Determination of pKa at the air/water interface by second harmonic generation

Zhao, Xiaolin; Subrahmanyan, Suchitra; Eisenthal, Kenneth B.

doi:10.1016/0009-2614(90)85263-c

Cited by 118 publications

(124 citation statements)

References 15 publications

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“…Because aqueous surfaces are usually charged relative to the bulk liquid (15), the thermodynamic requirement of uniform electrochemical activity throughout (including the interfacial regions) implies that the chemical activity of protons (pH) in IF could be different from that in the B liquid. Reduced hydration of ionic species at the interface could force acids and bases toward their undissociated forms (16).…”

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confidence: 99%

Brønsted basicity of the air–water interface

Mishra

Enami

Nielsen

et al. 2012

Proc. Natl. Acad. Sci. U.S.A.

168

203

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Differences in the extent of protonation of functional groups lying on either side of water-hydrophobe interfaces are deemed essential to enzymatic catalysis, molecular recognition, bioenergetic transduction, and atmospheric aerosol-gas exchanges. The sign and range of such differences, however, remain conjectural. Herein we report experiments showing that gaseous carboxylic acids RCOOH(g) begin to deprotonate on the surface of water significantly more acidic than that supporting the dissociation of dissolved acids RCOOH(aq). Thermodynamic analysis indicates that > 6 H 2 O molecules must participate in the deprotonation of RCOOH(g) on water, but quantum mechanical calculations on a model air-water interface predict that such event is hindered by a significant kinetic barrier unless OH − ions are present therein. Thus, by detecting RCOO − we demonstrate the presence of OH − on the aerial side of on pH > 2 water exposed to RCOOH(g). Furthermore, because in similar experiments the base (Me) 3 N(g) is protonated only on pH < 4 water, we infer that the outer surface of water is Brønsted neutral at pH ∼3 (rather than at pH 7 as bulk water), a value that matches the isoelectric point of bubbles and oil droplets in independent electrophoretic experiments. The OH − densities sensed by RCOOH(g) on the aerial surface of water, however, are considerably smaller than those at the (>1 nm) deeper shear planes probed in electrophoresis, thereby implying the existence of OH − gradients in the interfacial region. This fact could account for the weak OH − signals detected by surface-specific spectroscopies.gas-liquid reactions | surface potential | water surface acidity | interfacial proton transfer A cid-base chemistry at aqueous interfaces lies at the heart of major processes in chemistry and biology. Changes in the degree of dissociation of the acidic/basic residues upon translocation between aqueous and hydrophobic microenvironments orchestrate enzyme catalysis (1), drive proton/electron transport across biomembranes (2, 3), and mediate molecular recognition and self-assembly phenomena (4-6). Despite its importance, the characterization of acid-base chemistry at aqueous interfaces remains fraught with uncertainties (7-11). Basic questions linger about the thickness of interfacial layers (12), how acidity changes through the interfacial region (13), and the mechanistic differences between proton transfer (PT) across interfacial (IF) versus in bulk (B) water (10,14). Because aqueous surfaces are usually charged relative to the bulk liquid (15), the thermodynamic requirement of uniform electrochemical activity throughout (including the interfacial regions) implies that the chemical activity of protons (pH) in IF could be different from that in the B liquid. Reduced hydration of ionic species at the interface could force acids and bases toward their undissociated forms (16).These fundamental issues have been extensively investigated via electrostatic (17) and electrokinetic experiments (11), surface tension studies and analysis (1...

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confidence: 99%

Brønsted basicity of the air–water interface

Mishra

Enami

Nielsen

et al. 2012

Proc. Natl. Acad. Sci. U.S.A.

168

203

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“…Although the exact surface pH of water is debated (21-23), the surface is known to alter the pK a of surfaceactive molecules toward their neutral form (24,25), which aids in the promotion of peptide bond chemistry at the interface by reducing zwitterion formation and alleviating the kinetic constraint on peptide bond synthesis. The air-water interface has been reported in the literature to have a catalytic role in peptide bond formation (26-29) using synthetic long-chain amino acid esters Author contributions: E.C.G and V.V.…”

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In situ observation of peptide bond formation at the water–air interface

Griffith

Vaida

2012

Proc. Natl. Acad. Sci. U.S.A.

136

155

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We report unambiguous spectroscopic evidence of peptide bond formation at the air-water interface, yielding a possible mechanism providing insight into the formation of modern ribosomal peptide bonds, and a means for the emergence of peptides on early Earth. Protein synthesis in aqueous environments, facilitated by sequential amino acid condensation forming peptides, is a ubiquitous process in modern biology, and a fundamental reaction necessary in prebiotic chemistry. Such reactions, however, are condensation reactions, requiring the elimination of a water molecule for every peptide bond formed, and are thus unfavorable in aqueous environments both from a thermodynamic and kinetic point of view. We use the hydrophobic environment of the air-water interface as a favorable venue for peptide bond synthesis, and demonstrate the occurrence of this chemistry with in situ techniques using Langmuir-trough methods and infrared reflection absorption spectroscopy. Leucine ethyl ester (a small amino acid ester) first partitions to the water surface, then coordinates with Cu 2þ ions at the interface, and subsequently undergoes a condensation reaction selectively forming peptide bonds at the air-water interface.

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“…16,18 On the other hand, for an organic chromophore with long carbon chains, the distribution ratio in the water phase is negligible. 19 Therefore, for a liquidliquid interface between water and a non-polar solvent (e.g., cyclohexane) containing an insoluble chromophore, such as dioctadecyl-rhodamine B (Rh-C18), it is reasonable to suppose that only those dye molecules remaining at the interface contribute to the fluorescence signal.…”

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confidence: 99%