Double Minimum Proton Potential Research Articles

Hydrated Excess Protons in Acetonitrile/Water Mixtures: Solvation Species and Ultrafast Proton Motions

The solvation structure of protons in aqueous media is highly relevant to electric properties and to proton transport in liquids and membranes. At ambient temperature, polar liquids display structural fluctuations on femto- to picosecond time scales with a direct impact on proton solvation. We use two-dimensional infrared (2D-IR) spectroscopy to follow proton dynamics in acetonitrile/water mixtures with the Zundel cation H5O2+ prepared in neat acetonitrile as a benchmark. The 2D-IR spectra of the proton transfer mode of H5O2+ demonstrate stochastic large-amplitude motions in the double-minimum proton potential, driven by fluctuating electric fields. In all cases, the excess proton is embedded in a water dimer, forming an H5O2+ complex as the major solvation species. This observation is rationalized by quantum mechanics/molecular mechanics molecular dynamics simulations including up to four water molecules embedded in acetonitrile. The Zundel motif interacts with its closest water neighbor in an H7O3+ unit without persistent proton localization.

Car-Parrinello Molecular Dynamics Simulations of Infrared Spectra of Crystalline Vitamin C with Analysis of Double Minimum Proton Potentials for Medium-Strong Hydrogen Bonds.

We studied proton dynamics of a hydrogen bonds of the crystalline l-ascorbic acid. Our approach was based on the Car-Parrinello molecular dynamics. The focal point of our study was simulation of the infrared spectra of l-ascorbic acid associated with the O-H stretching modes that are very sensitive to the strength of hydrogen bonding. In the l-ascorbic acid there are four kinds of hydrogen bonds. We calculated their spectra by using anharmonic approximation and the time course of the dipole moment function as obtained from the Car-Parrinello simulation. The quantization of the nuclear motion of the protons was made to perform detailed analysis of strength and properties of hydrogen bonds. We presented double minimum proton potentials with small value of barriers for medium-strong hydrogen bonds. We have also shown the difference character of medium-strong hydrogen bonds compared to weaker hydrogen bonds in the l-ascorbic acid.

1H NMR studies of proton transfer equilibria in hydrogen bonds – the role of the entropy

The influence of the entropy on the proton transfer equilibria AH···B [Formula: see text]A–···H+B in the dimethylphosphinic acid (DMP)-N-base and in the methanesulfonic acid (MSA)-N-oxide hydrogen bonds is studied by 1H NMR spectroscopy as a function of the ΔpKa, i.e., the pKa of the base minus the pKa of the acid. It is shown that with increasing ΔpKa the proton transfers to the N-oxide acceptor. In the DMP + N-base family of systems, first the double-minimum proton potentials (ΔH = 0) occurs and only with further increasing ΔpKa do the POH···N [Formula: see text] PO–···H+N equilibria become symmetrical (ΔG = 0). ΔG and ΔH are connected by the Gibbs–Helmholtz equation, ΔG = ΔH –TΔS. Hence, the reason of this effect is the large negative interaction entropy term (ΔSI) arising from the large order around the polar structure, which shifts the equilibria strongly to the left-hand side. With the MSA-N-oxide hydrogen bonds a single-minimum proton potential was found. The minimum shifts with increasing ΔpKa from the donor to the acceptor of the O···H+···N hydrogen bonds. It is shown that with increasing ΔpKa the proton potential becomes on average symmetrical, whereas the proton is still largely on the left-hand side. This result is also explained by the large negative interaction entropy due to the order of the environment if the proton is at the acceptor B.Key words: entropy, hydrogen bonds, infrared, spectroscopy, proton transfer.

FT-IR Investigation of OH···N ⇌ O-···H+N Hydrogen Bonds with Large Proton Polarizability in Phosphinic Acid + N-Base Systems in the Middle and Far Infrared Region

Sixteen 1:1 dimethylphosphinic acid + N-base systems were studied in acetonitrile−chloroform (1:2) solutions as a function of the basicity of the N-bases. The complexes were measured in the middle infrared (MIR) and far-infrared (FIR) region at 20 °C and at −40 °C. The observed IR continua demonstrate that in the systems with the weaker bases an asymmetrical double minimum proton potential with the deeper well at the acid site is present. With increasing basicity the well at the base site becomes deeper and deeper and the proton potentials obtain a more symmetrical shape. The largest proton polarizability is attained with the system that shows the (on the average) most symmetrical proton potential, as indicated by the maximum bathochromic shift of the IR continuum. This shift toward lower wavenumbers is largest with the dimethylphosphinic acid + triallylamine complex. The most intense integrated absorbance of the IR continuum is also observed in this system. The characteristic intensity distribution at the symmetry point indicates medium-strong and relatively long OH···N ⇌ O-···H+N hydrogen bonds. With further increasing basicity the proton potential becomes asymmetrical again, but now with the deeper well at the base site. The proton transfer was studied considering the PO stretching vibration bands. The simultaneous observation of the acid as well as the acid anion PO bands reflect a proton-transfer equilibrium between the nonpolar and polar structure. The far-IR region demonstrates that in the most symmetrical cases the IR continua extend down to about 100 cm-1. Moreover, in contrast to the results with other families of systems, it was for the first time possible to distinguish the nonpolar and polar structure in the far-IR region. Furthermore, the position of the hydrogen bond vibration indicates a significant trend of the force constant. The precise evaluation shows that the system with the (on the average) most symmetrical hydrogen bonds is achieved with a little weaker base than triallylamine, showing the largest bathochromic shift of the IR continuum.

FTIR investigation of polarizable hydrogen bonds in carboxylic acid–pyridine complexes in the mid- and far-IR region

Twelve carboxylic acid–pyridine systems have been studied in chloroform solutions in the mid-IR and the far-IR regions as a function of the pKa of the carboxylic acids at 25 °C and at –40 °C. If the acidity of the carboxylic acid is small a strongly asymmetrical double-minimum proton potential is present. With increasing acidity the hydrogen bonds become stronger, i.e. the bond length shortens and the proton potentials become more symmetrical in character. In these cases the barrier of the potentials becomes very low and the situation may be compared with a broad, flat, asymmetrical single minimum. Therefore, the continua in these complexes are very similar to those in systems with broad, flat, single-minimum proton potentials. With increasing polarity of the complexes the hydrogen bonds lengthen again and the double minimum becomes still more symmetrical. The two proton-limiting structures OH⋯N ⇌ O–⋯H+N can be distinguished by different carbonyl bands; the band of the polar structure is found at relatively high wavenumbers indicating that the barrier is low. Almost symmetrical double-minimum proton potentials are now present. With the most acidic system at –40 °C, the polar structure has a larger weight.

Hydrogen-bonded chains with large proton polarizability as charge conductors in proteins Bacteriorhodopsin and the F 0 subunit of E. coli

Hydrogen bonds with a double minimum proton potential show large proton polarizability due to shifts of the proton. Particularly large proton polarizabilities are observed due to the collective motion of the protons within hydrogen-bonded chains. This result was demonstrated by experimental and theoretical treatments. The proton polarizability of hydrogen bonds is indicated by continua in the IR. Hydrogen-bonded chains with this property are very effective proton pathways since they may conduct protons within picoseconds. It is demonstrated that in the L 550 intermediate of bacteriorhodopsin such a pathway is present which conducts protons from the active site to the outside of the membrane. Furthermore, it is demonstrated that in the F 0 subunit a pathway with such properties is present. These pathways are discussed in detail.

Active site of bacteriorhodopsin. FTIR and 1H NMR studies using a model molecule

We obtained the 2-N-methylaminoethylguanidine amide, 4, of Kemp's triacid(all-cis-1,3,5-trimethylcyclohexane 1,3,5-tricarboxylic acid) as a model substance for the active site of bacteriorhodopsin. Compound 4 was synthesised from Kemp's triacid triethyl ester, 1, in three reactions. Compound 4 and its complex with tetrabutylammonium 4-methylphenolate were studied by FTIR and 1H NMR spectroscopy in acetonitrile solutions. In the case of compound 4, two types of hydrogen bond are formed: one is the CO2H⋯N⇌CO–2⋯H+N bond. In this case, the donor is one of the two carboxylic acid groups, and the acceptor, the guanidine group. A double-minimum proton potential is present in this bond and therefore it exhibits large proton polarizability. The second NH⋯OC hydrogen bond formed between the protonated guanidine (proton donor group) and the carbonyl O atom of the other carboxylic group is asymmetrical. The proton is localised at the guanidine residue. If a phenolate molecule is added to the solution of compound 4, the situation changes dramatically. A PhOH⋯N⇌PhO–⋯H+N bond with large proton polarizability is formed between the phenolate and guanidine groups. The polarizable carboxylic acid–guanidine hydrogen bond is broken and the asymmetrical NH⋯O bond between guanidine and the O atoms of carboxylic acid becomes much stronger. The results obtained with the model are compared with those obtained earlier with bacteriorhodopsin.

Intermolecular Li+and Na+bonds in N-(1,6-dihydro-6-oxopyrimidin-2-yl) amino acids: cation polarizabilities studied by Fourier-transform infrared spectroscopy

The tetrabutylammonium, Li+ and Na+ complexes of six N-(1,6-dihydro-6-oxopyrimidin-2-yl) amino acids (CAA) were studied by FTIR spectroscopy and compared with results obtained earlier with isomeric amino acids. In all amino acids an intramolecular OH⋯ON ⇌ O–⋯H+ON hydrogen bond with large proton polarizability has been observed. In the tetrabutylammonium salts of the amino acids the carboxylic acid group forms a strong asymmetrical NH⋯O– bond. In all Li+ compounds intramolecular Li+ bonds with double-minimum proton potentials and large ‘Li+ polarizability’ are found. In the case of the Na+ bonds flat, single-minimum Na+ potentials are present in the intramolecular Na+ bonds. The amplitude of the fluctuation, and hence the ‘Na+ polarizability’, is however small owing to the large mass of the Na+ ions.

Discussion of the proton potential with proton-transfer equilibria: thermodynamic data and infrared continua as a function of temperature

The infrared continua caused with intramolecular hydrogen bonds of the type AH⋯B ⇌ A–⋯H+B have been studied as a function of the ΔpKa of the hydrogen-bond donor and acceptor as well as a function of the temperature. From the behaviour of these continua it is shown that the shape of the proton potential may change with decreasing temperature from a double minimum with the deeper well at the donor to proton potential with the deeper well at the acceptor. The In (KPT) over 1/T plots result, however, always in negative apparent ‘ΔH° values’. Hence with all such proton-transfer hydrogen bonds the deeper well of the double-minimum proton potentials should always be at the acceptor group.

Change of the proton potential from a single to a double minimum in intramolecular hydrogen bonds with increasing p Ka of the phenolic group in 4-R-Mannich base N-oxides

Nine 4-R-Mannich base N-oxides have been studied by FTIR spectroscopy. The intramolecular hydrogen bonds formed between the phenolic and the N-oxide group show proton polarizability. If the phenolic group is relatively acidic a broad flat single minimum proton potential with the well at the NO group is found, whereas if the phenolic group is more basic a double minimum proton potential is observed. An analogous change of the shape of the proton potential was observed earlier in the case of carboxylic acid/trimethylammonium N-oxide systems if the acidity of the carboxylic acid increases. The reasons for this change of the proton potential are explained.

An intramolecular hydrogen bond with large proton polarizability within the head group of phosphatidylserine. An infrared investigation

Films of O-phospho-L-serine-P-ethylester (PSE) were studied by infrared spectroscopy. PSE films were studied pure and as 1:1 mixture with LiOH, NaOH, KOH, and Ca(OH)2 as a function of the degree of hydration. The same investigations were performed if (L-glu)n was added to the system (ratio 1:1, PSE/glu residue). In the PSE molecules an intramolecular (I) COOH...-OP in equilibrium with COO-...HOP (II) hydrogen bond is present. In this bond a double minimum proton potential occurs and it shows large proton polarizability. This hydrogen bond is relatively stable as shown by the neutralization experiments. At low degree of hydration the cations are present at the phosphate groups. The Li ions polarize the intramolecular hydrogen bonds much more than the other cations, i.e., the weight of the proton-limiting structure COOH...-OP is increased by Li ions. Regarding these results one has to assume that such a hydrogen bond is also present in the phosphatidylserine head groups. It is discussed that such hydrogen bonds could be part of a lateral charge-conducting system in the polar surfaces of biological membranes. Such systems could connect proton-creating and proton-consuming centers at the membrane surface and conduct positive charge at an extremely high rate.

Influence of phenol acidity and solvent polarity with phenol—retinal schiff base hydrogen bonds—thermodynamic parameters of bond formation and proton transfer

Phenol—butylamine Schiff base 1:1 systems are studied in methylcyclohexane solutions. The thermodynamic data (Δ H 0 and Δ S 0) of the hydrogen bond formation and of the proton transfer in the OH•••N ⇌ O −•••H +N bonds are determined using IR spectroscopy. In all systems studied these hydrogen bonds show large proton polarizability since double-minimum proton potentials are present, whereby the deeper well is at the N. The amounts of the negative Δ H 0 PT and Δ S 0 PT values increase in proportion to the Δp K a (p K a of the protonated Schiff base minus p K a of the phenol). For one phenol—Schiff base system the thermodynamic quantities were determined using a variety of solvents of different polarity. The amounts of the negative Δ H 0 PT and Δ S 0 PT values increase with increasing polarity of the solvent. A retinal Schiff base is the chromophore in visual pigment proteins (rhodopsin and bacteriorhodopsin). On the basis of our experimental results conclusions are presented concerning proton transfer processes between the side group of the amino acid tyrosine (which is a phenolic group) and the retinal Schiff base in the protein.

Thermodynamics of proton transfer in carboxylic acid-retinal Schiff base hydrogen bonds with large proton polarizability

During the photocycle of bacteriorhodopsin (BR) the chromophore, a retinal Schiff base, is deprotonated. Simultaneously an asp residue is protonated. These results suggest that this deprotonation occurs via a Schiff base - asp hydrogen bond. Therefore, we studied carboxylic acid - retinal Schiff base model systems in CCl4 using IR spectroscopy. The IR spectra show that double minimum proton potentials are present in the OH ... N in equilibrium with O- ... HN+ H-bonds formed and that the proton can easily be shifted in these bonds by local electrical fields. The thermodynamic data of H-bond formation and proton transfer within these H-bonds are determined. On the basis of these data a hypothesis is developed with regard to the molecular mechanism of the deprotonation of the Schiff base of BR.

31] Proton polarizability of hydrogen bonds: Infrared methods, relevance to electrochemical and biological systems

Publisher Summary This chapter describes the importance of infrared methods relevant to electrochemical and biological systems for proton polarizability of hydrogen bonds. Infrared spectroscopy is used for the identification of groups. The dissociation process of salts is studied by infra red (IR) spectroscopy if ions with degenerate vibrations are present. Completely different bands are observed in IR spectra if protons are added or removed from groups. The degree of dissociation is obtained from these band changes. Infrared bands are shifted by intramolecular and especially by intermolecular interactions because the potential curves are deformed. The special case discussed in the chapter is hydrogen bonds with double minimum proton potentials, or H-bonded systems with multiminimum proton potentials. The presence of H bonds and H-bonded systems showing large proton polarizability is demonstrated in the IR spectra by continuous absorptions. It is performed using poly (amino acid) + monomer systems, poly (amino acids), the copolymers of amino acids, the mixtures of acidic and basic amino acids, which are protected at their α-amino and 1-carboxylate groups, and other model systems. The instrumentation and experimental procedures for the studies of biological molecules are also reviewed in the chapter.

Analysis of proton tunnelling in KDP using a realistic proton potential

The authors argue that because of the skewness of the double-minimum proton potential and the magnitude of the ground-state splitting it is not possible to construct fully localised single-particle proton states in KDP. It follows the proton-proton interactions differ according to whether each proton is in its lowest symmetrical or antisymmetrical state, and that the transverse-field Ising-model Hamiltonian must be supplemented by a further interaction term bilinear in the pseudospin operator Sx. This argument is strongly supported in the crystallographic studies by Nelmes (1980) of the temperature variation of the atom positions in KDP and DKDP.

Intramolecular, negatively charged, easily polarizable hydrogen bonds in monosalts of diols. Infrared and nuclear magnetic resonance studies

Monotetraethylammonium salt solutions of o-bis(hydroxymethyl)benzene (compound 1), of 2,5-dimethylhexane-2,5-diol (compound 2), of 2,4-dimethyl-pentane-2,4-diol (compound 3), and of 2,3-dimethylbutane-2,3-diol (compound 4) were studied by n.m.r. and i.r. spectroscopies. In the mono salts, intramolecular OH…O–⇌–O…HO hydrogen bonds are formed quantitatively. Very intense continua in the i.r. spectra demonstrate that these hydrogen bonds are easily polarizable. With compounds 1 and 2 these continua extend from 3000 cm–1 over the whole region studied. Comparison with theory shows that these hydrogen bonds are of medium length, and that typical double-minimum proton potentials are present in these hydrogen bonds. This confirms the n.m.r. result that the proton is delocalized in these hydrogen bonds. In the series of compounds 2–4, owing to sterical conditions, these intramolecular hydrogen bonds become weaker, the intensity of the continua decreases a little, and at smaller wavenumbers it vanishes to a large extent.

A non-charged, easily polarizable, intramolecular, hydrogen bond in 2-(α-pyridyl N-oxide)ethane sulphonic acid

Non-charged intramolecular hydrogen bonds formed in 2-(α-pyridyl N-oxide)ethane sulphonic acid cause strong continuous absorption; thus, these hydrogen bonds are easily polarizable. A double minimum proton potential with a deeper well at the oxygen atom of the NO group is present in these hydrogen bonds.

Proton transfer in and polarizability of hydrogen bonds in proteins. Tyrosine-lysine and glutamic acid-lysine hydrogen bonds ? IR investigations

The OH ⋯ N ⇌ O− ⋯ H+N hydrogen bonds formed between tyrosine and lysine, and between glutamic acid and lysine residues are studied by infrared spectroscopy considering the following systems: (l-lys)n + phenol, copoly (l-lys, l-tyr)n, (l-lys)n + (l-tyr)n and (l-lys)n + (l-glu)n. The phenol-lysine hydrogen bonds are largely symmetrical in the average if the pKa of the protonated lysine is 2.2 units larger than that of the phenols. In the case of the hydrogen bonds between tyrosine and lysine residues in copoly (l-lys, l-tyr)n and (l-lys)n + (l-tyr)n, the weight of the proton limiting structure OH ⋯ N is 80–90%, and that of the polar O− ⋯ H+N structure 10–20%. Double minimum proton potentials occur but the proton is preferentially present at the tyrosine residues. In the (l-lys)n + (l-glu)n system, the protons are present at the lysine residues. Thus, these hydrogen bonds have very large dipole moments (about 10 D). With the lysine-phenole hydrogen bonds, hydration shifts the proton transfer equilibrium a little in favour of the polar proton limiting structure O− ⋯ H+N. These hydrogen bonds are broken to a large extent, however, when only about 3 water molecules are present per lysine residue. When less water is present, as in the copoly (l-lys, l-tyr)n and (l-lys)n + (l-tyr)n systems, these hydrogen bonds are, however, formed quantitatively. Thus — as discussed in this paper — the tyrosine-lysine hydrogen bonds can participate in proton conducting hydrogen bonded systems — as, for instance, present in bacteriorhodopsin — performing the proton transport through hydrophobic regions of biological membranes.

Estimating the barrier to proton or deuteron tunnelling in hydrogen-bonded crystals

Abstract We represent a symmetric double minimum proton potential by two Morse functions placed back-to-back. The wave-functions and energy eigenvalues are determined by asymptotic analysis. Using as input the crystallographic atom positions and the frequencies of two infrared transitions we can deduce the well parameters for CrOOD. Using the same well parameters (but somewhat different atom positions) for CrOOH, we derive an energy structure in good agreement with the observed mid infrared, far infrared and neutron inelastic scattering spectra. The model should be useful for estimating the tunnelling integral in KDP-type ferroelectrics, and its dependence on the oxygen-oxygen separation.

The trimethylammonium-trimethylamine complex cation

The existence of the symmetric hydrogen-bonded complex cation (trimethylamine/H/trimethylamine) + has been confirmed by infrared spectroscopy. The infrared spectra of this cation, and of the three other cations resulting from deuterium substitution in the base and at the bridge, have been examined in solution. There is no evidence for a double minimum proton potential from the v s band structure or from the appearance of internal modes of the trimethylamine moiety.