Observations on the origin of the non-linear van't Hoff behaviour of polypeptides in hydrophobic environments

Environmental chemistry response of beryllium to diverse soil-solution conditions at a waste disposal site

This study evaluated how the variation in different sorption conditions of beryllium (Be) in soil-water systems (electrolytes; ionic strengths; competing, counter, and co-existing ions; concentrations of Be and soil; and temperature) affected Be's environmental behaviour. For this reason, potentially contaminated soil was collected from a legacy waste site near Sydney, Australia. The sorption-desorption plateau for Be was found at >12.5 g L^-1 (soil/solution), considering higher sorption and limited desorption. Variable surface charges developed by different added ions (competing ions, counter ions, and co-existence of all ions) were not always correlated with Be sorption. However, effects of added ions in Be sorption (increased by counter ions and decreased by competing ions) primarily occurred at low pH, with no noticeable changes at pH > 6 due to the hydration and precipitation behaviour of Be at higher pH. Both laboratory data and modelling indicated the substantial effect of counter ions on increased sorption of Be. Relatively higher amounts of sorption under the co-existence of all added ions were suggested from synergistic actions. Sorption was favourable (K_L > 0, and 0 < R_L < 1) across all concentrations and temperatures at pH 5.5, and high retention (84-97%) occurred after four desorption cycles indicated specific sorption. The sorption process was exothermic (ΔH > -43 kJ mole^-1), while desorption was endothermic (ΔH > +78.4 kJ mole^-1). All sorption-desorption reactions were spontaneous (ΔG = -Ve), and executed without any structural deformation (ΔS = nearly zero) of soil particles. However, the effect of temperature on desorption was influenced by the concentrations of Be. Higher retention and different sorption-desorption parameters (K_d-desorption > K_d-sorption; K_f-desorption > K_f-sorption; n_desorption/n_sorption < 1) indicate limited mobility of Be and the presence of desorption hysteresis in the studied soil under the experimental conditions.

Investigations into the interaction thermodynamics of TRAP-related peptides with a temperature-responsive polymer-bonded porous silica stationary phase

The interaction thermodynamics of the thrombin receptor agonistic peptide (TRAP-1), H-Ser-Phe-Leu-Leu-Arg-Asn-Pro-OH, and a set of alanine scan substitution peptides, have been investigated with an n-octadecylacrylic polymer-bonded porous silica (Sil-ODA₁₈) and water-acetonitrile mobile phases at temperatures ranging from 5 to 80 °C in 5 °C increments. The retention of these peptides on the Sil-ODA₁₈ stationary phase decreased as the water content in the mobile phase was lowered from 80% (v/v) to ca. 45% (v/v) and reached a minimum value for each peptide at a specific water-acetonitrile composition. Further decreases in the water content of the mobile phase led to increased retention. The magnitude of the changes in enthalpy of interaction, Δ H a s s o c 0 , changes in entropy of interaction, Δ S a s s o c 0 , and changes in heat capacity, Δ C p 0 , were found to be dependent on the molecular properties of the mobile phase, the temperature, the structure/mobility of the stationary phase, and the conformation and solvation state of the peptides. With water-rich mobile phases, the retention behaviour of the TRAP analogues was dominated by enthalpic processes, consistent with the participation of strong hydrogen bonding effects, but became dominated by entropic effects with acetonitrile-rich mobile phases as the temperature was increased. These changes in the retention behaviour of these TRAP peptides are consistent with the generation of water or acetonitrile clusters in the mobile phase depending on the volume fractions of the organic solvent as the Sil-ODA₁₈ stationary phase transitions from its crystalline to its isotropic state.

Thermodynamic Insights into the Separation of Carotenoids in Reversed-Phase Liquid Chromatography

The retention mechanism of four major carotenoids, two xanthophylls (i.e., lutein and zeaxanthin) and two carotenes (i.e., lycopene and β-carotene), was investigated in reversed-phase liquid chromatography with the aim of thermodynamic analysis. The experimental variables considered in this study were the composition of mobile phase (MP) and the temperature. Chromatographic elutions were undertaken under linear, isocratic conditions by using a C18 stationary phase, four different MP compositions (by varying the ratio methanol/acetonitrile from 66.5/28.5 to 47.5/47.5 v/v), and column temperatures in the range 283–313 K. Traditional Van't Hoff analysis has been used to estimate changes of standard enthalpy (ΔH°) and Gibbs free energy (ΔG°) associated with the solute transfer from the mobile to the stationary phase at each mobile phase composition. The thermodynamic quantities have been correlated to the structure of investigated carotenoids and their interaction with the octadecyl silica stationary phase.

Mechanistic Modeling of Reversed-Phase Chromatography of Insulins within the Temperature Range 10-40 °C

In the many published theories on the retention in reversed-phase chromatography (RPC), the focus is generally on the effect of the concentration of the mobile phase modulator(s), although temperature is known to have a significant influence both on the retention and on the selectivity between the adsorbates. The aim of this study was to investigate and model the combined effects of the temperature and the modulator concentrations on RPC of three insulin variants. KCl and ethanol were used as mobile phase modulators, and the experiments were performed on two different adsorbents, with C₁₈ and C₄ ligands. The temperature dependence was investigated for the interval 10-40 °C and at two different concentrations of each modulator. The model is derived from the expression for the adsorption equilibrium, which assumes that ethanol is adsorbed to the ligands and displaced by the insulin molecules, similar to the displacement of counterions in the steric mass-action model for ion-exchange chromatography. A good model fit to the new linear-range retention data was achieved by only adding and calibrating three parameters for the temperature dependence of the equilibrium. We found that a lower temperature results in a longer retention time for all adsorbates, adsorbents, and modulator concentrations used in this study, indicating that the adsorption process is enthalpy-driven. A comparison of the different contributions to the temperature dependence revealed that the large contribution from the equilibrium constant is dampened by the significant contributions of the opposite sign from the changes in activity coefficients of insulins and ethanol. Neglect of these effects when comparing different adsorbents and modulators might yield incorrect conclusions because the equilibrium constant varies with both, whereas the activity coefficients should be independent of the adsorbent. As expected, the conditions that promote higher retention also give a higher selectivity between the adsorbates. Nonetheless, in relation to its effect on the retention, the influence of the KCl concentration on the selectivity was significantly stronger than that of the temperature or that of the ethanol concentration.

Non-linear van't Hoff behavior in pulmonary surfactant model membranes

Pulmonary surfactant exhibits phase coexistence over a wide range of surface pressure and temperature. Less is known about the effect of temperature on pulmonary surfactant models. Given the lack of studies on this issue, we used electron paramagnetic resonance (EPR) and nonlinear least-squares (NLLS) simulations to investigate the thermotropic phase behavior of the matrix that mimics the pulmonary surfactant lipid complex, i.e., the lipid mixture composed of dipalmitoyl phosphatidylcholine (DPPC), palmitoyl-oleoyl phosphatidylcholine (POPC) and palmitoyl-oleoyl phosphatidylglycerol (POPG). Irrespective of pH, the EPR spectra recorded from 5°C to 25°C in the DPPC/POPC/POPG (4:3:1) model membrane contain two spectral components corresponding to lipids in gel-like and fluid-like phases, indicating a coexistence of two domains in that range. The temperature dependence of the distribution of spin labels between the domains yielded nonlinear van't Hoff plots. The thermodynamic parameters evaluated were markedly different for DPPC and for the ternary DPPC/POPC/POPG (4:3:1) membranes and exhibited a dependence on chemical environment. While enthalpy and entropy changes for DPPC were always positive and presented a quadratic behavior with temperature, those of the ternary mixture were linearly dependent on temperature and changed from negative to positive values. Despite that, enthalpy-entropy compensation takes place in the two systems. The thermotropic process associated with the coexistence of the two domains is entropically-driven in DPPC and either entropically- or enthalpically-driven in the pulmonary surfactant membrane depending on the pH, ionic strength and temperature. The significance of these results to the structure and function of the pulmonary surfactant lipid matrix is discussed.

On-line determination by small angle X-ray scattering of the shape of hen egg white lysozyme immediately following elution from a hydrophobic interaction chromatography column

This study documents the use of an integrated approach, involving on-line HIC interfaced with SAXS measurements, to monitor the conformational status of proteins immediately upon elution from a chromatographic column.

Kinetics and thermodynamics of binding reactions as exemplified by anthrax toxin channel blockage with a cationic cyclodextrin derivative

The thermodynamics of binding reactions is usually studied in the framework of the linear van't Hoff analysis of the temperature dependence of the equilibrium constant. The logarithm of the equilibrium constant is plotted versus inverse temperature to discriminate between two terms: an enthalpic contribution that is linear in the inverse temperature, and a temperature-independent entropic contribution. When we apply this approach to a particular case-blockage of the anthrax PA(63) channel by a multicharged cyclodextrin derivative-we obtain a nearly linear behavior with a slope that is characterized by enthalpy of about 1 kcal/mol. In contrast, from blocker partitioning between the channel and the bulk, we estimate the depth of the potential well for the blocker in the channel to be at least 8 kcal/mol. To understand this apparent discrepancy, we use a simple model of particle interaction with the channel and show that this significant difference between the two estimates is due to the temperature dependence of the physical forces between the blocker and the channel. In particular, we demonstrate that if the major component of blocker-channel interaction is van der Waals interactions and/or Coulomb forces in water, the van't Hoff enthalpy of the binding reaction may be close to zero or even negative, including cases of relatively strong binding. The results are quite general and, therefore, of importance for studies of enzymatic reactions, rational drug design, small-molecule binding to proteins, protein-protein interactions, and protein folding, among others.

Effect of temperature on the self-assembly of the Escherichia coli ClpA molecular chaperone

Protein quality control pathways rely upon ATP-dependent proteases, such as Escherichia coli ClpAP, to perform maintenance roles in the cytoplasm of the cell. ATP-dependent proteases remove misfolded and partially synthesized proteins. This action is particularly important in situations where an unregulated accumulation of such proteins will have a deleterious effect on the cell. ClpAP is composed of a tetradecameric serine protease, ClpP (21.6 kDa monomer), and the ATPase/protein unfoldase ClpA (84.2 kDa monomer). ClpA also uses its protein unfolding activity to remodel proteins and protein complexes; thus, in the absence of the proteolytic component, ClpA is considered a molecular chaperone. Previous reports, by others, suggested that ClpA exists in a monomer-dimer equilibrium at 4 °C. In contrast, using a combination of sedimentation velocity, sedimentation equilibrium, and dynamic light scattering, we recently reported that ClpA exists in a monomer-tetramer equilibrium at 25 °C. Here we report an investigation of the effect of temperature on the self-association of the E. coli ClpA protein unfoldase using analytical ultracentrifugation techniques. The results of sedimentation velocity and sedimentation equilibrium experiments performed at multiple loading concentrations of ClpA over a range of temperatures from 3.9 to 38.2 °C are discussed. Sedimentation velocity experiments show a decrease in weight average s(20,w) at the extremes of temperature. This result, along with extensive sedimentation equilibrium data and analysis, suggests the presence of a dimeric intermediate of ClpA that is differentially populated as a function of temperature. Further, analysis of sedimentation equilibrium data as a function of temperature led us to propose a monomer-dimer-tetramer equilibrium to describe the temperature dependence of ClpA self-assembly in the absence of nucleotide.

Algorithmically designed peptides ameliorate behavioral defects in animal model of ADHD by an allosteric mechanism

This study exemplifies the use of three ADHD-relevant methodological innovations. (1) The use of novel, patented, computational peptide design techniques to generate peptides targeting the extra-cellular and para-transmembrane amino acid loops of the putatively ADHD-involved, D(2) dopamine receptor, D(2)DAR; (2) experimental evidence that these peptides in L-amino acid/ortho ordered or D-amino acid/reverse ordered (retro-inverso), D(2)DAR, hydrophobic eigenmode matched forms, evoked positive allosteric and indirect agonist influences on in vitro stably receptor transfected CHO and LtK cells and on in vivo, brain mediated activity; (3) a representative 15 residue all-D-amino acid, D(2) mode matched peptide, given parenterally, was found to "repair" a key aberrant ADHD behavioral characteristic in a standard animal model of ADHD, the Spontaneously Hypertensive Rat, SHR, relative to its progenitor species control, the Wistar-Kyoto rat, WKY. The representative, retro-inverso peptide, all-D-LLYKNKPRYPKRNRE, reversed SHR's relative deficiency in sensory motor gating (pre-pulse inhibition, PPI) while leaving SHR's nonselective attention (rearings), impulsive behavior (time in center), and activity level (timed total motor behavior) unchanged. Amphetamine also reversed SHRs sensory gating defect, but with significant increases in nonselective attention, impulsivity and hyperactivity. These preliminary results suggest the possibility of a new, "softer" pharmacological approach to ADHD: hydrophobic mode matched peptide allosteric augmentation of the activity of indigenous dopamine with respect to D(2)DAR mediated function, in place of stimulant drug-induced presynaptic dopamine release or impairment of dopamine uptake.

Designing allosteric peptide ligands targeting a globular protein

Patented signal analytic algorithms applied to hydrophobically transformed, numerical amino acid sequences have previously been used to design short, protein-targeted, L or D retro-inverso peptides. These peptides have demonstrated allosteric and/or indirect agonist effects on a variety of G-protein and tyrosine kinase coupled membrane receptors with 30% to over 80% hit rates. Here we extend these approaches to a globular protein target. We designed eight peptide ligands targeting an ELISA antibody responsive protein, beta-galactosidase, betaGAL. Three of the eight 14mer peptides allosterically activated betaGAL with ELISA methodology. Using Bayesian statistics, this 38% hit rate would have occurred 2 x 10(-9) by chance. These peptides demonstrated binding site competitive or noncompetitive interactions, suggesting allosteric site multiplicity with respect to their betaGAL binding-mediated ELISA signal. Kinetic studies demonstrated the temperature dependence of the betaGAL peptide binding functions. Using the van't Hoff relation, we found evidence for enthalpy-entropy compensation. This relation is often found for hydrophobic interactions in aqueous media, and is consistent with the postulated hydrophobic series encoding underlying our protein-targeted, peptide design methods. It appears that our algorithmic, hydrophobic autocovariance eigenvector template approach to the design of allosteric peptides targeting membrane receptors may also be applicable to the design of peptide ligands targeting nonmembrane involved globular proteins.

Binding behaviour and conformational properties of globular proteins in the presence of immobilised non-polar ligands used in reversed-phase liquid chromatography

The thermodynamic and extra-thermodynamic dependencies of five types of cytochrome c in water-acetonitrile mixtures of different composition in the presence of immobilised n-octyl ligands as a function of temperature from 278 K to 338 K have been investigated. The corresponding enthalpic, entropic and heat capacity parameters, deltaHdegrees assoc, deltaS degrees assoc and delta C degrees p, have been evaluated from the observed non-linear Van't Hoff plots of these globular proteins in these heterogeneous systems. The relationships between the free energy dependencies, various molecular parameters and extra-thermodynamic dependencies (empirical correlations) of these protein-non-polar ligand interactions have also been examined. Thus, the involvement of enthalpy-entropy compensation effects has been documented for the binding of these cytochrome cs to solvated n-octyl ligands. Moreover, the results confirm that this experimental approach permits changes in molecular surface area due to the unfolding of these proteins on association with non-polar ligands as a function of temperature to be correlated with other biophysical properties. This study thus provides a general procedure whereby the corresponding free energy dependencies of globular proteins on association with solvated non-polar ligands in heterogeneous two-phase systems can be quantitatively evaluated in terms of fundamental molecular parameters.

Peptide mapping by reversed-phase high-performance liquid chromatography employing silica rod monoliths

In this paper, a general procedure is described for the generation of peptide maps of proteins with monolithic silica-based columns. The peptide fragments were obtained by tryptic digestion of various cytochrome c species with purification of the tryptic fragments achieved by reversed-phase high-performance liquid chromatographic methods. Peak assignment of the various peptides was based on evaluation of the biophysical properties of the individual peptides and via mass spectrometric identification. The performance of several different monolithic sorbents prepared as columns of identical cross-sectional dimensions were investigated as part of these peptide mapping studies and the data evaluated by applying solvent strength theory. These studies revealed curvilinear dependencies in the corresponding relative resolution maps. These findings directly impact on the selection of specific sorbent types or column configurations for peptide separations with silica rod monoliths. Moreover, the influence of variations in the amino acid sequence of the cytochrome cs were evaluated with respect to their effect on intrinsic hydrophobicity, the number of experimental observed tryptic cleavage sites, detection limits of the derived fragments in relation to their molecular size, and the chromatographic selectivity and resolution of the various peptides obtained following enzymatic fragmentation of the parent protein. Finally, the scope of these approaches in method development was examined in terms of robustness and efficiency.

Cellular and behavioral effects of D2 dopamine receptor hydrophobic eigenmode-targeted peptide ligands

Patterns in G-protein-coupled receptors' hydrophobically transformed amino-acid sequences can be computationally characterized as hierarchies of autocorrelation waves, "hydrophobic eigenmodes", using autocovariance matrix decomposition and all poles power spectral and wavelet transformations. L- or D-amino acid (retro-inverso) 12-18 residue peptides targeting these modes can be designed using eigenvector templates derived from these computations. In all, 12 human long-form D(2) dopamine receptor eigenmode-targeted 15 mer peptides were designed, synthesized, and shown to modulate and/or indirectly activate the extracellular acidification response, EAR, in stably receptor-transfected CHO and LtK cells, with an 83% hit rate. Representative L- and D-amino-acid retro-inverso peptides injected bilaterally in the nucleus accumbens demonstrated changes in rat exploratory behavior and prepulse inhibition similar to those observed following parenteral amphetamine. In contrast with geometric models used for ligand design, such as pharmacophores, the hydrophobic eigenmode approach to lead modulatory peptide design targets hydrophobic eigenmode-bearing subsequences, including those not visible from X-ray and NMR studies such as extracellular segments and loops.

The CEC behaviour of several synthetic peptides related to the activin betaA-betaD subunits

The resolution of several structurally related synthetic peptides, derived from the loop 3 region of the activin betaA-betaD subunits, has been studied using capillary electrochromatography (CEC) with Hypersil n-octadecylsilica as the sorbent. The results confirm that the CEC migration of these peptides can be varied in a charge-state-specific manner as the properties of the background electrolyte, such as pH, salt concentration and content of organic modifier, or temperature are systematically changed. Acidic peptides followed similar trends in retention behaviour, which was distinctly different to that shown by more basic peptides. The CEC separation of these peptides with the Hypersil n-octadecyl-silica involved distinguishable contributions from both electrophoretic mobility and chromatographic retention. Temperature effects were reflected as variations in both the electro-osmotic flow and the electrophoretic mobility of the peptides. When the separation forces acting on the peptides were synergistic with the electro-osmotic flow, as, for example, with the positively charged peptides at a particular pH and buffer electrolyte composition, their retention coefficient, kappacec, decreased with increasing capillary temperature, whereas when the separation forces worked in opposite directions, as for example with negatively charged peptides, their kappacec values increased slightly with increasing temperature. Moreover, when the content of organic modifier, acetonitrile, was sufficiently high, e.g. > 40% (v/v) and nonpolar interactions with the Hypersil n-octadecyl-silica sorbent were suppressed, mixtures of both the basic and acidic synthetic peptides could be baseline resolved under isocratic conditions by exploiting the mutual processes of electrophoretic mobility and electrostatic interaction. A linear relationship between the ln kappacec values and the volume fractions, psi, of the organic modifier over a limited range of psi-values, was established for the negatively charged peptides under these isocratic conditions. These findings thus provide useful guidelines in a more general context for the resolution and analysis of structurally related synthetic peptides using CEC methods.

Thermodynamic assessment of the stability of thrombin receptor antagonistic peptides in hydrophobic environments

In this paper, a general procedure is described to determine thermodynamic parameters associated with the interaction of thrombin receptor antagonistic peptides (TRAPs) with immobilized nonpolar ligands. The results show that these interactions were associated with nonlinear van't Hoff dependencies over a wide temperature range. Moreover, changes in relevant thermodynamic parameters, namely the changes in Gibbs free energy of interaction, DeltaG(0)assoc, enthalpy of interaction, DeltaH(0)assoc, entropy of interaction, DeltaS(0)assoc, and heat capacity, DeltaC(0)p, have been related to the structural properties of these TRAP analogs. The implications of these investigations for the design of thrombin receptor agonists/antagonists with structures stabilized by intramolecular hydrophobic interactions are discussed.

Application of CEC procedures for the analysis of synthetic peptides: characterization of linear immunogenic peptides that mimic a HIV-1 gp120 epitope

In this study, we describe the application of a new analytical procedure based on capillary electrochromatographic(CEC) techniques for the characterization of different basic and acidic peptides using isocratic eluent conditions containing acetonitrile and ammonium acetate buffers of different molarities between pH 3.8 and 5.2. In particular,10 immunogenic peptide analogs with isoelectric points ranging from 3.7 to 10.1 were investigated; nine of these peptides, 1-9, were truncated analogs of the parent peptide, 10, which is a peptidomimetic related to a HIV-1 gp120 epitope. Several of these peptides have the propensity to form alpha-helical secondary structures in solution. Electrochromatographic separations of these peptides were achieved with packed fused silica capillaries(25 cm packed length, 100 microm i.d.) containing 3 microm n-octadecylsilica particles. The influence of temperature on the CEC elution behavior of these peptides, as well as the impact of changes in the eluent composition, e.g. pH, buffer concentration and acetonitrile content, were examined. The results confirm that improvements in the resolution and analysis of synthetic peptides by CEC procedures result from the increase inelectroosmotic flow (EOF) as the temperature is increased. These findings emphasize the dominant influence of the temperature-dependent viscosity parameter, eta, on the EOF and thus on peptide resolution in CEC. Moreover, these investigations have shown that eluent properties can be specifically chosen to favor either electrophoretic mobility or chromatographic retention, with the overall CEC selectivity peptides of different sequence or composition reflecting the summated contributions from both separation mechanisms. Over the pH range 4.0-5.0, and using eluents with ionic strengths ranging from 6.2 to 15 mM ammonium acetate but containing a fixed volume fraction, psi, of acetonitrile above psi = 0.40, the CEC retention behavior of peptides 1-10 correlated with a linear relationship linking the retention coefficient, kappta(cec), and the differential frictional size-to-mass ratio parameter, Xi(fric), of these peptides. However, using eluents with a low acetonitrile content and low pH values, linear correlations were also observed between the incremental retention coefficient, Delta(Kappa)cec, and the product term [-0.66(Delta(Sigma[Xn]) log(Mi/Mj)], which links the difference in intrinsic hydrophobicities and molecular masses of two peptides, Pi and Pj. This study thus demonstrates the power of CEC procedures in the analysis of synthetic bioactive peptides and provides a general experimental framework to evaluate,using CEC procedures, the influence of the key molecular attributes of peptides on their structure-retention dependencies.Finally, these studies provide additional, practical insights into the use of CEC procedures for the analysis, resolution and biophysical characterization of closely related peptide analogs derived from solid-state peptide synthesis under conditions of different eluent composition or temperature.

Thermal unfolding of proteins studied by coupled reversed-phase HPLC-electrospray ionization mass spectrometry techniques based on isotope exchange effects

In this study, a deuterium exchange procedure has been employed to evaluate the thermal stability of globular proteins under conditions that replicate their interactive behavior in reversed-phase high performance chromatographic (RP-HPLC) systems. In particular, this investigation has permitted the conformational stability of two proteins, hen egg white lysozyme (HEWL) and horse heart myoglobin (HMYO) to be examined under different temperature and low-pH solvent regimes. The results confirm that this experimental approach provides an efficient strategy to explore fundamental conformational features of polypeptides or proteins in their folded and partial unfolded states under these interactive conditions. In particular, this analytical procedure permits insight to be readily gained into the processes that occur when polypeptides and globular proteins interact with lipophilic liquid/ solid interfaces in the presence of water-organic solvent mixtures at different temperatures.

Experimental studies of pressure/temperature dependence of protein adsorption equilibrium in reversed-phase high-performance liquid chromatography

The effect of the average pressure and temperature of the column on the adsorption equilibrium of insulin variants on a C8 bonded silica was studied in isocratic reversed-phase HPLC. Analytical injections of samples of four different insulins (bovine, porcine. Lys-Pro and human recombinant) were carried out at constant flow-rate but under increased average pressure. The temperature dependence of the retention parameters over the range 25-50 degrees C was studied under two different average column pressures (47 and 147 bar). Substantial increases of the retention time (up to 300%) were observed when the pressure and/or the temperature were increased. Similar adsorption-induced changes in the partial molar volume at constant temperature (deltaVm approximately 102 ml/mol) were found for all the variants studied. Furthermore, deltaVm was revealed to be practically independent of the temperature, which suggests that the temperature has no or very little influence on the mechanism of the pressure induced perturbations in the molecular structure of the solute. This conclusion was also derived from the observed temperature dependence of the logarithm of the retention factor (k) measured under different pressures. The relation between the temperature and In k was nonlinear with a parabolic shape. Moreover, the shapes of the plots corresponding to the low and high pressures were found to be exactly the same, except that the curves were vertically shifted, due to the difference between the two average column pressures. These results indicate that pressure and temperature affect the retention behavior of insulins in a different and separate way.

Role of interfacial hydrophobic residues in the stabilization of the leucine zipper structures of the transcription factors c-Fos and c-Jun

This study documents a new and versatile experimental approach to study the relative stabilization energetics of recombinant polypeptide and protein mutants. In particular, the effect of temperature change over the range of T = 278-338 K on the thermodynamics of interaction of several leucine zipper coiled-coil polypeptides related to the transcription factors, c-Fos and c-Jun, following binding to immobilized n-octyl ligands has been determined. Plots of the change in heat capacity, DeltaC(p)0, versus T, in combination with the corresponding van't Hoff plots, allow the energetics of the interaction of polypeptides with n-octyl ligands to be rationalized and the respective mid-point transition temperatures, T(m) values, determined for the melting of their supramolecular structures. The derived experimental data correlated well with information available from other procedures, confirming that this new approach provides complementary insight into the interaction thermodynamics and the molecular nature of the thermal stability of recombinant polypeptides in non-polar or other types of chemical environments.

Capillary electrochromatography analysis of hormonal cyclic and linear peptides

The retention behavior of linear and cyclic peptides has been studied by capillary electrochromatography (CEC) with a variety of different n-alkyl silica reversed-phase sorbents and also with mixed-mode phases containing both strong cation-exchange (sulfonic acid) and n-alkyl groups bonded onto the silica surface, using eluents ranging from pH 2.0 to pH 5.0. Depending upon the amino acid sequence, electrochromatographic retention of the peptides was strongly affected by the composition of the eluent, its pH value, and the choice of sorbent packed into the capillaries. The dominant separation processes operating with these charged analytes could be modulated inter alia by the content of organic modifier, acetonitrile, in the eluent, with peptide resolution predominantly arising from electrophoretic migration processes at high acetonitrile content. As the concentration of acetonitrile was decreased, chromatographic retention processes became more pronounced. With the n-alkyl silica CEC columns used in this study, silanophilic interactions between the sorbents and the charged peptides could be suppressed by increasing the molarity of the buffer and by adjusting the pH of the eluent to lower values. On the other hand, electrostatic interactions between basic peptides and the surface of strong cation-exchanger, mixed-mode materials can be suppressed at low pH values by using higher ionic strength conditions in the eluent. Different selectivity behavior was achieved with desmopressin and the other peptides with Spherisorb C18/SCX and Hypersil mixed-mode materials when an identical eluent composition of 60% (v/v) acetonitrile with 7.6 mM triethylammonium phosphate, pH 3.0, was used. These findings confirm that the surface charge density of the sorbent fulfills an important role in the modulation of peptide selectivity in CEC. These studies also confirm that the dependency of the logarithm of the CEC retention coefficients, i.e., log Kcec, of a peptide separated with n-octadecyl silica sorbents under CEC conditions, on the volume fraction, psi, of the organic solvent modifier, acetonitrile, within the range of 0.20 < or = psi < or = 0.60, can be approximated by a linear relationship. Moreover, these studies show that the selectivity differences of peptides separated by CEC with nonpolar sorbents in packed capillary systems can be discussed in terms of semiempirical dependencies that link peptide retention behavior with their molecular descriptor properties, e.g., their hydrophobicity, surface charge anisotropy, surface area, molecular mass and intrinsic charge, and thus to their corresponding linear free energy relationships.

Peptide analysis by rapid, orthogonal technologies with high separation selectivities and sensitivities

This article examines the current status of peptide analysis by orthogonal micro-/nano-separation strategies, with emphasis on the complementary use of high performance capillary liquid chromatography (micro-HPLC), capillary zonal electrophoresis (HPCZE), open tubular capillary electrochromatography (ot -CEC) and packed capillary electrochromatography (p -CEC). The ability to interface these techniques with mass spectroscopic (MS) procedures has enabled substantial progress to be made in the analysis of very small quantities of peptides, as well as proteins and other bio-macromolecules. As a consequence, the staged application of these high resolution techniques as part of the standardisation of biological products via robust, sensitive protocols is rapidly becoming a reality. Recent conceptual and theoretical advances have also allowed improved levels of prediction and optimisation of these procedures. Since significant differences in selectivity can be achieved with micro-HPLC, HPCZE and HPCEC respectively, collectively these sophisticated techniques provide unprecedented opportunities for the rapid, orthogonal and sensitive separation of complex mixtures of peptides and proteins. Several advantages of using these technologies in tandem are highlighted.

HPLC of peptides and proteins: standard operating conditions

The standard operating conditions for the eight basic modes of HPLC are presented in this unit. They include: size-exclusion chromatography (HP-SEC), ion-exchange chromatography (HP-IEX), normal phase chromatography (HP-NPC), hydrophobic interaction chromatography (HP-HIC), reversed-phase chromatography (RP-HPLC), hydrophilic interaction chromatography (HP-HILIC), immobilized metal ion affinity chromatography (HP-IMAC), and biospecific/biomimetic affinity chromatography (HP-BAC). In addition, some subsets of these chromatographic modes, e.g., mixed mode chromatography (HP-MMC), charge transfer chromatography (HP-CTC), or ligand-exchange chromatography (HP-LEC) are described. Procedures for multimodal column switching are also included, as are guidelines for a systematic approach to method development. Example separations help illustrate the procedures. The standard operating conditions for the eight basic modes of HPLC are presented in this unit.

Influence of temperature on the behaviour of small linear peptides in capillary electrochromatography

The influence of temperature, T, on the retention times, peak widths, peak symmetry coefficients and theoretical plate numbers of two small linear peptides, [Met5]enkephalin and [Leu5]enkephalin, has been studied with capillary electrochromatography (CEC) capillary columns of 100 microm I.D. and 250 mm packed length with a total length of 335 mm, containing 3 microm Hypersil n-octadecyl bonded silica. With increasing column temperature from 15 to 60 degrees C, the electroosmotic flow (EOF) and the column efficiencies increased, whereas the retention coefficients (Kcec) of both peptides decreased. A linear relationship was found between the EOF value and the square root of the temperature over this temperature range, with a linear regression correlation of 0.998. Non linear Van 't Hoff plots (In Kcec versus 1/T) were observed for these peptides between 15 and 60 degrees C, suggesting that a phase-transition occurred with the n-octadecyl chains bonded on the silica surface, affecting the CEC retention behaviour of these peptides. In CEC systems, the Kcec values of peptides incorporate contributions from both electrophoretic migration and chromatographic retention. Positive and negative Kcec values can, in principle, thus arise with these charged analytes. However, the Kcec values of the enkephalin peptides under all temperature conditions studied were positive with an eluent composed of water-50 mM NH4OAc/AcOH, pH 5.2-acetonitrile (5:2:3, v/v) and therefore the chromatographic component dominates the retention process with these small peptides under these conditions.

Capillary electroendoosmotic chromatography of peptides

This review focuses on the current state of peptide separation by capillary electroendoosmotic chromatography (CEC). When carried out under optimised conditions, peptide separation by CEC methods represents an orthogonal and complementary technique to micro-HPLC (micro-HPLC) and high-performance capillary zone electrophoresis (HPCZE). The origin of the selectivity differences that can be achieved with these three separation techniques (CEC, micro-HPLC and HPCZE), respectively are discussed, and the current limits of performance with CEC methods documented. Peptide separations by CEC methods with n-alkyl bonded silicas or mixed-mode phases are also illustrated. The development of different variants of CEC and pressurised CEC (also commonly referred to in the literature as electrically-assisted micro-HPLC) are examined. The potential of coupling CEC systems to mass spectrometers for real-time analyses of peptides or protein digests has been examined. Several future directions for the application of this technique in phenotype/proteomic and zeomic mapping of naturally occurring peptides and proteins are highlighted.

Studies on the relationship between structure and electrophoretic mobility of alpha-helical and beta-sheet peptides using capillary zone electrophoresis

The electrophoretic behaviour of a series of 33 different synthetic peptides has been investigated using free solution high-performance capillary zonal electrophoretic (HPCZE) methods. The dependency of the electrophoretic mobility, mu(em), on the peptide charge, q, and on the charge-to-size ratio parameter, zeta, determined according to several electromobility models, have been examined. Significant divergences from linearity in the mu(em) vs. q or the mu(em) vs. zeta plots were noted for several peptides, possibly due to the proclivity of specific arrangements of their amino acid sequences to assume preferred alpha-helical or beta-sheet conformational features rather than random coil structures under the HPCZE conditions. These results provide further demonstration of the facility of HPCZE procedures to probe the effects of compositional, sequential and conformational differences of closely-related peptides and their consequences on their physicochemical behaviour in solution.

Comparison between the isocratic and gradient retention behaviour of polypeptides in reversed-phase liquid chromatographic environments

The isocratic and gradient elution behaviour of beta-endorphin and glucagon, two polypeptides known to exist in amphipathic alpha-helical conformations in lipophilic environments, have been examined under reversed-phase high-performance liquid chromatographic (RP-HPLC) conditions with low pH, aquo-acetonitrile mobile phases. The effects of changes in the volume fraction, psi, of the organic solvent modifier and temperature, T, on the magnitudes of the S and log k(o) values of these two polypeptides, obtained from the plots of logarithmic capacity factor (log k') vs. psi using isocratic elution conditions have been determined. These data have then been compared to the corresponding S and log k(o) values, obtained from the plots of logarithmic median capacity factor (log k) versus the median volume fraction of the organic solvent modifier (psi) derived from the linear gradient elution data, using the same n-butyl silica sorbent and related aquo-acetonitrile mobile phase conditions. As apparent from these studies, substantial differences occur in the temperature-dependent trends and magnitudes of the corresponding S and S values, or the log k(o) and log k(o) values, when these parameters are derived from experimental data acquired by these two different elution methods. Moreover, when gradient elution data for beta-endorphin and glucagon are utilised, the extrapolated values of the intercept and slope of the plots of log k vs. 1/T (corresponding to an apparent change in the median enthalpy of association, deltaH(o)assoc, or an apparent change in the median entropy of association, deltaS(o)assoc) substantially deviated from the values obtained for the thermodynamic parameters, deltaH(o)assoc and deltaS(o)assoc, derived from the log k' vs. 1/T plots using the corresponding isocratic data. These findings thus have important implications for biophysical and thermodynamic investigations when gradient elution data are employed to assess the molecular basis of the interaction of polypeptides with non-polar ligates.