Study of Short Peptide Adsorption on Solution Dispersed Inorganic Nanoparticles Using Depletion Method

Summary

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.

Abstract

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.

Introduction

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 engineering^1,2,3, and implantation^4,5,6,7. Contemporary bioresponsive devices, such as sensors and actuators, are based on functional proteins immobilized on oxide semiconducting surfaces^{8,9,10,11,12,13}. 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 substrates^15,16. 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 applications^18,19,20. 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 properties^21,22,23,24.

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 biointerfaces^5,13.

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 separation^{25,26,27,28,29,30}.

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 temperature^{32,33,34,35,36}.

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 parameters^37,38,39.

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 calibration^25,27,40. 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 surfaces^42,43. In situ monitoring of the fluorescence dynamics upon adsorption and desorption provide the adsorption kinetics and hence thermodynamic data^42,43.

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 labeling^47,48,49,50 or derivatization^51,52 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.

Protocol

1. Preparation of dipeptide stock solutions and dilutions

1. Preparation of 16 mM dipeptide solution
   1. Place 0.183 g of a dipeptide (Ile-His) (see Table of Materials) in a sterile polymeric test tube, dilute to 35 mL with double-distilled water (DDW), and dissolve at room temperature (RT) under vigorous stirring.
      NOTE: If the dipeptide does not dissolve in DDW while stirring, place the dipeptide solution into an ultrasonic bath and sonicate for a few minutes.
   2. Prepare a 50 mM solution of 2-(N-morpholino)ethanesulfonic acid (MES) buffer by dissolving 0.533 g of dry 2-(N-morpholino)ethanesulfonic acid in 50 mL of DDW in the sterile test tube. Prepare a 50 mM sodium hydroxide solution by dissolving 200 mg of sodium hydroxide in 100 mL of DDW.
   3. Adjust the pH of the predissolved dipeptide solution to 7.4 by carefully adding (microliter titration) 50 mM MES, or 50 mM sodium hydroxide to the 16 mM dipeptide solution, stirring at RT and monitoring the pH with a pH meter. After adjusting the pH, pour the solution into a measuring cylinder, rinse the test tube, and fill the measuring cylinder with DDW to 40 mL to make a final concentration of 16 mM.
2. Preparation of dipeptide dilutions from 16 mM stock solution
   1. Prepare peptide dilutions with concentrations between 0.4 and 12.0 mM by diluting the 16 mM dipeptide solution with DDW. For example, in order to prepare an 8 mM dipeptide solution, add 7 mL of DDW to 10 mL of the 16 mM dipeptide solution. After dilution, adjust the pH to 7.4 by adding 50 mM MES or 50 mM NaOH drop by drop to the dipeptide solution (see step 1.1.3). After adjusting the pH, pour the solution into a measuring cylinder, rinse the test tube, and fill the measuring cylinder up to 20 mL with DDW to make the dipeptide concentration 8 mM.
      NOTE: Other dilutions of 16 mM dipeptide stock solution with concentrations of 0.4, 0.8, 1.2, 1.6, 2.0, 3.0, 4.0, 8.0, and 12.0 mM, are prepared in accordance with Figure 1. The adjustment of each dipeptide solution pH to 7.4 is described in step 1.1.3.

2. Preparation of titania sol

1. Prepare a 10 mM solution of MES buffer by dissolving 1.066 g of MES in 500 mL of DDW. Adjust the pH to 7.4 with dry sodium hydroxide upon stirring and monitoring the pH with the pH meter.
2. Grind 200 mg of nanocrystalline TiO₂ in a mortar for at least 5 min (see Table of Materials).
3. Weigh 40 mg of the ground titanium dioxide nanoparticles into a laboratory flask. Put the flask into the sonication bath (see Table of Materials) using the laboratory stand.
4. Add 20 mL of 10 mM MES buffer into the flask with TiO₂ and sonicate in an ultrasonic bath (5 L, 40 kHz, 120 W) for 20 min.

3. Mixing and thermostating

1. Set the thermostat (see Table of Materials) to the desired temperatures (i.e., 0.00, 10.00, 20.00, 30.00, or 40.00 °C).
2. Add 1 mL of the sonicated sol of TiO₂ to the marked adsorption vials. Place the marked adsorption vials against corresponding dipeptide dilution in a makeshift flotation device made of extruded polystyrene foam. Place the flotation device with the marked vials and corresponding dipeptide dilutions into the thermostat for at least 5 min.
3. Add 1 mL of each dipeptide dilution to the corresponding marked adsorption vial, making sure all mixing solutions have the same temperature. Keep the series of obtained adsorption samples on the thermostat at 0.00, 10.00, 20.00, 30.00, or 40.00 °C for 24 h to achieve the adsorption equilibrium.
   NOTE: Cautiously shake all the samples of obtained dispersions prior to putting them into the thermostat.
4. Occasionally mix the TiO₂ dispersions by manually shaking them during thermostating.

4. Filtration of the thermostated samples

1. In order to avoid temperature-induced reequilibration take out one sample at a time from the thermostat for filtration.
2. Take a sample of the dipeptide solution from each glass vial with a syringe, through a syringe needle. Remove the needle from the syringe and put on the syringe filter (see Table of Materials) to filter the dipeptide solution into the glass vial. Repeat the filtration with the other samples.
3. Analyze the filtrate in accordance with section 5.
   NOTE: Do not centrifuge the samples, because it takes a few minutes and may cause a change in the concentration equilibrium.

5. Derivatization and HPLC analysis

1. Make a 50 mL solution of trifluoroacetic acid (TFA) in acetonitrile. Add 0.34 mL of TFA in the measuring cylinder and adjust the volume of the solution to 50 mL with acetonitrile at RT.
   CAUTION: Work with TFA under a fume hood with exhaust ventilation, because trifluoroacetic acid is harmful when inhaled, causes severe skin burns, and is toxic for aquatic organisms even at low concentrations⁵³.
2. Prepare the derivatization solution (i.e., Edman reagent⁵⁴) by placing 299 µL of phenyl isothiocyanate and 347 µL of triethylamine in a graduated cylinder and adjusting the volume of solution to 50 mL with acetonitrile at RT.
3. Prior to the high-performance liquid chromatography (HPLC) analysis, derivatize the samples with Edman's reagent in the chromatography vials. Mix 400 µL of the sample with 400 µL of Edman's reagent. Heat the sample at 60 °C for 15 min. After heating, neutralize the sample with 225 µL of the TFA solution and wait for a few minutes to cool the sample to RT.
4. Use HPLC analysis (see Table of Materials) to determine the concentration of the dipeptide solution before and after adsorption. Place the chromatography vials with the analyzed solutions into the HPLC autosampler and start analyzing the samples with the necessary conditions, which are set by the software (see Table of Materials).
   NOTE: The mobile phase consists of 0.1% TFA in deionized water and pure acetonitrile, with acetonitrile gradients from 20-90% at 286 nm for 13 min. Analyze each sample in triplicate. Measure the dipeptide solution concentration using the previously established calibration curve (Figure 2). For chromatography specifications see Shchelokov et al.⁵⁵.

Results

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

Discussion

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...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
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%