Sex determination, gonadal sex differentiation, and plasticity in vertebrate species.

A diverse array of sex determination (SD) mechanisms, encompassing environmental to genetic, have been found to exist among vertebrates, covering a spectrum from fixed SD mechanisms (mammals) to functional sex change in fishes (sequential hermaphroditic fishes). A major landmark in vertebrate SD was the discovery of the SRY gene in 1990. Since that time, many attempts to clone an SRY ortholog from nonmammalian vertebrates remained unsuccessful, until 2002, when DMY/dmrt1by was discovered as the SD gene of a small fish, medaka. Surprisingly, however, DMY/dmrt1by was found in only 2 species among more than 20 species of medaka, suggesting a large diversity of SD genes among vertebrates. Considerable progress has been made over the last 3 decades, such that it is now possible to formulate reasonable paradigms of how SD and gonadal sex differentiation may work in some model vertebrate species. This review outlines our current understanding of vertebrate SD and gonadal sex differentiation, with a focus on the molecular and cellular mechanisms involved. An impressive number of genes and factors have been discovered that play important roles in testicular and ovarian differentiation. An antagonism between the male and female pathway genes exists in gonads during both sex differentiation and, surprisingly, even as adults, suggesting that, in addition to sex-changing fishes, gonochoristic vertebrates including mice maintain some degree of gonadal sexual plasticity into adulthood. Importantly, a review of various SD mechanisms among vertebrates suggests that this is the ideal biological event that can make us understand the evolutionary conundrums underlying speciation and species diversity.

UV-A-Induced Photoisomerization and Photodimerization of Curcumin: An Ion Mobility Mass Spectrometry Study.

Curcumin, the bioactive compound present in spice plant turmeric, has been shown to exhibit selective phototoxic activities toward mammalian cancer cells, and it is being used extensively as a photosensitizer (PS) in photodynamic therapies (PDT). However, so far, the fate of curcumin toward photochemical transformations is not well understood. Here we report our findings of a number of novel photochemical reaction channels of curcumin in water-methanol mixture, like photoisomerization, photodimerization, and photooxidation (H2-loss). The reaction was performed by irradiating the curcumin solution with ultraviolet (UV) light of wavelength 350 nm, which is abundant in the earth's troposphere. Product identification and structure elucidation are done by employing an integrated method of drift tube ion mobility mass spectrometry (DTIMS) in combination with high-performance liquid chromatography (HPLC) and collision-induced dissociation (CID) of the mass-selected molecular ions. Two photoisomers of curcumin produced as a result of trans-cis configurational changes about C═C double bonds in the excited state have been identified, and it has been shown that they could serve as the precursors for formation of isomeric dimers via [2 + 2] cycloaddition and H2-loss products. Comparisons of the experimentally measured collision cross-section (CCS) values of the reactant and product ions obtained by the DTIMS method with those predicted by the electronic structure theory are found to be very effective for the discrimination of the produced photoisomers. The observed photochemical reaction channels are potentially significant toward uses of curcumin as a photosensitizer in photodynamic therapy.

SDF-1/CXCR4 signal is involved in the induction of Primordial Germ Cell migration in a model marine fish, Japanese anchovy (Engraulis japonicus).

Primordial germ cells (PGCs) are pivotal for gonadal development and reproductive success. Though artificial induction of sterility by targeting PGCs are gaining popularity due to its advantages in fish surrogacy and biodiversity management, it is often skill and time intensive. In this study, we have focused on understanding the role of PGCs and the chemotactic SDF-1/CXCR4 signaling on gonad development of Japanese anchovy (JA, Engraulis japonicus), an upcoming marine model organism with eco-commercial values, with an aim to develop a novel, easy, and versatile gonad sterilization method. Our data showed that PGC migration related genes, i.e., sdf-1a, sdf-1b, cxcr4a, cxcr4b and vasa, are phylogenetically closer relatives of respective herring (Clupea harengus) and zebrafish (Danio rerio) homolog. Subsequently, PGC marking and live tracing experiments confirmed that PGC migration in JA initiates from 16 hours post fertilization (hpf) followed by PGC settlement in the gonadal ridge at 44 hpf. We found that overexpression of zebrafish sdf-1a mRNA in the germ cell suppresses cxcr4a and increases cxcr4b transcription at 8 hpf, dose dependently disrupts PGC migration at 24-48 hpf, induces PGC death and upregulates sdf-1b at 5 days post hatching. 48 h of immersion treatment with CXCR4 antagonist (AMD3100, Abcam) also accelerated PGC mismigration and pushed the PGC away from gonadal ridge in a dose responsive manner, and further when grown to adulthood caused germ cell less gonad formation in some individuals. Cumulatively, our data, for the first time, suggests that JA PGC migration is largely regulated by SDF1/CXCR4 signaling, and modulation of this signaling has strong potential for sterile, germ cell less gonad preparation at a mass scale. However, further in-depth analysis is pertinent to apply this methodology in marine fish species to successfully catapult Japanese anchovy into a true marine fish model.

Tautomers and Rotamers of Curcumin: A Combined UV Spectroscopy, High-Performance Liquid Chromatography, Ion Mobility Mass Spectrometry, and Electronic Structure Theory Study.

The structures of tautomers and rotameric forms of curcumin, the bioactive compound present in spice plant turmeric, have been investigated using ion mobility mass spectrometry (IMMS) in conjunction with high-performance liquid chromatography (HPLC) and UV-visible spectroscopy. Two tautomeric forms of this ß-diketone compound, keto-enol and diketo, have been chromatographically separated, and the electronic absorption spectra for these two tautomeric forms in methanol solution have been recorded separately for the first time. The molecular identity of the HPLC-separated solution fractions is established unambiguously by recording the mass and fragmentation spectra simultaneously. The ion mobility spectrum for the deprotonated curcumin anion, [Cur-H]-, corresponding to the diketo tautomer, displays only one peak (P), and the collision cross-section (CCS) value is measured to be 185.9 Å2. However, the ion mobility spectrum corresponding to the HPLC-separated keto-enol tautomer shows three distinctly separated peaks, P, Q, and R, with CCS values of 185.9, 194.8, and 203.4 Å2, respectively, whereby peak R appears to be the most intense one, followed by peaks P and Q. The theoretically calculated CCS values of different isomers of [Cur-H]-, optimized by electronic structure theory methods, display satisfactory correlation with the experimentally observed values, corroborating our assignments. The spectral attributes also indicate the occurrence of structural rearrangements in the electrospray ionization process. With the aid of the electronic structure calculation, low-energy pathways for the occurrence of the structural isomerization to surpass the energy barrier are suggested, which are consistent with the assignments of the peaks observed in the IM spectra.

Sex Lethal Gene Manipulates Gonadal Development of Medaka, Oryzias latipes, through Estrogenic Interventions.

Germ cells are pivotal for gonadal sexuality maintenance and reproduction. Sex lethal (sxl), the somatic sex determining gene of Drosophila, is the known regulator and initiator of germ cell femininity in invertebrates. However, the role of the Sxl homologue has rarely been investigated in vertebrates. So, we used medaka to clarify the role of sxl in vertebrate gonadogenesis and sexuality and identified two Sxl homologues, i.e., Sxl1a and Sxl1b. We found that sxl1a specifically expresses in the primordial germ cells (PGC), ovary, (early gonia and oocytes), while sxl1b distributions are ubiquitous. An mRNA overexpression of sxl1a accelerated germ cell numbers in 10 DAH XY fish, and sxl1a knockdown (KD), on the other hand, induced PGC mis-migration, aberrant PGC structuring and ultimately caused significant germ cell reduction in XX fish. Using an in vitro promoter analysis and in vivo steroid treatment, we found a strong link between sxl1a and estrogenic germ cell-population maintenance. Further, using sxl1a-KD and erß2-knockout fish, we determined that sxl1 acts through erß2 and controls PGC sexuality. Cumulatively, our study highlights the novel role of sxl1a in germ cell maintenance and sexual identity assignment and thus might become a steppingstone to understanding the commonalities of animal sexual development.

Drought dynamics of Northwestern Teesta Floodplain of Bangladesh: a remote sensing approach to ascertain the cause and effect.

Drought is an affliction for a region that primarily depends on agriculture as economic activity. Commonly monitoring and characterizing of drought is performed by only analyzing the meteorological aspect, assuming precipitation as the primary source of water. However, in riverine Bangladesh, this can lead to an erroneous conclusion, as there is a multitude of available water sources. Consequently, in this study, vegetation condition (Standard Vegetation Index), soil moisture (Soil Moisture Index), and precipitation (Standard Precipitation Index) are separately investigated from 2003 to 2019, in the Northwestern Teesta floodplain. Subsequently, statistical regression analysis is performed to determine the relationships between different aspects of drought. In addition, information obtained from field visits and expert opinions has also been assimilated. Analysis of vegetation and soil moisture condition presents a progressively improving scenario. However, SPI shows an incessant decline in meteorological drought conditions, especially after 2007. Evidently, regression analysis does not provide any indication of an interrelationship between the studied agricultural and meteorological parameters. Presumably, this absence is instigated because the study area is highly irrigated as the groundwater table is suitably near the surface and the existence of nearby Teesta river allows for the utilization of surface water. Moreover, the cropping pattern is shifting toward crops that require much less water and to places where soil moisture is scarce. Thus, this study addresses the gap in knowledge regarding the nature of agricultural drought and the dynamics of different aspects of drought which will be invaluable for the water management and agricultural policy in the study area as well as other regions with a similar backdrop.

Probing the Endo/Exo Isomeric Variants of a Binary Complex of Cyclopropylamine with p-Fluorophenol by LIF Spectroscopy: A Comparative Study with Ammonia Complex.

Conformational isomers of an O-H···N hydrogen-bonded binary complex between para-fluorophenol (pFP) and a nonrigid primary amine base, cyclopropylamine (CPA), have been probed by means of laser-induced fluorescence (LIF) spectroscopy in a supersonic jet expansion. Two closely spaced electronic origin bands have been identified in the measured LIF excitation spectrum, and their assignments have been corroborated by making comparisons with the spectra of the parent pFP-NH3 complex recorded under the same expansion condition. The observation is consistent with the presence of endo and exo isomeric variants of the complex predicted by electronic structure theory methods, and the endo isomer is stabilized by ∼2 kcal/mol additionally owing to the formation of a C-H···O and a C-H···π type of weak hydrogen bonds between the two moieties. In the fluorescence excitation (FE) spectrum, the low-frequency bands for different intermolecular modes gain substantial intensity, and this spectral feature is in contrast to that of the pFP-NH3 complex. The Franck-Condon intensity of the bands has been simulated invoking Duschinsky rotation scheme, taking into consideration the ground- and excited-state geometries.

Medium-Dependent Crossover from the Red to Blue Shift of the Donor's Stretching Fundamental in the Binary Hydrogen-Bonded Complexes of CDCl3 with Ethers and Ketones.

Mid-infrared spectra for C-D···O hydrogen (H)-bonded binary complexes of CDCl3 with acetone (AC), cyclohexanone (CHN), diethyl ether (DEE), and tetrahydrofuran (THF) have been measured in the vapor phase at room temperature and in an argon matrix at 8 K. Remarkable matrix effect has been observed in each case with respect to the spectral shift of the donor group's stretching fundamental (ΔνC-D). In the case of complexes with AC and CHN, the sign of ΔνC-D changes from a few wavenumbers positive (blue shift) in the vapor phase to a few tens of wavenumbers negative (red shift) in the argon matrix. For the two ether complexes, although no apparent reversal in the sign of ΔνC-D occurs, but the magnitudes of the red shifts in the matrix are manifold larger, and the bands appear with large enhancement in transition intensity. The medium effect has been explained consistently in terms of the local hyperconjugative charge transfer interaction at the H-bonding sites of the complexes and its interplay with the H-bond distance that varies with the physical conditions of the medium. Under the matrix isolation condition, νC-D bands of CHN and THF complexes depict a large number of substructures, which has been interpreted in terms of matrix site effect as well as Fermi resonance enhancement of the fingerprint combination tones and trapping of more than one isomer of the complexes in the matrix sites.

Identification of an Emitting Metastable State of p-Fluorophenol-Ammonia 1:2 Complex by Laser-Induced Fluorescence Spectroscopy.

We have demonstrated here, for the first time to our knowledge, the formation of an emitting metastable species upon lowest electronic excitation (S1) of a hydrogen-bonded 1:2 complex of para-fluorophenol (pFP) with ammonia (NH3), which is known to be one of the smallest reactive complexes to undergo excited state H-atom transfer (HAT) reaction to produce •NH4(NH3) radical fragment. The emission spectrum of the species is characterized to be red-shifted, broad, and structureless. From the viewpoint of energy balance, an excited state proton transfer (ESPT) is unfavorable, but according to predicted electronic structure parameters, the metastable state species could be stabilized by charge transfer (CT) interaction at the hydrogen-bonded geometry of the complex. We propose that this species could act as an intermediate to the HAT process in the excited state. The observation of such a state could be valuable to understand the complex dynamics of similar events in biologically relevant systems.

Modulations of νO-H and νC═O Stretching Frequencies of Difluoroacetic Acid with Internal Rotation of CHF2 Rotor: A Combined Vapor Phase and Matrix Isolation Infrared Spectroscopy Study.

Mid-infrared spectra of difluoroacetic acid (DFAA) have been measured by isolating the molecule in argon and nitrogen matrices at 8 K and also in the vapor phase at room temperature. In argon matrix, the O-H stretching fundamental (νO-H) of -COOH group appears as a doublet with band maxima at 3554 and 3558 cm-1, and a similar doublet for C═O stretching fundamental appears at 1800 and 1810 cm-1. In the vapor phase, the νO-H transition is featured with multiple peaks, and the observed band shape has been deconvoluted as superposition of two transitions both having A-type rotational band contours. We have attributed these transitions to the two internal rotational isomers corresponding to the two distinct minima along -CHF2 torsional coordinate of the molecule. Natural bond orbital (NBO) analysis reveals that these torsional minima are the manifestations of different second order interactions involving bonding and antibonding orbitals corresponding to the rotor -CHF2 and COOH groups of the molecule. By use of the theoretically predicted rotational constants of the rotamers, the band profile for νO-H has been simulated satisfactorily by means of the PGOPHER method, and this has allowed estimating accurately the energy difference between the two rotamers as 0.54 kcal/mol. The predicted energy barrier for interconversion between the rotamers is very small, ∼0.5 kcal/mol from rotamer II to rotamer I, which implies that the molecule could hop almost freely between the two rotameric forms at room temperature. As a result, the frequencies of the key stretching vibrational modes, like νO-H, νC═O, and νC-H, undergo modulation with internal rotation of the rotor -CHF2 group. Such modulation of high frequency modes could be an efficient mechanism for acceleration of rotor-induced IVR (intramolecular vibrational redistribution) well documented in the literature. Furthermore, the spectra measured in matrix isolated environment show signatures for an energetically higher third rotamer, where -OH and -C═O groups are in anti orientation. It has also been shown that DFAA can easily form weak hydrogen bonded dimeric complexes with molecular nitrogen (N2), which causes νO-H to undergo a red shift of ∼30 cm-1 in argon matrix for all three DFAA monomers.

Molecular cloning, characterization and expression analysis of complement components in red sea bream (Pagrus major) after Edwardsiella tarda and red sea bream Iridovirus (RSIV) challenge.

The complement system plays an important role in immune regulation and acts as the first line of defense against any pathogenic attack. To comprehend the red sea bream (Pagrus major) immune response, three complement genes, namely, pmC1r, pmMASP and pmC3, belonging to the classical, lectin and alternative complement cascade, respectively, were identified and characterized. pmC1r, pmMASP, and pmC3 were comprised of 2535, 3352, and 5735 base mRNA which encodes 732, 1029 and 1677 aa putative proteins, respectively. Phylogenetically, all the three studied genes clustered with their corresponding homologous clade. Tissue distribution and cellular localization data demonstrated a very high prevalence of all the three genes in the liver. Both bacterial and viral infection resulted in significant transcriptional alterations in all three genes in the liver with respect to their vehicle control counterparts. Specifically, bacterial challenge affected the pmMASP and pmC3 expression, while the viral infection resulted in pmC1r and pmC3 mRNA activation. Altogether, our data demonstrate the ability of pmC1r, pmMASP and pmC3 in bringing about an immune response against any pathogenic encroachment, and thus activating, not only one, but all the three complement pathways, in red sea bream.

Barrierless Proton Transfer in the Weak C-H···O Hydrogen Bonded Methacrolein Dimer upon Nonresonant Multiphoton Ionization in the Gas Phase.

Intermolecular proton transfer (IMPT) in a C-H···O hydrogen bonded dimer of an α,ß-unsaturated aldehyde, methacrolein (MC), upon nonresonant multiphoton ionization by 532 nm laser pulses (10 ns), has been investigated using time-of-flight (TOF) mass spectrometry under supersonic cooling condition. The mass peaks corresponding to both the protonated molecular ion [(MC)H+] and intact dimer cation [(MC)2]•+ show up in the mass spectra, and the peak intensity of the former increases proportionately with the latter with betterment of the jet cooling conditions. The observations indicate that [(MC)2]•+ is the likely precursor of (MC)H+ and, on the basis of electronic structure calculations, IMPT in the dimer cation has been shown to be the key reaction for formation of the latter. Laser power dependences of ion yields indicate that at this wavelength the dimer is photoionized by means of 4-photon absorption process, and the total 4-photon energy is nearly the same as the predicted vertical ionization energy of the dimer. Electronic structure calculations reveal that the optimized structures of [(MC)2]•+ correspond to a proton transferred configuration wherein the aldehydic hydrogen is completely shifted to the carbonyl oxygen of the neighboring moiety. Potential energy scans along the C-H···O coordinate also show that the IMPT in [(MC)2]•+ is a barrierless process.

Conformational Preference Determined by C-H···π Interaction of an O-H···O Hydrogen-Bonded Binary Complex of p-Fluorophenol with 2,5-Dihydrofuran: A Laser-Induced Fluorescence Spectroscopy Study.

Conformational preferences of a binary hydrogen-bonded complex between p-fluorophenol (pFP) and 2,5-dihydrofuran (DHF) have been studied by means of laser induced fluorescence (LIF) spectroscopy in a supersonic jet expansion. Calculation predicts two major conformers for this complex, one having a nearly linear geometry in which the two molecular moieties are bound only by an O-H···O H-bond, but in the other an additional C-H···π type interaction between an ortho C-H group of pFP and ethylene group of DHF contributes to the binding stabilization and results in a folded geometry for the complex with respect to a global view, although the H-bond angle of the latter is relatively larger. This prediction is realized experimentally by identifying transitions corresponding to the two discrete conformers in a vibrationally resolved LIF excitation spectrum of the complex, and the red shifts of S1-S0 origin band of pFP moiety of the two conformers are 542 and 659 cm-1, respectively. The assignments are corroborated by means of dispersed fluorescence (DF) spectroscopy. In comparison, the LIF spectral bands for the pFP-tetrahydrofuran complex can be corresponded to only one conformer, whose S1-S0 origin transition shows a red shift (563 cm-1) somewhat similar to the linear conformer of pFP-DHF complex. Such similarities in spectral shifting behavior is consistent with the predictions of electronic structure calculations. The DF spectra also reveal that the energy threshold and pathways of vibrational dynamics in S1 of the two conformers show different behavior. Excitation to 6a1 level of pFP moiety of the folded conformer displays signatures of restricted intramolecular vibrational energy redistribution (IVR), whereas the linear form displays the emission feature for dissipative IVR.

Antagonistic Interplay Between an Intermolecular CH···O and an Intramolecular OH···O Hydrogen Bond in a 1:1 Complex Between 1,2-Cyclohexanedione and Chloroform: A Combined Matrix Isolation Infrared and Quantum Chemistry Study.

Matrix isolation infrared spectra of a weak C-H···O hydrogen-bonded complex between the keto-enol form of 1,2-cyclohexanedione (HCHD) and chloroform have been measured. The spectra reveal that the intramolecular O-H···O H-bond of HCHD is weakened as a result of complex formation, manifesting in prominent blue shift (∼23 cm-1) of the νO-H band and red shifts (∼7 cm-1) of νC═O bands of the acceptor (HCHD). The νC-H band of donor CHCl3 undergoes a large red shift of ∼33 cm-1. Very similar spectral effects are also observed for formation of the complex in CCl4 solution at room temperature. Our analysis reveals that out of several possible iso-energetic conformational forms of the complex, the one involving antagonistic interplay between the two hydrogen bonds (intermolecular C-H···O and intramolecular O-H···O) is preferred. The combined experimental and calculated data presented here suggest that in condensed media, conformational preferences are guided by directional hyperconjugative charge transfer interactions at the C-H···O hydrogen bonding site of the complex.

Hydrogen bond induced HF elimination from photoionized fluorophenol dimers in the gas phase.

In this paper, we report finding of a remarkable chemical effect of hydrogen bonding, elimination of hydrogen fluoride (HF) from the hydrogen bonded dimers of 2-fluorophenol (2-FP) and 3-fluorophenol (3-FP), in a supersonic jet expansion upon multi-photon ionization using 4th harmonic wavelength (266 nm) of a Q-switched Nd:YAG laser, and the reaction has been probed by time-of-flight mass spectrometry. No HF elimination is observed to occur by such means from the monomer of 3-FP, but it occurs with a small yield from the monomer of 2-FP. On the other hand, upon dimerization the reaction is triggered on for 3-FP, and for 2-FP it becomes so facile that no intact dimer cation survives and only the HF eliminated product ion appears in the mass spectra. Electronic structure calculation shows that in the cationic ground (D0) state, although the reaction for 2-FP dimer is exothermic, the associated barrier is significantly high (2.75 eV) and for its occurrence, absorption of three photons (2+1 type) is required. However, the reaction is predicted barrierless in the intermediate S1 state of this dimer, and HF loss dimer cation mass peak could appear in the mass spectrum due to an effective two-photon (1+1) ionization process. In the case of 3-FP dimer, the energy barriers both in S1 (neutral) and D0 (ionic) states are high, and it is suggested that for occurrence of HF elimination, dimer cation needs to absorb an additional photon. For facilitation of HF loss from this dimer cation, a rearrangement of the geometry and formation of an intermediate adduct have been suggested, and it is argued that the latter could be produced by nucleophilic attack of the neutral moiety at the ortho site of the cationic counterpart.

Matrix Isolation Infrared Spectroscopy of an O-H···π Hydrogen-Bonded Complex between Formic Acid and Benzene.

Mid-infrared spectra of an O-H···π hydrogen-bonded 1:1 complex between formic acid and benzene were measured by isolating the complex in an argon matrix at a temperature of 8 K. The O-H stretching fundamental of formic acid (νO-H) undergoes a red shift of 120 cm(-1), which is the largest among the known π-hydrogen bonded complexes of an O-H donor with respect to benzene as acceptor. Electronic structure theory methods were used extensively to suggest a suitable geometry of the complex that is consistent with a recent study performed at CCSD(T)/CBS level by Zhao et al. (J. Chem. Theory Comput. 2009, 5, 2726-2733), as well as with the measured IR spectral shifts of the present study. It has been determined that density functional theory (DFT) D functionals as well as parametrized DFT functionals like M06-2X, in conjunction with modestly sized basis sets like 6-31G (d, p), are sufficient for correct predictions of the spectral shifts observed in our measurement and also for reproducing the value of the binding energy reported by Zhao et al. We also verified that these low-cost methods are sufficient in predicting the νO-H spectral shifts of an analogous O-H···π hydrogen-bonded complex between phenol and benzene. However, some inconsistencies with respect to shifts of νO-H arise when diffuse functions are included in the basis sets, and the origin of this anomaly is shown to lie in the predicted geometry of the complex. Natural bond orbital (NBO) and atoms-in-molecule (AIM) analyses were performed to correlate the spectral behavior of the complex with its geometric parameters.

LIF Spectroscopy of p-Fluorophenol···Water Complex: Hydrogen Bond Vibrations, Fermi Resonance, and Vibrational Relaxation in the Excited State.

Laser-induced fluorescence (FE) and vibrationally resolved dispersed fluorescence (DF) spectra of a 1:1 water complex of p-fluorophenol (pFP) have been measured in a supersonic jet expansion. The hydrogen bond stretching fundamental (σ01) of the complex appears in the FE spectrum as a doublet with band maxima at 155 and 161 cm-1. Emission spectra recorded upon excitations of the two components reveal that a Fermi resonance between σ1 and a combination involving a low-frequency intramolecular mode of pFP (mode 11) and a bending mode of water at the hydrogen bonded interface (mode ρ1) is responsible for the observed splitting. The DF spectra of the Franck-Condon active 6a01 band (000 + 427 cm-1) of pFP reveals signatures of hydrogen bond induced vibrational energy relaxation (VER) predominantly from the bright (6a01) to a dark (9b1) level in S1. The relative intensities of the emission bands from the locally excited and relaxed levels indicate that VER for excitation up to this level occurs at a time scale similar to the fluorescence decay time of the complex. However, complete VER at a much faster time scale occurs for excitation beyond 822 cm-1 above S1 origin.

Photo-oxidation of Acetone to Formic Acid in Synthetic Air and Its Atmospheric Implication.

Acetone photo-oxidation in synthetic air under exposure of 311 nm ultraviolet light has been studied, and the photo-oxidation products are identified by means of infrared spectroscopy. Analysis reveals that formic acid is one of the major products, although there have been debates in the past concerning the authenticity of formation of this acid in synthetic air via the photo-oxidation pathway. The quantum yield of formation of this acid is similar to that of other major photoproducts like methanol, formaldehyde, and carbon monoxide. The reaction yield, however, decreases with an increase in total air pressure in the reaction cell, but it is still significant at pressures relevant to tropospheric conditions. A kinetic model has been used to simulate the measured reaction kinetics, and the quantum yields predicted by the model are found to be consistent with the measured yields for different durations of light exposure. The same model has also been used to investigate the effect of atmospheric nitric oxide on the fate of formation of this acid in the troposphere. Although nitric oxide is known to be a quencher of peroxy radicals, the precursors of formaldehyde and formic acid in acetone photo-oxidation, but our model predicts that this oxide plays a positive role in the overall reaction kinetics for production of this acid in the troposphere.

On the origin of donor O-H bond weakening in phenol-water complexes.

Matrix isolation infrared spectroscopy has been used to investigate intermolecular interactions in a series of binary O-H⋯O hydrogen bonded phenol-water complexes where water is the common acceptor. The interaction at the binding site has been tuned by incorporating multiple fluorine substitutions at different aromatic ring sites of the phenol moiety. The spectral effects for the aforesaid chemical changes are manifested in the infrared spectra of the complexes as systematic increase in spectral shift of the phenolic O-H stretching fundamental (ΔνO-H). While νO-H bands of the monomers of all the fluorophenols appear within a very narrow frequency range, the increase in ΔνO-H of the complexes from phenol to pentafluorophenol is very large, nearly 90%. The observed values of ΔνO-H do not show a linear correlation with the total binding energies (ΔEb) of the complexes, expected according to Badger-Bauer rule. However, in the same ΔνO-H vs ΔEb plot, nice linear correlations are revealed if the complexes of ortho-fluorophenols are treated separately from their meta/para-substituted analogues. The observations imply that in spite of having the same binding site (O-H⋯O) and the same chemical identities (phenolic), the complexes of ortho and non-ortho fluorophenols do not belong, from the viewpoint of detailed molecular interactions, to a homologous series. Linear correlations of ΔνO-H are, however, observed with respect to the electrostatic component of ΔEb as well as the quantum mechanical charge transfer interaction energy (ECT). From quantitative viewpoint, the latter correlation along with the associated electronic structure parameters appears more satisfactory. It has also been noted that the observed ΔνO-H values of the complexes display a linear relationship with the aqueous phase pKa values of the respective phenol derivatives.

Electronic spectra and excited state dynamics of pentafluorophenol: Effects of low-lying πσ(∗) states.

Multiple fluorine atom substitution effect on photophysics of an aromatic chromophore has been investigated using phenol as the reference system. It has been noticed that the discrete vibronic structure of the S1←S0 absorption system of phenol vapor is completely washed out for pentafluorophenol (PFP), and the latter also shows very large Stokes shift in the fluorescence spectrum. For excitations beyond S1 origin, the emission yield of PFP is reduced sharply with increase in excess vibronic energy. However, in a collisional environment like liquid hydrocarbon, the underlying dynamical process that drives the non-radiative decay is hindered drastically. Electronic structure theory predicts a number of low-lying dark electronic states of πσ(∗) character in the vicinity of the lowest valence ππ(∗) state of this molecule. Tentatively, we have attributed the excitation energy dependent non-radiative decay of the molecule observed only in the gas phase to an interplay between the lowest ππ(∗) and a nearby dissociative πσ(∗) state. Measurements in different liquids reveal that some of the dark excited states light up with appreciable intensity only in protic liquids like methanol and water due to hydrogen bonding between solute and solvents. Electronic structure theory methods indeed predict that for PFP-(H2O)n clusters (n = 1-11), intensities of a number of πσ(∗) states are enhanced with increase in cluster size. In contrast with emitting behavior of the molecule in the gas phase and solutions of nonpolar and polar aprotic liquids, the fluorescence is completely switched off in polar protic liquids. This behavior is a chemically significant manifestation of perfluoro effect, because a very opposite effect occurs in the case of unsubstituted phenol for which fluorescence yield undergoes a very large enhancement in protic liquids. Several dynamical mechanisms have been suggested to interpret the observed photophysical behavior.