A tetrazole-based fluorescence "turn-on" sensor for Al(III) and Zn(II) ions and its application in bioimaging.

A tetrazole derivative 1-[(1H-tetrazol-5-ylimino)methyl]naphthalen-2-ol (H2L) as a fluorescent chemosensor for Al(3+) in DMSO and Zn(2+) in DMF was designed and synthesized. From (1)H NMR data, the Job plot and the ESI-MS spectrum, 1 : 1 stoichiometric complexation between H2L and Al(3+)/Zn(2+) was found in DMSO and DMF, respectively. The theoretical calculations at the level of B3LYP/6-311G** for the ground state and TD-B3LYP/6-311G** for the excited state revealed the sensing mechanism is the inhibition of excited state intramolecular proton transfer (ESIPT). And the possible fluorescent species formed in the DMSO and DMF solutions were deduced to be [Al(HL)(OH)(NO3)(H2O)(DMSO)] and [Zn(HL)(OH)(H2O)(DMF)]. What's more, it is confirmed that H2L could be used to detect Al(3+) and Zn(2+) in cells by bioimaging.

Theoretical investigation of the first-shell mechanism of acetylene hydration catalyzed by a biomimetic tungsten complex.

The reaction mechanism of the hydration of acetylene to acetaldehyde catalyzed by [W(IV)O(mnt)(2)](2-) (where mnt(2-) is 1,2-dicyanoethylenedithiolate) is studied using density functional theory. Both the uncatalyzed and the catalyzed reaction are considered to find out the origin of the catalysis. Three different models are investigated, in which an aquo, a hydroxo, or an oxo coordinates to the tungsten center. A first-shell mechanism is suggested, similarly to recent calculations on tungsten-dependent acetylene hydratase. The acetylene substrate first coordinates to the tungsten center in an η(2) fashion. Then, the tungsten-bound hydroxide activates a water molecule to perform a nucleophilic attack on the acetylene, resulting in the formation of a vinyl anion and a tungsten-bound water molecule. This is followed by proton transfer from the tungsten-bound water molecule to the newly formed vinyl anion intermediate. Tungsten is directly involved in the reaction by binding and activating acetylene and providing electrostatic stabilization to the transition states and intermediates. Three other mechanisms are also considered, but the associated energetic barriers were found to be very high, ruling out those possibilities.

Modeling, preparation, and characterization of a dipole moment switch driven by Z/E photoisomerization.

We report the results of a multidisciplinary research effort where the methods of computational photochemistry and retrosynthetic analysis/synthesis have contributed to the preparation of a novel N-alkylated indanylidene-pyrroline Schiff base featuring an exocyclic double bond and a permanent zwitterionic head. We show that, due to its large dipole moment and efficient photoisomerization, such a system may constitute the prototype of a novel generation of electrostatic switches achieving a reversible light-induced dipole moment change on the order of 30 D. The modeling of a peptide fragment incorporating the zwitterionic head into a conformationally rigid side chain shows that the switch can effectively modulate the fluorescence of a tryptophan probe.

Theoretical studies on pyridoxal 5'-phosphate-dependent transamination of alpha-amino acids.

Density functional methods have been applied to investigate the irreversible transamination between glyoxylic acid and pyridoxamine analog and the catalytic mechanism for the critical [1,3] proton transfer step in aspartate aminotransferase (AATase). The results indicate that the catalytic effect of pyridoxal 5'-phosphate (PLP) may be attributed to its ability to stabilize related transition states through structural resonance. Additionally, the PLP hydroxyl group and the carboxylic group of the amino acid can shuttle proton, thereby lowering the barrier. The rate-limiting step is the tautomeric conversion of the aldimine to ketimine by [1,3] proton transfer, with a barrier of 36.3 kcal/mol in water solvent. A quantum chemical model consisting 142 atoms was constructed based on the crystal structure of the native AATase complex with the product L-glutamate. The electron-withdrawing stabilization by various residues, involving Arg386, Tyr225, Asp222, Asn194, and peptide backbone, enhances the carbon acidity of 4'-C of PLP and Calpha of amino acid. The calculations support the proposed proton transfer mechanism in which Lys258 acts as a base to shuttle a proton from the 4'-C of PLP to Calpha of amino acid. The first step (proton transfer from 4'-C to lysine) is shown to be the rate-limiting step. Furthermore, we provided an explanation for the reversibility and specificity of the transamination in AATase.

Highly efficient and site-selective [3 + 2] cycloaddition of carbene-derived ambident dipoles with ketenes for a straightforward synthesis of spiro-pyrrolidones.

The [3 + 2] cycloaddition reaction of 2-arylthiocarbamoyl benzimidazolium, -imidazolinium, and -triazolium inner salts (the ambident C-C-N and C-C-S 1,3-dipoles derived from carbenes) with ketenes proceeded efficiently in a highly site-selective manner to produce the C-C-N cycloaddition products benzimidazoline-, imidazolidine-, or triazoline spiro-pyrrolidones in 58-93% yields. Theoretical calculation suggests a stepwise mechanism for the reaction and indicates that the C-C-N cycloaddition of the dipoles with ketenes is both a dynamically and thermodynamically favored reaction pathway. Their easy availability, high reactivity, and reaction selectivity render the benzimidazolium, -imidazolinium, and -triazolium inner salts powerful and versatile 1,3-dipoles in the construction of novel spiro heterocyclic systems, which are not easily accessible by other methods.

Water-assisted transamination of glycine and formaldehyde.

A computational study on the transamination reaction of molecular complexes that consist of NH2CH2COOH + CH2O + nH2O, where n = 0, 1, 2, is presented. This work has allowed the description of the geometries of all the intermediates and transition states of the reactions, which can be described by five steps: carbinolamine formation, dehydration, 1,3 proton transfer, hydrolysis, and carbinolamine elimination. Among the five steps of the reaction, hydrolysis and elimination occur with the existence of general acid catalysis related to the carboxylic group. The water molecules can be involved in the reaction by performing as a proton-transfer carrier and a stabilizing zwitterion. It can be predicted from our calculations that in the transamination between alpha-amino acids and alpha-keto acids, the carbinolamine is formed with small barrier or even barrierless while the dehydration occurs easily at room temperature. However, without heating the 1,3 proton transfer could not occur as the barrier is 26.7 kcal/mol relative to the reactant complex when including two water molecules. Our results are in good agreement with experimental conclusions.

Combined nonadiabatic transition-state theory and ab initio molecular dynamics study on selectivity of the alpha and beta bond fissions in photodissociation of bromoacetyl chloride.

The selectivity of the alpha C-Cl and beta C-Br bond fissions upon n-->pi(*) excitation of bromoacetyl chloride has been investigated with combined nonadiabatic Rice-Ramsperger-Kassel-Marcus theory and ab initio molecular dynamics calculations, which are based on the potential energy profiles calculated with the complete active space self-consistent field and multireference configuration interaction methods. The Zhu-Nakamura [J. Chem. Phys. 101, 10630 (1994); 102, 7448 (1995)] theory is chosen to calculate the nonadiabatic hopping probability. It is found that nonadiabatic effect plays an important role in determining selective dissociations of the C-Cl and C-Br bonds. The calculated rate constants are close to those from experimentally inferred values, but the branching ratio of the alpha C-Cl and beta C-Br bond fissions is different from the experimental findings. The direct molecular dynamics calculations predict that fission of the C-Cl bond occurs on a time scale of picoseconds and cleavage of the beta C-Br bond proceeds with less probability within the same period. This reveals that the initial relaxation dynamics is probably another important factor that influences the selectivity of the C-Cl and C-Br bond fissions in photodissociation of BrCH(2)COCl at 248 nm.

Insights into the photochemical processes of ClC(O)SCl from ab initio calculations.

All possible unimolecular processes upon photolysis of ClC(O)SCl in the UV-visible region have been characterized in the present paper through the optimized stationary structures and computed potential-energy profiles of the S0, S1, T2, and S2 states with the MP2, B3LYP, CASSCF, and MR-CI methods in conjugation with the cc-pVDZ basis set. Upon photoexcitation in the range of 300-400 nm, the ClC(O)SCl molecules are excited to the S1 state. From this state, the dissociation into ClC(O)S + Cl takes place immediately and subsequently Cl2 and SCO are formed. The C-Cl and C-S bond fissions that start from the S2 state are the dominant channels upon photodecomposition of ClC(O)SCl in the gas and condensed phases in the wavelength range of 200-248 nm. The formed Cl, C(O)SCl, ClCO, and SCl radicals are very reactive, and the Cl2, SCO, CO, and SCl2 molecules are subsequently produced as stable products in the condensed phase.

Striving to understand the photophysics and photochemistry of thiophosgene: a combined CASSCF and MR-CI study.

The potential energy surfaces for Cl(2)CS dissociation into ClCS + Cl in the five lowest electronic states have been determined with the combined complete active space self-consistent field (CASSCF) and MR-CI method. The wavelength-dependent photodissociation dynamics of Cl(2)CS have been characterized through computed potential energy surfaces, surface crossing points, and CASSCF molecular dynamics calculations. Irradiation of the Cl(2)CS molecules at 360-450 nm does not provide sufficient internal energy to overcome the barrier on S(1) dissociation, and the S(1)/T(2) intersection region is energetically inaccessible at this wavelength region; therefore, S(1) --> T(1) intersystem crossing is the dominant process, which is the main reason S(1)-S(0) fluorescence breaks off at excess energies of 3484-9284 cm(-1). Also, the S(1) --> T(2) intersystem crossing process can take place via the S(1)-T(2) vibronic interaction in this range of excess energies, which is mainly responsible for the quantum beats observed in the S(1) emission. Both S(2) direct dissociation and S(2) --> S(3) internal conversion are responsible for the abrupt breakoff of S(2)-S(0) fluorescence at higher excess energies. S(2) direct dissociation leads to the formation of the fragments of Cl(X(2)P) + ClCS(A(2)A' ') in excited electronic states, while S(2) --> S(3) internal conversion followed by direct internal conversion to the ground electronic state results in the fragments produced in the ground state.