Miguel Costas

Effect of molecular size on the W-shaped excess heat capacities: oxaalkane–alkane systems

Excess molar heat capacities, CEp, throughout the entire concentration range have been determined at 25 °C for the following oxaalkane–alkane systems: 2,5,8,11,14-pentaoxapentadecane (tetraglyme) with decane, nonane, octane and hexane; 2,5,8,11-tetraoxadodecane (triglyme) with hexadecane, pentadecane, tetradecane, dodecane, decane and hexane; 2,5,8-trioxanonane (diglyme) with 2,6,10,15,19,23-hexamethyltetracosane (squalane), 2,6,10,14-tetramethylpentadecane (pristane), hexadecane and decane; 2,5-dioxahexane (monoglyme) with squalane, pristane, decane and heptane; and 1,4-dioxacyclohexane (p-dioxane) with cyclohexane. For all these tures CEp has a W-shaped concentration dependence (two minima separated by a maximum). By increasing the molecular size of either component, the maximum in CEp is enhanced and displaced towards high concentrations of the smaller component. CEp behaviour correlates with degrees of non-randomness in the mixtures as quantified by the concentration–concentration correlation function Scc which is calculated using the Flory–Huggins theory with a group-interaction model and assuming interaction between molecular surfaces.

Self-association of alcohols in inert solvents. Apparent heat capacities and volumes of linear alcohols in hydrocarbons

Apparent molar heat capacities, ϕC, and volumes, ϕV, have been measured for methanol, hexan-1-ol and decan-1-ol in n-alkane solvents between xROH= 0.001 and 0.2 at 10, 25 and 40 °C. The apparent molar heat capacities show a maximum against concentration which increases and moves to lower alcohol concentrations as the temperature decreases. This leads to a negative dϕC/dT at low alcohol concentrations, changing sign at higher alcohol concentrations. The Treszczanowicz–Kehiaian model for self-associated liquids + inert solvents explains these concentration and temperature dependences in terms of alcohol self-association through hydrogen bonds. Tetramers are the predominant species, dimers being almost absent even at very low alcohol concentrations. The excess heat capacity, CEp, and dCEp/dT of the mixtures are of different sign in the following approximate concentration ranges: (I) for xROH > 0.01, CEp and dCEp/dT > 0, (II) for 0.005 0 and dCEp/dT < 0 and (III) for xROH < 0.005, CEp and dCEp/dT < 0. This behaviour is explained quantitatively by the Treszczanowicz–Kehiahian theory and is believed to occur in all associated + inert liquid systems. The apparent molar volumes increase rapidly as the concentration of alcohol decreases, corresponding to a destruction of the tetramers. ϕC and ϕV have also been measured for methanol dissolved in an active solvent, methyl acetate. The radically different results indicate that hydrogen bonding between the alcohol and the solvent has replaced self-association as the predominant influence on the thermodynamics.

Structural Basis of Human Triosephosphate Isomerase Deficiency MUTATION E104D IS RELATED TO ALTERATIONS OF A CONSERVED WATER NETWORK AT THE DIMER INTERFACE

Human triosephosphate isomerase deficiency is a rare autosomal disease that causes premature death of homozygous individuals. The most frequent mutation that leads to this illness is in position 104, which involves a conservative change of a Glu for Asp. Despite the extensive work that has been carried out on the E104D mutant enzyme in hemolysates and whole cells, the molecular basis of this disease is poorly understood. Here, we show that the purified, recombinant mutant enzyme E104D, while exhibiting normal catalytic activity, shows impairments in the formation of active dimers and low thermostability and monomerizes under conditions in which the wild type retains its dimeric form. The crystal structure of the E104D mutant at 1.85 Å resolution showed that its global structure was similar to that of the wild type; however, residue 104 is part of a conserved cluster of 10 residues, five from each subunit. An analysis of the available high resolution structures of TIM dimers revealed that this cluster forms a cavity that possesses an elaborate conserved network of buried water molecules that bridge the two subunits. In the E104D mutant, a disruption of contacts of the amino acid side chains in the conserved cluster leads to a perturbation of the water network in which the water-protein and water-water interactions that join the two monomers are significantly weakened and diminished. Thus, the disruption of this solvent system would stand as the underlying cause of the deficiency.

Towards an understanding of the heat capacity of liquids. A simple two-state model for molecular association

A model for the temperature dependence of the isobaric heat capacity of associated pure liquids C(p,m)(o)(T) is proposed. Taking the ideal gas as a reference state, the residual heat capacity is divided into nonspecific C(p) (res,ns) and associational C(p) (res,ass) contributions. Statistical mechanics is used to obtain C(p)(res,ass) by means of a two-state model. All the experimentally observed C(p,m)(o)(T) types of curves in the literature are qualitatively described from the combination of the ideal gas heat capacity C(p)(id)(T) and C(p)(res,ass)(T). The existence of C(p,m)(o)(T) curves with a maximum is predicted and experimentally observed, for the first time, through the measurement of C(p,m)(o)(T) for highly sterically hindered alcohols. A detailed quantitative analysis of C(p,m)(o)(T) for several series of substances (n-alkanes, linear and branched alcohols, and thiols) is made. All the basic features of C(p,m)(o)(T) at atmospheric and high pressures are successfully described, the model parameters being physically meaningful. In particular, the molecular association energies and the C(p)(res,ns) values from the proposed model are found to be in agreement with those obtained through quantum mechanical ab initio calculations and the Flory model, respectively. It is concluded that C(p,m)(o)(T) is governed by the association energy between molecules, their self-association capability and molecular size.

Heat capacity and corresponding states in alkan-1-ol–n-alkane systems

The apparent molar heat capacities, ΦC, and excess heat capacities, CpE, have been obtained at 25 °C for the following alkan-1-ols, Cm H2m+1 OH at low mole fraction, x1 < 0.1, in n-alkanes, Cn H2n+2, (m,n): (1,6), (1,7), (2,10), (4,10), (6,6), (6,9), (6,10), (6,12), (6,16), (7,10), (10,6), (10,10), (10,16), (11,10), (12,10), (14,6), (16,6) and (16,16). The Treszczanowicz–Kehiaian (TK) association theory was applied to the associative part of the apparent heat capacity, ΦC(assoc), obtaining ΔH° for hydrogen-bond formation and the volume-fraction equilibrium constant, K4Φ, for formation of tetramers which are the dominant multimers. K4Φ are related to K4, an equilibrium constant for hydrogen-bond formation in tetramers, which is found to be independent of alkan-1-ol chain length from m= 4 to 16, while K4 for m= 2 and 1 are larger. A common K4 implies a single corresponding states curve (CSC) of ΦC(assoc) either against ψ1, the number of moles of alcohol per mole of segments in the solution, as introduced by Pouchly, or against molarity. The CSC is obeyed by the dilute solution data, with some deviation for methanol and ethanol systems, and also by ΦC(assoc) measured throughout the whole concentration range at 25 °C for m= 6 with n= 7, 8, 12 and 16 and for m= 1 (high and low concentration only), 4, 6 and 10 with n= 7 and m= 2, 4, 6 and 10 with n= 12 (again with m= 2 as exception). The CSC in reduced form allows the determination of the size of the dominant multimer from experimental ΦC data and should be followed by alkan-1-ol–inert solvent data at any temperature. CpE for constant m increases with n, and this is a consequence of the adherence to a CSC as well as being predicted by the TK theory. Together with literature results for other systems it is found that for n= constant, CpE increases as m decreases, reaching a maximum at m= 3, then decreasing again for m= 2 and 1. The behaviour of m= 10–3 is a consequence of the CSC, whereas m= 2 and 1 represent deviations from the CSC.

Between-Species Variation in the Kinetic Stability of TIM Proteins Linked to Solvation-Barrier Free Energies

Theoretical, computational, and experimental studies have suggested the existence of solvation barriers in protein unfolding and denaturation processes. These barriers are related to the finite size of water molecules and can be envisioned as arising from the asynchrony between water penetration and breakup of internal interactions. Solvation barriers have been proposed to play roles in protein cooperativity and kinetic stability; therefore, they may be expected to be subject to natural selection. We study the thermal denaturation, in the presence and in the absence of chemical denaturants, of triosephosphate isomerases (TIMs) from three different species: Trypanosoma cruzi, Trypanosoma brucei, and Leishmania mexicana. In all cases, denaturation was irreversible and kinetically controlled. Surprisingly, however, we found large differences between the kinetic denaturation parameters, with T. cruzi TIM showing a much larger activation energy value (and, consequently, much lower room-temperature, extrapolated denaturation rates). This disparity cannot be accounted for by variations in the degree of exposure to solvent in transition states (as measured by kinetic urea m values) and is, therefore, to be attributed mainly to differences in solvation-barrier contributions. This was supported by structure-energetics analyses of the transition states and by application of a novel procedure to estimate from experimental data the solvation-barrier impact at the entropy and free-energy levels. These analyses were actually performed with an extended protein set (including six small proteins plus seven variants of lipase from Thermomyces lanuginosus and spanning a wide range of activation parameters), allowing us to delineate the general trends of the solvation-barrier contributions. Overall, this work supports that proteins sharing the same structure and function but belonging to different organisms may show widely different solvation barriers, possibly as a result of different levels of the selection pressure associated with cooperativity, kinetic stability, and related factors.

Structural energetics of protein–carbohydrate interactions: Insights derived from the study of lysozyme binding to its natural saccharide inhibitors

High‐sensitivity isothermal titration calorimetry was used to characterize the binding of the glycohydrolitic enzyme hen egg‐white lysozyme to its natural saccharide inhibitors, chitobiose and chitrotriose. Measurements were done at a pH of 4.7, in the 15°C −45°C temperature range. Using a structural‐energetic parameterization derived previously for lectin‐carbohydrate associations, both binding enthalpies and entropies for the present systems and for the complex of chitobiose with turkey egg‐white lysozyme from the literature were correctly accounted for. These observations suggest that both lysozymes and lectins follow the same structural‐energetic behavior in the binding to their ligands. From the analysis of lysozyme data in conjunction with other binding data reported in the literature, an ad hoc parameterization of ΔCp for protein–carbohydrate complexes was derived for the first time. The novel parameters for both polar and apolar surface areas differed significantly from correlations obtained previously from model compounds and protein‐folding data. As ΔCp is extremely sensitive to changes in solvent structure, this finding indicates that protein–carbohydrate complexes have distinctive hydration properties. According to our analysis, the dehydration of polar groups is the major cause for the observed decrease in ΔCp, which implies that these groups behave hydrophobically. The contribution of apolar surface areas was found of the expected sign, but their specific weight is much smaller than those obtained in other correlations. This small contribution to ΔCp is consistent with Lemieuxs hypothesis of a low degree of hydration of apolar surfaces on carbohydrates.

General thermodynamic analysis of the dissolution of non-polar molecules into water. Origin of hydrophobicity

The Gibbs energy, enthalpy, entropy and heat capacity of transfer from the pure non-polar liquid into water are analysed in detail. It is found that if the combinatorial contribution to the Gibbs energy and entropy of transfer is subtracted from the experimental values, all non-polar solutes in water behave in a universal manner, i.e. all of their thermodynamic transfer functions can be studied with their molecular surface area as the only parameter. This is illustrated with the alkylbenzene series, for which experimental Gibbs energies of transfer in a wide temperature range have been obtained recently. A new interpretation scheme for the thermodynamic transfer functions is presented and contrasted with that due to Privalov and Gill. It is considered that water molecules around the solute undergo a relaxation process which lowers the Gibbs energy, enthalpy and entropy of the system and is responsible for the large heat capacity of transfer. This relaxation process is described here using a two-state model for water molecules obtained from first principles. The negative relaxation contribution to the Gibbs energy promotes solubility, but is overcome by a large positive contribution arising from the creation of a cavity in water and the large differences between solute–solute, water–water and solute–water interactions. The origin of hydrophobicity lies then in the high cohesive energy of water. The proposed interpretation scheme is used to (a) predict the solubility of alkanes in water, (b) understand the origin of the solubility minimum appearing in aqueous solutions of non-polar solutes, (c) rationalize the experimental finding that the enthalpy of transfer becomes zero in a narrow temperature range for many non-polar solutes, (d) discuss the significance of entropy of transfer vs. heat capacity of transfer plots often used to understand the nature of the hydrophobicity of non-polar solutes and proteins, and (e) account for the expected change in sign (with temperature) of the water proton NMR chemical shifts discussed earlier in the literature.

Similarities and differences between cyclodextrin-sodium dodecyl sulfate host-guest complexes of different stoichiometries: molecular dynamics simulations at several temperatures.

An extensive dynamic and structural characterization of the supramolecular complexes that can be formed by mixing α-, β-, and γ-cyclodextrin (CD) with sodium dodecyl sulfate (SDS) in water at 283, 298, and 323 K was performed by means of computational molecular dynamics simulations. For each CD at the three temperatures, seven different initial conformations were used, generating a total of 63 trajectories. The observed stoichiometries, intermolecular distances, and relative orientation of the individual molecules in the complexes, as well as the most important interactions which contribute to their stability and the role of the solvent water molecules were studied in detail, revealing clear differences and similarities between the three CDs. Earlier reported findings in the inclusion complexes field are also discussed in the context of the present results. For any of the three native cyclodextrins, the CD(2)SDS(1) species in the head-to-head conformation appears to be a promising building block for nanotubular aggregates both in the bulk and at the solution/air interface, as earlier suggested for the case of α-CD. Moreover, the observed noninclusion arrangements involving β-CD are proposed as the seed for the premicellar (β-CD)-induced aggregation of SDS described in the literature.

Interactions between DMPC liposomes and the serum blood proteins HSA and IgG.

The interaction between two serum blood proteins, namely human serum albumin (HSA) and human immunoglobulin G (IgG), with 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine (DMPC) liposomes has been studied in detail using dynamic light scattering, flow cytometry, enzyme-linked immunosorbent assay (ELISA), electrophoretic mobility, differential scanning calorimetry (DSC), and surface tension measurements. HSA and IgG interact with liposomes forming molecular aggregates that remain stable at protein concentrations beyond those of total liposome coverage. Both HSA and IgG penetrate into the liposome bilayer. An ELISA assay indicates that the Fc region of IgG is the one that is immersed in the DMPC membrane. The liposome-protein interaction is mainly of electrostatic nature, but an important hydrophobic contribution is also present.