X-ray structure and mutagenesis analyses of Clostridioides difficile endolysin Ecd09610 glucosaminidase domain.

Clostridioides difficile endolysin (Ecd09610) consists of an unknown domain at its N terminus, followed by two catalytic domains, a glucosaminidase domain and endopeptidase domain. X-ray structure and mutagenesis analyses of the Ecd09610 catalytic domain with glucosaminidase activity (Ecd09610CD53) were performed. Ecd09610CD53 was found to possess an α-bundle-like structure with nine helices, which is well conserved among GH73 family enzymes. The mutagenesis analysis based on X-ray structures showed that Glu405 and Asn470 were essential for enzymatic activity. Ecd09610CD53 may adopt a neighboring-group mechanism for a catalytic reaction in which Glu405 acted as an acid/base catalyst and Asn470 helped to stabilize the oxazolinium ion intermediate. Structural comparisons with the newly identified Clostridium perfringens autolysin catalytic domain (AcpCD) in the P1 form and a zymography analysis demonstrated that AcpCD was 15-fold more active than Ecd09610CD53. The strength of the glucosaminidase activity of the GH73 family appears to be dependent on the depth of the substrate-binding groove.

Biochemical Characterizations of the Putative Amidase Endolysin Ecd18980 Catalytic Domain from Clostridioides difficile.

Clostridioides difficile is the major causative pathogen of pseudomembranous colitis, and novel antimicrobial agents are required for treatment. Phage-derived endolysins exhibiting species-specific lytic activity have potential as novel antimicrobial agents. We surveyed the genome of C. difficile strain 630 and identified a gene encoding an endolysin, Ecd18980, which has an amidase_3 domain at the N-terminus but unknown C-terminal domain. The genes encoding Ecd18980 and its catalytic domain (Ecd18980CD) were cloned and expressed in Escherichia coli as N-terminal histidine-tagged proteins. These purified proteins showed lytic activity against C. difficile. Ecd18980CD showed higher lytic activity than the wild-type enzyme and near-specific lytic activity against C. difficile. This species specificity is thought to depend on substrate cleavage activity rather than binding. We also characterized the biochemical properties of Ecd18980CD, including optimal pH, salt concentration, and thermal stability.

Structural and biochemical characterizations of the novel autolysin Acd24020 from Clostridioides difficile and its full-function catalytic domain as a lytic enzyme.

Autolysin is a lytic enzyme that hydrolyzes peptidoglycans of the bacterial cell wall, with a catalytic domain and cell wall-binding (CWB) domains, to be involved in different physiological functions that require bacterial cell wall remodeling. We identified a novel autolysin, Acd24020, from Clostridioides (Clostridium) difficile (C. difficile), with an endopeptidase catalytic domain belonging to the NlpC/P60 family and three bacterial Src-homology 3 domains as CWB domains. The catalytic domain of Acd24020 (Acd24020-CD) exhibited C. difficile-specific lytic activity equivalent to Acd24020, indicating that Acd24020-CD has full-function as a lytic enzyme by itself. To elucidate the specific peptidoglycan-recognition and catalytic reaction mechanisms of Acd24020-CD, biochemical characterization, X-ray structure determination, a modeling study of the enzyme/substrate complex, and mutagenesis analysis were performed. Acd24020-CD has an hourglass-shaped substrate-binding groove across the molecule, which is responsible for recognizing the direct 3-4 cross-linking structure unique to C. difficile peptidoglycan. Based on the X-ray structure and modeling study, we propose a dynamic Cys/His catalyzing mechanism, in which the catalytic Cys299 and His354 residues dynamically change their conformations to complement each step of the enzyme catalytic reaction.

Structural and biochemical characterization of the Clostridium perfringens-specific Zn2+-dependent amidase endolysin, Psa, catalytic domain.

Phage-derived endolysins, enzymes that degrade peptidoglycans, have the potential to serve as alternative antimicrobial agents. Psa, which was identified as an endolysin encoded in the genome of Clostridium perfringens st13, was shown to specifically lyse C. perfringens. Psa has an N-terminal catalytic domain that is homologous to the Amidase_2 domain (PF01510), and a novel C-terminal cell wall-binding domain. Here, we determined the X-ray structure of the Psa catalytic domain (Psa-CD) at 1.65 Å resolution. Psa-CD has a typical Amidase_2 domain structure, consisting of a spherical structure with a central ß-sheet surrounded by two α-helix groups. Furthermore, there is a Zn2+ at the center of Psa-CD catalytic reaction site, as well as a unique T-shaped substrate-binding groove consisting of two grooves on the molecule surface. We performed modeling study of the enzyme/substrate complex along with a mutational analysis, and demonstrated that the structure of the substrate-binding groove is closely related to the amidase activity. Furthermore, we proposed a Zn2+-mediated catalytic reaction mechanism for the Amidase_2 family, in which tyrosine constitutes part of the catalytic reaction site.

X-ray structures of Clostridium perfringens sortase C with C-terminal cell wall sorting motif of LPST demonstrate role of subsite for substrate-binding and structural variations of catalytic site.

Pili of Gram-positive bacteria are flexible rod proteins covalently attached to the bacterial cell wall, that play important roles in the initial adhesion of bacterial cells to host tissues and bacterial colonization. Pili are formed by the polymerization of major and minor pilins, catalyzed by class C sortase (SrtC), a family of cysteine transpeptidases. The Gram-positive bacterium Clostridium perfringens has a major pilin (CppA), a minor pilin (CppB), and SrtC (CpSrtC). CpSrtC recognizes the C-terminal cell wall sorting signal motifs with five amino acid residues, LPSTG of CppA and LPETG of CppB, for the polymerization of pili. Here, we report biochemical analysis to detect the formation of Clostridium perfringens pili in vivo, and the X-ray structure of a novel intermolecular substrate-enzyme complex of CpSrtC with a sequence of LPST at the C-terminal site. The results showed that CpSrtC has a subsite for substrate-binding to aid polymerization of pili, and that the catalytic site has structural variations, giving insights into the enzyme catalytic reaction mechanism and affinities for the C-terminal cell wall sorting signal motif sequences.

Ventrolateral Striatal Medium Spiny Neurons Positively Regulate Food-Incentive, Goal-Directed Behavior Independently of D1 and D2 Selectivity.

The ventral striatum is involved in motivated behavior. Akin to the dorsal striatum, the ventral striatum contains two parallel pathways: the striatomesencephalic pathway consisting of dopamine receptor Type 1-expressing medium spiny neurons (D1-MSNs) and the striatopallidal pathway consisting of D2-MSNs. These two genetically identified pathways are thought to encode opposing functions in motivated behavior. It has also been reported that D1/D2 genetic selectivity is not attributed to the anatomical discrimination of two pathways. We wanted to determine whether D1- and D2-MSNs in the ventral striatum functioned in an opposing manner as previous observations claimed, and whether D1/D2 selectivity corresponded to a functional segregation in motivated behavior of mice. To address this question, we focused on the lateral portion of ventral striatum as a region implicated in food-incentive, goal-directed behavior, and recorded D1 or D2-MSN activity by using a gene-encoded ratiometric Ca2+ indicator and by constructing a fiberphotometry system, and manipulated their activities via optogenetic inhibition during ongoing behaviors. We observed concurrent event-related compound Ca2+ elevations in ventrolateral D1- and D2-MSNs, especially at trial start cue-related and first lever press-related times. D1 or D2 selective optogenetic inhibition just after the trial start cue resulted in a reduction of goal-directed behavior, indicating a shared coding of motivated behavior by both populations at this time. Only D1-selective inhibition just after the first lever press resulted in the reduction of behavior, indicating D1-MSN-specific coding at that specific time. Our data did not support opposing encoding by both populations in food-incentive, goal-directed behavior.SIGNIFICANCE STATEMENT An opposing role of dopamine receptor Type 1 or Type 2-expressing medium spiny neurons (D1-MSNs or D2-MSNs) on striatum-mediated behaviors has been widely accepted. However, this idea has been questioned by recent reports. In the present study, we measured concurrent Ca2+ activity patterns of D1- and D2-MSNs in the ventrolateral striatum during food-incentive, goal-directed behavior in mice. According to Ca2+ activity patterns, we conducted timing-specific optogenetic inhibition of each type of MSN. We demonstrated that both D1- and D2-MSNs in the ventrolateral striatum commonly and positively encoded action initiation, whereas only D1-MSNs positively encoded sustained motivated behavior. These findings led us to reconsider the prevailing notion of a functional segregation of MSN activity in the ventral striatum.

Real-time observation of the photoionization-induced water rearrangement dynamics in the 5-hydroxyindole-water cluster by time-resolved IR spectroscopy.

Solvation plays an essential role in controlling the mechanism and dynamics of chemical reactions in solution. The present study reveals that changes in the local solute-solvent interaction have a great impact on the timescale of solvent rearrangement dynamics. Time-resolved IR spectroscopy has been applied to a hydration rearrangement reaction in the monohydrated 5-hydroxyindole-water cluster induced by photoionization of the solute molecule. The water molecule changes the stable hydration site from the indolic NH site to the substituent OH site, both of which provide a strongly attractive potential for hydration. The rearrangement time constant amounts to 8 ± 2 ns, and is further slowed down by a factor of more than five at lower excess energy. These rearrangement times are slower by about three orders of magnitude than those reported for related systems where the water molecule is repelled from a repulsive part of the interaction potential toward an attractive well. The excess energy dependence of the time constant is well reproduced by RRKM theory. Differences in the reaction mechanism are discussed on the basis of energy relaxation dynamics.

Synthesis and Antimicrobial Activity of 2-Trifluoroacetonylbenzoxazole Ligands and Their Metal Complexes.

Three 2-fluoroacetonylbenzoxazole ligands 1a-c and their new Zn(II) complexes 2a-c have been synthesized. In addition, syntheses of new metal [Mg(II), Ni(II), Cu(II), Pd(II), and Ag(I)] complexes from 1a have been also described. The molecular and crystal structures of six metal complexes 2b and 2d-h were determined by single-crystal X-ray diffraction analyses. Their antibacterial activities against six Gram-positive and six Gram-negative bacteria were evaluated by minimum inhibitory concentrations (MIC), which were compared with those of appropriate antibiotics and silver nitrate. The results indicate that some metal compounds have more antibacterial effects in comparison with free ligands and have preferred antibacterial activities that may have potential pharmaceutical applications. Noticeably, the Ag(I) complex 2h exhibited low MIC value of 0.7 µM against Pseudomonas aeruginosa, which was even superior to the reference drug, Norfloxacin with that of 1.5 µM. Against P. aeruginosa, 2h is bacteriostatic, exerts the cell surface damage observed by scanning electron microscopy (SEM) and is less likely to develop resistance. The new 2h has been found to display effective antimicrobial activity against a series of bacteria.

X-ray structure of Clostridium perfringens sortase B cysteine transpeptidase.

The pathogenesis and infectivity of Gram-positive bacteria are mediated by many surface proteins that are covalently attached to peptidoglycans of the cell wall. The covalent attachment of these proteins is catalyzed by sortases (Srts), a family of cysteine transpeptidases, which are classified into six classes, A - F, based on their amino acid sequences and biological roles. Clostridium perfringens, one of the pathogenic clostridial species, has a class B sortase (CpSrtB) with 249 amino acid residues. X-ray structures of CpSrtB and its inactive mutant form were determined at 2.2 Å and 1.8 Å resolutions, respectively. CpSrtB adopts a typical sortase-protein fold, and has a unique substrate-binding groove formed by three ß-strands and two helices creating the sidewalls of the groove. The position of the catalytic Cys232 of CpSrtB is significantly different from those commonly found in Srts structures. The modeling study of the CpSrtB/peptide complex suggested that the position of Cys232 found in CpSrtB is preferable for the catalytic reaction to occur. Structural comparison with other class B sortases demonstrated that the catalytic site likely converts between two forms. The movement of Cys232 between the two forms may help His136 deprotonate Cys232 to be activated as a thiolate, which may the catalytic Cys-activated mechanism for Srts.

Elevation of the Energy Threshold for Isomerization of 5-Hydroxyindole-(tert-butyl alcohol)1 Cluster Cations.

Isomerization between two hydrogen-bonded (H-bonded) isomers of 5-hydroxyindole-(tert-butyl alcohol)1 cluster cations ([5HI-(t-BuOH)1]+) was investigated in the gas phase. In the S0 state, jet-cooled 5HI-(t-BuOH)1 has two structural isomers, 5HI(OH)-(t-BuOH)1 and 5HI(NH)-(t-BuOH)1, in which the t-BuOH molecule is bound to the OH or the NH group of 5HI. The IR photodissociation spectrum of [5HI-(t-BuOH)1]+ generated by two-color resonant two-photon ionization (2C-R2PI) via the S1-S0 origin of 5HI(NH)-(t-BuOH)1 provided evidence of both [5HI(OH)-(t-BuOH)1]+ and [5HI(NH)-(t-BuOH)1]+ coexisting in the D0 state, indicating that [5HI(NH)-(t-BuOH)1]+ isomerizes to [5HI(OH)-(t-BuOH)1]+ after 2C-R2PI of 5HI(NH)-(t-BuOH)1. The lower limit of the energy threshold for the isomerization of [5HI(NH)-(t-BuOH)1]+ to [5HI(OH)-(t-BuOH)1]+ was experimentally determined to be 3362 ± 30 cm-1, and the corresponding energy threshold for the isomerization of [5HI(NH)-(H2O)1]+ to [5HI(OH)-(H2O)1]+ has been reported to be 2127 ± 30 cm-1. Thus, the energy threshold for the isomerization is elevated by at least 1200 cm-1 when the solvent molecule changes from H2O to t-BuOH. The elevation of the energy threshold is explained by the difference in the stabilization energies of [5HI-(t-BuOH)1]+ and [5HI-(H2O)1]+ in the initial and transition states owing to the larger proton affinity of t-BuOH than H2O.

Identification of Characteristic Phenolic Constituents in Mousouchiku Extract Used as Food Additives.

Mousouchiku extract is prepared from the bamboo-sheath of Phyllostachys heterocycla MITF. (Poaceae), and is registered as a food manufacturing agent in the List of Existing Food Additives in Japan. This study describes the chromatographic evaluation of characteristic components of this extract to obtain the chemical data needed for standardized specifications. We isolated 12 known compounds from this extract: 5-hydroxymethyl-2-furfural, 4-hydroxybenzoic acid, trans-p-coumaric acid, trans-ferulic acid, N,N'-diferuloylputrescine, 4'-hydroxypropiophenone, ß-arbutin, tachioside, isotachioside, 3,4'-dihydroxypropiophenone 3-O-glucoside, koaburaside, and (+)-lyoniresinol 9'-O-glucoside. Moreover, a new propiophenone glycoside, propiophenone 4'-O-(6-ß-D-xylosyl)-ß-D-glucoside (propiophenone 4'-O-primeveroside), was isolated. The structure of each isolated compound was elucidated based on NMR and MS data or direct HPLC comparisons with authentic samples. Among the isolates, (+)-lyoniresinol 9'-O-glucoside was found to be the major ingredients of the extract as observed using HPLC analysis. However, 2,6-dimethoxy-1,4-benzoquinone, which is considered the main constituent of mousouchiku extract, was only detected as a trace constituent and not isolated in this study.

A Novel Methodology for Synthesis of 1,5,6-Trisubstituted 2(1H)-Pyrazinones of Biological Interest.

In this report, we describe a new method for the synthesis of densely functionalized 2(1H)-pyrazinones. Treatment of mesoionic 1,3-oxazolium-5-olates with carbanions derived from activated methylene isocyanides (p-toluenesulfonylmethyl isocyanide (TosMIC) and ethyl isocyanoacetate) causes a novel ring transformation affording 2(1H)-pyrazinones in moderate to high yields. The cytotoxicity and antibacterial activity of some of the obtained products were studied and some of the products exhibited tumor-specific cytotoxicity.

Whisker experience-dependent mGluR signaling maintains synaptic strength in the mouse adolescent cortex.

Sensory experience-dependent plasticity in the somatosensory cortex is a fundamental mechanism of adaptation to the changing environment not only early in the development but also in adolescence and adulthood. Although the mechanisms underlying experience-dependent plasticity during early development have been well documented, the corresponding understanding in the mature cortex is less complete. Here, we investigated the mechanism underlying whisker deprivation-induced synaptic plasticity in the barrel cortex in adolescent mice. Layer 4 (L4) to L2/3 excitatory synapses play a crucial role for whisker experience-dependent plasticity in rodent barrel cortex and whisker deprivation is known to depress synaptic strength at L4-L2/3 synapses in adolescent and adult animals. We found that whisker deprivation for 5 days or longer decreased the presynaptic glutamate release probability at L4-L2/3 synapses in the barrel cortex in adolescent mice. This whisker deprivation-induced depression was restored by daily administration of a positive allosteric modulator of the type 5 metabotropic glutamate receptor (mGluR5). On the other hand, the administration of mGluR5 antagonists reproduced the effect of whisker deprivation in whisker-intact mice. Furthermore, chronic and selective suppression of inositol 1,4,5-trisphosphate (IP3 ) signaling in postsynaptic L2/3 neurons decreased the presynaptic release probability at L4-L2/3 synapses. These findings represent a previously unidentified mechanism of cortical plasticity, namely that whisker experience-dependent mGluR5-IP3 signaling in the postsynaptic neurons maintains presynaptic function in the adolescent barrel cortex.

Excited-state intramolecular proton transfer and conformational relaxation in 4'-N,N-dimethylamino-3-hydroxyflavone doped in acetonitrile crystals.

The effect of intermolecular interactions on excited-state intramolecular proton transfer (ESIPT) in 4'-N,N-dimethylamino-3-hydroxyflavone (DMHF) doped in acetonitrile crystals was investigated by measuring its temperature dependence of steady-state fluorescence excitation and fluorescence spectra and picosecond time-resolved spectra. The relative intensity of emission from the excited state of the normal form (N*) to that from the excited state of the tautomer form (T*) and spectral features changed markedly with temperature. Unusual changes in the spectral shift and spectral features were observed in the fluorescence spectra measured between 200 and 218 K, indicating that a solid-solid phase transition of DMHF-doped acetonitrile crystals occurred. Time-resolved fluorescence spectra suggested conformational relaxation of the N* state competed with ESIPT after photoexcitation and the ESIPT rate increased with temperature in the low-temperature phase of acetonitrile. However, the intermolecular interaction of N* with acetonitrile in the high-temperature phase markedly stabilized the potential minimum of the fluorescent N* state and slowed the ESIPT. This stabilization can be explained by reorganization of acetonitrile originating from the strong electric dipole-dipole interaction between DMHF and acetonitrile molecules.

Rearrangements of a Water Molecule in Both Directions between Two Hydrogen-Bonding Sites of 5-Hydroxyindole Cation: Experimental Determination of the Energy Threshold for the Rearrangement.

Rearrangements of a water molecule in both directions between two hydrogen-bonding (H-bonding) sites of the 5-hydroxyindole (5HI) cation was investigated in the gas phase. IR-dip spectra of jet-cooled 5HI-(H2O)1 revealed that two structural isomers, 5HI(OH)-(H2O)1 and 5HI(NH)-(H2O)1, in which a water molecule is bound to either the OH group or the NH group of 5HI, were formed in the S0 state. The IR photodissociation spectrum of [5HI-(H2O)1](+) generated by two-color resonant two-photon ionization (2C-R2PI) via the S1-S0 origin of 5HI(NH)-(H2O)1 clearly showed that [5HI(OH)-(H2O)1](+) and [5HI(NH)-(H2O)1](+) coexist in the D0 state. The appearance of [5HI(OH)-(H2O)1](+) after R2PI via the S1-S0 origin of 5HI(NH)-(H2O)1 is explained by isomerization of [5HI(NH)-(H2O)1](+) to [5HI(OH)-(H2O)1](+), which corresponds to the rearrangement of the water. In addition, isomerization in the opposite direction was also observed when [5HI-(H2O)1](+) was generated via the S1-S0 origin of 5HI(OH)-(H2O)1. The upper limit of the energy threshold for the rearrangement of the water in [5HI(NH)-(H2O)1](+) was experimentally determined to be 2127 ± 30 cm(-1) from the adiabatic ionization energy of 5HI(NH)-(H2O)1. Above the energy threshold, the water molecule in [5HI-(H2O)1](+) may fluctuate between the two preferential H-bonding sites of 5HI(+).

Calcium-dependent N-cadherin up-regulation mediates reactive astrogliosis and neuroprotection after brain injury.

Brain injury induces phenotypic changes in astrocytes, known as reactive astrogliosis, which may influence neuronal survival. Here we show that brain injury induces inositol 1,4,5-trisphosphate (IP3)-dependent Ca(2+) signaling in astrocytes, and that the Ca(2+) signaling is required for astrogliosis. We found that type 2 IP3 receptor knockout (IP3R2KO) mice deficient in astrocytic Ca(2+) signaling have impaired reactive astrogliosis and increased injury-associated neuronal death. We identified N-cadherin and pumilio 2 (Pum2) as downstream signaling molecules, and found that brain injury induces up-regulation of N-cadherin around the injured site. This effect is mediated by Ca(2+)-dependent down-regulation of Pum2, which in turn attenuates Pum2-dependent translational repression of N-cadherin. Furthermore, we show that astrocyte-specific knockout of N-cadherin results in impairment of astrogliosis and neuroprotection. Thus, astrocytic Ca(2+) signaling and the downstream function of N-cadherin play indispensable roles in the cellular responses to brain injury. These findings define a previously unreported signaling axis required for reactive astrogliosis and neuroprotection following brain injury.

X-ray structure of a novel endolysin encoded by episomal phage phiSM101 of Clostridium perfringens.

Gram-positive bacteria possess a thick cell wall composed of a mesh polymer of peptidoglycans, which provides physical protection. Endolysins encoded by phages infecting bacteria can hydrolyse peptidoglycans in the bacterial cell wall, killing the host bacteria immediately. The endolysin (Psm) encoded by episomal phage phiSM101 of enterotoxigenic Clostridium perfringens type A strain SM101 exhibits potent lytic activity towards most strains of Clostridium perfringens. Psm has an N-terminal catalytic domain highly homologous to N-acetylmuramidases belonging to the glycoside hydrolase 25 family, and C-terminal tandem repeated bacterial Src homology 3 (SH3_3) domains as the cell wall-binding domain. The X-ray structure of full-length Psm and a catalytic domain of Psm in complex with N-acetylglucosamine were determined to elucidate the catalytic reaction and cell wall recognition mechanisms of Psm. The results showed that Psm may have adopted a neighbouring-group mechanism for the catalytic hydrolysing reaction in which the N-acetyl carbonyl group of the substrate was involved in the formation of an oxazolinium ion intermediate. Based on structural comparisons with other endolysins and a modelling study, we proposed that tandem repeated SH3_3 domains of Psm recognized the peptide side-chains of peptidoglycans to assist the catalytic domain hydrolysing the glycan backbone.

Simultaneous Interaction of Hydrophilic and Hydrophobic Solvents with Ethylamino Neurotransmitter Radical Cations: Infrared Spectra of Tryptamine(+)-(H2O)m-(N2)n Clusters (m,n ≤ 3).

Solvation of biomolecules by a hydrophilic and hydrophobic environment strongly affects their structure and function. Here, the structural, vibrational, and energetic properties of size-selected clusters of the microhydrated tryptamine cation with N2 ligands, TRA(+)-(H2O)m-(N2)n (m,n ≤ 3), are characterized by infrared photodissociation spectroscopy in the 2800-3800 cm(-1) range and dispersion-corrected density functional theory calculations at the ωB97X-D/cc-pVTZ level to investigate the simultaneous solvation of this prototypical neurotransmitter by dipolar water and quadrupolar N2 ligands. In the global minimum structure of TRA(+)-H2O generated by electron ionization, H2O is strongly hydrogen-bonded (H-bonded) as proton acceptor to the acidic indolic NH group. In the TRA(+)-H2O-(N2)n clusters, the weakly bonded N2 ligands do not affect the H-bonding motif of TRA(+)-H2O and are preferentially H-bonded to the OH groups of the H2O ligand, whereas stacking to the aromatic π electron system of the pyrrole ring of TRA(+) is less favorable. The natural bond orbital analysis reveals that the H-bond between the N2 ligand and the OH group of H2O cooperatively strengthens the adjacent H-bond between the indolic NH group of TRA(+) and H2O, while π stacking is slightly noncooperative. In the larger TRA(+)-(H2O)m clusters, the H2O ligands form a H-bonded solvent network attached to the indolic NH proton, again stabilized by strong cooperative effects arising from the nearby positive charge. Comparison with the corresponding neutral TRA-(H2O)m clusters illustrates the strong impact of the excess positive charge on the structure of the microhydration network.

External Electric Field Effects on Excited-State Intramolecular Proton Transfer in 4'-N,N-Dimethylamino-3-hydroxyflavone in Poly(methyl methacrylate) Films.

The external electric field effects on the steady-state electronic spectra and excited-state dynamics were investigated for 4'-N,N-(dimethylamino)-3-hydroxyflavone (DMHF) in a poly(methyl methacrylate) (PMMA) film. In the steady-state spectrum, dual emission was observed from the excited states of the normal (N*) and tautomer (T*) forms. Application of an external electric field of 1.0 MV·cm(-1) enhanced the N* emission and reduced the T* emission, indicating that the external electric field suppressed the excited-state intramolecular proton transfer (ESIPT). The fluorescence decay profiles were measured for the N* and T* forms. The change in the emission intensity ratio N*/T* induced by the external electric field is dominated by ESIPT from the Franck-Condon excited state of the N* form and vibrational cooling in potential wells of the N* and T* forms occurring within tens of picoseconds. Three manifolds of fluorescent states were identified for both the N* and T* forms. The excited-state dynamics of DMHF in PMMA films has been found to be very different from that in solution due to intermolecular interactions in a rigid environment.

Weak hydrogen bonding motifs of ethylamino neurotransmitter radical cations in a hydrophobic environment: infrared spectra of tryptamine(+)-(N2)n clusters (n ≤ 6).

Size-selected clusters of the tryptamine cation with N2 ligands, TRA(+)-(N2)n with n = 1-6, are investigated by infrared photodissociation (IRPD) spectroscopy in the hydride stretch range and quantum chemical calculations at the ωB97X-D/cc-pVTZ level to characterize the microsolvation of this prototypical aromatic ethylamino neurotransmitter radical cation in a nonpolar solvent. Two types of structural isomers exhibiting different interaction motifs are identified for the TRA(+)-N2 dimer, namely the TRA(+)-N2(H) global minimum, in which N2 forms a linear hydrogen bond (H-bond) to the indolic NH group, and the less stable TRA(+)-N2(π) local minima, in which N2 binds to the aromatic π electron system of the indolic pyrrole ring. The IRPD spectrum of TRA(+)-(N2)2 is consistent with contributions from two structural H-bound isomers with similar calculated stabilization energies. The first isomer, denoted as TRA(+)-(N2)2(2H), exhibits an asymmetric bifurcated planar H-bonding motif, in which both N2 ligands are attached to the indolic NH group in the aromatic plane via H-bonding and charge-quadrupole interactions. The second isomer, denoted as TRA(+)-(N2)2(H/π), has a single and nearly linear H-bond of the first N2 ligand to the indolic NH group, whereas the second ligand is π-bonded to the pyrrole ring. The natural bond orbital analysis of TRA(+)-(N2)2 reveals that the total stability of these types of clusters is not only controlled by the local H-bond strengths between the indolic NH group and the N2 ligands but also by a subtle balance between various contributing intermolecular interactions, including local H-bonds, charge-quadrupole and induction interactions, dispersion, and exchange repulsion. The systematic spectral shifts as a function of cluster size suggest that the larger TRA(+)-(N2)n clusters with n = 3-6 are composed of the strongly bound TRA(+)-(N2)2(2H) core ion to which further N2 ligands are weakly attached to either the π electron system or the indolic NH proton by stacking and charge-quadrupole forces.