Neutral Formic Acid Research Articles

Evidence for bidirectional formic acid translocation in vivo via the Escherichia coli formate channel FocA

Pentameric FocA permeates either formate or formic acid bidirectionally across the cytoplasmic membrane of anaerobically growing Escherichia coli. Each protomer of FocA has its own hydrophobic pore, but it is unclear whether formate or neutral formic acid is translocated in vivo. Here, we measured total and dicyclohexylcarbodiimide (DCCD)-inhibited proton flux out of resting, fermentatively grown, stationary-phase E. coli cells in dependence on FocA. Using a wild-type strain synthesizing native FocA, it was shown that using glucose as a source of formate, DCCD-independent proton efflux was ∼2.5 mmol min−1, while a mutant lacking FocA showed only DCCD-inhibited, FOF1-ATPase-dependent proton-efflux. A strain synthesizing a chromosomally-encoded FocAH209N variant that functions exclusively to translocate formic acid out of the cell, showed a further 20 % increase in FocA-dependent proton efflux relative to the parental strain. Cells synthesizing a FocAT91A variant, which is unable to translocate formic acid out of the cell, showed only DCCD-inhibited proton efflux. When exogenous formate was added, formic acid uptake was shown to be both FocA- and proton motive force-dependent. By measuring rates of H2 production, potassium ion flux and ATPase activity, these data support a role for coupling between formate, proton and K+ ion translocation in maintaining pH and ion gradient homeostasis during fermentation. FocA thus plays a key role in maintaining this homeostatic balance in fermenting cells by bidirectionally translocating formic acid.

Formic acid as a sacrificial agent for byproduct suppression in glucose dehydration to 5-hydroxymethylfurfural using NaY zeolite catalyst

Biomass-derived 5-hydroxymethylfurfural (HMF) holds potential for applications in green materials, but its conventional synthesis is hindered by undesired side reactions. This study presents a catalytic system that effectively suppresses the formation of byproducts, thus enhancing HMF yield. The system demonstrated synergistic effects between Lewis acid NaY zeolite and formic acid sacrificial agent for the production of HMF from glucose. The results indicate that formic acid reacts with reactive intermediates from glucose decomposition, preventing their interactions with other sugar-derived species in the dehydration path to HMF. Such effect originates from the neutral formic acid species rather than the dissociated acidic proton normally observed in Brønsted acid-catalyzed reactions. The NaY/formic acid catalysts in isopropanol/water achieved a 57% HMF yield, a significant improvement over 31% and 27% yields with NaY or formic acid alone, respectively. Moreover, performance of the spent catalysts was easily restored to the original state via a simple NaCl wash.

Infrared action spectroscopy of the deprotonated formic acid trimer, trapped in helium nanodroplets.

Hydrogen bonding interactions are essential in the structural stabilization and physicochemical properties of complex molecular systems, and carboxylic acid functional groups are common participants in these motifs. Consequently, the neutral formic acid (FA) dimer has been extensively investigated in the past, as it represents a useful model system to investigate proton donor-acceptor interactions. The analogous deprotonated dimers, in which two carboxylate groups are bound by a single proton, have also served as informative model systems. In these complexes, the position of the shared proton is mainly determined by the proton affinity of the carboxylate units. However, very little is known about the nature of the hydrogen bonding interactions in systems containing more than two carboxylate units. Here we report a study on the deprotonated (anionic) FA trimer. IR spectra are recorded in the 400-2000 cm-1 spectral range by means of vibrational action spectroscopy of FA trimer ions embedded in helium nanodroplets. Characterization of the gas-phase conformer and assignment of the vibrational features is achieved by comparing the experimental results with electronic structure calculations. To assist in the assignments, the 2H and 18O FA trimer anion isotopologues are also measured under the same experimental conditions. Comparison between the experimental and computed spectra, especially the observed shifts in spectral line positions upon isotopic substitution of the exchangeable protons, suggests that the prevalent conformer, under the experimental conditions, exhibits a planar structure that resembles the crystalline structure of formic acid.

Mechanism of formate-nitrite transporters by dielectric shift of substrate acidity.

Bacterial formate-nitrite transporters (FNTs) regulate the metabolic flow of small, weak mono-acids. Recently, the eukaryotic PfFNT was identified as the malaria parasite's lactate transporter and novel drug target. Despite crystal data, central mechanisms of FNT gating and transport remained unclear. Here, we show elucidation of the FNT transport mechanism by single-step substrate protonation involving an invariant lysine in the periplasmic vestibule. Opposing earlier gating hypotheses and electrophysiology reports, quantification of total uptake by radiolabeled substrate indicates a permanently open conformation of the bacterial formate transporter, FocA, irrespective of the pH Site-directed mutagenesis, heavy water effects, mathematical modeling, and simulations of solvation imply a general, proton motive force-driven FNT transport mechanism: Electrostatic attraction of the acid anion into a hydrophobic vestibule decreases substrate acidity and facilitates protonation by the bulk solvent. We define substrate neutralization by proton transfer for transport via a hydrophobic transport path as a general theme of the Amt/Mep/Rh ammonium and formate-nitrite transporters.

Nonequimolar Mixture of Organic Acids and Bases: An Exception to the Rule of Thumb for Salt or Cocrystal.

Formation of salt and/or cocrystal from organic acid-base mixtures has significant consequences in the pharmaceutical industry and its related intellectual property rights (IPR). On the basis of calculations using periodic dispersion corrected DFT (DFT-D2) on formic acid-pyridine adduct, we have demonstrated that an equimolar stoichiometric ratio (1:1) exists as a neutral cocrystal. On the other hand, the nonequimolar stoichiometry (4:1) readily forms an ionic salt. While the former result is in agreement with the ΔpKa rule between the base and the acid, the latter is not. Calculations reveal that, within the equimolar manifold (n:n; n = 1-4), the mixture exists as a hydrogen bonded complex in a cocrystal-like environment. However, the nonequimolar mixture in a ratio of 5:1 and above readily forms salt-like structures. Because of the cooperative nature of hydrogen bonding, the strength of the O-H···N hydrogen bond increases and eventually transforms into O(-)···H-N(+) (complete proton transfer) as the ratio of formic acid increases and forms salt as experimentally observed. Clearly, an enhanced polarization of formic acid on aggregation increases its acidity and, hence, facilitates its transfer to pyridine. Motion of the proton from formic acid to pyridine is shown to follow a relay mechanism wherein the proton that is far away from pyridine is ionized and is subsequently transferred to pyridine via hopping across the neutral formic acid molecules (Grotthuss type pathway). The dynamic nature of protons in the condensed phase is also evident for cocrystals as the barrier of intramolecular proton migration in formic acid (leading to tautomerism), ΔH(⧧)tautomer = 17.1 kcal/mol in the presence of pyridine is half of that in free formic acid (cf. ΔH(⧧)tautomer = 34.2 kcal/mol). We show that an acid-base reaction can be altered in the solid state to selectively form a cocrystal or salt depending on the strength and nature of aggregation.

Theoretical investigation on the ruthenium catalyzed dehydrogenation of formic acid and ligand effect

Density functional theory (DFT) calculations were used to study the mechanisms of dehydrogenation of formic acid catalyzed by two Ru complexes, including P(CH2CH2PPh2)3RuH+ (P(CH2CH2PPh2)3=tris[2-(diphenylphosphino)ethyl]phosphine) and P(CH2CH2CH2PiPr2)3RuH+ (P(CH2CH2CH2PiPr2)3=tris[3-(diisopropylphosphino)propyl]phosphine). Two competing catalytic cycles (I and II) have been explored. In cycle I, the catalytic reaction starts with a direct hydride transfer from HCOO− to a Ru center, releasing CO2 in the first place, followed by H2 production, while in cycle II, neutral formic acid approaches Ru catalyst to produce H2 molecule prior to CO2 generation via β-hydride elimination. The computational results show that cycle I is more accessible than cycle II, regardless of the ligands surrounding Ru center. The calculated overall Gibbs free energy barriers for formic acid dehydrogenation catalyzed by P(CH2CH2PPh2)3RuH+ (14.3kcal/mol) is significantly lower than its iron analogue in the previous experimental and theoretical studies (18.9kcal/mol) at the same level of theory. More interestingly, isopropyl group, a more electron-donating group than phenyl group, leads to an even lower reaction barrier of P(CH2CH2CH2PiPr2)3RuH+ (12.9kcal/mol) than P(CH2CH2PPh2)3RuH+, indicating a more positive role the ligand could potentially play in the performance of the catalysts. Our results pave a new way to design more efficient catalysts for formic acid dehydrogenation.

CONTRIBUTIONS FROM GRAIN SURFACE AND GAS PHASE CHEMISTRY TO THE FORMATION OF METHYL FORMATE AND ITS STRUCTURAL ISOMERS

Both grain surface and gas phase chemistry have been invoked to explain the disparate relative abundances of methyl formate and its structural isomers acetic acid and glycolaldehyde in the Sgr B2(N) star-forming region. While a network of grain surface chemistry involving radical-radical reactions during the warm-up phase of a hot core is the most chemically viable option proposed to date, neither qualitative nor quantitative agreement between modeling and observation has yet been obtained. In this study, we seek to test additional grain surface and gas phase processes to further investigate methyl formate-related chemistry by implementing several modifications to the Ohio State University gas/grain chemical network. We added two new gas phase chemical pathways leading to methyl formate, one involving an exothermic, barrierless reaction of protonated methanol with neutral formic acid; and one involving the reaction of protonated formic acid with neutral methanol to form both the cis and trans forms of protonated methyl formate. In addition to these gas phase processes, we have also investigated whether the relative product branching ratios for methanol photodissociation on grains influence the relative abundances of methyl formate and its structural isomers. We find that while the new gas phase formation pathways do not alter the relative abundances of methyl formate and its structural isomers, changes in the photodissociation branching ratios and adjustment of the overall timescale for warm-up can be used to explain their relative ratios in Sgr B2(N).

ION-ICE ASTROCHEMISTRY: BARRIERLESS LOW-ENERGY DEPOSITION PATHWAYS TO HCOOH, CH3OH, AND CO2ON ICY GRAIN MANTLES FROM PRECURSOR CATIONS

A new family of very favorable reaction pathways is explored involving the deposition of ions on icy grain mantles with very low energies. Quantum chemical cluster calculations at the MP2/6-31+G** level in 4H2O clusters and at the B3LYP/6-31+G** level in 17H2O clusters indicate that HCO+ and CH3+ are able to react spontaneously with one of the water molecules in the cluster to form protonated formic acid (HCOOH2+) and protonated methanol (CH3OH2+), respectively. It is furthermore found that these initial adducts spontaneously transfer their excess protons to the cluster to form neutral formic acid and methanol, plus solvated hydronium, H3O+. In the final case, if a CO molecule is bound to the surface of the cluster, OH+ may react with it to form protonated carbon dioxide (HCO2+), which then loses its proton to yield CO2 and H3O+. In the present model, all of these processes were found to occur with no barriers. Discussion includes the analogous gas-phase processes, which have been considered in previous studies, as well as the competitive abstraction pathway for HCO+ + H2O.

Single photon ionization of hydrogen bonded clusters with a soft x-ray laser: (HCOOH)x and (HCOOH)y(H2O)z

Pure, neutral formic acid (HCOOH)n+1 clusters and mixed (HCOOH)(H2O) clusters are investigated employing time of flight mass spectroscopy and single photon ionization at 26.5 eV using a very compact, capillary discharge, soft x-ray laser. During the ionization process, neutral clusters suffer little fragmentation because almost all excess energy above the vertical ionization energy is taken away by the photoelectron, leaving only a small part of the photon energy deposited into the (HCOOH)n+1+ cluster. The vertical ionization energy minus the adiabatic ionization energy is enough excess energy in the clusters to surmount the proton transfer energy barrier and induce the reaction (HCOOH)n+1+-->(HCOOH)nH+ +HCOO making the protonated (HCOOH)nH+ series dominant in all data obtained. The distribution of pure (HCOOH)nH+ clusters is dependent on experimental conditions. Under certain conditions, a magic number is found at n=5. Metastable dissociation rate constants of (HCOOH)nH+ are measured in the range (0.1-0.8)x10(4) s(-1) for cluster sizes 4<n<9. The rate constants display an odd/even alternating behavior between monomer and dimer loss that can be attributed to the structure of the cluster. When small amounts of water are added to the formic acid, the predominant signals in the mass spectrum are still (HCOOH)nH+ cluster ions. Also observed are the protonated mixed cluster series (HCOOH)n(H2O)mH+ for n=1-8 and m=0-4. A magic number in the cluster series n=5, m=1 is observed. The mechanisms and dynamics of formation of these neutral and ionic clusters are discussed.

Weak hydrogen bonds in complexes pairing monohalomethanes with neutral formic acid

Ab initio calculations are used to analyze the interaction between formic acid and CH 3X, for X equal to each of F, Cl, and Br. All minima are cyclic in that they contain more than one H-bond. The most strongly bound contain a OH⋯X bond, along with CH⋯O, and the others contain CH⋯X and CH⋯O interactions. Alterations of the covalent bond lengths within each subunit, and vibrational frequency shifts, coupled with electronic charge shifts, reveal fundamental features of these complexes, and the nature of the interactions. The OH⋯X bond is the strongest of those examined here, followed by CH⋯X and CH⋯O.

Strong Decrease of the Benzene−Ammonium Ion Interaction upon Complexation with a Carboxylate Anion

The ternary complex between benzene, ammonium ion, and formate ion has been investigated computationally. The total interaction energy is found to be much lower than the sum of the three pairwise interactions, due to nonadditive polarizations (intra- and intermolecular). The interaction with a benzene cannot stabilize the ammonium−formate complex sufficiently to avoid gas-phase collapse to the neutral formic acid−ammonia complex.

On the Origin of Substrate Directing Effects in the Epoxidation of Allyl Alcohols with Peroxyformic Acid

The reactant cluster and transition state for epoxidation of allyl alcohol with peroxyformic acid have been located at the MP2/6-31G(d) level of theory. The free energy of activation (Δ = 19.8 kcal/mol) predicted at the MP4//MP2/6-31G(d) level is quite comparable with experimental data for epoxidation of 3-hydroxycyclohexene (Δ = 19.7 kcal/mol). A spiro transition state (TS) was found where the plane defined by the peroxyacid moiety is oriented at 89° to the CC bond axis. Intrinsic reaction coordinate analysis suggests that after the barrier is crossed a 1,4-hydrogen migration of the peroxyacid hydrogen to the carbonyl oxygen takes place in concert with O−O bond cleavage affording the epoxide of allyl alcohol hydrogen bonded to the neutral formic acid leaving group. The activation parameters calculated at the B3LYP/6-311G(d,p) level are in excellent agreement with the MP4//MP2 values. The transition structure with the allyl alcohol O−C−CC dihedral angle of 16.4° is 2.1 kcal/mol lower in energy than a tran...

Use of weak acids to determine the bulk diffusion limitation of H+ ion conductance through the gramicidin channel

The addition of 2 M formic acid at pH 3.75 increased the single channel H+ ion conductance of gramicidin channels 12-fold at 200 mV. Other weak acids (acetic, lactic, oxalic) produce a similar, but smaller increase. Formic acid (and other weak acids) also blocks the K+ conductance at pH 3.75, but not at pH 6.0 when the anion form predominates. This increased H+ conductance and K+ block can be explained by formic acid (HF) binding to the mouth of the gramicidin channel (Km = 1 M) and providing a source of H+ ions. A kinetic model is derived, based on the equilibrium binding of formic acid to the channel mouth, that quantitatively predicts the conductance for different mixtures of H+, K+, and formic acid. The binding of the neutral formic acid to the mouth of the gramicidin channel is directly supported by the observation that a neutral molecule with a similar structure, formamide (and malonamide and acrylamide), blocks the K+ conductance at pH 6.0. The H+ conductance in the presence of formic acid provides a lower bound for the intrinsic conductance of the gramicidin channel when there is no diffusion limitation at the channel mouth. The 12-fold increase in conductance produced by formic acid suggests that greater than 90% of the total resistance to H+ results from diffusion limitation in the bulk solution.

Negative ion-molecule reactions in formic acid.

Negative ion-molecule reaction has been investigated for formic acid utilizing a conventional type mass spectrometer. At elevated pressure negative ions react with neutral species in the ion source of mass spectrometer. It was found that a negative ion of mass 91 is produced by the reaction of the HCOO- ion with neutral formic acid asHCOO-+HCOOH→HCOOHCOOH-This reaction has two processes, i.e. a dissociative resonance electron capture (DREC) process and a high energy electron process. From the studies of HCOOH gas pressure dependence it was observed that the intensities of the HCOO- and HCOOHCOOH- ions in a DREC process increase with first and second order of the HCOOH gas pressure, whereas in a high energy electron process, they increase with fourth and seventh order of the pressure, respectively. The reaction rate constants of two processes were determined as (6.8 ±1.5) ×10-13 cc/molecule.sec for a DREC process and (1.3 ±0.5) ×10-37 cc3/molecule3·sec for a high energy electron process.