)H (G?G) and the (N3)H (T) sites. Based on measurements with 2′‐deoxynucleosides at 25 °C and 47 °C, they were transferred to pKa values valid in water at 25 °C and I=0.1 M . Intramolecular stacks between the nucleobases A1 and G2 as well as most likely also between G2 and G3 are formed. For HNPP three pKa clusters occur, that is those encompassing the pKa values of 2.44, 2.97, and 3.71 of G2(N7)H+, G3(N7)H+, and A1(N1)H+, respectively, with overlapping buffer regions. The tautomer populations were estimated, giving for the release of a single proton from five‐fold protonated H5(HNPP)±, the tautomers (G2)N7, (G3)N7, and (A1)N1 with formation degrees of about 74, 22, and 4 %, respectively. Tautomer distributions reveal pathways for proton‐donating as well as for proton‐accepting reactions both being expected to be fast and to occur practically at no “cost”. The eight pKa values for H5(HNPP)± are compared with data for nucleosides and nucleotides, revealing that the nucleoside residues are in part affected very differently by their neighbors. In addition, the intrinsic acidity constants for the RNA derivative r(A1?G2?G3? C4?C5?U6), where U=uridine, were calculated. Finally, the effect of metal ions on the pKa values of nucleobase sites is briefly discussed because in this way deprotonation reactions can easily be shifted to the physiological pH range.  相似文献   

3.
The microscopic ionization behavior of piroxicam was investigated using two different approaches, i.e., direct UV spectroscopy and an indirect analogue approach (deductive method). The best microscopic pKa values (pKa12 = 4.60, pKa21 = 5.40, pKa22 = 2.72, and pKa11 = 1.92) were obtained by the deductive method using as pKa22 the pKa of the enolic O-methylated piroxicam 2 . The results show remarkable electrostatic effects in the protonation/deprotonation equilibria, a marked increase in the acidity of the enolic function (2.68 pKa units) being caused by the pyridinium group. The electronic structure of piroxicam was studied based on 1H-NMR chemical shifts at various ionization states, indicating an extended electron conjugation through the molecule. The partition measurements in octan-1-ol/H2O of zwitterionic compound 3 (the pyridyl N-methyl derivative of piroxicam ( 1 )) suggest that the two opposite charges in zwitterionic piroxicam are indeed in a close intramolecular proximity.  相似文献   

4.
1,3-Diaryl-2-propen-1-ones, I, reacted with guanidine hydrochloride (II) in the presence of 3 moles of sodium hydroxide to give the corresponding 2-amino-4,6-diarylpyrimidines, III. The structure and configuration of the products are based on chemical and spectroscopic evidence. The protonation constants of these compounds (series A and series B) have been determined in 50 volume percent ethanol-water medium. Excellent linear correlations are obtained when pKa values of the two series of 2-amino-4,6-diarylpyrimidines, IIIa-j and IIIk-r, are plotted against the substituent constant, σx, and the polar substituent constant, σ* xC6H4, for substituted phenyl groups. The pKa values have also been correlated with the extended Hammett equation. The correlation follows the equations: Series A; pKa = 3.273 - 0.820σI,X - 0.662σR,X Series B; pKa = 3.169 - 0.424σI,X - 0.137σR,X  相似文献   

5.
Annelated derivatives of 2-phenylquinoline, 2-(2′-pyridyl)quinoline, and 2-phenyl-1,8-naphthyridine have been prepared where the bridging unit contains from one to four methylene units. The conformational properties of these molecules have been analyzed by 1H nmr and uv spectroscopy as well as by pKa determinations.  相似文献   

6.
The effect of the macrocyclic host, cucurbit[7]uril (CB7), on the photophysical properties of the 2‐(2′‐hydroxyphenyl)benzimidazole (HPBI) dye have been investigated in aqueous solution by using ground‐state absorption and steady‐state and time‐resolved fluorescence measurements. All three prototropic forms of the dye (cationic, neutral, and anionic) form inclusion complexes with CB7, with the largest binding constant found for the cationic form (K≈2.4×106 M ?1). At pH≈4, the appearance of a blue emission band upon excitation of the HPBI cation in the presence of CB7 indicates that encapsulation into the CB7 cavity retards the deprotonation process of the excited cation, and hence reduces its subsequent conversion to the keto form. Excitation of the neutral form (pH≈8.5), however, leads to an increase in the keto form fluorescence, indicating an enhanced excited‐state intramolecular proton‐transfer process for the encapsulated dye. In both the ground and excited states, the two pKa values of the HPBI dye show upward shifts in the presence of CB7. The prototropic equilibrium of the CB7‐complexed dye is represented by a six‐state model, and the pH‐dependent changes in the binding constants have been analyzed accordingly. It has been observed that the calculated pKa values using this six‐state model match well with the values obtained experimentally. The changes in the pKa values in the presence of CB7 have been corroborated with the modulation of the proton‐transfer process of the dye within the host cavity.  相似文献   

7.
Heterylation of 3-R1-5-R2-1'2'4-triazoles (pK a 3-12) with N-alkyl-, N-alkenyl-, N-alkoxy-carbonyl-, N-oxoalkyl-, N-nitroxyalkyl, N-nitroaminoalkyl-3'5-dinitro-1'2'4-triazoles results insubstitution of a nitro group in 5 position of the dinitro compound yielding 1-R-methyl-3-nitro-5-(3-R1-5-R2-1,2,4-triazolyl)-1,2,4-triazoles. The side processes: Hydroxide-ion attack on C5 and (or) N1 of the ring both in the substrate and in the target compound afford 1-R-methyl3-nitro-1,2,4-triazol-5-ones, 3,5-dinitro-1,2,4-triazole and NH-acids of N-C-bitriazole series. Optimal reaction media are aprotic dipolar substances, and for compounds prone to heterolysis ethyl acetate-water systems. The azole pK a is the decisive factor controlling the composition and the ratio of reaction products. The process is promising for azoles with pK a > 5, and the optimal range of pK a is 8-10.  相似文献   

8.
A useful 2J(N?H) coupling‐based NMR spectroscopic approach is proposed to unveil, at the molecular level, the contribution of the imidazole groups of histidines from RNA/DNA‐binding proteins on the modulation of binding to nucleic acids by pH. Such protonation/deprotonation events have been monitored on the single His96 located at the second RNA/DNA recognition motif (RRM2) of T‐cell intracellular antigen‐1 (TIA‐1) protein. The pKa values of the His96 ionizable groups were substantially higher in the complexes with short U‐rich RNA and T‐rich DNA oligonucleotides than those of the isolated TIA‐1 RRM2. Herein, the methodology applied to determine changes in pKa of histidine side chains upon DNA/RNA binding, gives valuable information to understand the pH effect on multidomain DNA/RNA‐binding proteins that shuttle among different cellular compartments.  相似文献   

9.
The protonation‐deprotonation equilibrium of 6‐benzylaminopurine (6‐BAP) and its derivatives was studied by potentiometry and voltammetry. The effect of Cl‐ or OCH3‐group in position 2′, 3′ and 4′ of the benzene ring of 6‐BAP on both pKa values was investigated. To determine the enthalpy and entropy, the temperature dependence of pKa was employed. It was found that with increasing temperature the pKa decreased. In comparison with 6‐BAP the chloro‐ or methoxy‐ group resulted in pKa increase. The first pKa values were also determined by linear sweep (LSV) and elimination voltammetry with linear scan (EVLS). New approaches were shown not only for the determination of pKa from voltammetric titration curves but also for the evaluation of the reduction processes of benzylaminopurines.  相似文献   

10.
Bromophilic attack by the transition metal carbonyl anion, [Re(CO)5]Na (pKa = 21.1), on 2-(1-bromoalkylidene)thiazolidin-4-ones is significantly faster than abstraction of an acidic lactam hydrogen (pKa ∼17-18), when the generated carbanion is stabilized by an α-CN or α-PhCO group. The bromophilic reaction of 2-(1-bromoalkylidene)thiazolidin-4-one, having an α-CN electron-withdrawing group, resulted in formation of a new metallacyclic anionic complex. With less reactive vinyl bromides, containing an α-CONHPh or α-CO2Et group, only deprotonation is observed. The role of the metal carbonyl anion is highlighted by a comparison with the 9-methylfluorenide carbanion (pKa of 9-methylfluorene is 22.3), which reacts exclusively via a deprotonation pathway.  相似文献   

11.
Base hydrolysis of N-acetylpyrrole ( 1a ) involves formation of an anionic tetrahedral intermediate ( 2a ). The equilibrium constant between these two species can be estimated by extrapolation based on the equilibrium constants for hydration of N-trichloroacetyl- and N-trifluoroacetylpyrrole and the estimated pKa for deprotonation of the hydrates of these compounds. Inductive effects upon hydration and deprotonation of the hydrates were estimated by analogy with inductive effects upon the equilibrium reactions of chloral and acetaldehyde. The free energies of activation for formation and return of 2a are approximately 16 and 12.5 Kcal mole−1 respectively and for conversion of 2a to products 11 Kcal mole−1 in aqueous 1M OH.  相似文献   

12.
The H-bonded complexes between 2-dimethylamino-3,3-dimethyl-1-azirine (TMAAZ) and some phenol derivatives have been studied by IR spectrometry in carbon tetrachloride. The equilibrium constants at 300, 315 and 328 K and the ? ΔH°, ? ΔνOH and ΔνCN values have been determined. The comparison with previously studied OH … N complexes shows that TMAAZ forms stronger complexes than would be expected from the pKa value. These facts are discussed in terms of steric, hybridization, solvation and charge transfer effects. From a pKa of 8, the IR spectrum shows the appearance of ion pairs N+-H …?O. A predominance of protonated species can be calculated for a pKa of 6. This can explain why 1,2-ring cleavage whose first step is probably the formation of an azirinium cation, occurs for acids characterized by pKa values lower than 5 but not for enolizable ketones having pKa values higher than 9.  相似文献   

13.
A viologen derivative carrying a benzimidazole group ( V-P-I 2+; viologen–phenylene–imidazole V-P-I ) can be dimerized in water using cucurbit[8]uril (CB[8]) in the form of a 2:2 complex resulting in a negative shift of the guest pKa, by more than 1 pH unit, contrasting with the positive pKa shift usually observed for CB-based complexes. Whereas 2:2 complex protonation is unclear by NMR, silver cations have been used for probing the accessibility of the imidazole groups of the 2:2 complexes. The protonation capacity of the buried imidazole groups is reduced, suggesting that CB[8] could trigger proton release upon 2:2 complex formation. The addition of CB[8] to a solution containing V-P- I 3+ indeed released protons as monitored by pH-metry and visualized by a coloured indicator. This property was used to induce a host/guest swapping, accompanied by a proton transfer, between V-P-I 3+ ⋅ CB[7] and a CB[8] complex of 1-methyl-4-(4-pyridyl)pyridinium. The origin of this negative pKa shift is proposed to stand in an ideal charge state, and in the position of the two pH-responsive fragments inside the two CB[8] which, alike residues engulfed in proteins, favour the deprotonated form of the guest molecules. Such proton release triggered by a recognition event is reminiscent of several biological processes and may open new avenues toward bioinspired enzyme mimics catalyzing proton transfer or chemical reactions.  相似文献   

14.
Risedronate inhibits bone resorption in diseases like osteoporosis, Paget’s disease, tumor bone diseases or the malfunction of phosphocalcium metabolism. The acid-base properties of risedronate in an aqueous solution have been studied in a pH range from 2 to 12 and can be described in terms of four dissociation steps: pK a,2, pK a,4, pK a,5 (related to the dissociation of POH groups) and pK a,3 related to the dissociation of protonated amino group NH3 +. The mixed dissociation constants were determined at different ionic strengths I = 0.02 to 0.20 mol dm−3 KCl and of 25°C and 37°C using pH-spectrophotometric and pH-potentiometric titration methods. Determination of group parameters L 0, H T might lead to false estimates of common parameters p K a;therefore, the computational strategy employed is important. A comparison between the two programs ESAB and HYPERQUAD demonstrated that the ESAB program provides a better fit of potentiometric titration curve. The thermodynamic dissociation constants pK aT were estimated by a nonlinear regression of (pK a, I) data and a Debye-Hückel equation at 25°C and 37°C, pK a,2T = 2.37(1) and 2.44(1), pK a,3T = 6.29(3) and 6.26(1), pK a,4T = 7.48(1) and 7.46(2) and pK a,5T = 9.31(7) and 8.70(3) at 25°C and 37°C using pH-spectroscopic data and pK a,2T = 2.48(3) and 2.43(1), pK a,3T= 6.12(2) and 6.10(2), pK a,4T = 7.25(2) and 7.23(1) and pK a,5T = 12.04(5) and 11.81(2) at 25°C and 37°C. The ascertained estimates of three dissociation constants pK a,3, pK a,4, pK a,5 are in agreement with the predicted values obtained using PALLAS   相似文献   

15.
The physicochemical properties of polymeric complexing sorbents with the o,o′-dihydroxy-(1-azo-1′)-functional complexing group were studied. Optimal sorption parameters for chromium(III) and manganese(II) (medium acidity, temperature, time), the sorption capacity of the sorbents, and the stability constants (pK a′)of the complexes were determined. Correlation relations were established between the dissociation constants (pK a′) of the sorbent functional group and pH50 of the complexation of the considered elements; between and the stability of the formed complexes (logβ); and between pK a′ and the deprotonation energy (E depr) of the sorbent hydroxy group located in the para-position to the introduced substituent. The established correlation relations are useful for targeted prediction of the physicochemical parameters of the sorbents, their complexes, and the sorption of manganese(II) and chromium(III) aimed at recovery and pre-concentration of these ions from objects with various chemical composition.  相似文献   

16.
At one extreme of the proton‐transfer spectrum in cocrystals, proton transfer is absent, whilst at the opposite extreme, in salts, the proton‐transfer process is complete. However, for acid–base pairs with a small ΔpKa (pKa of base ? pKa of acid), prediction of the extent of proton transfer is not possible as there is a continuum between the salt and cocrystal ends. In this context, we attempt to illustrate that in these systems, in addition to ΔpKa, the crystalline environment could change the extent of proton transfer. To this end, two compounds of salicylic acid (SaH) and adenine (Ad) have been prepared. Despite the same small ΔpKa value (≈1.2), different ionization states are found. Both crystals, namely adeninium salicylate monohydrate, C5H6N5+·C7H5O3?·H2O, I , and adeninium salicylate–adenine–salicylic acid–water (1/2/1/2), C5H6N5+·C7H5O3?·2C5H5N5·C7H6O3·2H2O, II , have been characterized by single‐crystal X‐ray diffraction, IR spectroscopy and elemental analysis (C, H and N) techniques. In addition, the intermolecular hydrogen‐bonding interactions of compounds I and II have been investigated and quantified in detail on the basis of Hirshfeld surface analysis and fingerprint plots. Throughout the study, we use crystal engineering, which is based on modifications of the intermolecular interactions, thus offering a more comprehensive screening of the salt–cocrystal continuum in comparison with pure pKa analysis.  相似文献   

17.
Using spectrophotometric methods, the protopysis constant of the 5.ClDMPAP reagent (pKa1 = −0.19; pKa2 = 1.97; pKa3 = 11.98) and the stability constant of its vanadic complex (6.0 ± 0.11) × 1014 were determined. A high-sensitivity spectrophotometric method was developed to determine V(V) using 0.1–1.2 ppm and pH = 3.8. ε586 = 55,300 ± 400 liters · mol−1 · cm−1. A study on the most important interferences and the way to eliminate them was carried out. The method can be applied to the determination of the element in steels and ferrovanadiums.  相似文献   

18.
王蕊林海  林华宽 《中国化学》2007,25(11):1646-1651
Two long-chain multidentate ligands: 2,9-di-(n-2',5',8'-triazanonyl)-1,10-phenanthroline (L^1) and 2,9-di- (n-4',7',10'-triazaundecyl)-1,10-phenanthroline (L^2) were synthesized. The hydrolytic kinetics of 2-hydroxypropyl p-nitrophenyl phosphate (HPNP) catalyzed by the complexes of L^1 or L^2 with La(Ⅲ) or Gd(Ⅲ) have been studied in aqueous solution at (298.2±0.1) K, I=0.10 mol·dm^-3 KNO3 in pH 7.5-9.1, respectively, finding that the catalytic effect of GdL^1 was the best among the four complexes for hydrolysis of HPNP. Its kLnLH-1, kLnLand pKa are 0.047 mol^-1·L·s^-1, 0.000074 mol^-1·L·s^-1 and 8.90, respectively. This paper expounded the studied result with the structure of the ligands and the properties of the metal ions, and deduced the catalysis mechanism.  相似文献   

19.
The deprotonation of 5,7-dihydroxy-2-(4-hydroxyphenyl)chroman-4-one (naringenin) was studied in aqueous solutions of ethanol and 0.1 mol L?1 sodium perchlorate at 25°C. The chemical species that contributed to deprotonation were evaluated together with their pure spectral characteristics and concentration profiles by some chemometric methods. The deprotonation constants assigned by pK 1, pK 2, and pK 3 were determined by multivariate curve analysis of spectral data at different pcH values. The pure spectral analysis concordant with the theoretical prediction of deprotonation constants indicates that the acidity of hydroxyl groups in naringenin decreases in the order: 7-OH, 4′-OH, 5-OH. The effects of the solvent on deprotonation were analysed in terms of the linear solvation energy relationships using the model of Kamlet, Abboud, and Taft (KAT). Multiple linear regressions were aimed towards correlating the deprotonation constants with the microscopic parameters containing hydrogen-bond acidity (α), dipolarity/polarisability (π*), and hydrogen-bond basicity (β). The most significant parameter was found to be the hydrogen-bond acidity of binary mixtures.  相似文献   

20.
We have obtained 5-phenyltetrazol-2-ylalkanoic acids and their derivatives containing terminal nitrile, amide, and tetrazol-5-yl groups. Tetrazolylalkanoic acids with two (pK a 4.93) and three (pK a 5.45) bridging methylene groups are weaker acids than the corresponding ditetrazoles pK a 4.68 and 5.29 respectively). However, the acidity of 5-phenyltetrazol-2-ylacetic acid (pK a 3.12), is higher than acidity of the corresponding ditetrazole (pK a 3.27).  相似文献   

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