Structure of the Mycobacterium tuberculosis type VII secretion system chaperone EspG5 in complex with PE25–PPE41 dimer

The growth or virulence of Mycobacterium tuberculosis bacilli depends on homologous type VII secretion systems, ESX‐1, ESX‐3 and ESX‐5, which export a number of protein effectors across membranes to the bacterial surface and environment. PE and PPE proteins represent two large families of highly polymorphic proteins that are secreted by these ESX systems. Recently, it was shown that these proteins require system‐specific cytoplasmic chaperones for secretion. Here, we report the crystal structure of M. tuberculosis ESX‐5‐secreted PE25–PPE41 heterodimer in complex with the cytoplasmic chaperone EspG₅. EspG₅ represents a novel fold that is unrelated to previously characterized secretion chaperones. Functional analysis of the EspG₅‐binding region uncovered a hydrophobic patch on PPE41 that promotes dimer aggregation, and the chaperone effectively abolishes this process. We show that PPE41 contains a characteristic chaperone‐binding sequence, the hh motif, which is highly conserved among ESX‐1‐, ESX‐3‐ and ESX‐5‐specific PPE proteins. Disrupting the interaction between EspG₅ and three different PPE target proteins by introducing different point mutations generally affected protein secretion. We further demonstrate that the EspG₅ chaperone plays an important role in the ESX secretion mechanism by keeping aggregation‐prone PE–PPE proteins in their soluble state.     

Study of optical properties of electropolymerized melanin films by photopyroelectric spectroscopy

Photopyroelectric (PPE) spectroscopy, in the 350–1,075 nm wavelength range, was used to study the optical properties of electropolymerized melanin films on indium tin oxide (ITO) coated glass. The PPE intensity signal as a function of the wavelength λ, V _n(λ) and its phase F _n(λ) were independently measured. Using the PPE signal intensity and the thermal and optical properties of the pyroelectric detector, we were able to calculate the optical absorption coefficient β of melanin in the solid-state. We believe this to be the first such measurement of its kind on this material. Additionally, we found an optical gap in these melanin films at 1.70 eV.     

Synthesis of poly(phenylacetylene)s containing chiral phenylethyl carbamate residues as coated-type CSPs with high solvent tolerability

Two novel helical poly(phenylacetylene) derivatives containing chiral phenylethyl carbamate residues in the end of each side chain ( PPA-S and PPA-R ) were synthesized by polymerization of the corresponding phenylacetylene monomers using Rh(nbd)BPh₄ as a catalyst in DMF. The enantioseparation properties of the polymers were evaluated as coated-type chiral stationary phases (CSPs) for high-performance liquid chromatography (HPLC). Under the same chromatographic conditions, PPA-S and PPA-R showed different enantioseparation properties, indicating that the different interactions between the analytes and the polymers, which result from the different chiral phenylethyl carbamate groups in the end of each side chains. Racemates 1 , 7 , and 8 could be better resolved on PPA-S , while racemate 6 was separated on PPA-R more efficiently. In addition, the coated-type CSPs showed good solvent tolerability and could work without any damage by introducing the polar solvents, such as CHCl₃ and THF, in eluent. Moreover, some racemates could be better resolved on these coated-type CSPs with the addition of THF to the eluent.     

Molecular theory of the helix–coil transition in polyamino acids. V. Explanation of the different conformational behavior of valine,isoleucine, and leucine in aqueous solution

Even though poly(L -valine) and poly(L -isoleucine) both contain residues that are branched at their β-carbon atoms, they exhibit a different behavior of their Zimm-Bragg helix-growth parameter s in aqueous solution. This quantity increases with temperature for poly(L -valine) but decreases for poly(L -isoleucine). The origin of this behavioral difference was examined by computing theoretical values of s versus temperature from interatomic interaction energies, taking solvent (hydrophobic and hydrophilic) effects into account. The calculated s versus temperature curves for both homopolymers are consistent with the observed experimental behavior. The two homopolymers behave differently because of differences in the change in the number of hydration-shell water molecules accompanying their helix–coil transitions. The larger isoleucine side chains are more crowded together in both the α-helical and coil forms than are those of valine. Therefore, there is a smaller change in hydration of the isoleucine side chains compared to that of the valine side chains in the helix–coil transition. By analyzing the effects of hydration on the s versus temperature curves, it is possible to account also for the experimental curve for poly(L -leucine), which exhibits an intermediate behavior between those for poly(L -valine) and poly(L -isoleucine).     

Interplay Between Side Chain Pattern,Polymer Aggregation,and Charge Carrier Dynamics in PBDTTPD:PCBM Bulk‐Heterojunction Solar Cells

Poly(benzo[1,2‐b:4,5‐b′]dithiophene–alt–thieno[3,4‐c]pyrrole‐4,6‐dione) (PBDTTPD) polymer donors with linear side‐chains yield bulk‐heterojunction (BHJ) solar cell power conversion efficiencies (PCEs) of about 4% with phenyl‐C₇₁‐butyric acid methyl ester (PC₇₁BM) as the acceptor, while a PBDTTPD polymer with a combination of branched and linear substituents yields a doubling of the PCE to 8%. Using transient optical spectroscopy it is shown that while the exciton dissociation and ultrafast charge generation steps are not strongly affected by the side chain modifications, the polymer with branched side chains exhibits a decreased rate of nongeminate recombination and a lower fraction of sub‐nanosecond geminate recombination. In turn the yield of long‐lived charge carriers increases, resulting in a 33% increase in short circuit current (J _sc). In parallel, the two polymers show distinct grazing incidence X‐ray scattering spectra indicative of the presence of stacks with different orientation patterns in optimized thin‐film BHJ devices. Independent of the packing pattern the spectroscopic data also reveals the existence of polymer aggregates in the pristine polymer films as well as in both blends which trap excitons and hinder their dissociation.     

Cis–trans peptide-bond equilibria in cyclic hexapeptides

Data are presented on the position of the equilibria of cyclo(Xxx-Pro-Yyy)₂ backbones between forms with two cis Xxx-Pro peptide bonds and forms with only trans peptide bonds. These data are interpreted in terms of two factors: a solvent-independent steric interaction between the Xxx and Pro side chains, and the ability of solvent to influence the transannular electrostatic interaction between N? H and C?O groups of the Xxx units in the all-trans form.     

The influence of asymmetric carbon atoms of the side chains on the conformational properties of polypeptides: Comparison of diastereomeric homooligopeptides of L-isoleucine and D-alloisoleucine

The conventionally protected oligopeptides of the two homologous series Boc-(L -Ile)_n-OMe and Boc-(D -aIle)_n-OMe (n = 2–6) were synthesized in a standard stepwise fashion and their uv and CD spectra in 2,2,2-trifluoroethanol, and solid-state ir spectra were investigated. In addition, two oligomeric products derived from the NCAs of L -isoleucine and of D -allo-isoleucine and having a DP of 20 and 12, respectively, were studied in the solid state by x-ray and ir. No substantial differences between the properties of the diastereomeric oligomers in the solid state were noticed, a β-structure being very likely at least for the Boc-protected hexapeptides and the higher oligomers. In contrast, differences were observed between the spectroscopic properties of the diastereomeric oligopeptides, and especially of the hexapeptides, in trifluoroethanol solution. The different properties of the hexapeptides in solution were related to the existence, in the case of Boc-(L -Ile)₆-OMe, of soluble molecular aggregates in which the peptide chains assume the β-conformation. These results provide an additional example of the influence of the configuration of asymmetric carbon atoms of the side chains on the conformational properties of peptide molecules in solution.     

In Situ Analysis of Solvent and Additive Effects on Film Morphology Evolution in Spin‐Cast Small‐Molecule and Polymer Photovoltaic Materials

To elucidate the details of film morphology/order evolution during spin‐coating, solvent and additive effects are systematically investigated for three representative organic solar cell (OSC) active layer materials using combined in situ grazing incidence wide angle x‐ray scattering (GIWAXS) and optical reflectance. Two archetypical semiconducting donor (p‐type) polymers, P3HT and PTB7, and semiconducting donor small‐molecule, p‐DTS(FBTTh₂)₂ are studied using three neat solvents (chloroform, chlorobenzene, 1,2‐dichlorobenzene) and four processing additives (1‐chloronaphthalene, diphenyl ether, 1,8‐diiodooctane, and 1,6‐diiodohexane). In situ GIWAXS identifies several trends: 1) for neat solvents, rapid crystallization occurs that risks kinetically locking the material into multiple crystal structures or crystalline orientations; and 2) for solvent + additive processed films, morphology evolution involves sequential transformations on timescales ranging from seconds to hours, with key divergences dependent on additive/semiconductor molecular interactions. When π‐planes dominate the additive/semiconductor interactions, both polymers and small molecule films follow similar evolutions, completing in 1–5 min. When side chains dominate the additive/semiconductor interactions, polymer film maturation times are up to 9 h, while initial crystallization times <10 s are observed for small‐molecule films. This study offers guiding information on OSC donor intermediate morphologies, evolution timescales, and divergent evolutions that can inform OSC manufacture.     

Cyclo-hexa-peptides at the water/cyclohexane interface: a molecular dynamics simulation

Molecular dynamic (MD) simulations have been performed to study the behaviors of ten kinds of cyclo-hexa-peptides (CHPs) composed of amino acids with the diverse hydrophilic/hydrophobic side chains at the water/cyclohexane interface. All the CHPs take the “horse-saddle” conformations at the interface and the hydrophilicity/hydrophobicity of the side chains influences the backbones’ structural deformations. The orientations and distributions of the CHPs at the interface and the differences of interaction energies (ΔΔE) between the CHPs and the two liquid phases have been determined. RDF analysis shows that the H-bonds were formed between the O_C atoms of the CHPs’ backbones and H_w atoms of water molecules. N atoms of the CHPs’ backbones formed the H-bonds or van der Waals interactions with the water solvent. It was found that there is a parallel relationship between ΔΔE and the lateral diffusion coefficients (D _xy ) of the CHPs at the interface. The movements of water molecules close to the interface are confined to some extent, indicating that the dynamics of the CHPs and interfacial water molecules are strongly coupled.

Structure and dynamics of Candida rugosa lipase: the role of organic solvent

The effect of organic solvent on the structure and dynamics of proteins was investigated by multiple molecular dynamics simulations (1 ns each) of Candida rugosa lipase in water and in carbon tetrachloride. The choice of solvent had only a minor structural effect. For both solvents the open and the closed conformation of the lipase were near to their experimental X-ray structures (C rms deviation 1–1.3 Å). However, the solvents had a highly specific effect on the flexibility of solvent-exposed side chains: polar side chains were more flexible in water, but less flexible in organic solvent. In contrast, hydrophobic residues were more flexible in organic solvent, but less flexible in water. As a major effect solvent changed the dynamics of the lid, a mobile element involved in activation of the lipase, which fluctuated as a rigid body about its average position. While in water the deviations were about 1.6 Å, organic solvent reduced flexibility to 0.9 Å. This increase rigidity was caused by two salt bridges (Lys85–Asp284, Lys75–Asp79) and a stable hydrogen bond (Lys75–Asn 292) in organic solvent. Thus, organic solvents stabilize the lid but render the side chains in the hydrophobic substrate-binding site more mobile. Figure Superimposition of open (black, PDB entry 1CRL) and closed (gray, PDB entry 1TRH) conformers of C. rugosa lipase. The mobile lid is indicatedThis revised version was published online in October 2004 with corrections to the Graphical Abstract.     

The dissolution and characterization of Bombyx mori silk fibroin in calcium nitrate-methanol solution and the regeneration of films

Bombyx mori silk fibroin from the silkworm was found to be soluble in a calcium nitrate-methanol system. Fibroin dissolves in 75% w/v Ca(NO₃)₂/MeOH solution at a temperature of 67°C. The viscometric behavior of the fibroin-salt solution was analyzed and the fibroin's secondary structures were determined via ¹³C solution nmr. Fourier transform ir, solid state ¹³C-nmr, x-ray diffraction, differential scanning calorimetry, scanning electron microscopy (SEM), and polarizing microscopy were used to characterize regenerated films and fibers. A compositional phase diagram of fibroin in the salt solution was constructed. Viscosity data indicate that there is aggregation of fibroin chains within the salt solution. The extremely high value of intrinsic viscosity of 8.7 dL/g at 298 K may be due to aggregation. Aggregation may be caused by the complexing of calcium ions with the fibroin chains at their amide linkages. The energy required for viscous flow for the fibroin solution (ΔH_vis = 9.03 kcal/mol) is greater than that of the solvent (ΔH_vis = 7.01 kcal/mol). Chain entanglements may be hindering the free motion of chains thus increasing the energy required for the viscous flow. ¹³C-nmr shows that fibroin chains exist in two independent conformational environments. While most of the molecule is in a random coil conformation, there is evidence of some order within the chains of fibroin. In as-cast regenerated films, the fibroin chains are in a random coil/α-helix conformation with some β-sheet content. Crystallinity induced by immersion of thin films in methanol is evidenced via x-ray diffraction, which shows lattice spacings at 4.042 Å. Thin films have a fibrillar morphology that is clearly shown under the SEM and the polarizing microscope. Fibers were hand pulled from the concentrated fibroin-salt solutions and coagulated with acetone and methanol. A microscopic analysis was done using the polarizer. © 1997 John Wiley & Sons, Inc. Biopoly 42: 61–74, 1997     

Interpretation of energy-transfer experiments by theoretical studies of model compounds using semiempirical potential functions. II. Monte Carlo calculations on oligopeptides

The properties relevant to nonradiative energy transfer have been computed in the unperturbed chain model for oligopeptides composed of from 4 to 21 residues of the formula Tyr-(Ala)_n-Tyr and Trp-(X)_n-Tyr, X being either Ala or Gly. A Monte Carlo method has been used for the generation of the chains. The relation between the distribution functions of the distances between the luminophores and the various properties in energy transfer has been examined for chains of different lengths and compositions. The averge of the orientation factor κ² has been computed as a functions of chain length both for molecules in a randomly coiled state and for molecules with backbones in a well-defined three-dimensional structure. The various averaging regimes of energy-transfer efficiency and of fluorescence decay are compared. Theoretical curves relating experimental efficiencies to the mean distance between the luminophores are proposed.     

Conformation of oligopeptides with L-leucyl-L-leucyl-L-alanyl and L-methionyl-L-methionyl-L-leucyl repeating units

The conformation of oligopeptides with hydrophobic side chains, Nps-(L -Leu-L -Leu-L -Ala)_n-OEt and Nps-(L -Met-L -Met-L -Leu)_n-OEt(n = 1–6), in the solid state, obtained either by evaporation of the solvent or by precipitation with diethyl ether from a 1,1,1,3,3,3-hexafluoropropan-2-ol (HFIP) solution, has been studied with ir spectroscopy and x-ray powder-diffraction measurements. The conformation of these peptides in the HFIP solution has been studied by CD spectroscopy. Due to a strong preference of the amino acids to form an α helix, the peptides begin forming α helices at the dodecapeptide in the HFIP solution, and in the solid state by evaporation. In the solid state, with precipitation, the α-helical conformation is first observed at the octadecapeptide and the lower peptides assume a β structure. The conformational change, from the α helix to the β structure of the peptides with 12 to 15 amino acid residues, during the precipitation process, is due to a strong tendency of the amino acids to form the β-structure in rather short peptide lengths.     

Substrate Specificity of Aqualysin I Altered by an Organic Solvent,DMSO

Aqualysin I is the alkaline serine protease isolated from an extreme thermophile, Thermus aquaticus YT-1. We have analyzed the kinetic properties of aqualysin I, using thirty-one kinds of chromogenic succinyl-tripeptide p-nitroanilides as substrates in the presence of 10% dimethylsulfoxide (DMSO). Aqualysin I hydrolyzed many peptides in a DMSO-containing mixture, however the substrate specificity was different from that in the absence of DMSO. The K_m for each peptide was raised by the addition of 10% DMSO. Also, the P3- as well as P2-specificity of aqualysin I was altered. These results suggested that the side chains of the P2 and P3 residues are exposed to the solvent, and the hydrophobic interactions between the side chain of the substrate and the solvent may take a part in the substrate recognition of the enzyme.     

Alkyl Chain Engineering of Solution‐Processable Star‐Shaped Molecules for High‐Performance Organic Solar Cells

The impact of alkyl side‐chain substituents on conjugated polymers on the photovoltaic properties of bulk heterojunction (BHJ) solar cells has been studied extensively, but their impact on small molecules has not received adequate attention. To reveal the effect of side chains, a series of star‐shaped molecules based on a triphenylamine (TPA) core, bithiophene, and dicyanovinyl units derivatized with various alkyl end‐capping groups of methyl, ethyl, hexyl and dodecyl is synthesiyed and studied to comprehensively investigate structure‐properties relationships. UV‐vis absorption and cyclic voltammetry data show that variations of alkyl chain length have little influence on the absorption and highest occupied molecular orbital (HOMO)‐lowest unoccupied molecular orbital (LUMO) levels. However, these seemingly negligible changes have a pronounced impact on the morphology of BHJ thin films as well as their charge carrier separation and transportation, which in turn influences the photovoltaic properties of these small‐molecule‐based BHJ devices. Solution‐processed organic solar cells (OSCs) based on the small molecule with the shortest methyl end groups exhibit high short circuit current (J_sc) and fill factor (FF), with an efficiency as high as 4.76% without any post‐treatments; these are among the highest reported for solution‐processed OSCs based on star‐shaped molecules.     

Curious structure in “canonical” alanine-based peptides

We have performed all atom simulations of blocked peptides of the form (AAXAA)₃, where X = Gln, Asn, Glu, Asp, Arg, and Lys with explicit water molecules to examine the interactions between side chains spaced i,i–5 in the sequence. Although side chains in this i,i–5 arrangement are commonly believed to be noninteracting, we have observed the formation of unusual i,i–5 main chain hydrogen bonding in such sequences with positively charged residues (Lys) as well as polar uncharged groups (Gln). Our results are consistent with the unusual percentage of hydrogen bonding curves produced by amide exchange measurements on the well-studied sequence acetyl-(AAQAA)₃-amide in water (Shalongo, W., Dugad, L., Stellwagen, E. J. Am. Chem. Soc. 116:8288–8293, 1994). Analysis of our simulations indicated that the glutamine side chain showed the greatest propensity to support π helix formation and that the i,i–5 intramolecular hydrogen bonds were stabilized by water-bridging side chain interactions. This intermittent formation of the unusual π helix structure was observed for up to 23% of the total simulation time in some residues in (AAQAA)₃. Control studies on peptides with glutamine side chains spaced i,i–3, i,i–4, and i,i–6 did not reveal similar unique structures, providing stronger evidence for the unique role side chain interactions with i,i–5 spacing. © 1997 Wiley-Liss Inc.     

β‐sheet assembly in amyloidogenic glutamic acid nanostructures: Insights from X‐ray scattering and infrared nanospectroscopy

Glutamic acid–rich peptides are crucial to a variety of biological processes, including glutamatergic neurotransmission and immunological defense. Glutamic acid sequences often exhibit unusual organization into β₂‐type sheets, where bifurcated H bonds formed between glutamic acid side chains and NH in amide bonds on adjacent β‐strands play a paramount role for stabilizing the molecular assembly. Herein, we investigate the self‐assembly and supramolecular structure of simplified models consisting of alternating glutamic acid/phenylalanine residues. Small‐angle X‐ray scattering and atomic force microscopy show that the aggregation pathway is characterized by the formation of small oligomers, followed by coalescence into nanofibrils and nanotapes. Amyloidogenic features are further demonstrated through fiber X‐ray diffraction, which reveal molecular packing according to cross‐β patterns, where β‐strands appear perpendicularly oriented to the long axis of nanofibrils and nanotapes. Nanoscale infrared spectroscopy from individual nanoparticles on dried samples shows a remarkable decrease of β₂‐sheet content, accompanied by growth of standard β‐sheet fractions, indicating a β₂‐to‐β₁ transition as a consequence of the release of solvent from the interstices of peptide assemblies. Our findings highlight the key role played by water molecules in mediating H‐bond formation in β₂‐sheets commonly found in amyloidogenic glutamic acid–rich aggregates.     

Proton magnetic resonance studies on conformation and association of oligo-γ-benzyl-L-glutamates in solution

The proton magnetic resonance spectra of o-nitrophenylthio-tetra- and hexa-γ-benzyl-L -glutamate ethylamides have been measured at different concentrations in CDCl₃ and CD₂₂C1. The NH and α-CH resonances of the tetrapeptide show downfield shifts with increasing concentration, accompanying disappearance of their fine structure and line broadening. The apparent feature of chemical shifts against concentration is sigmoidal, and it can be interpreted by assuming the presence of a step or more of association–dissociation equilibria of tetrapeptide. With increasing concentration, small aggregates are formed by intermolecular hydrogen bonding, the size of which is not sufficiently large to exhibit critical micelle concentrations. In contrast to the tetrapeptide, the hexapeptide has constant chemical shifts of the NH and α-CH resonances, independent of concentration, which implies that only the unassociated molecules show observable sharp resonances. In the hexapeptide, the phenyl CH and benzyl CH₂ groups of the side chains exhibit new resonances above certain critical concentrations, indicating the restriction of rotational freedom of the side chains in the aggregated states.     

N‐terminal diproline and charge group effects on the stabilization of helical conformation in alanine‐based short peptides: CD studies with water and methanol as solvent

Protein folding problem remains a formidable challenge as main chain, side chain and solvent interactions remain entangled and have been difficult to resolve. Alanine‐based short peptides are promising models to dissect protein folding initiation and propagation structurally as well as energetically. The effect of N‐terminal diproline and charged side chains is assessed on the stabilization of helical conformation in alanine‐based short peptides using circular dichroism (CD) with water and methanol as solvent. A1 (Ac–Pro–Pro–Ala–Lys–Ala–Lys–Ala–Lys–Ala–NH₂) is designed to assess the effect of N‐terminal homochiral diproline and lysine side chains to induce helical conformation. A2 (Ac–Pro–Pro–Glu–Glu–Ala–Ala–Lys–Lys–Ala–NH₂) and A3 (Ac–d Pro–Pro–Glu–Glu–Ala–Ala–Lys–Lys–Ala–NH₂) with N‐terminal homochiral and heterochiral diproline, respectively, are designed to assess the effect of Glu...Lys (i , i  + 4) salt bridge interactions on the stabilization of helical conformation. The CD spectra of A1 , A2 and A3 in water manifest different amplitudes of the observed polyproline II (PPII) signals, which indicate different conformational distributions of the polypeptide structure. The strong effect of solvent substitution from water to methanol is observed for the peptides, and CD spectra in methanol evidence A2 and A3 as helical folds. Temperature‐dependent CD spectra of A1 and A2 in water depict an isodichroic point reflecting coexistence of two conformations, PPII and β‐strand conformation, which is consistent with the previous studies. The results illuminate the effect of N‐terminal diproline and charged side chains in dictating the preferences for extended‐β, semi‐extended PPII and helical conformation in alanine‐based short peptides. The results of the present study will enhance our understanding on stabilization of helical conformation in short peptides and hence aid in the design of novel peptides with helical structures. Copyright © 2017 European Peptide Society and John Wiley & Sons, Ltd.     

Étude thermodynamique de la transition hélice–pelote statistique en solvants mixtes de différents esters de l'acide poly(L-glutamique)

Helix–coil transition of poly(γ-methyl-L -glutamate), poly(γ-ethyl-L -glutamate), and poly(γ-benzyl-L -glutamate) has been studied in mixed solvents by calorimetry, polarimetry, and viscometry. The experimental data have allowed the evaluation of solvation enthalpy Δh_b, equilibrium constant K for hydrogen bond formation between the active solvent component and CO and NH groups, and the cooperativity parameter σ. The conformational transition of polypeptides in solution in a mixed solvent containing enough active solvent to maintain the coiled conformation has been produced by dilution with the helix-supporting solvent for the measurements of enthalpy of transition Δh_s. The average value for Δh_s is 3550 ± 300 J/mol and is practically independent of the nature of the side chain for the dichloroacetic acid-ethylene dichloride solvent pair at 25°C. A noticeable concentration effect exists in the case of poly(γ-benzyl-L -glutamate). The helical conformation is less stable for poly(γ-ethyl-L -glutamate), and this is explained by a steric effect hindering the access of dichloroacetic acid to side chains. Constant K has been calculated using polarimetric data and also from values of Δh_s obtained at different temperatures using the Bixon and Lifson theory on the one hand and that of Sayama and coworkers on the other hand. Values of σ for poly(γ-ethyl-L -glutamate) have been calculated according to both theories mentioned, and the results show that the two sets of values are quite similar. The constant σ depends on the nature of the active solvent, on temperature, and on the binary-solvent composition. These conclusions are confirmed by viscometric results. Values of Δh_b calculated from constant K are 5230 J/mol when Bixon and Lifson theory is used and 5569 J/mol when the theory at Sayama and coworkers is used. In both cases the value for Δh_b is much lower than that of an intramolecular hydrogen bond. Experimental results suggest that the solvation mechanism would proceed in a manner so that mechanisms described in both theories are involved.