The first step in comprehending biomolecule-inorganic solid phase interaction is revealing fundamental physicochemical constants that may be evaluated by establishing adsorption isotherms. Adsorption from the liquid phase is restricted by kinetics, surface capacity, pH, and competitive adsorption, which all should be cautiously considered before setting an adsorption experiment.
Fundamentals of inorganic-organic interactions are critically important in the discovery and development of novel biointerfaces amenable for utilization in biotechnology and medicine. Recent studies indicate that proteins interact with surfaces through limited adsorption sites. Protein fragments such as amino acids and peptides can be used for interaction modeling between complex biological macromolecules and inorganic surfaces. During the last three decades, many valid and sensitive methods have been developed to measure the physical chemistry fundamentals of those interactions: isothermal titration calorimetry (ITC), surface plasmon resonance (SPR), quartz crystal microbalance (QCM), total internal reflection fluorescence (TIRF), and attenuated total reflectance spectroscopy (ATR).
The simplest and most affordable technique for the measurement of adsorption is the depletion method, where the change in sorbate concentration (depletion) after contact with solution-dispersed sorbent is calculated and assumed to be adsorbed. Adsorption isotherms based on depletion data provide all basic physicochemical data. However, adsorption from solutions requires longer equilibration times due to kinetic restrictions and sorbents with a high specific surface area, making it almost inapplicable to macroscopic fixed plane surfaces. Moreover, factors such as the instability of sols, nanoparticle aggregates, sorbent crystallinity, nanoparticle size distribution, pH of the solution, and competition for adsorption, should be considered while studying adsorbing peptides. Depletion data isotherm construction provides comprehensive physical chemistry data for literally every soluble sorbate yet remains the most accessible methodology, as it does not require expensive setups. This article describes a basic protocol for the experimental study of peptide adsorption on inorganic oxide and covers all critical points that affect the process.
For the last 50 years the interaction between inorganic surfaces and peptides has drawn a lot of attention due to its high importance in material science and medicine. Biomedical research is focused on the compatibility and stability of bioinorganic surfaces, which have direct implications for regenerative medicine, tissue engineering1,2,3, and implantation4,5,6,7. Contemporary bioresponsive devices, such as sensors and actuators, are based on functional proteins immobilized on oxide semiconducting surfaces8,9,10,11,12,13. Modern purification practices for protein production often rely on biomolecule interaction properties in downstream purification and separation14.
Among multiple inorganic oxides, titanium dioxide remains the most utilized in combination with biologically relevant substrates15,16. Research in the area of TiO2-based biointerfaces has concentrated on establishing strong and specific binding of proteins and peptides without changing their biological and structural properties. Ultimately, the major objective is a high surface density layer of biomolecules with high stability and increased functionality that will advance the creation of titanium-based biotechnological and medical applications17.
Titanium and its alloys have been used extensively as a surgical implant material for at least six decades because a surface TiO2 layer with a thickness of a few nanometers is corrosion resistant and exhibits a high level of biocompatibility in many in vivo applications18,19,20. Titanium dioxide is also widely considered an inorganic substrate produced in biomineralization, where nucleation and inorganic phase growth accompanied by proteins and peptides may provide materials with promising catalytic and optical properties21,22,23,24.
Given the high relevance of the interaction between inorganic materials and biomolecules in general and protein-TiO2 interactions in particular, there has been a lot of research to address the manipulation and control of the adsorption of proteins on TiO2. Due to these studies, some fundamental properties of this interaction have been revealed, such as adsorption kinetics, surface coverage, and biomolecule conformation, giving substantial support for further advances in biointerfaces5,13.
However, protein complexity adds considerable restrictions on full determination and understanding of a protein's molecular level interaction with inorganic surfaces. Assuming that the biomolecules interact with the inorganic surfaces through limited sites, some proteins with known structures and amino acid sequences have been reduced to their components-peptides and amino acids-which are studied separately. Some of these peptides have demonstrated significant activity, making them a unique subject of adsorption studies without the need for previous protein separation25,26,27,28,29,30.
Quantitative characterization of peptide adsorption on TiO2 or other inorganic surfaces can be accomplished by means of physical methods that have been adapted specifically for biomolecules for the past few decades. These methods include isothermal titration calorimetry (ITC), surface plasmon resonance (SPR), quartz crystal microbalance (QCM), total internal reflection fluorescence (TIRF), and attenuated total reflectance spectroscopy (ATR), all of which allow for the detection of the adsorption strength by providing key thermodynamic data: The binding constant, Gibbs free energy, enthalpy, and entropy31.
The adsorption of biomolecules to the inorganic material may be accomplished in two ways: 1) ITC as well as the depletion method use particles dispersed in a solution binding to fixed macroscopic surfaces; 2) SPR, QCM, TIRF, and ATR use macroscopic surfaces modified with inorganic material, such as gold-coated glass or metal chips, quartz crystals, zinc sulfide crystals, and PMMA chips, respectively.
Isothermal titration calorimetry (ITC) is a label-free physical method that measures the heat produced or consumed upon titration of solutions or heterogeneous mixtures. Sensitive calorimetric cells detect heat effects as small as 100 nanojoules, making the measurement of adsorption heat on nanoparticle surfaces possible. Thermal behavior of the sorbate during continuous addition- titration, provides a full thermodynamic profile of the interaction revealing enthalpy, binding constant, and entropy at a given temperature32,33,34,35,36.
Surface plasmon resonance (SPR) spectroscopy is a surface-sensitive optical technique based on the measurement of the refractive index of the media in close proximity to the studied surface. It is a real-time and label-free method for monitoring reversible adsorption and adsorbed layer thickness. The binding constant can be calculated from the association and dissociation rates. Adsorption experiments performed at different temperatures may provide information about the temperature dependence of the activation energy and sequentially other thermodynamic parameters37,38,39.
The quartz crystal microbalance (QCM) method measures the change in the oscillating frequency of piezoelectric crystals during the adsorption and desorption processes. The binding constant may be evaluated from the ratio of the adsorption and desorption rate constants. QCM is used for relative mass measurements and therefore, needs no calibration25,27,40. QCM is used for adsorption from both gas and liquid. The liquid technique allows QCM to be used as an analysis tool to describe deposition on variously modified surfaces41.
Total internal reflection fluorescence (TIRF) is a sensitive optical interfacial technique based on the measurement of the fluorescence of adsorbed fluorophores excited with internally reflected evanescent waves. The method allows for the detection of fluorescent molecules covering the surface with thicknesses on the order of tens of nanometers, which is why it is used in the study of macromolecular adsorption on various surfaces42,43. In situ monitoring of the fluorescence dynamics upon adsorption and desorption provide the adsorption kinetics and hence thermodynamic data42,43.
Attenuated total reflectance (ATR) was used by Roddick-Lanzilotta to establish lysine adsorption isotherms based on the lysine spectral bands at 1,600 and 1,525 cm-1. This is the first time that the binding constant for a peptide on TiO2 was determined using an in situ infrared method44. This technique was effective in establishing adsorption isotherms for polylysine peptides45 and acidic amino acids46.
Unlike the abovementioned methods, where the adsorption parameter is measured in situ, in a conventional experiment the amount of the adsorbed biomolecules is measured by the concentration change after the surface contacted the solution. Because the concentration of a sorbate decays in a vast majority of adsorption cases, this method is referred to as the depletion method. Concentration measurements require a validated analytical assay, which may be based on an intrinsic analytical property of the sorbate or based on the labeling47,48,49,50 or derivatization51,52 thereof.
Adsorption experiments using QCM, SPR, TIRF, or ATR require special surface preparation of the chips and sensors used for adsorption studies. Prepared surfaces should be used once and require change upon switching the adsorbate, due to the inevitable hydration of the oxide surface or possible chemisorption of a sorbate. Only one sample at a time can be run using ITC, QCM, SPR, TIRF, or ATR, whereas in the depletion method one can run dozens of samples, for which the quantity is only limited by the thermostat capacity and sorbent availability. This is especially important when processing large sample batches or libraries of bioactive molecules. Importantly, the depletion method does not require costly equipment but solely a thermostat.
However, despite its obvious advantages the depletion method requires complex procedural features that may seem cumbersome. This article presents how to perform a comprehensive physicochemical study of dipeptide adsorption on TiO2 using the depletion method and addresses issues that researchers may face when performing relevant experiments.
1. Preparation of dipeptide stock solutions and dilutions
2. Preparation of titania sol
3. Mixing and thermostating
4. Filtration of the thermostated samples
5. Derivatization and HPLC analysis
Adsorption of a dipeptide on nanocrystalline titanium dioxide was studied at the biocompatible conditions in a temperature range of 0−40 °C. Experimental dipeptide adsorption (A, mmol/g) on the surface of a titanium dioxide was evaluated as
Where C0 and Ce are the dipeptide starting and equilibrium concentrations in millimoles, respectively; V is the volume of a dipeptide solution in liters; and m is the weight of the sorbent in grams.
The measurements of the dipeptide adsorption were data processed using the Henry model. This isotherm model assumes adsorption at relatively low concentrations with the sorbate molecules isolated from each other on a sorbent surface and is suitable for describing the experimental data (Figure 3). Note, however, that this model can only be applied in the case of reversible adsorption, which should be confirmed as well. IR-spectroscopy of the material rinsed multiple times is suitable for this purpose. The obtained equilibrium peptide amounts on the TiO2 and solution are related in accordance with the linear equation:
where KH is Henry's adsorption constant.
The equilibrium binding constant KH was obtained from the slope of the dependence of dipeptide adsorption (A) on the dipeptide equilibrium concentration (Ce). The standard Gibbs free energy (ΔG, kJ/mol) for each temperature T was determined through the Van't Hoff equation:
where R is the ideal gas constant in J/mol*K, and T is the temperature of the adsorption process in Kelvin.
Dipeptide Gibbs free energies determined at each temperature (Figure 4) disclosed enthalpy (ΔH) as an interception of the linear regression with the axis. The regression variable, the entropy of the process (ΔS), was derived from the fundamental equation:
The calculated values of the equilibrium binding constant (KH), standard Gibbs energy (ΔG), enthalpy (ΔH), and entropy (ΔS) for Ile-His are presented in Table 1.
Figure 1: Dilution of the 16 mM dipeptide stock solution. Please click here to view a larger version of this figure.
Figure 2: Calibration curve at different dipeptide concentration. The dipeptide concentrations were between 0.4-16.0 mM. Please click here to view a larger version of this figure.
Figure 3: The dipeptide adsorption isotherms calculated by the Henry model for each temperature. Dipeptide adsorption isotherms at (A) 0 °C (B) 10 °C (C) 20 °C (D) 30 °C, and (E) 40 °C, respectively. The calculated correlation coefficients (R2) fell into a 0.96−0.99 range for all obtained Henry model isotherms. Error bars represent the 95% confidence interval for each sample concentration measured in triplicate. Please click here to view a larger version of this figure.
Figure 4: Dependence of the standard Gibbs free energy of the dipeptide adsorption on temperature. Error bars represent the 95% confidence interval for Gibbs free energy as indirect measurement based on Henry Model. Please click here to view a larger version of this figure.
T, K | KH | ΔG0, kJ/mol | ΔH0, kJ/mol | ΔS0, kJ/mol K |
273.15 | 0.32 ± 0.01 | 2.6 ± 0.0 | – 41 ± 9 | – 0.16 ± 0.03 |
283.15 | 0.25 ± 0.01 | 3.2 ± 0.1 | ||
293.15 | 0.17 ± 0.06 | 4.3 ± 0.9 | ||
303.15 | 0.050 ± 0.002 | 7.6 ± 0.1 | ||
313.15 | 0.037 ± 0.002 | 8.3 ± 0.1 |
Table 1: Thermodynamic parameters of dipeptide adsorption.
Adsorption from solutions for isotherm construction requires a longer time for equilibration due to kinetic restrictions and sorbents with a high specific surface area. Moreover, instability of sols, nanoparticle aggregates, crystallinity, nanoparticle size distribution, pH of the solution, and competition for adsorption should be considered while adsorbing amino acids. However, adsorption isotherm construction using the depletion method remains the most available methodology, because it does not require expensive setups, and yet it provides exhaustive physical chemistry data for literally every soluble sorbate.
A distinction has to be made between adsorption modes (i.e., solution dispersed particles or on a fixed surface) when a crystalline material is used as a sorbent. One should expect a substantial difference in the distribution of crystalline faces on macroscopically flat surfaces and on particles. Resulting thermodynamic parameters determined from adsorption of peptides on nanoparticles may not correspond to the thermodynamic parameters of peptide adsorption to macroscopically flat surfaces.
The average amount of peptides adsorbed on the inorganic surfaces is extremely low. At room temperature, this value is about several hundreds of micrograms per square meter28. This small amount of adsorbate requires accurate measurement methods and solids with well-developed surfaces. Therefore, small particle substances with a large specific surface (hundreds of square meters) should be used for adsorption experiments43,56,57,58,59,60.
Peptides are, like proteins, unstable, and retain their functionality at a narrow range of conditions. Adsorption experiments were performed on nanocrystalline titanium dioxide at biocompatible temperatures of 0 °C-40 °C (273.15 K-313.15 K), which are similar to those of a normal, functioning, living organism. Adsorption at higher or lower temperatures are irrelevant and should not be considered for the experiment.
Multifunctional biologically active compounds also exhibit high susceptibility to pH of the media, as it affects the surface charge and therefore Coulomb interactions between charged functional groups61,62,63. The sorbent charge of oxide materials is also pH-dependent due to active proton exchange at the hydrated surface64. In order to establish pH stable conditions for adsorption equilibrium use of a buffer is required. In this study, MES buffer is used for its noncoordinating property65, so it would not compete with the peptide for adsorption on the metal oxide surface, unlike phosphate buffers66.
This recent test of amino acid adsorption shows that the major binding site on the nanoparticle is the surface defect55. Defect distribution on the surface is one of the least controllable features of the nanocrystalline substrates, hence one should use sorbent from the same batch in order to maintain consistency in adsorption studies.
QCM, plasmon resonance, and ITC are genuine methods with subtle sensitivity that in a combination of spectroscopic methods reveal structural peculiarities of the adsorbate during interplay with the surface. However, they do not overcome kinetic restrictions and still require considerable time to achieve adsorption equilibration. Furthermore, only one sample at a time can be processed, which makes batch sample analysis challenging. On the other hand, the depletion method presented is simple and only restricted to the thermostat capacity, making the processing of a large number of samples possible.
The thermostated samples should be filtered as soon as they are removed from the thermostat in order to avoid temperature-induced reequilibration. Although equilibration at a new temperature may take up to a few hours, keeping the adsorption samples at a different temperature should be minimized. Centrifugation of the samples for supernatant separation is also not recommended, because it takes up to a few minutes and may cause a change in the concentration equilibrium. The choice of the filter material depends on the sorbate nature and should reduce possible filter-binding for maximum recovery. It is best to follow vendor instructions and recommendations when choosing specific filters.
Additionally, one should bear in mind that concentration change in the adsorption studies should be monitored using validated quantification method using mass spectrometry, radio-spectroscopy, or UV-visible spectroscopy. The analysis is easy if the adsorbate is spectroscopically active, otherwise additional labeling or derivatization of the adsorbate is required.
The authors have nothing to disclose.
This work was financially supported by the Russian Foundation for Basic Research (Grant No. 15-03-07834-a).
2-(N-Morpholino)ethanesulfonic acid | TCI Chemicals | 4432-31-9 | MES, >98% |
Acetonitrile | Panreac AppliChem | HPLC grade | |
Chromatography vials | glass | ||
Dipeptide Ile-His | Bachem | 4000894 | |
Double-distilled water | DDW was obtained on spot | ||
Heating cleaning bath "Ultrasons-HD" | J.P. Selecta | 3000865 | 5 L, 40 kHz, 120 Watts |
High-performance liquid chromatograph system equipped with a UV−vis detector | Shimadzu, LC-20 Prominence | HPLC | |
Isopropanol | Sigma-Aldrich (Merck) | 67-63-0 | 99.70% |
LabSolutions Lite | Shimadzu | 223-60410 | Software for high-performance liquid chromatography system |
Nanocrystalline TiO2 | Pure anatase with at least 99% crystallinity. Average particle size 10.62 ± 3.31 nm. Specific surface 131.9 m2/g (BET). See Langmuir 2019, 35, 538−550, for details. | ||
Phenyl isothiocyanate | Acros Organics | 103-72-0 | PITC, 98% |
Reversed-phase Zorbax column | ZORBAX LC | 150×2.5 mm i.d. with a mean particle size of 5 μm | |
Syringe filter | Vladfilter | 25 mm, 0.2 μm pore, cellulose acetate | |
Test sterile polymeric tube | polypropylene | ||
Thermostat TC-502 | Brookfield | Refrigerating/heating circulating bath with the programmable controller for the sample derivatization | |
Triethylamine | Sigma-Aldrich (Merck) | 121-44-8 | TEA; 99% |
Trifluoroacetic acid | Panreac AppliChem | 163317 | TFA, 99% |