Isoelectric Point Of Bovine Serum Albumin Research Articles

BSA Adsorption and Antibiofilm Properties of (N-Cyclohexylacrylamide-co-Acrylamide/AMPSNa) Hydrogels

The hydrogels poly (N-cyclohexylacrylamide-co-acrylamide/2-acrylamido-2-methyl-1-propanesodiumsulfonate) i.e.poly(NCA-co-AM/AMPSNa) (HG41,HG42,HG43&HG44) were synthesised via free-radical copolymerization of NCA and (AM) in a fixed proportion (50:50), but varying the ionic monomer-AMPSNa (0.1g,0.3g,0.5 d 0.7g) in a medium of mixture of water and methanol at 60°C in an oil bath, Potassium persulfate (KPS) was used as an initiator, while N,N'-methylene-bisacrylamide (MBA) was used as a cross-linker. FT-IR spectral data, SEM, XRD, and TGA techniques were used to characterise the synthesised hydrogel (HG43). On changing pH(3.0, 5.0, 7.0 & 9.0), the amount of Bovine Serum Albumin (BSA) adsorption efficiency by these hydrogels was evaluated. At pH 5.0, which is near to BSA's isoelectric point (4.7)., the maximum adsorption was found. BSA adsorption increased as the amount of AMPSNa increased. SEM and XRD were used to examine the BSA-adsorbed hydrogel (HG43). The antibiofilm abilities of the hydrogel (HG43) by Microtiter plate Assay(MTP), Fluorescence microscopy and SEMl against Staphylococcus aureus and Pseudomonas aureginosa displayed outstanding efficacy.

The effect of grafting monomer charge on the antifouling performance of poly(ether ether ketone) hollow fiber membrane by ultraviolet irradiation polymerization

AbstractTo improve the antifouling properties of poly(ether ether ketone) (PEEK) membrane, negatively charged acrylic acid (AA), neutral ethyl hydroxyacrylate (HEA) and positively charged acryloxyethyl trimethylammonium chloride (DAC) as hydrophilic monomers were respectively grafted onto the PEEK hollow fiber membrane surface via ultraviolet irradiation polymerization. The effects of the grafted monomers on surface chemical structure, surface morphology, zeta potential, hydrophilic performance, permeability and the antifouling properties of the membrane were investigated in detail. The results show that the charge state of the membrane surface is closer to the monomer charge characteristic, which obviously affects the permeability and antifouling performance of the membrane. The pure water flux of the membrane grafted with neutral hydrophilic HEA monomer is increased and the antifouling properties to bovine serum albumin (BSA) and sodium alginate (SA) are also enhanced, while the pure water flux of the membranes grafted with negatively charged AA monomer or positively charged DAC monomer is decreased, with different antifouling properties to BSA and SA. Furthermore, the antifouling experiment with BSA under different pH conditions revealed that membrane fouling depends on the pH value, and membrane fouling was the most serious at the isoelectric point of BSA. © 2020 Society of Chemical Industry

Glycation from α-dicarbonyl compounds has different effects on the heat-induced aggregation of bovine serum albumin and β-casein

α-Dicarbonyl compounds are generated in large amounts during heat treatment in food production. This work compared the influence of glycation by α-dicarbonyl on the hydrothermal aggregation of bovine serum albumin (BSA) and of β-casein (β-CN). Glycation by α-dicarbonyl compounds was found to be more efficient than glycation by glucose in reducing the free amino groups, surface hydrophobicity and isoelectric point of BSA, thus greatly inhibited the hydrothermal aggregation of BSA. In addition, glycation by α-dicarbonyl greatly transformed the rigid BSA aggregates into flexible structures, based on analysis by fluorescence spectrum, transmission electron microscope and small-angle X-ray scattering. In contrast, both the aggregation process and aggregates conformation of β-CN were found to be minimally affected by glycation, possibly due to the intrinsic disorder of β-CN. This work highlights the substantial influences of α-dicarbonyl on dietary proteins during heat treatment depending on the protein structural characteristics.

Associative interactions between pullulan and negatively charged bovine serum albumin in physiological saline solutions

In the present study, the behavior of bovine serum albumin (BSA) and pullulan (PULL) mixtures was investigated in physiological saline solution at 37 °C and pH = 7.4 (above the isoelectric point of BSA), by viscometry and membrane osmometry. The new Wolf approach was used for modeling the viscometric behavior of the PULL/BSA saline solutions. From the viscometric data, the contributions of binary, ternary and quaternary intersegmental contacts were determined. Osmotic pressure measurements were carried out as a function of PULL/BSA mixture concentration and composition. In dilute 0.15 M NaCl solutions, the electrostatic repulsion between the BSA molecules is partially screened and attractive PULL/protein interactions are mediated by Na ions. Thus, large deviations from ideal behavior were observed for BSA containing systems. This effect could be attributed to complexes formation as the protein content increases in the solution. The complexation was depicted as the adsorption protein molecules on the PULL chains through the electrolyte molecules. The results obtained by viscometry and osmometry were corroborated, allowing the prediction of the phase behavior of the mixtures. The reported experimental results provide useful information for the design of new biomaterials starting from pullulan and globular proteins.

PH-responsive double network alginate/kappa-carrageenan hydrogel beads for controlled protein release: Effect of pH and crosslinking agent

Bovine Serum Albumin (BSA) was encapsulated into pH-responsive alginate/kappa(κ)-carrageenan double network hydrogel beads at different pHs and CaCl2/KCl ratios. Controlled release of BSA was performed in the simulated gastric fluid (SGF) and then in simulated intestinal fluid (SIF). Alginate reinforced the mechanical properties while double network with kappa-carrageenan made slow down the release of BSA into SIF. Sulfate groups on kappa-carrageenan improved the resistivity against phosphate buffer solution. Encapsulation and BSA release was manipulated by varying pH and crosslinking concentration. The highest encapsulation efficiency (83–89%) was obtained with the hydrogel beads prepared at the lower pH than isoelectric point of BSA, then 67–72% of them released in SIF in 4–5 h while lower than 10% of BSA released in SGF, hence, protein delivery targeted intestinal was achieved. BSA release exhibited a coupling of diffusion and polymer relaxation mechanism without burst release.

Magnetic restricted access carbon nanotubes for smooth Cu and Zn extraction from Cu, Zn-superoxide dismutase

In this paper we proposed the synthesis, characterization and application of magnetic restricted access carbon nanotubes (M-RACNTs) to smoothly capture Cu and Zn from Cu, Zn superoxide dismutase (Cu,Zn-SOD). Fe3O4 magnetic nanoparticles were incorporated on the external surface of carbon nanotubes. The obtained material was encapsulated with a chemically crosslinked bovine serum albumin (BSA) layer, resulting in the M-RACNTs. The BSA capsule acts as a biocompatible barrier that prevents the proteins binding on the M-RACNTs surface. In a pH higher than the isoelectric point of BSA, the proteins from the sample and from the BSA layer, are negatively charged, resulting in an exclusion mechanism by electrostatic repulsion. The characterization tests confirmed the presence of the Fe3O4 magnetic nanoparticles in the interior, and the BSA layer on the surface of the M-RACNTs. The kinetic and isotherm tests were carried out for the Cu and Zn sorption by M-RACNTs, and the results confirmed the fast mass transference rate, and the metal retention in monolayers (Sips model, Cu and Zn maximum adsorption capacities of about 65 mg g−1), respectively. M-RACNTs were able to remove 64 and 62% of Cu and Zn, respectively, from a 0.140 mol L−1 Cu,Zn-SOD aqueous solution, with only one magnetic dispersive solid phase extraction cycle. During this process, M-RACNTs retained less than 5% of the enzyme, attesting the efficiency of the BSA layer to prevent protein binding. The Cu,Zn-SOD submitted to the extraction lost about 15% of its activity, which was totally recovered when Cu and Zn were reincorporated in the enzyme structure by addiction of an aqueous solution of both metals. Thus, we are sure that the M-RACNTs are promising sorbents to obtain apoproteins from metalloproteins in biotechnological applications.

Effect of pH on the adsorption and interactions of Bovine Serum Albumin with functionalized silicon nitride surface.

Force-distance curves between atomic force microscopy (AFM) tip (Si3N4 non-functionalized) and bovine serum Albumin (BSA) immobilized on Si3N4 substrates have been performed with the purpose to understand how multiple interactions between the protein and the tip were favored in different pHs (4, 6 and 10). In this work, 100 silicon wafer samples were used to deposit a layer of Si3N4. Protein immobilization consisted of the silanization of the substrates with 3-aminopropyltriethoxysilane (APTES) and crosslinking with glutaraldehyde (GA). All functionalization steps were evaluated by contact angle, X-Ray electron spectroscopy (XPS) and AFM. AFM images showed increase of roughness following functionalization. At pH 4, it was possible to note that small forces (49.1 ± 2.4 pN) were needed to stretch BSA, with a contour length of CL = (30.0 ± 1.1 nm). At pH 6, the force applied was higher (101.5 ± 5.0 pN) with a higher molecule stretch CL = (75.6 ± 3.8 nm) because the pH is close to the BSA isoelectric point where the folding of the protein is favored as surfaces charges are minimized leading to lower attractive intramolecular forces. Young's Modulus were also calculated and the lowest value (265 kPa) was observed at pH 10.

Adsorption of BSA on carbon-coated Fe3O4 microspheres activated with 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide hydrochloride

Carbon-coated Fe3O4 (Fe3O4/C) microspheres activated with 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide hydrochloride (EDC) were prepared, characterized and applied to adsorb bovine serum albumin (BSA). The prepared magnetic microspheres had spherical core-shell structure with a uniform and continuous carbon coating coupled with activation by EDC, and possessed superparamagnetic characteristics. The experimental results showed that the adsorption amount of BSA on the EDC-activated Fe3O4/C (Fe3O4/C-EDC) microspheres was higher than that on the Fe3O4/C microspheres. The maximum adsorption of BSA on Fe3O4/C-EDC microspheres occurred at pH 4.7, which was the isoelectric point of BSA. At low concentrations (below 1.0 M), salt had no noticeable effect on BSA adsorption. The BSA adsorption of Fe3O4/C-EDC microspheres had a better fit to the Langmuir model than the Freundlich isotherm and Temkin isotherm model, and the kinetic data were well described by the pseudo-second-order model. The adsorption equilibrium could be reached within 20 min. High desorption efficiency (97.6%) of BSA from Fe3O4/C-EDC microspheres was obtained with 0.5 M Na2HPO4 (pH 9.4) as the desorbent.

Model protein BSA adsorption onto novel magnetic chitosan/PVA/laponite RD hydrogel nanocomposite beads

Chitosan-based magnetic beads were developed by solution-mixing method. Firstly, the Fe3O4 nanoparticles were in situ immobilized on laponite RD sheets. The magnetic laponite RD was then dispersed in PVA and mixed with chitosan solution. PVA was aimed to prevent the disintegration of chitosan under acidic media due to its ability to form hydrogel network through freezing-thawing method. The manufactured magnetic chitosan/PVA/laponite RD beads were utilized for adsorption study of a model protein, bovine serum albumin (BSA). The adsorption of BSA on beads was pH-dependent where smaller mass of protein was adsorbed at pH values lower than isoelectric point of BSA. Moreover, it was discovered that introduction of magnetic laponite RD can improve the adsorption capacity of magnetic beads for BSA in which hydrogel with the highest content of magnetic laponite RD demonstrated the maximum adsorption capacity for BSA (qm=240.5mg/g). Langmuir model described the isotherm data better than Freundlich model.

Impact of multiple quaternary ammonium salts on dynamic properties of BSA adsorption layer at different pH values.

The interaction mechanism of multiple quaternary ammonium salts (MQAS) with bovine serum albumin (BSA) was examined by the fluorescence quenching method and circular dichroism (CD) spectra. Moreover, the effects of MQAS on the dynamic properties of BSA adsorption layers at different pH values were investigated using dilational interfacial rheology. Results show that the quenching constants increase with an increase in pH values and decrease with an increase in the experiment temperature at pH 5.3. The quenching mechanism is static quenching, and the electrostatic force dominates the interaction between MQAS and BSA at pH 5.3. Due to three positive head groups, MQAS can significantly affect the dynamic interfacial activity of BSA molecules at a relatively low concentration. At pH 4.3, the electrostatic repulsion is unfavorable for the formation of MQAS/BSA complexes. Consequently, MQAS molecules will replace BSA molecules from the interface by competitive adsorption. At the pH value above the isoelectric point of BSA, the electrostatic attraction is better for the formation of MQAS/BSA complexes, which exhibit a rapid adsorption rate and an enhanced interfacial activity. Moreover, the kinetic dependencies of interfacial dilational elasticity for the MQAS/BSA mixtures become nonmonotonous. The appearance of the maximum interfacial elasticity values can be attributed to the formation of tails and loops, which suggests that the addition of MQAS destroys the secondary and tertiary structure of protein in the bulk phase. In addition, the effects of MQAS on the secondary structure of protein were demonstrated by CD spectra.

Preparation of MnO2-impregnated carbon-coated Fe3O4 nanocomposites and their application for bovine serum albumin adsorption

MnO2-impregnated carbon-coated Fe3O4 (Fe3O4/C/MnO2) nanocomposites with a good core–shell structure were prepared by direct oxidation of carbon-coated Fe3O4 (Fe3O4/C) microspheres with KMnO4 in alkaline solution and applied to adsorb bovine serum albumin (BSA). X-ray diffraction (XRD), transmission electron microscope (TEM), Fourier transform infrared spectrometer (FTIR), vibrating sample magnetometer (VSM) and thermogravimetric analyzer (TGA) tests show that Fe3O4/C microspheres were newly functionalized via the oxidation by KMnO4. Fe3O4/C/MnO2 nanocomposites exhibit a higher adsorption capacity for BSA than Fe3O4/C microspheres and the maximum adsorption of BSA on them occurs at pH 4.7, which is the isoelectric point of BSA. Langmuir isotherm model describes the adsorption of BSA better than Freundlich model and Temkin model, and the kinetics data fit well with the pseudo-second-order model.

Filtration of protein colloids by fibrous membranes: A mechanistic investigation using packed bed filtration approach

Abstract This research aimed to use packed bed filtration as a novel approach to investigate fibrous membrane filtration of aqueous particles, a novel environmental application of fibrous materials and a new frontier for engineering colloidal science. For this purpose, bovine serum albumin (BSA) was filtered through a pre-characterized glass bead column to determine attachment efficiencies (α) of BSA onto glass beads. Subsequently, BSA was filtered through a glass-fiber membrane possessing similar surface elemental compositions as the glass beads. By simulating the glass fiber membrane as a packing of spherical glass particles with a diameter equivalent to fiber diameter, α values pertinent to BSA-glass fiber attachment were calculated using the clean bed efficiency model. A side-by-side comparison of the α values show that membrane filtration exhibited greater α values than column filtration, the α values calculated for membrane filtration were always about 1.5 times of those determined under similar conditions in column filtration. However, α values for the two systems varied with solution chemistry in similar trends and the maximal α values for both systems were both found at the isoelectric point of BSA (pHIEP = 4.78). This finding implies that fibrous membrane filtration may be studied using classical packed bed filtration theories.

Fouling of forward osmosis membrane by protein (BSA): effects of pH, calcium, ionic strength, initial permeate flux, membrane orientation and foulant composition

In this study, bovine serum albumin (BSA) was selected to represent proteins of secondary wastewater effluent. The role of various physical and chemical interactions, such as calcium concentration, ionic strength, solution pH, feed foulant composition, initial permeate flux, and membrane orientation, in BSA fouling of forward osmosis (FO) membranes was investigated. Fouling experiments showed that membrane fouling by BSA was enhanced with increasing calcium concentration and ionic strength. The former was mainly due to the complexes formed by the interaction of Ca2+ and carboxylic functional groups of BSA, and the latter resulted from the decreasing electrostatic repulsion among BSA molecules and between BSA molecules and membrane. Moreover, FO membrane fouling became much more significant at solution pH 4.7 (the BSA isoelectric point), where BSA molecules were neutrally charged and had no electrostatic repulsion among themselves. It was also demonstrated that the presence of alginate (a model polysaccharide) as co-foulant aggravated the BSA fouling of FO membrane, which could be attributed to the remarkable contribution of the alginateBSACa2+ complexes within the fouling layer to the total membrane resistance. The fouled membranes were examined by scanning electron microscopy to further sustain the conclusion. In addition, the size distribution of foulant molecules in various FS was measured and used as a reference to judge and control the behavior of BSA fouling. The present paper is contributed to better understanding of FO membrane fouling caused by protein (BSA) and has instructive significance for the future development.

Adsorption and thermal properties of the bovine serum albumin–silicon dioxide system

The adsorption, electrokinetic, thermal and stability properties of the silicon dioxide (silica, SiO2)/bovine serum albumin (BSA) system were determined. All measurements were carried out as a function of solution pH value. The highest amount of BSA absorbed on the silica surface was observed at pH 4.6 [the value close to the BSA isoelectric point (pI)], which is primarily related to the packed albumin conformation and the lack of adsorbent–adsorbate electrostatic repulsion. At pH 4.6, largest mass decrease was also noticed (thermogravimetric measurements). At pH 3, 7.6 and 9, the adsorption levels were much lower. This phenomenon is associated with the electrostatic repulsion between the BSA macromolecules and the silica particles as well as the expanded BSA structure. During biopolymer adsorption, the whole solid surface is coated with the albumin macromolecules. Then, the properties of the silica particles become similar to those of the BSA macromolecules. In the presence of albumin, the silica pHiep point is identical to the BSA pI value. It should also be noted that the albumin adsorption affects the SiO2 suspension stability. The greatest change was observed at pH 3. Under these conditions, the BSA addition causes electrosteric system stabilization.

Comparative SERS study carried out on unsilanized and silanized oxidized porous silicon surface coated by small gold nanoparticles

Porous silicon (PSi) was formed by electrochemical anodization and was oxidized before silanization and immersion in colloidal gold nanoparticles (GNP) (5 nm size) solution. The oxidized PSi (OPSi) coated with nanoparticles was used as surface enhanced Raman scattering (SERS) substrate, where bovine serum albumin (BSA) was used as a target molecule. Also, rhodamine 6G (R6G) dyes were used to confirm the SERS efficiency. Firstly, the bio-conjugation between protein and GNP was investigated by localized surface plasmon resonance spectroscopy. Then a comparative SERS study from silanized and unsilanized OPSi was performed and showed a good vibrational BSA bands resolution with the unsilanized one. The silanized surface did not show any improvement on the SERS enhancement of the BSA due to the interferences between the vibrational modes of 3-aminopropyltriethoxysilane and BSA. Furthermore, we have investigated the BSA solution pH effect on the SERS results in the case of unsilanized OPSi surface. The results prove the efficiency of the SERS substrate at pH 4.9, corresponding to BSA iso-electric point. A detection limit of about 10−8 M was obtained for both BSA and R6G molecules.

Determination of the second virial coefficient of bovine serum albumin under varying pH and ionic strength by composition-gradient multi-angle static light scattering.

Composition-gradient multi-angle static light scattering (CG-MALS) is an emerging technique for the determination of intermolecular interactions via the second virial coefficient B22. With CG-MALS, detailed studies of the second virial coefficient can be carried out more accurately and effectively than with traditional methods. In addition, automated mixing, delivery and measurement enable high speed, continuous, fluctuation-free sample delivery and accurate results. Using CG-MALS we measure the second virial coefficient of bovine serum albumin (BSA) in aqueous solutions at various values of pH and ionic strength of a univalent salt (NaCl). The systematic variation of the second virial coefficient as a function of pH and NaCl strength reveals the net charge change and the isoelectric point of BSA under different solution conditions. The magnitude of the second virial coefficient decreases to 1.13 x 10(-5) ml*mol/g(2) near the isoelectric point of pH 4.6 and 25 mM NaCl. These results illuminate the role of fundamental long-range electrostatic and van der Waals forces in protein-protein interactions, specifically their dependence on pH and ionic strength.

Impact of ionic liquid-type imidazolium surfactant addition on dynamic properties of BSA adsorption layers at different pH.

The dynamic interfacial properties of mixed solutions of bovine serum albumin (BSA) and the ionic liquid-type imidazolium surfactant ([C16mim]Br) were measured as a function of the interface age, surfactant concentration and solution pH. Three BSA conformers were investigated: the normal N form as well as the fast F and aged A forms, corresponding to the different solution pH, respectively. The interfacial tension and the interfacial dilational elasticity isotherms for the mixed adsorption layers indicate that the addition of [C16mim]Br to the different structures of BSA isomers influences the properties of the adsorption layer at the decane/water interface. The addition of [C16mim]Br does not influence the structure of the protein at pH below the isoelectric point of BSA, but at higher solution pH, the addition of surfactants significantly influences the dynamic interfacial properties of BSA solutions due to the electrostatic interaction between the components.

Influence of sodium polystyrene sulfonate on dynamic surface properties of bovine serum albumin solutions

Dynamic surface elasticity of solutions of bovine serum albumin (BSA)/sodium polystyrene sulfonate (PSS) complexes has been measured as depending on the age of a surface, polyelectrolyte concentration, and solution pH by the oscillating-ring method. At pH values below the isoelectric point of BSA, the rate of variations in the surface properties increases due to a decrease in the electrostatic adsorption barrier as a result of a reduction in the total charge of the protein/polyelectrolyte complex. Therewith, a local maximum arises in the kinetic dependences of the surface elasticity, this maximum indicating the onset of the breakage of the tertiary structure of the protein in the surface layer. In the pH range corresponding to like charges of the protein and polyelectrolyte, variations in the surface properties slow down. In this case, the BSA/PSS complex is also formed via the interaction of PSS with those domains of globule surface that carry a charge opposite to the total charge of a protein molecule. A higher negative charge of the complex than that of protein globules increases the electrostatic adsorption barrier and decelerates variations in the surface properties. At the same time, the dependences of the surface elasticity on the surface pressure coincide with the dependences for the protein solution. Hence, the polyelectrolyte-protein interaction affects only the adsorption kinetics, while the surface properties in the vicinity of equilibrium are governed by adsorbed protein globules.

Adsorption Characteristics of Bovine Serum Albumin on Cationic Grafted Particles QPDMAEMA/SiO<sub>2</sub> with Brush Structure

以微米级硅胶微粒为基质, 通过接枝聚合和大分子反应, 制备了具有刷状结构的阳离子性接枝微粒, 深入研究了其对牛血清白蛋白(BSA)的强吸附能力、吸附机理和吸附热力学. 首先使含叔胺基团的单体甲基丙烯酸二甲基氨基乙酯(DMAEMA)在硅胶微粒表面发生接枝聚合, 制得接枝微粒PDMAEMA/SiO 2 , 然后以氯乙胺为试剂, 使接枝大分子PDMAEMA链中的叔胺基团发生季铵化反应, 获得了具有刷状结构的阳离子聚电解质的功能接枝微粒QPDMAEMA/SiO 2 . 测定了微粒QPDMAEMA/SiO 2 的zeta 电位, 实施了对BSA的等温吸附实验, 考察了介质pH值、离子强度及温度对吸附作用的影响, 研究了吸附热力学. 研究结果表明, 功能接枝微粒QPDMAEMA/SiO 2 比接枝微粒PDMAEMA/SiO 2 具有更高的zeta 电位, 在静电相互作用驱动下, 微粒QPDMAEMA/SiO 2 对BSA具有很强的吸附能力. 吸附容量随介质pH值的增大呈现先增大后减小的变化趋势,当pH值等于BSA的等电点(pI=4.7)时, 具有最高的吸附容量(高达112 mg·g -1 ). 以等电点为界, 离子强度对吸附容量会产生完全相反的影响作用: 当介质pH值小于BSA的等电点时, 电解质浓度增大, 吸附容量增高; 当介质pH值等于BSA的等电点时, 吸附容量几乎不随电解质的浓度发生变化. 吸附过程熵值减小而且放出热量,是一个焓驱动的吸附过程.

Influence of polyelectrolyte on dynamic surface properties of BSA solutions

The dynamic surface elasticity of mixed solutions of bovine serum albumin (BSA) and poly(diallyldimethylammonium chloride) (PDADMAC) was measured as a function of the surface age, polyelectrolyte concentration and solution pH. The addition of polyelectrolyte does not influence the dynamic surface properties of protein solution at pH below the isoelectric point of BSA. At the same time, one can observe significant changes of the kinetic dependencies of the surface pressure and surface elasticity under the influence of polyelectrolyte at higher pH indicating the interaction between the components. The acceleration of the changes of the surface properties can be connected with the decrease of the electric adsorption barrier due to the decrease of the total charge of the protein/polyelectrolyte complex. The local maximum of the kinetic dependencies of the dynamic surface elasticity at pH higher than the isoelectric point indicates partial destruction of the protein tertiary structure at high surface pressures.