Nonspecific Forces Research Articles

Non-specific (entropic) forces as major determinants of the structure of mammalian chromosomes

Four specific forces (H-bonds, van der Waals forces, hydrophobic and charge interactions) shape the structure of proteins, and many biologists assume they will determine the shape of all structures in the cell. However, as the mass and contour length of a human chromosome are ~7 orders of magnitude larger than those of a typical protein, additional forces can become significant. We review evidence that additional non-specific (entropic) forces are major determinants of chromosomal shape and position. They are sufficient to drive the segregation (de-mixing) of newly replicated DNA to the poles of bacterial cells, while an entropic centrifuge can both form human chromosomes into territories and position them appropriately in nuclei; more locally, a depletion attraction can loop bacterial and human genomes.

Probing In Vitro Interactions between Lactococcus lactis and Mucins Using AFM

This work was devoted to the first AFM investigation of the adhesion force to pig gastric mucin (PGM) using Lactococcus lactis as the model for lactic acid bacteria. The PGM coating on polystyrene was characterized using a complementary set of multiscale analytical methods, including AFM (HarmoniX mode), XPS, and the sessile drop method. The PGM layer, which was mainly composed of C-O, C-N, COOH, CONH, and sulfur-related species (protein core and oligosaccharide side chains), was quite homogeneous and hydrophilic, with an estimated thickness of 3.4 nm. L. lactis cells were immobilized on the AFM tip (lacto probe) and used as a force probe to measure the interaction forces between bacteria and PGM-coated polystyrene on the nanoscale. After mucin adsorption, adhesion force levels were lower because of the interplay of electrostatic, hydrophilic, and steric repulsions. For example, the adhesion forces of the lacto probe to bare and PGM-coated polymer were 0.74 +/- 0.10 and 0.12 +/- 0.06 nN, respectively. The shape analysis of retraction force-distance curves highlighted the contribution of both nonspecific and specific forces (ligand/receptor bonding). The lacto probe concept and the associated AFM measurements may now provide a powerful framework for understanding interaction mechanisms between mucins and lactic acid bacteria.

Membrane Mediated Sorting

Inclusions in biological membranes may interact via deformations they induce on the shape of that very membrane. Such deformations are a purely physical effect, resulting in nonspecific forces between the inclusions. In this Letter we show that this type of interaction can organize membrane domains and hence may play an important biological role. Using a simple analytical model we predict that membrane inclusions sort according to the curvature they impose. We verify this prediction by both numerical simulations and experimental observations of membrane domains in phase separated vesicles.

Entropic organization of interphase chromosomes

Chromosomes are not distributed randomly in nuclei. Appropriate positioning can activate (or repress) genes by bringing them closer to active (or inactive) compartments like euchromatin (or heterochromatin), and this is usually assumed to be driven by specific local forces (e.g., involving H bonds between nucleosomes or between nucleosomes and the lamina). Using Monte Carlo simulations, we demonstrate that nonspecific (entropic) forces acting alone are sufficient to position and shape self-avoiding polymers within a confining sphere in the ways seen in nuclei. We suggest that they can drive long flexible polymers (representing gene-rich chromosomes) to the interior, compact/thick ones (and heterochromatin) to the periphery, looped (but not linear) ones into appropriately shaped (ellipsoidal) territories, and polymers with large terminal beads (representing centromeric heterochromatin) into peripheral chromocenters. Flexible polymers tend to intermingle less than others, which is in accord with observations that gene-dense (and so flexible) chromosomes make poor translocation partners. Thus, entropic forces probably participate in the self-organization of chromosomes within nuclei.

Measuring the force of single protein molecule detachment from surfaces with AFM

Atomic force microscopy (AFM) was used to measure the non-specific detachment force of single fibrinogen molecules from glass surfaces. The identification of single unbinding events was based on the characteristics of the parabolic curves, recorded during the stretching of protein molecules. Fibrinogen molecules were covalently bound to Si 3N 4 AFM tips, previously modified with 3-aminopropyl-dimethyl-ethoxysilane, through a homobifunctional poly(ethylene glycol) linker bearing two hydroxysulfosuccinimide esters. The most probable detachment force was found to be 210 pN, when the tip was retracting with a velocity of 1400 nm/s, while the distribution of the detachment distances indicated that the fibrinogen chain can be elongated beyond the length of the physical conformation before detachment. The dependence of the most probable detachment force on the loading rate was examined and the dynamics of fibrinogen binding to the surface were found amenable to the simple expression of the Bell–Evans theory. The theory's expansion, however, by incorporating the concept of the rupture of parallel residue–surface bonds could only describe the detachment of fibrinogen for a small number of such bonds. Finally, the mathematical expression of the Worm-Like Chain model was used to fit the stretching curves before rupture and two interpretations are suggested for the description of the AFM curves with multiple detachment events.

Poisson Analysis of Streptococcal Bond Strengthening on Stainless Steel with and without a Salivary Conditioning Film

Poisson analysis of retract force-distance curves in atomic force microscopy (AFM) has yielded a new dimension to the decoupling of individual bond forces into a hydrogen bonding and nonspecific force component. Accordingly, bacterial adhesion forces have been decoupled into a hydrogen bonding and nonspecific Lifshitz-Van der Waals contribution. Due to the forced nature of AFM contact, the nonspecific force contribution has hitherto turned out to be repulsive in the analysis of bacterial adhesion forces on nonconducting surfaces. In this study, we present the results of a Poisson analysis of adhesion forces for streptococci adhering to a conducting surface. Adhesion forces measured between stainless steel, both in the absence and presence of an adsorbed salivary conditioning film, increased with increasing contact time between the streptococcal AFM probe and the surface. Concurrent with the increase in adhesion force, there was an increase in the number of minor force peaks in the retract force-distance curves. Poisson analyses of the adhesion forces indicated repulsive nonspecific Lifshitz-Van der Waals forces for streptococci adhering to saliva-coated stainless steel, but interestingly and for the first time, attractive nonspecific forces were revealed on stainless steel in the absence of a salivary conditioning film. We tentatively attribute this to attraction between the negatively charged streptococci and their positive image charges in the conducting material, which cannot develop in a nonconducting material or in the presence of a nonconductive protein layer on the stainless steel surface.

Characterisation of the Ion Exchange Reaction Between Propranolol-H+ or K+ with Amberlite™ IRP 69 Resin by Both, Isothermal Titration Calorimetry and (Flame) Photometric Equilibrium Analysis

The model system propranolol hydrochloride (PROP-HCl)/Amberlite™ was studied in terms of equilibrium concentrations and amount drug bound to the ion exchanger, and compared to the system potassium chloride (KCl)/Amberlite™. It was found that both the affinity and exchange capacity of Amberlite™ for PROP-H + ions is higher than for K + . Specific heat effects of the exchange reaction measured by isothermal calorimetry and spectroscopy revealed that even at the lowest degree of loading with propranolol, the heat effects surprisingly change continuously with the de- gree of loading, while they are constant for potassium. Involvement of non-specific forces in the binding of propranolol- H + to Amberlite™ is discussed as a possible reason for this behaviour.

Bond-Strengthening in Staphylococcal Adhesion to Hydrophilic and Hydrophobic Surfaces Using Atomic Force Microscopy

Time-dependent bacterial adhesion forces of four strains of Staphylococcus epidermidis to hydrophobic and hydrophilic surfaces were investigated. Initial adhesion forces differed significantly between the two surfaces and hovered around -0.4 nN. No unambiguous effect of substratum surface hydrophobicity on initial adhesion forces for the four different S. epidermidis strains was observed. Over time, strengthening of the adhesion forces was virtually absent on hydrophobic dimethyldichlorosilane (DDS)-coated glass, although in a few cases multiple adhesion peaks developed in the retract curves. Bond-strengthening on hydrophilic glass occurred within 5-35 s to maximum adhesion forces of -1.9 +/- 0.7 nN and was concurrent with the development of multiple adhesion peaks upon retract. Poisson analysis of the multiple adhesion peaks allowed separation of contributions of hydrogen bonding from other nonspecific interaction forces and revealed a force contribution of -0.8 nN for hydrogen bonding and +0.3 nN for other nonspecific interaction forces. Time-dependent bacterial adhesion forces were comparable for all four staphylococcal strains. It is concluded that, on DDS-coated glass, the hydrophobic effect causes instantaneous adhesion, while strengthening of the bonds on hydrophilic glass is dominated by noninstantaneous hydrogen bond formation.

Methods for reducing nonspecific interaction in antibody–antigen assay via atomic force microscopy

We developed a method to measure the rupture forces between antibody and antigen by atomic force microscopy (AFM). Previous studies have reported that in the measurement of antibody–antigen interaction using AFM, the specific intermolecular forces are often obscured by nonspecific adhesive binding forces between antibody immobilized cantilever and substrate surfaces on which antigen or nonantigen are fixed. Here, we examined whether detergent and nonreactive protein, which have been widely used to reduce nonspecific background signals in ordinary immunoassay and immunoblotting, could reduce the nonspecific forces in the AFM measurement. The results showed that, in the presence of both nonreactive protein and detergent, the rupture forces between anti-ferritin antibodies immobilized on a tip of cantilever and ferritin (antigen) on the substrate could be successfully measured, distinguishing from nonspecific adhesive forces. In addition, we found that approach/retraction velocity of the AFM cantilever was also important in the reduction of nonspecific adhesion. These insights will contribute to the detection of specific molecules at nanometer scale region and the investigation of intermolecular interaction by the use of AFM.

Specific and Nonspecific Interaction Forces Between Esch-erichia coli and Silicon Nitride, Determined by Poisson Statistical Analysis

ADVERTISEMENT RETURN TO ISSUEPREVCorrectionORIGINAL ARTICLEThis notice is a correctionSpecific and Nonspecific Interaction Forces Between Esch-erichia coli and Silicon Nitride, Determined by Poisson Statistical AnalysisNehal I. Abu-Lail and Terri A. CamesanoCite this: Langmuir 2008, 24, 8, 4420Publication Date (Web):March 12, 2008Publication History Published online12 March 2008Published inissue 1 April 2008https://doi.org/10.1021/la800552sCopyright © 2008 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views366Altmetric-Citations5LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (8 KB) Get e-AlertsSUBJECTS:Bacteria,Fundamental interactions,Nitrides Get e-Alerts

Interaction Forces between Salivary Proteins and Streptococcus mutans with and without Antigen I/II

The antigen I/II family of surface proteins is expressed by oral streptococci, including Streptococcus mutans, and mediates specific binding to, among others, salivary films. The aim of this study was to investigate the interaction forces between salivary proteins and S. mutans with (LT11) and without (IB03987) antigen I/II through atomic force microscopy (AFM) and to relate these interaction forces with the adhesion of the strains to saliva-coated glass in a parallel plate flow chamber. Upon approach of the bacteria toward a saliva-coated AFM tip, both strains experienced a similar repulsive force that was significantly smaller at pH 6.8 (median 3.0 and 3.1 nN for LT11 and IB03987, respectively) than at pH 5.8 (median 4.6 and 4.7 nN). The decay length of these repulsive forces was between 19 and 37 nm. Upon retraction at pH 6.8, the combined specific and nonspecific adhesion forces were significantly stronger for the parent strain LT11 (median -0.4 nN) than for the mutant strain IB03987 (median 0.0 nN), whereas at pH 5.8 the median of the adhesion forces measured was 0.0 nN for both strains. Moreover, at pH 6.8, the parent strain LT11 adhered in significantly higher numbers (9.6 x 106 cm-2) to a salivary coating than the mutant strain IB03987 (2.5 x 106 cm-2). Similar to the difference in adhesion forces between both strains at pH 5.8, the difference in adhesion between both strains also disappeared at pH 5.8, which suggests the involvement of attractive electrostatic forces in the interaction between antigen I/II and salivary coatings. In summary, this study shows that antigen I/II at the surface of S. mutans LT11 is responsible for its increased adhesion to salivary coatings under flow through an additional attractive electrostatic force.

Conditions limiting the influence of thiol–disulphide interchange reactions on the heat-induced aggregation kinetics of β-lactoglobulin

The effects of calcium and N-ethylmaleimide addition on the heat-induced denaturation/aggregation of β-lactoglobulin at neutral pH were examined. Aggregation of unfolded β-lactoglobulin was found to be rate limiting on heating aqueous protein solutions, while stoichiometric additions of calcium indicated that the unfolding reaction was limiting. Increased binding of calcium reduced the zeta potential of β-lactoglobulin to a minimum of −5 mV, resulting in first-order denaturation/aggregation kinetics at pH 7.0. Thiol–disulphide interchange reactions had a positive effect on the aggregation of unfolded β-lactoglobulin at low protein concentrations and low ionic strength. In the presence of calcium, the absence of thiol–disulphide interchange reactions did not affect the protein aggregation kinetics. Thiol–disulphide interchange reactions had little effect on storage modulus development of heated β-lactoglobulin (3%, w/w) acid gels; however, they enhanced the strain stability. Results showed the importance of non-specific forces in controlling the heat-induced aggregation of unfolded β-lactoglobulin.

Intermolecular forces and enthalpies in the adhesion of Streptococcus mutans and an antigen I/II-deficient mutant to laminin films.

The antigen I/II family of surface proteins is expressed by most oral streptococci, including Streptococcus mutans, and mediates specific adhesion to, among other things, salivary films and extracellular matrix proteins. In this study we showed that antigen I/II-deficient S. mutans isogenic mutant IB03987 was nearly unable to adhere to laminin films under flow conditions due to a lack of specific interactions (0.8 x 10(6) and 1.1 x 10(6) cells cm(-2) at pH 5.8 and 6.8, respectively) compared with parent strain LT11 (21.8 x 10(6) and 26.1 x 10(6) cells cm(-2)). The adhesion of both the parent and mutant strains was slightly greater at pH 6.8 than at pH 5.8. In addition, atomic force microscopy (AFM) experiments demonstrated that the parent strain experienced less repulsion when it approached a laminin film than the mutant experienced. Upon retraction, combined specific and nonspecific adhesion forces were stronger for the parent strain (up to -5.0 and -4.9 nN at pH 5.8 and 6.8, respectively) than for the mutant (up to -1.5 and -2.1 nN), which was able to interact only through nonspecific interactions. Enthalpy was released upon adsorption of laminin to the surface of the parent strain but not upon adsorption of laminin to the surface of IB03987. A comparison of the adhesion forces in AFM with the adhesion forces reported for specific ligand-receptor complexes resulted in the conclusion that the number of antigen I/II binding sites for laminin on S. mutans LT11 is on the order of 6 x 10(4) sites per organism and that the sites are probably arranged along exterior surface structures, as visualized here by immunoelectron microscopy.

Regulation of the Function of αvβ3 in Platelets by a Designed Peptide Targeting the αv Transmembrane Domain.

Integrins reside on cell surfaces in an equilibrium between inactive and active conformations. When inactive, the transmembrane (TM) domains of integrin α and β subunits interact, but the domains separate when integrins assume their active conformation. Although this conformational change has not been shown for αvβ3, we hypothesized that a peptide designed to bind to the αv TM domain might activate αvβ3 in platelets by disrupting the TM domain heterodimer of the inactive molecule. To design such a peptide, we used CHAMP (Computed Helical Anti-Membrane Protein) methodology. In the CHAMP method, the αv TM helix was scanned for motifs likely to mediate αvβ3 TM domain interactions. Next, the backbone conformation for an αv-binding peptide was selected based on known structural preferences for the motifs identified in the αv helix. Finally, the sequence for the peptide was designed computationally using a side-chain repacking algorithm. The CHAMP peptide, anti-αv, and its TM helix target, αv-TM, were synthesized by solid phase synthesis. We also synthesized anti-αvmut in which the putative anti-αv-TM binding motif, GXXXG, was mutated to LXXXL. Lys2 dipeptides and short polyethylene glycol sequences were appended to the C- and N-termini of the peptides, respectively, to facilitate their solubility and insertion into membranes. CD spectroscopy revealed that both anti-αv and αv-TM were helical in micelles and phospholipid vesicles. Measurement of Trp fluorescence intensity revealed that anti-αv rapidly inserted into unilamellar POPC/POPG vesicles. Analytical ultracentrifuge and fluorescence resonance energy transfer experiments demonstrated that anti-αv bound to the αv-TM, but not to a homologous αIIb-TM domain peptide. In addition, there was negligible interaction between αv-TM and anti-αvmut. Anti-αv also formed heteromeric complexes with the αv TM domain in bacterial membranes but not with the TM domains of αIIb, α2, β1, or β3. αvβ3 mediates the adhesion of agonist-stimulated platelets to the matrix protein osteopontin (OPN). We found that anti-αV at μM concentrations induced platelet adhesion to OPN. Adhesion was prevented by the divalent cation chelator EDTA, consistent with an integrin-mediated process, but was only minimally affected by pre-incubating the platelets with PGE1, implying that anti-αv-induced adhesion by interacting directly with the αv TM domain. Force spectroscopy using laser tweezers confirmed the specificity of the interaction of anti-αv with platelets: anti-αv induced specific rupture forces between platelets and OPN-coated beads, but not between platelets and fibrinogen-coated beads. Moreover, only non-specific rupture forces were detected between OPN-coated beads and platelets incubated with anti-αvmut. These results demonstrate the successful application of computational methods to design a soluble peptide that specifically recognizes the TM domain of αv in platelets membranes, even when a 400-fold excess of the homologous integrin subunit αIIb is present. Further, because the peptide binds to the site on the αv TM domain that interacts with the β3 TM helix and activates αvβ3, these results strongly support the hypothesis that separation of the αv and β3 TM domains regulates the function of this integrin.

Bimodal Polymer Mushrooms: Compressive Forces and Specificity toward Receptor Surfaces

End-grafted poly(ethylene glycol) (or PEG) polymer chains are used to extend the in vivo circulation time of targeted liposomes and nanoparticles; however, the most efficacious structure for also imparting high target specificity remains unknown. Using the surface force apparatus, we have measured the specific and nonspecific forces between bimodal mixtures of grafted polymer mushrooms and model receptor surfaces. Specifically, supported lipid membranes anchoring 2000 or 5000 Da PEG with a controlled fraction of PEG(2000) bearing biotin ligands were compressed against opposing streptavidin surfaces. The presence of the longer 5000 Da chain increased the steric repulsion of the bimodal mushroom layer and thus decreased the net adhesive force when shorter chains were ligated. However, the 5000 Da chain did not detectably alter the distance where ligand-receptor binding occurs and adhesion begins. This latter result is in good agreement with theoretical predictions based on summing the repulsive steric and attractive bridging forces. Further, all ligated structures adhered to receptors under both static and dynamic fluid flow conditions. The dynamic movement of the flexible PEG tethers permitted ligand-receptor bonds to form far beyond the equilibrium edge of the bimodal mushroom layer. This work demonstrates that liposome targeting should be enhanced by grafting ligands to liposomes with a tether that has a contour length longer than the equilibrium height of the bimodal mushroom layer.

Multilayer Interactions Determine the Golgi Localization of GRIP Golgins

Golgin-97, RanBP2alpha, Imh1p and p230/golgin-245 (GRIP) domain golgins are targeted to the Golgi membrane through their GRIP domains. By analyzing more than 30 mutants of golgin-97 and golgin-245 GRIP domains for their properties of dimerization, interaction with ARF like protein 1 (Arl1)-GTP and Golgi targeting, we found hierarchically organized three-tier interactions governing the Golgi targeting of GRIP domain golgins. GRIP domain self-dimerization is necessary for bivalent interaction with Arl1-GTP. Unexpectedly, however, these two interactions are not sufficient for Golgi targeting, as a third group of residues, including positive-charged arginine between alpha1 and alpha2 and hydrophobic residues C-terminal to the GRIP domain, turn out to be essential. Surface plasmon resonance analysis indicates that GRIP domain interacts directly with membrane lipid, partially through the third group of residues such as W744 of golgin-97. This third tier of interaction with the membrane could be mediated by non-specific hydrophobic and electrostatic forces.

Specific and Nonspecific Interaction Forces Between Escherichia coli and Silicon Nitride, Determined by Poisson Statistical Analysis

The nature of the physical interactions between Escherichia coli JM109 and a model surface (silicon nitride) was investigated in water via atomic force microscopy (AFM). AFM force measurements on bacteria can represent the combined effects of van der Waals and electrostatic forces, hydrogen bonding, steric interactions, and perhaps ligand-receptor type bonds. It can be difficult to decouple these forces into their individual components since both specific (chemical or short-range forces such as hydrogen bonding) and nonspecific (long-range colloidal) forces may be present in the overall profiles. An analysis is presented based on the application of Poisson statistics to AFM adhesion data, to decouple the specific and nonspecific interactions. Comparisons with classical DLVO theory and a modified form of a van der Waals expression for rough surfaces were made in order to help explain the nature of the interactions. The only specific forces in the system were due to hydrogen bonding, which from the Poisson analysis were found to be -0.125 nN. The nonspecific forces of 0.155 nN represent an overall repulsive interaction. These nonspecific forces are comparable to the forces calculated from DLVO theory, in which electrostatic-double layer interactions are added to van der Waals attractions calculated at the distance of closest approach, as long as the van der Waals model for "rough" spherical surfaces is used. Calculated electrostatic-double layer and van der Waals interactions summed to 0.116 nN. In contrast, if the classic (i.e., smooth) sphere-sphere model was used to predict the van der Waals forces, the sum of electrostatic and van der Waals forces was -7.11 nN, which appears to be a large overprediction. The Poisson statistical analysis of adhesion forces may be very useful in applications of bacterial adhesion, because it represents an easy way to determine the magnitude of hydrogen bonding in a given system and it allows the fundamental forces to be easily broken into their components.

Entropy-Driven Genome Organization

DNA and RNA polymerases active on bacterial and human genomes in the crowded environment of a cell are modeled as beads spaced along a string. Aggregation of the large polymerizing complexes increases the entropy of the system through an increase in entropy of the many small crowding molecules; this occurs despite the entropic costs of looping the intervening DNA. Results of a quantitative cost/benefit analysis are consistent with observations that active polymerases cluster into replication and transcription “factories” in both pro- and eukaryotes. We conclude that the second law of thermodynamics acts through nonspecific entropic forces between engaged polymerases to drive the self-organization of genomes into loops containing several thousands (and sometimes millions) of basepairs.

Interaction between non-specific electrostatic forces and humoral factors in haemocyte attachment and encapsulation in the edible cockle,Cerastoderma edule

In invertebrates, encapsulation is the common immune defence reaction towards foreign bodies, including multicellular parasites, which enter the haemocoel and are too large to be phagocytosed. This immune response has been most extensively studied in insects, in which it is highly complex, involving a diversity of cellular and molecular processes, but little is known of this process in bivalve molluscs. Non-specific physicochemical properties are known to influence parasite-haemocyte interactions in many invertebrates, and these may provide the common basis of encapsulation on which highly specific biochemical interactions are imposed. The present study uses synthetic beads and thread to mimic inactive metacercarial cysts of trematodes, and thus investigates factors involved in the basic, non-specific mechanisms of cell attachment and encapsulation in the edible cockle, Cerastoderma edule. Results showed that positively charged targets stimulated the most vigorous response, and further detailed experiments revealed that non-specific electrostatic forces and humoral plasma factors have a synergistic role in haemocyte attachment and the encapsulation response of C. edule.

Measurement of the surface energy of lubricated pharmaceutical powders by inverse gas chromatography

The objective of the study was to determine whether lubrication of pharmaceutical powders with magnesium stearate (MgSt) results in a change in the surface energy of the powder, and to assess whether surface energy changes, if any, are correlated to lubricant concentration and blend time. The surface energies of microcrystalline cellulose (MCC), lactose, and blends of each material with MgSt, prepared at a range of concentrations and blending times were measured using inverse gas chromatography. The physical distribution of MgSt in the blend was mapped by energy dispersive spectrometry. Overall, there was a reduction in the dispersive surface energy of MCC–MgSt blends with increase in MgSt concentration, that was attributed to increasing coverage of the high-energy sites on microcrystalline cellulose by magnesium stearate. MgSt concentration had a larger effect on dispersive energy than the blending time of the powder with lubricant. X-ray maps of blend samples indicated a heterogeneous distribution of the lubricant in the blend and on the excipient particles. Measurement of the specific component of surface energy indicated that MgSt interacts with excipient powders through non-specific forces rather than acid–base interactions. No distinction among lactose–MgSt blends could be made on the basis of dispersive energy because of similar surface energies of the native materials.