Site-specific solvation of the photoexcited protochlorophyllide a in methanol: formation of the hydrogen-bonded intermediate state induced by hydrogen-bond strengthening

THEORETICAL STUDY OF THE HYDROGEN BOND CHARACTER BETWEEN THE FNO AND HO2 RADICAL

The hydrogen-bonding characters between FNO and HO2 radical are studied systematically with the MPWB1K method and 6-311++G (d, p), aug-cc-PVDZ as well as aug-cc-PVTZ basis sets. The relevant geometrical characteristics, energy properties, and the characters of the intramolecular hydrogen bonds have been reported in this work. In addition, the changes of electrostatic potential density are presented for further understanding the nature of hydrogen bonds and the preference of F atom as the hydrogen-bonding site.

A MOLECULAR RECOGNITION MODEL FOR ENANTIOSELECTIVITY AND AUTOINDUCTION IN CYANOHYDRIN FORMATION CATALYZED BY CYCLO[(S)-HIS-(S)-PHE]

A molecular recognition mechanism based on dimeric model for cyclic dipeptide Cyclo[(S)-His-(S)-Phe] (abridged CHP) catalyzed autoinduction is proposed according to the inference of previous experimental findings, which is supported by theoretical calculation with Oniom(B3LYP/3-21G*:AM1) method. The most unstable CHP dimer whose intermolecular hydrogen bonds are immensely lessened by two intramolecular hydrogen bonds is defined as the highest active component (IIa) existing in solid among the three possible dimers (Ia, IIa, and IIb). The carbonyl group of benzaldehyde coordinates to CHP dimer (IIa) by a hydrogen bond with Phe–NαH rather than His–NαH and HCN interacted with the imidazole moiety of His residue to form cyanide ion. In view of the theoretical calculation and experimental results, the structures of the nine-ring complexes derived from interaction between catalytic active dimer CHP(IIa) and cyanohydrins were postulated to explain the enantioselective autoinduction: The structure of no nitrile involved six-ring complex derived from interaction between catalytic active dimer CHP(IIa) and cyanohydrins were postulated to explain the elimination of enantioselective autoinduction.

MOLECULAR DYNAMICS SIMULATION ON THE STRUCTURE AND DYNAMICS OF WATER IN THE 1-BUTYL-3-METHYLIMIDAZOLIUM TETRAFLUOROBORATE/WATER MIXTURE

The micro-structure, and IR spectrum of water molecules in 1-butyl-3-methylimi- dazolium tetrafluoroborate( [Bmim]BF4 )/water mixture with different concentrations (x1 = 25.0%, 50.0%, 75.0%, and 90.0%) were studied with molecular dynamics simulation at room temperature. It was shown that water molecules tend to be isolated from each other in mixtures with more ions than water molecules in pure water. With the increase of the molar fraction of water in the mixture, the rotation bands and the bending bands of water display red shift from 566.2 to 651.4 cm-1 and from 1638.4 to 1683.2 cm-1 respectively, whereas the O–H stretch bands show blue shift from 3519.8 to 3452 cm-1, which agree well with the experimental results. This suggests that the molecules are hindered and their motions are difficult and slow, due to the hydrogen-bond interactions and the inharmonic interactions between the inter- or intra-molecular modes of water molecules.

CALCULATOR FOR FINDING COMPOSITION OF (Me1)x1(Me2)x2(CcHhNnOo)x3(NO3)x4(H2O)x5(Cl)x6 TYPE COMPLEX FROM ELEMENTAL ANALYSIS DATA

The paper presents the method and numerical program along with graphical user interface for finding the probable composition of Schiff base complexes based on elemental analysis data. The applied algorithm uses sorting procedure of possible variations of number of groups under condition of electrical neutrality. The program was tested for a few examples and its usefulness was discussed.

Experimental and theoretical study of the exited state of aminostyryl terpyridine derivatives: hydrogen-bonding effects

The synthesis, structure and photophysical behavior of the aminostyryl terpyridine derivatives, named 4'-(4-{2-[4-(N,N-dimethylaniline)]vinyl}phenyl)-2,2':6',2″-terpyridine (M(1)) and the related model compounds 4'-(4-{2-[4-(N,N-diphenylammino)phenyl]vinyl}phenyl)-2,2':6',2″-terpyridine (M(2)), respectively, are reported. Large solvatochromic shifts of the first excited-state fluorescence maximum suggest the intramolecular charge transfer characters for both compounds. In addition, with N,N-dimethyl substituents, its fluorescence is quenched a lot in protic solvents. This is consisted with the decay of its S(1) state, through nonradiative internal conversion, to the ground state, which is facilitated by the formation of the hydrogen bond between M(1) and alcohols. Whereas, the introduction of N,N-diphenyl substituents has been proved to be a hydrogen-bond-free case with the unchanged Φ(f). Furthermore, the formation of TICT state via diffusive twisting motion of the dimethyl/phenylamino group is the major relaxation process, which is proved by the ultrafast relaxation dynamics experiment and theoretically conformational optimization of the first excited-state of both the compounds.

TD-DFT study on fluoride-sensing mechanism of 2-(2'-phenylureaphenyl)benzoxazole: the way to inhibit the ESIPT process

The fluoride-sensing mechanism of the sensor 2-(2'-phenylurea-phenyl)benzoxazole (PUBO) has been investigated by means of the TD-DFT method. The present theoretical study indicates that there is an excited-state intramolecular proton transfer (ESIPT) process in the sensor PUBO. The added fluoride anion could capture the proton in the free N-H moiety instead of the hydrogen-bonding one. The experimental UV/Vis and fluorescence spectra (J. Org. Chem. 2007, 72, 62) are well reproduced by the calculated vertical excitation energies in the ground state and the first singlet excited state. For example, the calculated emission wavelength of PUBO at 534 nm is very close to the fluorescence band at 554 nm. Furthermore, we theoretically confirmed that the added fluoride anions could inhibit the ESIPT process in PUBO. But different from the classical ESIPT-inhibition mechanism, the ESIPT process in the sensor PUBO is inhibited by the high energy barrier of its deprotonated form rather than by the absence of the transferred proton.

Protein-induced excited-state dynamics of protochlorophyllide

The light-driven NADPH:protochlorophyllide oxidoreductase (POR) is a key enzyme of chlorophyll biosynthesis in angiosperms. POR's unique requirement for light to become catalytically active makes the enzyme an attractive model to study the dynamics of enzymatic reactions in real time. Here, we use picosecond time-resolved fluorescence and femtosecond pump-probe spectroscopy to examine the influence of the protein environment on the excited-state dynamics of the substrate, protochlorophyllide (PChlide), in the enzyme/substrate (PChlide/POR) and pseudoternary complex including the nucleotide cofactor NADP(+) (PChlide/NADP(+)/ POR). In comparison with the excited-state processes of unbound PChlide, the lifetime of the thermally equilibrated S(1) excited state is lengthened from 3.4 to 4.4 and 5.4 ns in the PChlide/POR and PChlide/NADP(+)/POR complex, whereas the nonradiative rates are decreased by ∼30 and 40%, respectively. This effect is most likely due to the reduced probability of nonradiative decay into the triplet excited state, thus keeping the risk of photosensitized side reactions in the enzyme low. Further, the initial reaction path involves the formation of an intramolecular charge-transfer state (S(ICT)) as an intermediate product. From a strong blue shift in the excited-state absorption, it is concluded that the S(ICT) state is stabilized by local interactions with specific protein sites in the catalytic pocket. The possible relevance of this result for the catalytic reaction in the enzyme POR is discussed.

Excited-state hydrogen bonding and deprotonation of esculetin in solution: a DFT/TDDFT study

The combined density functional theory (DFT) and time-dependent density functional theory (TDDFT) method was used to study the electronic spectral properties of different deprotonated forms of esculetin. By comparing the experimental absorption and fluorescence bands with the calculated electronic spectra, it is evidently demonstrated that the minor absorption and fluorescence bands observed at slightly longer wavelengths than the principal bands in experiments are predominantly from the de-H3 form of the esculetin monomer. Furthermore, we clarified the relationship between electronic spectral shifts and electronic excited-state intramolecular hydrogen bonding changes: the strengthening of intramolecular hydrogen bond can induce an electronic spectral blueshift while the intramolecular hydrogen bond weakening can result in an electronic spectral redshift.

Excited-state dynamics of protochlorophyllide revealed by subpicosecond infrared spectroscopy

To gain a better understanding of the light-induced reduction of protochlorophyllide (PChlide) to chlorophyllide as a key regulatory step in chlorophyll synthesis, we performed transient infrared absorption measurements on PChlide in d4-methanol. Excitation in the Q-band at 630 nm initiates dynamics characterized by three time constants: τ₁ = 3.6 ± 0.2, τ₂ = 38 ± 2, and τ₃ = 215 ± 8 ps. As indicated by the C13'=O carbonyl stretching mode in the electronic ground state at 1686 cm⁻¹, showing partial ground-state recovery, and in the excited electronic state at 1625 cm⁻¹, showing excited-state decay, τ₂ describes the formation of a state with a strong change in electronic structure, and τ₃ represents the partial recovery of the PChlide electronic ground state. Furthermore, τ₁ corresponds with vibrational energy relaxation. The observed kinetics strongly suggest a branched reaction scheme with a branching ratio of 0.5 for the path leading to the PChlide ground state on the 200 ps timescale and the path leading to a long-lived state (>>700 ps). The results clearly support a branched reaction scheme, as proposed previously, featuring the formation of an intramolecular charge transfer state with ∼25 ps, its decay into the PChlide ground state with 200 ps, and a parallel reaction path to the long-lived PChlide triplet state.

Reaction mechanism and chemoselectivity of intermolecular cycloaddition reactions between phenyl-substituted cyclopropenone ketal and methyl vinyl ketone

In this paper, the mechanisms of the intermolecular [3+2] and [1+2] cycloaddition reactions of 1,1/1,3-dipolar π-delocalized singlet vinylcarbenes, which is obtained from cyclopropenone, with an electron-deficient C═O or C═C dipolarophile, to generate five-membered ring products are first disclosed by the density functional theory (DFT). Four reaction pathways, including two concerted [3+2] cycloaddition reaction pathways and two stepwise reaction pathways (an initial [1+2] cycloaddition and then a rearrangement from the [1+2] cycloadducts to the final [3+2] cycloadducts), are investigated at the B3LYP/6-31G(d,p) level of theory. The calculated results reveal that, in contrast to the concerted C═O [3+2] cycloaddition reaction pathway, which is 7.1 kcal/mol more energetically preferred compared with its stepwise reaction pathway, the C═C dipolarophile favors undergoing [1+2] cycloaddition rather than concerted [3+2] cycloaddition (difference of 5.3 kcal/mol). The lowest free energy barrier of the C═O concerted [3+2] cycloaddition reaction pathway shows that it predominates all other reaction pathways. This observation is consistent with the finding that the C═O [3 + 2] cycloadduct is the main product under experimental conditions. In addition, natural bond orbital second-order perturbation charge analyses are carried out to explain the preferred chemoselectivity of C═O to the C═C dipolarophile and the origins of cis-stereoselectivity for C═C [1+2] cycloaddition. Solvent effects are further considered at the B3LYP/6-31G(d,p) level in the solvents CH(3)CN, DMF, THF, CH(2)Cl(2), toluene, and benzene using the PCM model. The results indicate that the relative reaction trends and the main products are insensitive to the polarity of the reaction solvent.

Single and multi-exciton dynamics in aqueous protochlorophyllide aggregates

In plants, the oxidoreductase enzyme POR reduces protochlorophyllide (Pchlide) into chlorophyllide (Chlide), using NADPH as a cofactor. The reduction involves the transfer of two electrons and two protons to the C17═C18 double bond of Pchlide, and the reaction is initiated by the absorption of light by Pchlide itself. In this work we have studied the excited state dynamics of Pchlide dissolved in water, where it forms excitonically coupled aggregates, by ultrafast time-resolved transient absorption and fluorescence experiments performed in the 480-720 nm visible region and in the 1780-1590 cm(-1) mid-IR region. The ground state visible absorption spectrum of aqueous Pchlide red shifts and broadens in comparison to the spectrum of monomeric Pchlide in organic solvents. The population of the one-exciton state occurs at low excitation densities, of <1 photon per aggregate. We characterized the multiexciton manifolds spectra by measuring the absorption difference spectra at increasingly higher photon densities. The multiexciton states are characterized by blue-shifted stimulated emission and red-shifted excited state absorption in comparison to those of the one-exciton manifold. The relaxation dynamics of the multiexciton manifolds into the one-exciton manifold is found to occur in ∼10 ps. This surprisingly slow rate we suggest is due to the intrinsic charge transfer character of the PChlide excited state that leads to solvation, stabilizing the CT state, and subsequent charge recombination, which limits the exciton relaxation.

Excited-state proton transfer via hydrogen-bonded acetic acid (AcOH) wire for 6-hydroxyquinoline

Spectroscopic studies on excited-state proton transfer (ESPT) of hydroxyquinoline (6HQ) have been performed in a previous paper. And a hydrogen-bonded network formed between 6HQ and acetic acid (AcOH) in nonpolar solvents has been characterized. In this work, a time-dependent density functional theory (TDDFT) method at the def-TZVP/B3LYP level was employed to investigate the excited-state proton transfer via hydrogen-bonded AcOH wire for 6HQ. A hydrogen-bonded wire containing three AcOH molecules at least for connecting the phenolic and quinolinic -N- group in 6HQ has been confirmed. The excited-state proton transfer via a hydrogen-bonded wire could result in a keto tautomer of 6HQ and lead to a large Stokes shift in the emission spectra. According to the results of calculated potential energy (PE) curves along different coordinates, a stepwise excited-state proton transfer has been proposed with two steps: first, an anionic hydrogen-bonded wire is generated by the protonation of -N- group in 6HQ upon excitation to the S(1) state, which increases the proton-capture ability of the AcOH wire; then, the proton of the phenolic group transfers via the anionic hydrogen-bonded wire, by an overall "concerted" process. Additionally, the formation of the anionic hydrogen-bonded wire as a preliminary step has been confirmed by the hydrogen-bonded parameters analysis of the ESPT process of 6HQ in several protic solvents. Therefore, the formation of anionic hydrogen-bonded wire due to the protonation of the -N- group is essential to strengthen the hydrogen bonding acceptance ability and capture the phenolic proton in the 6HQ chromophore.

Time-dependent density functional theory study on excited-state dihydrogen bonding O-H···H-Ge of the dihydrogen-bonded phenol-triethylgermanium complex

Intermolecular dihydrogen bond O-H···H-Ge in the electronically excited state of the dihydrogen-bonded phenol-triethylgermanium (TEGH) complex was studied theoretically using time-dependent density functional theory. Analysis of the frontier molecular orbitals revealed a locally excited S(1) state in which only the phenol moiety is electronically excited. In the predicted infrared spectrum of the dihydrogen-bonded phenol-TEGH complex, the O-H stretching vibrational mode shifts to a lower frequency in the S(1) state in comparison with that in ground state. The Ge-H stretching vibrational mode demonstrates a relatively smaller redshift than the O-H stretching vibrational mode. Upon electronic excitation to the S(1) state, the O-H and Ge-H bonds involved in the dihydrogen bond both get lengthened, whereas the C-O bond is shortened. With an increased binding energy, the calculated H···H distance significantly decreases in the S(1) state. Thus, the intermolecular dihydrogen bond O-H···H-Ge of the dihydrogen-bonded phenol-TEGH complex becomes stronger in the electronically excited state than that in the ground state.

Protochlorophyllide excited-state dynamics in organic solvents studied by time-resolved visible and mid-infrared spectroscopy

Protochlorophyllide (PChlide) is a precursor in the biosynthesis of chlorophyll. Complexed with NADPH to the enzyme protochlorophyllide oxidoreductase (POR), it is reduced to chlorophyllide, a process that occurs via a set of spectroscopically distinct intermediate states and is initiated from the excited state of PChlide. To obtain a better understanding of these catalytic events, we characterized the excited state dynamics of PChlide in the solvents tetrahydrofuran (THF), methanol, and Tris/Triton buffer using ultrafast transient absorption in the visible and mid-infrared spectral regions and time-resolved fluorescence emission experiments. For comparison, we present time-resolved transient absorption measurements of chlorophyll a in THF. From the combined analysis of these experiments, we derive that during the 2-3 ns excited state lifetime an extensive multiphasic quenching of the emission occurs due to solvation of the excited state, which is in agreement with the previously proposed internal charge transfer (ICT) character of the S1 state ( Zhao , G. J. ; Han , K. L. Biophys. J. 2008 , 94 , 38 ). The solvation process in methanol occurs in conjunction with a strengthening of a hydrogen bond to the Pchlide keto carbonyl group. We demonstrate that the internal conversion from the S2 to S1 excited states is remarkably slow and stretches out on to the 700 fs time scale, causing a rise of blue-shifted signals in the transient absorption and a gain of emission in the time-resolved fluorescence. A triplet state is populated on the nanosecond time scale with a maximal yield of approximately 23%. The consequences of these observations for the catalytic pathway and the role of the triplet and ICT state in activation of the enzyme are discussed.