Novel indole based colorimetric and "turn on" fluorescent sensors for biologically important fluoride anion sensing

A novel indole Hydrazone receptor 1 has been synthesized by one step of condensation, which can act as an efficient colorimetric and "turn on" fluorescent sensor for fluoride anions; Benesi-Hildebrand equation indicates that 1 associates with F(-) in a 1:1 stoichiometry; [TBA]OH and (1)H NMR titration experiments indicate that the deprotonation process involved upon addition of fluoride anions.     

The rate ladder of proton-coupled tyrosine oxidation in water: a systematic dependence on hydrogen bonds and protonation state

Proton coupled electron transfer (PCET) from tyrosine covalently linked to Ru(bpy)32+ has been studied with laser flash-quench techniques. Two new complexes with internal hydrogen bonding bases to the phenolic proton have been synthesized. Depending on the hydrogen bonding and protonation situation the rate constant of PCET spanned over 5 orders of magnitude and revealed a systematic dependence on pH. This resulted in a previously predicted "rate ladder" scheme: (i) pH dependent concerted electron-proton transfer (CEP) with deprotonation to bulk water, giving low PCET rates, (ii) pH independent CEP with deprotonation to the internal base, giving intermediate PCET rates, and (iii) pure electron transfer from tyrosinate, giving high rates. This behavior is reminiscent of Yz oxidation in Mn-depleted and native photosystem II. The study also revealed important differences in rates between phenols with strong and weak hydrogen bonds, and for the latter a hydrogen bond-gated PCET was observed.     

Anodic oxidation mechanism of 3-arylaminoindoles at a rotating and at a (new type of) pulsing platinum electrode in dipolar aprotic solvents

The oxidation of 2-phenyl-3-arylaminoindoles (1a–e) has been studied in acetonitrile, dimethylformamide and propylene carbonate at a new type of platinum electrode with periodical renewal of the diffusion layer and at a rotating platinum electrode by cyclic voltammetry, d.c. and a.c. voltammetry, controlled potential coulometry and spectrometry (e.s.r. and u.v.). In the examined aprotic solvents and in the presence of a base (like water or diphenylguanidine) amines 1a–e undergo a two-electron oxidation to the corresponding imines. In a protophobic medium (acetonitrile or propylene carbonate) 1a–e are oxidized in two one-electron steps, the first of which leads to the formation of a cation radical (identified by e.s.r. spectrum), oxidizable at more positive potentials (second step). The second electron transfer, however, must be simultaneous with, or be immediately followed by, a very fast deprotonation reaction. The same e.e.c. sequence explains the observed bielectronic step of 1a–e in a protophilic aprotic solvent, like dimethylformamide.     

Absorption and Emission Sensitivity of 2‐(2′‐Hydroxyphenyl)benzoxazole to Solvents and Impurities

2‐(2′‐Hydroxyphenyl)benzoxazole (HBO) is known for undergoing intramolecular proton transfer in the excited state to result in the emission of its tautomer. A minor long‐wavelength absorption band in the range 370–420 nm has been reported in highly polar solvents such as dimethylsulfoxide (DMSO). However, the nature of this species has not been entirely clarified. In this work, we provide evidence that this long‐wavelength absorption band might have been caused by base or metal salt impurities that are introduced into the spectral sample during solvent transport using glass Pasteur pipettes. The contamination by base or metal salt could be avoided by using borosilicate glass syringes or nonglass pipettes in sample handling. Quantum chemical calculations conclude that solvent‐mediated deprotonation is too energetically costly to occur without the aid of a base of an adequate strength. In the presence of such a base, the deprotonation of HBO and its effect on emission are investigated in dichloromethane and DMSO, the latter of which facilitates deprotonation much more readily than the former. Finally, the absorption and emission spectra of HBO in 13 solvents are reported, from which it is concluded that ESIPT is hindered in polar solvents that are also strong hydrogen bond acceptors.     

A unique mechanism for base catalyzed hydrolysis of pentaaminecobalt(III) complexes containing picolyl residues

A novel [Co(pentaamine)Cl](2+) complex having all tertiary amine or pyridine donors has been synthesized (pentaamine = 1,4-bis(2'-pyridyl)-7-methyl-1,4,7-triazacyclononane). This asym-[Co(dmpmetacn)Cl](2+) species has been completely characterized through 1D and 2D NMR studies, and through the X-ray structure for the ZnCl(4)(2)(-) salt. Despite the lack of an activating NH center, remarkably its hydrolysis to [Co(pentaamine)OH](2+) is base catalyzed (k(OH) 0.70 M(-)(1) s(-)(1), 25 degrees C, I = 1.0 M, NaCl). Detailed NMR studies reveal that the base catalyzed substitution leads to the exchange of just one deuterium in one of the two -CH(2)- pyridyl arms, that is approximately trans to the leaving group, and this occurs during and not after base hydrolysis. Quenching experiments for the reaction of asym-[Co(dmpmetacn)Cl](2+) and control experiments on H/D exchange for the product asym-[Co(dmpmetacn)OD](2+) in OD(-) show that each act of deprotonation at the acidic methylene leads to loss of Cl(-). This is the first established case of base catalyzed substitution for a complex where the effective site of deprotonation is at a pyridyl group. A pronounced kinetic isotope effect is observed for the species perdeuterated at the pyridyl methylenes (k(H)/k(D) = 5.0), consistent with rate limiting deprotonation which is a rare event in Co(III) substitution chemistry. The activation afforded by the carbanion is discussed in terms of a new process coined the pseudo-aminate mechanism.     

Photoassisted "gate-lock" fluorescence "turn-on" in a new Schiff base and coordination ability of E-Z isomers

Rapid photoresponse (1.0-8.0 min) through fluorescence "turn-on" signaling displayed by a novel Schiff base (L) creating "gate lock" via intramolecular C-H···N interaction in photoisomerized product (L') has been described. Coordination chemistry of pre- and postirradiated species demonstrated a drastic change in the reactivity which has been supported by NMR, HRMS, UV-vis, emission, electrochemical, and complexation studies.     

Proton diffusion at phospholipid assemblies

A new scanning electrochemical microscopy proton feedback method has been developed for investigating lateral proton diffusion at phospholipid assemblies: specifically Langmuir monolayers at the water/air interface. In this approach, a base is electrogenerated by the reduction of a weak acid (producing hydrogen) at a "submarine" ultramicroelectrode (UME) placed in the aqueous subphase of a Langmuir trough close to a monolayer. The electrogenerated base diffuses to and titrates monolayer-bound protons and is converted back to its initial form, so enhancing the current response at the UME. Local deprotonation of the monolayer creates a concentration gradient for lateral proton diffusion. A numerical model has been developed, taking into account the potential-dependent association/dissociation constant of the interfacial acid groups. A comparison is made of monolayers comprising either acidic DL-alpha-phosphatidyl-L-serine, dipalmitoyl (DPPS) or zwitterionic L-alpha-phosphatidylcholine, dipalmitoyl (DPPC) monolayers at a range of surface pressures. It is demonstrated that lateral proton fluxes at DPPS are significant, but the lateral proton diffusion coefficient is lower than in bulk solution. In contrast, lateral proton diffusion cannot be detected at DPPC, suggesting that the acid/base character of the phospholipid is important in determining the magnitude of interfacial proton fluxes.     

弱碱柔能克刚:动力学去质子官能化反应

碳氢键的去质子官能化反应是碳碳键构建最常用的方法,是一种重要的碳氢键活化方式.近年来,碱催化碳碳键形成反应在含弱酸性碳氢键化合物作为亲核试剂的底物拓展方面取得了重要进展.强碱性试剂或催化剂是实现这些弱酸性碳氢键官能化反应的关键.根据酸碱平衡理论,相对较强的碱才能够对弱酸性碳氢键发生去质子化反应,形成较大浓度的碳负离子中间体,进而发生亲核反应.相对较弱的碱不足以对弱酸性碳氢键进行去质子化反应,然而尽管碳负离子中间体可能浓度很低,但应该仍然存在于反应体系中.如果可以选择性地进行热力学有利的化学转化,碳负离子中间体的浓度将会下降并引起去质子化平衡的重新构建.结合碳负离子中间体不可逆的转化和去质子平衡的重新构建,弱酸性碳氢键就可以在弱碱条件下实现缓慢却持续不断的去质子官能化反应.为区别于强碱条件下、通过热力学稳定碳负离子中间体的传统碳氢键去质子官能化反应,我们将这种在弱碱条件下、通过热力学不利的碳负离子中间体转化和酸碱平衡重新构建实现的弱酸性碳氢键的官能化反应称为动力学去质子官能化反应.本文总结了碳氢键去质子官能化反应研究现状和本研究团队近年来在弱碱条件下的动力学去质子官能化反应研究进展.     

Electric-Field Effects in 13-Demethyl-11,14-Epoxyretinal-Bacteriorhodopsin Films

Abstract— We report the results of experiments on the application of electric fields across thin, dry Alms of a bacteriorho-dopsin (BR) analog pigment in which the retinal chro-mophore has been replaced with 13-demethyl-11,14-epoxyretinal. As previously observed in other BR variants with low Schiff-base pK values, this pigment exhibits protonation and deprotonation of the Schiff base under an applied electric field, depending on the initial Schiff-base protonation state or effective pH. At low effective pH, a fast (<200 μs) deprotonation reaction dominates. At high pH, an apparently different mechanism leads to Schiff-base protonation on the time scale of seconds. Comparison of our results with a simple model suggests that the external field causes a shift in the pK of the Schiff base by1–2 pH units.     

Cesium carbonate as a mediated inorganic base in some organic transformations

Cesium carbonate is a type of alkali carbonate salt having a remarkable number of applications and has been demonstrated to be a mild inorganic base in organic synthesis. It has received much attention for its utility in C, N, O alkylation and arylation reactions. Cesium carbonate not only promotes successful carbonylation of alcohols and carbamination of amines, but also suppresses common side reactions traditionally encountered with other protocols. It is used also in six-membered annulation, intramolecular and intermolecular cyclizations, Suzuki coupling, aza-Henry, nucleophilic substitution, cross coupling and different cycloaddition reactions.     

Calcium carbonate phase transformations during the carbonation reaction of calcium heavy alkylbenzene sulfonate overbased nanodetergents preparation

The preparation and application of overbased nanodetergents with excess alkaline calcium carbonate is a good example of nanotechnology in practice. The phase transformation of calcium carbonate is of extensive concern since CaCO(3) serves both as an important industrial filling material and as the most abundant biomineral in nature. Industrially valuable overbased nanodetergents have been prepared based on calcium salts of heavy alkylbenzene sulfonate by a one-step process under ambient pressure, the carbonation reaction has been monitored by the instantaneous temperature changes and total base number (TBN). A number of analytical techniques such as TGA, DLS, SLS, TEM, FTIR, and XRD have been utilized to explore the carbonation reaction process and phase transformation mechanism of calcium carbonate. An enhanced understanding on the phase transformation of calcium carbonate involved in calcium sulfonate nanodetergents has been achieved and it has been unambiguously demonstrated that amorphous calcium carbonate (ACC) transforms into the vaterite polymorph rather than calcite, which would be of crucial importance for the preparation and quality control of lubricant additives and greases. Our results also show that a certain amount of residual Ca(OH)(2) prevents the phase transformation from ACC to crystalline polymorphs. Moreover, a vaterite nanodetergent has been prepared for the first time with low viscosity, high base number, and uniform particle size, nevertheless a notable improvement on its thermal stability is required for potential applications.     

DFT/TDDFT investigation of the stepwise deprotonation in tetracycline: pK<Subscript>a</Subscript> assignment and UV–vis spectroscopy

Tetracyclines are a class of derivatives of polycyclic naphthacene carboxamide, which have attracted wide interest in the pharmaceutical field for their use as antibiotics. These molecules are characterized by a substantial conformational flexibility and by the presence of different binding sites which endow tetracycline with a noticeable capability in binding biological targets. A salient property of tetracyclines is the presence of multiple acidic groups: four equilibrium constants have been measured for the fully protonated tetracycline (TCH₃ ⁺) but so far no clear information concerning the pK_as of the various sites has been reported. We present here a computational investigation on the correlation between the acid–base and the spectroscopic properties of this important class of compounds. Starting from the TCH₃ ⁺ species, the pK_a of all the possible deprotonation sites has been computed by DFT calculations. The computed pK_as nicely compare with the experimental data, within 1 pK_a unit, allowing us to individuate the products of the first deprotonation. This procedure has been iteratively repeated using as starting species the products singled out from the previous deprotonation, thus individuating the stepwise products of each deprotonation step. Then, the optical absorption spectra have been computed for all the species involved in the protonation/deprotonation equilibria, comparing the results with the experimental data. The good agreement between theory and experiment has allowed us to rationalize the correlation between the solution pH and the absorption spectra.     

STM-based molecular detection of "catch-and-release" of protons for bipyridine bound to phenylene-ethynylene thiol

The protonation/deprotonation response of a novel bipyridine containing (phenylene-ethynylene) thiol adsorbed to a Au surface was investigated with scanning tunneling microscopy (STM), showing reversible changes in the average heights (approximately 50 spots) and the height distribution arising from protonation/deprotonation.     

Acid-base and spectroelectrochemical properties of doubly N-confused porphyrins

The cis-doubly N-confused porphyrin, H2N2CP, containing two adjacent confused pyrrole rings has been investigated from the point of view of its acid-base and electrochemical behavior in dichloromethane. This novel porphyrin isomer can form two metal-carbon bonds in the central core, stabilizing metal ions in unusually high oxidation states. Furthermore, the two outside N-pyrrole atoms remain available for acid-base and specific solvent interactions. Protonation of the pyrrole N atoms proceeds according to two successive steps, while only a single deprotonation step has been observed in the presence of bases. Similarly, in the case of the silver and copper complexes the protonation and deprotonation of the outer pyrrole rings have been detected, confirming the structure of the metalated species as M(III)-HN2CP. The electrochemical reduction of the metal ions (III/II redox process) and oxidation of the macrocycle ring have been detected respectively at -0.9 and 1.4 V based on spectroelectrochemical measurements in conjunction with the acid/base equilibrium studies. Additional waves observed around -0.5 and 1.3 V have been assigned to redox processes involving water molecules associated with the doubly N-confused porphyrins.     

THE EFFECT OF IONIC STRENGTH AND pH ON THE PROTONATED SCHIFF BASE AND TYROSINE DEPROTONATION KINETICS DURING THE BACTERIORHODOPSIN PHOTOCYCLE

Abstract— The deprotonation kinetics of tyrosine and the protonated Schiff base during the bacteriorho-dopsin photocycle were studied under different perturbations by transient absorption spectroscop Native purple membrane, as well as samples which were deionized (blue) then restored with Na⁺ or La³⁺ were used at pH's ranging from 7 to 10 at very low salt concentrations. The results were compared with previous studies at higher ionic strength. The important conclusions can be summarized as follows: (a) The rate constants of both the Schiff base and tyrosine deprotonation are not very sensitive to the changes of conditions. (b) An almost linear relationship is observed between the relative amplitudes of the tyrosine deprotonated during the cycle and the slow component of the Schiff base deprotonation under the different perturbations studied. This was taken to support the two site model for the protonated Schiff base, one near tyrosine and the other near its ionized form. (c) The pK_a value determined from the ratio of the amplitude of the fast to the slow component of the Schiff base deprotonation is found to decrease with increasing ionic strength of the medium. At extremely low ionic strength, it was found to equal that of the tyrosine phenolic group in solution.     

Methodology for the preparation of C1-monoalkylated 1,2-dihydro[C70)] derivatives: formation of the "other" regioisomer

Deprotonation of 1,2-C(70)H(2) with TBAOH, followed by alkylation with methyl bromoacetate, results in formation of a C1-monoalkylated 1,2-dihydro-C(70) derivative. The position of the alkyl group (C1) was established by NMR spectroscopy and comparison with literature spectra of C2-monoalkylated analogs. Presumably, C1-alkylation is the major process due to selective deprotonation of 1,2-C(70)H(2) at C1. Substitution of benzyl bromide for methyl bromoacetate results in rapid dialkylation, unless the amount of base is carefully controlled, in which case C1-monobenzylation is the major process. This methodology for alkylation at C1 is complimentary to methods for the C2-monoalkylation of C(70) with Zn and methyl bromoacetate.     

Secondary deuterium kinetic isotope effect for aquation, solvolysis, and isomerization reactions of trans-[Co(en)2(OSMe2)N3]2+, and the resolution of a mechanistic anomaly

The two closely spaced NH signals in the (1)H NMR spectrum of trans-[Co(en)(2)(OSMe(2))(N(3))](2+) have been reassigned using 2D NMR and other techniques. Thus, the unusual syn to anti (to Co-N(3)) NH rearrangement on base catalyzed substitution of the selectively deuterated complex in ND(3)(l) has been reinterpreted as "normal", with inversion of the effective deprotonation site accompanying the act of substitution. The re-examination of this system required a repeat study of the secondary isotope effect for the acid hydrolysis reaction, previously used to assign syn and anti amine sites, and this has been extended to other solvents (Me(2)SO, MeCN). The relative NH proton exchange rates are also reconsidered. A systematic rate reduction for Me(2)SO substitution is observed for deuterium incorporation into the cis-NH centers, irrespective of whether these are syn or anti, and the effect is much greater in Me(2)SO than in water. The results are interpreted in terms of zero point energy effects and coupled vibrations.     

Cs2CO3 Promoted coupling reactions for the preparation of skipped diynes

An improved methodology for the preparation of copper(I) alkynides has been developed using cesium carbonate as base. In the presence of cesium carbonate and CuI various 1-alkynes were converted smoothly to the corresponding copper alkynides which in situ reacted with propargylic halides in order to synthesize skipped diynes.     

Catalytic monoalkylation of 1,2-diols

A catalytic monoalkylation of 1,2-diols by using a weak base has been developed. Copper(II) dichloride and boronic acids are effective catalysts for activating 1,2-diols in the presence of potassium carbonate as a base. Various 1,2-diols were selectively monoalkylated with allyl-, benzyl- and alkyl- halides in DMF by choosing a suitable catalyst for each 1,2-diols.     

An efficient synthesis of 3,4-dihydropyrimidin-2(1<Emphasis Type="Italic">H</Emphasis>)-one and 5,6-diphenylpyrimidine derivatives under solvent-free and base conditions

An efficient and convenient Biginelli-like reaction one-pot synthesis of a series of 4-aryl-5,6-diphenyl-3,4-dihydropyrimidin-2(1H)-one and 4-aryl-5,6-diphenylpyrimidine derivatives under solvent-free conditions from the reaction of aromatic aldehydes, 1,2-diphenylethanone, urea, guanidine carbonate or acetamidine hydrochloride has been reported. This methodology has the advantages of short reaction time, mild reaction conditions, easy work-up and environmental friendliness. Moreover, 4-aryl-5,6-diphenylpyrimidine derivatives were first reported in this process. The structures of the title compounds were further determined by X-ray diffraction. More importantly, different from general Biginelli reaction, this reported method was carried out under base conditions.