New expressions to describe solution nonideal osmotic pressure, freezing point depression, and vapor pressure

Water Compartments in Cells

Human experience in the macrobiological world leads scientists to visualize water compartments in cells analogous to the bladder in the human pelvis or ventricles in the brain. While such water-filled cellular compartments likely exist, the volume contributions are insignificant relative to those of biomolecular hydration compartments. The purpose of this chapter is to identify and categorize the molecular water compartments caused by proteins, the primary macromolecular components of cells. The categorical changes in free energy of water molecules on proteins cause these compartments to play dominant roles in osmoregulation and provide important adjuncts to fundamental understanding of osmosensing and osmosignaling mechanisms. Water compartments possess differences in molecular motion, enthalpy, entropy, freezing point depression, and other properties because of electrostatic interaction of polar water molecules with electric fields generated by covalently bound pairs of opposite charge caused by electronegative oxygen and nitrogen atoms of the protein. Macromolecules, including polypeptides, polynucleotides, and polysaccharides, are stiff molecular chains with restricted folding capacities due to inclusion of rigid ring structures or double amide bonds in the backbone sequence. This creates "irreducible spatial charge separation" between positive and negative partial charges, causing elevated electrostatic energy. In the fully hydrated in vivo state of living cells the high dielectric coefficient of water reduces protein electrostatic free energy by providing polar "water bridge networks" between charges, thereby creating four measurably different compartments of bound water with distinct free energy differences.

Photon correlation spectroscopy investigations of proteins

Physical principles of photon correlation spectroscopy (PCS), mathematical treatment of the PCS data (converting autocorrelation functions to distribution functions or average characteristics), and PCS applications to study proteins and other biomacromolecules in aqueous media are described and analysed. The PCS investigations of conformational changes in protein molecules, their aggregation itself or in consequence of interaction with other molecules or organic (polymers) and inorganic (e.g. fumed silica) fine particles as well as the influence of low molecular compounds (surfactants, drugs, salts, metal ions, etc.) reveal unique capability of the PCS techniques for elucidation of important native functions of proteins and other biomacromolecules (DNA, RNA, etc.) or microorganisms (Escherichia coli, Pseudomonas putida, Dunaliella viridis, etc.). Special attention is paid to the interaction of proteins with fumed oxides and the impact of polymers and fine oxide particles on the motion of living flagellar microorganisms analysed by means of PCS.

Osmotic Pressure, Small-Angle X-Ray, and Dynamic Light Scattering Studies of Human Serum Albumin in Aqueous Solutions

Osmotic pressure measurements of human serum albumin (HSA) dissolved in water and in 0.01, 0.1, and 1.0 M phosphate buffer are reported as a function of the protein concentration. Two different forms of the protein were studied: defatted HSA (HSA1) and HSA with fatty acids (HSA2). The measured values of the osmotic coefficient were well below 1, indicating large deviations from ideality even for dilute protein solutions. The measured values increased with increasing HSA concentration and the increase was a function of pH. For higher concentrations of added phosphate buffer, the pH of solution had less influence on the measured osmotic pressure. The osmotic pressure of HSA1 in water was found to be considerably lower than that of the HSA2 modification. This effect was ascribed to formation of dimers in the HSA1 solution. The osmotic measurements were complemented by the small-angle X-ray scattering (SAXS) and dynamic light scattering (DLS) studies of dilute HSA solutions in water. The SAXS and DLS data confirmed the dimerization of HSA1 molecules under these conditions. Detailed analysis of the SAXS data suggested a parallel orientation of two protein molecules in a dimer. Copyright 2001 Academic Press.

Understanding Nonidealities of the Osmotic Pressure of Concentrated Bovine Serum Albumin

Previously Vilker et al. (J. Colloid Interface Sci. 79(2), (1981)) reported the osmotic pressure of concentrated bovine serum albumin (BSA) up to 475 g/L in 0.15 M sodium chloride at pH 4.5, 5.4, and 7.4. The authors used a semiempirical model based on Donnan theory to predict the osmotic pressure with good agreement. However, the formal application of a three-term virial expansion with the coefficients determined from the potential energy of interaction between BSA molecules resulted in poor agreement with their data. In this study, modeling of the osmotic pressure was reexamined using a free-solvent model that considered average solute-solvent and microion-solute interactions in a mole fraction concentration variable. The resulting fits were excellent for all three pH. The model is designed with no fitted parameters; however, the model results were highly sensitive to the selected hydration and microion binding. Therefore the hydration was further regressed from its initial estimate of 1 g H2O/g BSA (based on water-17O magnetic resonance studies of other globular proteins) to minimize the least-squares error between the predicted values and data. The resulting average hydration was determined to be 1.14 +/- 0.03 g H2O/g BSA for all pH values. However, the standard error in hydration for each pH was no greater than +/-0.0063 g H2O/g BSA. These results demonstrate that solvent-solute interaction and the concentration variable may be critical factors when evaluating osmotic pressure data of concentrated protein solutions. Copyright 1998 Academic Press.

Multiple-quantum filtered 17Q and 23Na NMR analysis of mitochondrial suspensions

The fraction of strongly- and weakly-bound water molecules within mitochondrial suspensions, determined using three-quantum filtered 17O NMR relaxation analysis, was found to be large in comparison with that in erythrocytes and concentrated solutions of bovine serum albumen. It is suggested that bound water, together with regulation of mitochondrial matrix volume, may be an important controlling factor in the modulation of enzymic activity in the matrix. A spin I = 5/2 Jeener-Broekaert experiment and a four-quantum filtration experiment were used to demonstrate the absence of orientationally ordered water molecules within the mitochondrion. In contrast, the mitochondrial sodium environment was shown to be highly ordered using a spin I = 3/2 Jeener-Broekaert experiment.

Physicochemical and immunochemical properties of recombinant human serum albumin from Pichia pastoris

We analyzed and compared the physicochemical and immunochemical properties of recombinant human serum albumin (rHSA) from Pichia pastoris with those of plasma-derived human serum albumin (pHSA). The second virial coefficient of rHSA, obtained from colloid osmotic pressure measurements at pH 6.7 +/- 0.1 was not significantly different from that of pHSA (P > 0.05). A 25% rHSA solution exhibited Newtonian flow, and the viscosity of 25% rHSA at 20 +/- 0.02 degrees C was not significantly different from that of 25% pHSA (P > 0.05). We analyzed the long- and medium-chain fatty acid composition of rHSA by reverse-phase HPLC using 9-anthryldiazomethane as the fluorescent labeling reagent. The total amount of fatty acid was higher for pHSA than for rHSA. The fatty acid composition of the rHSA preparation was the same as that of the pHSA preparation. However, the amounts of palmitic acid (C16:0) and stearic acid (C18:0) in rHSA were much lower than those in pHSA. Interestingly, we found that P. pastoris produced linolenic acid (C18:3) because it was detected in rHSA. The immunochemical properties of rHSA were analyzed by a parallel line assay method using anti-pHSA polyclonal antibody, and were identical to those of pHSA (P > 0.05).

Free-Solvent Model of Osmotic Pressure Revisited: Application to Concentrated IgG Solution under Physiological Conditions

The osmotic pressure measurements of bovine immuno-gamma globulin in phosphate-buffered solution at pH 7.4 and 0.13 M total salt concentration were extended to near saturation concentrations for ambient temperature. The osmotic pressure at the highest measured concentration of 424 g/L was 4.18 psi (28.3 kPa). A free-solvent model, considering solute-solvent interaction in the concentration variable, provided an excellent fit to observed osmotic pressure nonideality at even the highest protein concentration. The calculated mass of hydrated solvent compared with amounts determined from water-17O magnetic resonance for other globular proteins. This model provides an improved correlation to the data over virial equations (truncated to the third term) when only solute-solute interactions are considered. The use of mole fraction as the composition variable was critical in obtaining the excellent fit of the free-solvent model. A combination of the free-solvent correction for the concentration variable coupled with models incorporating solute-solute interaction, such as a virial expansion, will be necessary to generally describe the osmotic pressure of protein solutions for all concentrations. Copyright 1998 Academic Press. Copyright 1998Academic Press

Probing protein hydration and conformational states in solution

The addition of polyethylene glycol (PEG), of various molecular weights, to solutions bathing yeast hexokinase increases the affinity of the enzyme for its substrate glucose. The results can be interpreted on the basis that PEG acts directly on the protein or indirectly through water activity. The nature of the effects suggests to us that PEG's action is indirect. Interpretation of the results as an osmotic effect yields a decrease in the number of water molecules, delta Nw, associated with the glucose binding reaction. delta Nw is the difference in the number of PEG-inaccessible water molecules between the glucose-bound and glucose-free conformations of hexokinase. At low PEG concentrations, delta Nw increases from 50 to 326 with increasing MW of the PEG from 300 to 1000, and then remains constant for MW-PEG up to 10,000. This suggests that up to MW 1000, solutes of increasing size are excluded from ever larger aqueous compartments around the protein. Three hundred and twenty-six waters is larger than is estimated from modeling solvent volumes around the crystal structures of the two hexokinase conformations. For PEGs of MW > 1000, delta Nw falls from 326 to about 25 waters with increasing PEG concentration, i.e., PEG alone appears to "dehydrate" the unbound conformation of hexokinase in solution. Remarkably, the osmotic work of this dehydration would be on the order of only one k T per hexokinase molecule. We conclude that under thermal fluctuations, hexokinase in solution has a conformational flexibility that explores a wide range of hydration states not seen in the crystal structure.

Acute hemodynamic effects of recently developed monomer and dimer magnetic resonance imaging contrast media: a comparative study

RATIONALE AND OBJECTIVES

We evaluated and compared the acute cardiovascular effects of equiosmolar doses of recently developed nonionic monomer and macrocyclic dimer magnetic resonance (MR) imaging contrast media with the clinically available ionic and nonionic MR contrast media.

METHODS

Normotensive adult Sprague-Dawley rats were divided into six groups of seven rats per group. Group 1 received the nonionic monomer Gd-CMPA-BMPA (500 mmol/l solution); group 2 received the nonionic dimer Gd(2)2(O)DO3A (500 mmol/l solution); group 3 also received Gd(2)2(O)DO3A but at a higher concentration (1,000 mmol/l solution); group 4 received gadopentetate dimeglumine (500 mmol/l solution); and group 5 received gadodiamide (500 mmol/l solution). Each rat received a rapid (1-2 sec) bolus intravenous injection of 0.1, 0.25, and 0.5 mmol/kg of each contrast agent. Group 6 was used to test the peak effects of quiosmolar glucose solutions (500, 1,000, and 2,000 mOsm/kg water). Data were acquired at baseline, 20 sec (peak effect) after injection, and 1, 3, 5, and 10 min after injection. Peripheral (systolic, diastolic, and mean) pressure, central venous pressure, left ventricular (LV) pressure (peak systolic and end diastolic) pressure, first derivative of left ventricular pressure (+/-dP/dt), rate pressure product, and heart rate were measured.

RESULTS

Bolus administration (0.1, 0.25, and 0.5 mmol/kg) of Gd-CMPA-BMPA and gadodiamide (500 mmol/l) had no significant effects on the monitored cardiovascular parameters. Bolus injection of 0.25 and 0.5 mmol/kg Gd(2)2(O)DO3A (500 and 1,000 mmol/l) and gadopentetate dimeglumine (500 mmol/l) caused transient cardiovascular depression, including decreased peripheral blood pressure, LV systolic pressure, peak positive and negative dP/dt, and rate pressure product, but an increased LV end diastolic pressure. These cardiovascular effects were slightly less profound than those produced by gadopentetate dimeglumine.

CONCLUSION

Gd-CMPA-BMPA and gadodiamide have no adverse cardiovascular effects. Gd(2)2(O)DO3A and gadopentetate dimeglumine cause vasodilation and reduced cardiac performance. Therefore, presuming similar effects, if Gd(2)2(O)DO3A and gadopentetate dimeglumine are to be used at high doses for the MR quantification of blood volume or as a bolus for perfusion study, appropriate consideration should be given to possible adverse physiologic changes.

Osmotic pressure method to measure salt induced folding/unfolding of bovine serum albumin

A new approach has been developed to monitor protein folding by utilizing osmotic pressure and a range of salt concentrations in a well characterized protein, bovine serum albumin (BSA). It is hypothesized that both the 'effective' osmotic molecular weight, Ae, and the solute/solvent interaction parameter, I, in the empirical relation Msolvent/Msolute = (RT rho/Ae)1/pi + I [1] can be used as measures of protein folding. I is a measure of solvent perturbed by the solute and is thought to depend directly upon the solvent accessible surface area (ASA). It is reasoned that larger solvent accessible surface area of an unfolded or denatured protein should perturb more water and produce larger I-values. Thus I-values allow calculation of a unfolded protein fraction, fua, due to changes in relative solvent accessible surface area. It has been observed that Ac decreases for filamentous, denatured proteins due to segmental motion of the molecule [2]. This allows calculation of unfolded protein fraction from the effective molecular weight, fum. Colloid osmotic pressure of BSA was measured in a range of salt concentrations at 25 degrees C, and pH = 7 (above the isoelectric point of BSA at pH = 5.4). Both S and I were used to monitor protein folding as the salt concentration was varied. In general, larger and variable I-values and smaller Ae were observed at salt concentrations less than 50 mmolal NaCl (Imax = 8.9), while constant I = 4.1 and Ae = 66,500 were observed above 50 mmolal NaCl. The two expressions for fractional unfolding (fua and fum) are in general agreement. Small differences in the parameters below 50 mmolal salt concentration are explained with well known shifts in the relative amounts of alpha-helix, beta-sheet and random coil in denatured BSA. The relative amounts of these shifts agree with predictions in the literature attributed to continuous BSA expansion rather than an 'all-or-none' conversion.

Antagonistic effects of hydrostatic pressure and osmotic pressure on cytochrome P-450cam spin transition

The combined effects of hydrostatic pressure and osmotic pressure, generated by polyols, on the spin equilibrium of fenchone-bound cytochrome P-450cam were investigated. Hydrostatic pressure indices a high spin to low spin transition, whereas polyols induce the reversed reaction. Of the four solutes used, glycerol, glucose, stachyose, and sucrose, only the last two would act on the spin transition by osmotic stress. The spin volume changes measured by both techniques are different, 29 and -350 ml/mol for hydrostatic pressure and osmotic pressure, respectively. It suggests that even if the two are perturbing water molecules, different properties are probed. From the volume change induced by osmotic stress, 19 water molecules are deduced that would be implicated in the spin transition of the fenchone-bound protein. This result suggests that water molecules other than the well defined ones located in the active site play a key role in modulating the spin equilibrium of cytochrome P-450cam.

Solution nonideality related to solute molecular characteristics of amino acids

By measuring the freezing-point depression for dilute, aqueous solutions of all water-soluble amino acids, we test the hypothesis that nonideality in aqueous solutions is due to solute-induced water structuring near hydrophobic surfaces and solute-induced water destructuring in the dipolar electric fields generated by the solute. Nonideality is expressed with a single solute/solvent interaction parameter I, calculated from experimental measure of delta T. A related parameter, I(n), gives a method of directly relating solute characteristics to solute-induced water structuring or destructuring. I(n)-values correlate directly with hydrophobic surface area and inversely with dipolar strength. By comparing the nonideality of amino acids with progressively larger hydrophobic side chains, structuring is shown to increase with hydrophobic surface area at a rate of one perturbed water molecule per 8.8 square angstroms, implying monolayer coverage. Destructuring is attributed to dielectric realignment as described by the Debye-Hückel theory, but with a constant separation of charges in the amino-carboxyl dipole. By using dimers and trimers of glycine and alanine, this destructuring is shown to increase with increasing dipole strength using increased separation of fixed dipolar charges. The capacity to predict nonideal solution behavior on the basis of amino acid characteristics will permit prediction of free energy of transfer to water, which may help predict the energetics of folding and unfolding of proteins based on the characteristics of constituent amino acids.

Correction for solute/solvent interaction extends accurate freezing point depression theory to high concentration range

The authors describe empirical corrections to ideally dilute expressions for freezing point depression of aqueous solutions to arrive at new expressions accurate up to three molal concentration. The method assumes non-ideality is due primarily to solute/solvent interactions such that the correct free water mass Mwc is the mass of water in solution Mw minus I.M(s) where M(s) is the mass of solute and I an empirical solute/solvent interaction coefficient. The interaction coefficient is easily derived from the constant in the linear regression fit to the experimental plot of Mw/M(s) as a function of 1/delta T (inverse freezing point depression). The I-value, when substituted into the new thermodynamic expressions derived from the assumption of equivalent activity of water in solution and ice, provides accurate predictions of freezing point depression (+/- 0.05 degrees C) up to 2.5 molal concentration for all the test molecules evaluated; glucose, sucrose, glycerol and ethylene glycol. The concentration limit is the approximate monolayer water coverage limit for the solutes which suggests that direct solute/solute interactions are negligible below this limit. This is contrary to the view of many authors due to the common practice of including hydration forces (a soft potential added to the hard core atomic potential) in the interaction potential between solute particles. When this is recognized the two viewpoints are in fundamental agreement.

A study of the molecular sources of nonideal osmotic pressure of bovine serum albumin solutions as a function of pH

The nonideal osmotic pressure of bovine serum albumin (BSA) solutions was studied extensively by Scatchard and colleagues. The extent of pH- and salt-dependent nonideality changes are large and unexplained. In 1992, Fullerton et al. derived new empirical expressions to describe solution nonideal colligative properties including osmotic pressure (Fullerton et al. 1992. Biochem. Cell Biol. 70:1325-1331). These expressions are based on the concepts of volume occupancy and hydration force. Nonideality is accurately described by a solute/solvent interaction parameter I and an "effective" osmotic molecular weight Ae. This paper uses the interaction-corrected nonideal expressions for osmotic pressure to calculate the hydration I values and "effective" osmotic molecular weight of BSA, Ae, as a function of pH. Both factors vary in a predictable manner due to denaturing of the BSA molecule. Both contribute to an increase in osmotic pressure for the same protein concentration as the solution pH moves away from the isoelectric point. Increased nonideality is caused by larger hydration resulting from larger solvent-accessible surface areas and by the decrease in "effective" osmotic molecular weight, Ae, due to segmental motion of denatured (filamentous) molecules.

Solute/solvent interaction corrections account for non-ideal freezing point depression

A new highly accurate curve-fitting technique for looking at freezing-point depression data was proposed by Fullerton et al. (Biochem. Cell Biol., in press). The method involve plotting mass solvent to mass solute ratio (Mw/M(s)) vs. 1/delta T (i.e. the inverse change in freezing point). A measured molecular weight and a solute/solvent interaction parameter (called I value) are inferred from the resultant linear plot. The accuracy of the molecular weight method was first demonstrated with the monomers of ethylene glycol, glycerol, propanol, mannitol, glucose and sucrose to show a mean molecular weight error of 0.02% with root mean square (RMS) error 0.9%. The RMS error (0.9%) is our best estimate of the molecular weight measurement accuracy for the method applied to a monomer. This error is consistent with the experimental precision (approximately 1%) which implies no systematic error. Non-ideality is described with a single constant, I. Polyethylene glycol (PEG) polymers of increasing length (vendor designation 200 to 10,000 Da) were analyzed to show monotonically increasing non-ideality (I values of 0.12 to 3.67) with increasing molecular weight. The measured molecular weights agreed with the end-point titration value for the three smallest polymers (where the number of polymeric units was less than or equal to 7). The method underestimates the vendor molecular weights for longer polymers. This disagreement is assigned to segmental motion (internal entropy) of longer, more flexible, PEG molecules.