Bulk Aqueous Phase Research Articles

Separating and Sorting Membrane Species using a Supported Bilayer Extractor Composed of Patterned Lipid Phases

The role of lipid rafts is important in understanding cell processes and disease states; however, identifying the key components of these lipid domains is difficult and impedes research progress. Here we present a new platform to separate, sort, and characterize membrane-bound molecules based on their affinity for raft phase domains using a heterogeneous supported lipid bilayer (SLB) consisting of patterned lipid raft and non-raft domains. Adjacent SLBs consisting of liquid-ordered and liquid-disordered phases are formed inside a microfluidic channel and maintain phase separation. Membrane-bound biomolecules loaded into the device are convected laterally in the two-dimensional plane along the heterogeneous supported bilayer by a shear force induced from hydrodynamic flow of the bulk aqueous phase. During axial convection, the membrane-bound biomolecules diffuse across the microchannel within the 2-D heterogeneous bilayer plane, partitioning into their preferred lipid phase. Patterns for the lipid phases are designed to facilitate the sorting and collection of separated species. The main advantages of this new method over existing methods used to identify and assay raft species include separating membrane species within a membrane environment, near physiological conditions, and without artifacts associated with detergent, high salt, or alkaline pH. These criteria are particularly crucial to the separation of lipid-linked proteins and transmembrane proteins to avoid denaturation. This new capability to separate membrane-bound species near native conditions based on affinity for certain lipid phases can be used to identify intrinsic membrane raft residents and to characterize how post-translational modifications shift the affinity of analogs to a particular lipid phase.

A Review of the Aqueous Aerosol Surface Chemistry in the Atmospheric Context

In this review the surface chemistry and properties of aqueous atmospheric aerosols are explored. Water plays a major role in scavenging pollutants. Reactions occur on thin water films in atmospheric aerosols. The study of the aerosol wa- ter surface is important to properly account for chemical transformations in the troposphere. The thermodynamics of adsorption of organic molecules and oxidant species on the aqueous surface and, the techniques employed to quantify the adsorption isotherms are summarized. Experimental techniques for elucidating the reactions on the water surface are described. Field and laboratory data for oxidation reactions of compounds at the air-water interface are summarized. The Langmuir-Hinshelwood reaction mechanism is useful in quantifying the reaction rate on the aqueous aerosol sur- face. A hypothesis for the large heterogeneous reaction rate on the water surface over the homogeneous bulk aqueous phase reaction is presented.

Experimental Methods and Prediction with COSMO-RS to Determine Partition Coefficients in Complex Surfactant Systems

Surfactant-based separation processes are a promising alternative to conventional organic solvent processes. A crucial parameter to describe the efficiency of such processes is the partition coefficient between the surfactant aggregates (micelles) and the aqueous bulk phase. In this work, several experimental methods to determine these partition coefficients (micellar liquid chromatography, micellar enhanced ultrafiltration, and cloud point extraction) are evaluated and compared. In addition, these results are compared to predictions with the thermodynamic model COSMO-RS. In particular, systems with the nonionic surfactant TritonX-100 are studied. The partition equilibria of various solutes (pyrene, naphthalene, phenanthrene, phenol, 3-methoxyphenol, and vanillin) and the influence of different additives (alcohols) are investigated. All experimental methods show very good reproducibility. Moreover, the results from different methods are in good agreement, supplementing one another concerning the temperature ranges. Notably, the COSMO-RS model is capable of predicting partition coefficients between micelles and water in the investigated temperature range and at different alcohol concentrations. The results demonstrate the potential of the model COSMO-RS to facilitate the selection of optimized process parameters for a given separation problem. By predicting partition equilibria in multicomponent systems, the selection of surfactant, temperature, and appropriate additives can be facilitated.

Thermodynamic analysis of protegrin-1 insertion and permeation through a lipid bilayer.

Molecular dynamics (MD) simulations are used to study the pathway for the insertion of the cationic antimicrobial peptide protegrin-1 (PG1) into mixed anionic lipid bilayers composed of palmitoyl-oleoyl-phosphatidylglycerol (POPG) and palmitoyl-oleoyl-phosphatidylethanolamine (POPE) in a 1:3 ratio (POPG/POPE). We calculate the potential of mean force (PMF) during the transfer of the peptide from the bulk aqueous phase to the transmembrane (TM) configuration using the adaptive biasing force (ABF) method. We find that the PMF has two energy minima separated by an energy barrier. One minimum corresponds to the fully transmembrane inserted state, with a free energy of -20.1 kcal/mol. The second PMF minimum, which corresponds to adsorption to the membrane surface, has a value of -2.5 kcal/mol. The PMF also shows the existence of a free energy barrier of +6.3 kcal/mol for the insertion process. Using the Kramers theory Langevin equation and the Grote-Hynes theory generalized Langevin equation, we calculated the transmission coefficient for PG1 diffusion through the potential barrier. We focus on the use of the PMF and the time correlation function of the fluctuation of the instantaneous force to calculate the rate constants for insertion/deinsertion of PG1 from the mixed POPG/POPE membrane. The influence of the activation free energy barrier on the dynamics of the insertion and permeation of peptides through the membrane are discussed.

Modeling colloid transport and retention in saturated porous media under unfavorable attachment conditions

A mathematical model is presented for colloid transport and retention in saturated porous media under unfavorable attachment conditions. The model accounts for colloid transport in the bulk aqueous phase and adjacent to the solid surface, and rates of colloid collision, interaction, release, and immobilization on the solid phase. Model parameters were estimated using (1) filtration theory; (2) calculated interaction energies in conjunction with the Maxwellian kinetic energy model of diffusion; (3) information about the velocity magnitude and distribution adjacent to the solid phase that was obtained from pore scale water flow simulations; (4) colloid and collector sizes; (5) the balance of applied hydrodynamic and resisting adhesive torques; and (6) time dependent filling of retention locations using a Langmuirian approach. The presented theory constrains the model parameters and output to physically realistic values in many instances, and minimizes the need for parameter optimization. Example simulations demonstrate that our modeling formulation is qualitatively consistent with observed trends for retention with colloid size and concentration, grain size, and velocity for many systems. The model provides a clear conceptual explanation for the causes of hyperexponential, exponential, uniform, and nonmonotonic retention profiles without invoking hypotheses with regard to colloid heterogeneity, aggregation, or multiple deposition rates. Furthermore, the model formulation and research presented herein helps to identify areas where additional research and theory development are still needed.

Miniaturizing Biocatalysis: Enzyme‐Catalyzed Reactions in an Aqueous/Organic Segmented Flow Capillary Microreactor

AbstractA segmented flow capillary microreactor was used to perform the enzyme‐catalyzed conversion of 1‐heptaldehyde to 1‐heptanol in a two liquid‐liquid phase system. These reactor formats are established for chemical reactions but so far data describing the relevant system parameters for enzymatic catalysis are lacking. This work now addresses the impact of important parameters such as capillary diameter, flow velocity, phase ratio, and enzyme as well as substrate concentration on the performance of the enzymatic reaction under segmented flow conditions. All key parameters governing reaction performance have been correlated in a novel operational window for an easy assessment of the various system constraints. Such systems are characterized by high productivities and easy phase separation facilitating downstream processing. This work underscores the importance of segmented flow systems as a promising tool to perform multiphasic enzymatic catalysis. Abbreviations/Nomenclature: Da: Damköhler number; kcat: turnover number (s−1); eo: enzyme concentration (mM); φ: phase ratio; kL: mass transfer coefficient (m s−1); a: interfacial area per volume (m−1); CAe: equilibrium substrate concentration in the aqueous phase (mM); CAL: substrate concentration in the bulk aqueous phase (mM); rA: rate of reaction in the aqueous phase; mA: substrate mass transfer into the aqueous phase; STY: space time yield.

Transformation of ActoHMM Assembly Confined in Cell-Sized Liposome

To construct a simple model of a cellular system equipped with motor proteins, cell-sized giant liposomes encapsulating various amounts of actoHMM, the complexes of actin filaments (F-actin) and heavy meromyosin (HMM, an actin-related molecular motor), with a depletion reagent to mimic the crowding effect of inside of living cell, were prepared. We adapted the methodology of the spontaneous transfer of water-in-oil (W/O) droplets through a phospholipid monolayer into the bulk aqueous phase and successfully prepared stable giant liposomes encapsulating the solution with a physiological salt concentration containing the desired concentrations of actoHMM, which had been almost impossible to obtain using currently adapted methodologies such as natural swelling and electro-formation on an electrode. We then examined the effect of ATP on the cytoskeleton components confined in those cell-sized liposomes, because ATP is known to drive the sliding motion for actoHMM. We added α-hemolysin, a bacterial membrane pore-forming toxin, to the bathing solution and obtained liposomes with the protein pores embedded on the bilayer membrane to allow the transfer of ATP inside the liposomes. We show that, by the ATP supply, the actoHMM bundles inside the liposomes exhibit specific changes in spatial distribution, caused by the active sliding between F-actin and HMM. Interestingly, all F-actins localized around the inner periphery of liposomes smaller than a critical size, whereas in the bulk solution and also in larger liposomes, the actin bundles formed aster-like structures under the same conditions.

A new high-throughput method utilizing porous silica-based nano-composites for the determination of partition coefficients of drug candidates

We show that highly porous silica-based nanoparticles prepared via micro-emulsion and sol-gel techniques are stable colloids in aqueous solution. By incorporating a magnetic core into the porous silica nano-composite, it is found that the material can be rapidly separated (precipitated) upon exposure to an external magnetic field. Alternatively, the porous silica nanoparticles without magnetic cores can be equally separated from solution by applying a high-speed centrifugation. Using these silica-based nanostructures a new high-throughput method for the determination of partition coefficient for water/n-octanol is hereby described. First, a tiny quantity of n-octanol phase is pre-absorbed in the porous silica nano-composite colloids, which allows an establishment of interface at nano-scale between the adsorbed n-octanol with the bulk aqueous phase. Organic compounds added to the mixture can therefore undergo a rapid partition between the two phases. The concentration of drug compound in the supernatant in a small vial can be determined by UV-visible absorption spectroscopy. With the adaptation of a robotic liquid handler, a high-throughput technology for the determination of partition coefficients of drug candidates can be employed for drug screening in the industry based on these nano-separation skills. The experimental results clearly suggest that this new method can provide partition coefficient values of potential drug candidates comparable to the conventional shake-flask method but requires much shorter analytical time and lesser quantity of chemicals.

Ethanol-Assisted Graphene Oxide-Based Thin Film Formation at Pentane–Water Interface

Graphene oxide (GO) can be viewed as an amphiphilic soft material, which form thin films at organic solvent-water interfaces. However, organic solvent evaporation provides little driving force, which results in slow GO transfer in aqueous phase, thus dawdling GO film formation processes for various potential applications. We present an ethanol-assisted self-assembly method for the quick formation of GO or GO-based composite thin films with tunable composition, transmittance, and surface resistivity at pentane-water interface. The thickness of pure GO and reduced GO (rGO) films ranging from ~1 nm to more than 10 nm can be controlled by the concentration of GO in bulk solution. The transmittance of rGO films can be tuned from 72% to 97% at 550 nm while the surface resistivity changes from 8.3 to 464.6 kΩ sq(-1). Ethanol is essential for achieving quick formation of GO thin films. When ethanol is injected into GO aqueous dispersion, it serves as a nonsolvent, compromising the stability of GO and providing driving force to allow GO sheets aggregate at the water-pentane interface. On the other hand, neither the evaporation of pentane nor the mixing between ethanol and water provides sufficient driving forces to allow noteworthy amount of GO sheets to migrate from the bulk aqueous phase to the interface. This method can also be extended to prepare GO-based composites thin films with tunable composition, such as GO/single walled carbon nanotube (SWCNT) composite thin films investigated in this work. Reduced GO/SWCNT composite films show much lower surface resistivity compared to pure rGO thin films. This ethanol-assisted self-assembly method opens opportunities to design and fabricate new functional GO-based hybrid materials for various potential applications.

Real-time UV imaging of drug diffusion and release from Pluronic F127 hydrogels

The objective of this study was to introduce and evaluate UV imaging technology for real-time characterization of drug diffusion in and release from hydrogels. Piroxicam and human serum albumin diffusion in Pluronic F127 hydrogel was monitored by measuring the absorbance of light passing through the diffusion cell at 26 °C, thus providing real-time concentration maps (7 × 3 mm imaging area) within the gel as a function of time. Apparent diffusion coefficients were obtained on the basis of Fick’s second law. Piroxicam and human serum albumin diffusivities in 20% (w/w) F127 gel were 8 and 24 times lower than those determined in the phosphate buffer (pH 7.4). The effect of increasing polymer concentration (20%, 25% and 30% (w/w)) on piroxicam diffusion was further investigated. The decreasing diffusion rate with increasing F127 concentration agreed well with small-angle X-ray scattering (SAXS) measurements. UV imaging was also successfully applied to monitor piroxicam release from 30% (w/w) F127 gel into a stirred aqueous buffer solution, providing simultaneous information on gel dissolution rate, change in thickness of gel–aqueous boundary layer as well as the release of piroxicam into bulk aqueous phase. The current study indicates that UV imaging has great potential for measuring drug diffusion in and release from gel matrices. Compared to the currently used conventional techniques, this technology has several advantages including high information content, non-intrusive measurements without the need for labeling, flexibility with respect to experimental design and simplicity of operation.

The Role of Water H-Bond Imbalances in B-DNA Substate Transitions and Peptide Recognition Revealed by Time-Resolved FTIR Spectroscopy

The conformational substates B(I) and B(II) of the phosphodiester backbone in B-DNA are thought to contribute to DNA flexibility and protein recognition. We have studied by rapid scan FTIR spectroscopy the isothermal B(I)-B(II) transition on its intrinsic time scale. Correlation analysis of IR absorption changes occurring within seconds after a reversible incremental growth of the DNA hydration shell identifies water populations w(1) (PO(2)(-)-bound) and w(2) (non-PO(2)(-)-bound) exhibiting weaker and stronger H-bonds, respectively, than those dominating in bulk water. The B(II) substate is stabilized by w(2). The water H-bond imbalance of 3-4 kJ mol(-1) is equalized at little enthalpic cost upon formation of a contiguous water network (at 12-14 H(2)O molecules per DNA phosphate) of reduced ν(OH) bandwidth. In this state, hydration water cooperatively stabilizes the B(I) conformer via the entropically favored replacement of w(2)-DNA interactions by additional w(2)-water contacts, rather than binding to B(I)-specific hydration sites. Such water rearrangements contribute to the recognition of DNA by indolicidin, an antimicrobial 13-mer peptide from bovine neutrophils which, despite little intrinsic structure, preferentially binds to the B(I) conformer in a water-mediated induced fit. The FTIR spectra resolve sequential steps leading from PO(2)(-)-solvation to substate transition and eventually to base stacking changes in the complex. In combination with CD-spectral titrations, the data indicate that, in the absence of a bulk aqueous phase, as in molecular crowded environments, water relocation within the DNA hydration shell allows for entropic contributions similar to those assigned to water upon DNA ligand recognition in solution.

Giant Vesicles with Membranous Microcompartments

Incubation of a cell-sized lipid membrane vesicle (giant vesicle, GV) in a diluted aqueous solution of neutral phosphate buffer salts or glucose transformed the GV to an oligovesicular vesicle (OVV) that encapsulates one or more smaller GVs. During the incubation, the membrane of flaccid vesicle invaginated and closed to form the inner vesicle of an OVV engulfing a part of the bulk aqueous phase. Using the GV-to-OVV transformation, an OVV that has different aqueous contents in each membranous microcompartment was constructed.

Influence of Nanoreactor Environment and Substrate Location on the Activity of Horseradish Peroxidase in Olive Oil Based Water-in-Oil Microemulsions

Oxidative enzymatic reactions using horseradish peroxidase (HRP) were carried out in water-in-oil (w/o) microemulsions composed of olive oil/lecithin/1-propanol/water, a model biomimetic system. The substrates used (gallic acid, octyl gallate and 2,2'-azino-bis[3-ethylbenzo-thiazoline-6-sulfonic acid] (ABTS)) have different hydrophobicities and possible locations in the microemulsion system. HRP reactivity with reference to substrate hydrophobicity and structural characteristics of the microemulsions is discussed. The nature of the enzyme microenvironments was examined using dynamic light scattering (DLS), differential scanning calorimetry (DSC) and diffusion NMR (DOSY) methodologies while the location of various enzymatic substrates in the microemulsion phase was assessed by solubility measurements and by taking pressure-area isotherms of mixed monolayers of the substrates with dipalmitoyl-phosphatidylcholine (DPPC), which is a major constituent of lecithin. In contrast to the bulk aqueous phase, in the severely restricted environment of the polar domains of the microemulsion HRP reacted faster with octyl gallate, a substrate that is solubilized at the lipid interfaces. HRP was deactivated in the olive oil microemulsions within a few hours, a phenomenon that has also been observed in other microemulsion systems.

Bilayer heating in magnetite nanoparticle–liposome dispersions via fluorescence anisotropy

Temperature measurements have been made within magnetite (Fe 3O 4) nanoparticle–liposome dispersions subjected to electromagnetic field at radiofrequency (RF) heating based on the fluorescence anisotropy of diphenylhexatriene (DPH) embedded within the bilayer. Incorporating cholesterol within dipalmitoylphosphatidylcholine (DPPC) bilayers broadened the anisotropy window associated with lipid melting. Cryogenic transmission electron microscopy showed that the dispersions contained magnetoliposomes with nanoparticle aggregates at both low and high encapsulation densities. RF heating results demonstrated the ability to measure the temperature of the ML bilayer with on/off RF cycles using DPH anisotropy. These measurements reflected the temperature of the bulk aqueous phase, which is consistent with previous work showing rapid heat dissipation from a nanoparticle surface during RF heating and a negligible difference between surface and bulk temperature.

Successful preferential formation of a novel macromolecular assembly—Trilayered polymeric micelle—That can incorporate hydrophilic compounds: The optimization of factors affecting the micelle formation from amphiphilic block copolymers

We have developed a novel macromolecular assembly, trilayered polymeric micelle, which can incorporate hydrophilic compounds. The micelle can be prepared from the amphiphilic block copolymers without regard to their properties such as the copolymer's charges and the homogeneity of the copolymers forming the micelle's inner and outer parts. In this study, we investigated the optimal condition for the preferential formation of the trilayered polymeric micelle. GPC results clarified that the composition of the block copolymer, the concentration of PVA in the aqueous bulk phase, and the temperature during the preparation were the important preparation factors affecting preferential formation of the trilayered polymeric micelles. We successfully achieved the preferential formation of the trilayered polymeric micelles under optimal conditions. Furthermore, we confirmed that the model hydrophilic compound, FITC-dextran, was successfully encapsulated into the hydrophilic core of the trilayered polymeric micelles. The novel micelle that can incorporate hydrophilic compounds can have a variety of future medical applications such as a protein delivery-based therapy.

DRIFTS studies on the photosensitized transformation of gallic acid by iron(iii) chloride as a model for HULIS in atmospheric aerosols

Little is known about the interfacial photochemistry of transition metal cations and chromophores relevant to atmospheric aerosols. We report herein water uptake and in situ surface-sensitive spectroscopic studies on the photosensitized transformation of solid gallic acid (GA), externally mixed with FeCl(3) as a photosensitizer, under dry and humid conditions of <1% and 30% relative humidity (RH), respectively. GA is a hydrolysis product of tannic acid, a model macromolecule for humic-like substances (HULIS) in aerosols and aged polyaromatic hydrocarbons (PAH). Water uptake on GA and GA/FeCl(3) mixture films was quantified using a quartz crystal microbalance (QCM) as a function of %RH (<1-60%). Results indicate continuous multilayer formation of adsorbed water with no phase transitions, with a monolayer of adsorbed water forming around 30 and 12%RH, respectively. Photochemical studies were conducted using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Spectra were collected as a function of irradiation time (4 h), mass fraction of FeCl(3) (0.5-3%) using irradiance of simulated solar light equivalent to 120 Wm(-2) at 555 nm. Difference absorbance spectra show changes to GA functional groups suggesting the formation of organochlorine compounds in the condensed phase with their signature v(C=C) at 1601 cm(-1), and release of CO(2). Potential halogenation pathways of GA in the presence of Fe(3+) are discussed based on well-known aqueous phase chemistry. These pathways along with our results also suggest the release of volatile organochlorine compounds and Cl(2) gas. Apparent first order rate constants, k(s), of the photosensitized reactions were derived from kinetic curves of the most intense positive and negative spectral features at 1601 and 1381 cm(-1), respectively. Values of k(s) at 120 Wm(-2) are found to be higher than those reported from UV photo-Fenton reactions of GA in bulk aqueous phases containing H(2)O(2), Fe(2+) or Fe(3+). The implication of our studies on the aging of multicomponent aerosols containing organic matter, transition metals and halide ions due to heterogeneous photochemistry is discussed.

Unique emulsions based on biotechnically produced hydrophobins

The emphasis of this manuscript is on emulsions with gel-like properties based on biotechnically produced hydrophobins. These emulsions are compared to emulsions based on surfactants. Even though the preparation conditions for both emulsion types were the same, the structure and the properties were very different. Homogeneous, gel-like emulsions could be obtained with a protein concentration between 0.02 and 1 wt% and an oil mass fraction of more than 0.65. The gelified state is formed because the protein-covered droplets behave like sticky spheres even when the globules are ionically charged and the long range interaction is repulsive. Conductivity and microscopy measurements showed that the emulsions were of the oil in water (o/w) type. The size of the emulsion droplets depends on the mixing apparatus. With a vortex shaker oil droplets of up to 100 μm diameter were obtained indicating some protein remained in the bulk aqueous phase. With a high pressure homogenizer the emulsion droplets got much smaller and the protein was completely adsorbed at the droplet interface. Interestingly the emulsions aged with time without changing their structure. The aging was a result of the increase of the storage modulus G′. In the case of surfactants no homogeneous stable emulsions could be obtained under the same conditions.

Estimate of the potential difference across metal/water interfaces and across the lipid bilayer moiety of biomimetic membranes: an approach

Mercury is a particularly advantageous support of lipid, thiol and disulfide self-assembled monolayers (SAMs), because it provides them with a perfectly smooth and fluid surface and allows their gradual expansion in aqueous solution with progressive tilt of the adsorbed molecules, without water incorporation. These unique advantageous features permit an estimate of the extrathermodynamic absolute potential difference, ϕ, across the Hg|water interface and of the surface dipole potential of mercury-supported SAMs. Two different models of metal-supported tethered bilayer lipid membranes (tBLMs) incorporating a cation-selective channel predict that the ϕ value at the inflection point of a plot of the in-phase component, Y′, of the electrochemical admittance against the applied potential E is almost coincident with the surface dipole potential, χs, of the hydrophilic spacer moiety of the tBLM. This prediction allows an estimate of the absolute potential difference, ϕ, across the interface between any metal capable of supporting tBLMs and the bulk aqueous phase, provided the χs value of the tBLM is known. Moreover, the potential difference across the lipid bilayer moiety of the tBLM (i.e., the transmembrane potential) is shown to be practically equal to χs at the inflection point of the corresponding Y′ vs.E plot. This approach is applied to polycrystalline Au and Ag(111).

Gas exsolution and flow during supersaturated water injection in porous media: I. Pore network modeling

Degassing and in situ development of a mobile gas saturation take place when an aqueous phase saturated with gas at a pressure higher than the subsurface pressure is injected in water-saturated porous media. In the first part of this work, a pore network model is used to study the key physical aspects of this novel and hitherto unexplored way of introducing a gas phase in the subsurface. Following heterogeneous nucleation, growth of gas phase clusters driven by convective diffusion of solute from the bulk aqueous phase, is shown to result in a ramified pattern of gas-occupied pores, which is controlled by capillary and buoyancy forces. The interplay between mass transfer and immiscible displacement processes, namely gas cluster coalescence, mobilization under the action of buoyancy forces and fragmentation resulting from capillary instabilities, is seen to favour the propagation of a stable gas saturation front. Pore network model predictions of the macroscopic mass transfer rate coefficient are in fair agreement with a recently published empirical correlation.

Zinc Interactions with Glucosamine-Functionalized Fused Silica/Water Interfaces

The adsorption of Zn2+ to glucosamide-functionalized fused silica/water interfaces is studied using second harmonic generation (SHG). We characterize each step of the surface functionalization using vibrational sum frequency generation (SFG) and X-ray photoelectron spectroscopy (XPS), where specific vibrational modes in the C−H region and binding energies in the C1s and N1s region are determined to be indicative of glucose covalently tethered to the surface. We employ the SHG χ(3) technique to track Zn2+ adsorption and desorption directly at the glucosamide-functionalized fused silica/aqueous interface at pH 7 and 10 mM NaCl and determine the electrostatic and thermodynamic binding parameters using standard electrical double layer models to quantify the change in interfacial potential upon zinc adsorption. The results presented here allow for the possibility of 2:1 to 3:1 carbohydrate:metal coordination complexes and suggest the possibility for multivalent interactions which have not been observed with glucose in the bulk aqueous phase, where 1:1 complexes dominate. These findings suggest that interactions between metal ions and carbohydrate arrays may be much stronger at interfaces as opposed to the bulk phase, with direct implications for controlling and predicting coordination chemistry.