A Thermodynamic Description of the Adsorption of Simple Water

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A Thermodynamic Description of the Adsorption of Simple Water-Soluble Peptoids to Silica Anna L Calkins, Jennifer Yin, Jacenda L Rangel, Madeleine R. Landry, Amelia A. Fuller, and Grace Y. Stokes Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b02804 • Publication Date (Web): 18 Oct 2016 Downloaded from http://pubs.acs.org on October 21, 2016

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A Thermodynamic Description of the Adsorption of Simple Water-Soluble Peptoids to Silica Anna L. Calkins, Jennifer Yin, Jacenda L. Rangel, Madeleine R. Landry, Amelia A. Fuller, Grace Y. Stokes* Department of Chemistry and Biochemistry, Santa Clara University, 500 El Camino Real, Santa Clara, CA 95053

ABSTRACT: The first report of a water-soluble peptoid adsorbed to silica monitored by second harmonic generation (SHG) at the liquid/solid interface is presented here. The molecular insights gained from these studies will inform the design and preparation of novel peptoid coatings. Simple 6- and 15-residue peptoids were dissolved in phosphate buffered saline and adsorbed to bare silica surfaces. Equilibrium binding constants and relative surface concentrations of adsorbed peptoids were determined from fits to the Langmuir model. Complementary fluorescence spectroscopy studies were used to quantify the maximum surface excess. Binding constants, determined here by SHG, were comparable to those previously reported for cationic proteins and small molecules. Enthalpies and free energies of adsorption were determined to elucidate thermodynamic driving forces. Circular dichroism spectra confirm that minimal

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conformational changes occur when peptoids are adsorbed to silica while pH studies indicate that electrostatic interactions impact adsorption.

I. INTRODUCTION Peptoids are peptide-mimicking N-substituted glycine oligomers (Figure 1) with diverse potential applications, including serving as surface coatings.1-5 In part because peptoids are more robust than peptides against degradation by proteases found in the human body,6,7 peptoid coatings have been explored as materials to alter the surfaces of artificial implants, medical devices, water purification systems, and hospital equipment to prevent bacterial growth and resist biofilm adhesion.8-11 This promising application motivates the study of peptoids at the aqueous liquid/solid interface. Ellipsometry and X-ray photoelectron spectroscopy have been previously used to characterize peptide-linked peptoids on TiO2-coated silicon wafers at the air/solid interface.8,12 Additionally, near-edge X-ray absorption fine structure (NEXAFS) and sum frequency generation (SFG) have been used to characterize dried films of peptoid-based polymers on silica (SiO2) and silicon wafers.13-15 To complement studies at the air/solid interface, thin films were hydrated and studied with contact angle and SFG.13-15 Prior studies of peptoid-based polymers and peptide-peptoid hybrids on oxide surfaces have revealed that oligomer length and sequence influence surface coverage and anti-fouling properties.12,13,15 Only the mass-based method, optical waveguide lightmode spectroscopy (OWLS), has been used to quantify peptoids adsorbed at the liquid/solid interface.8,12 However, a detailed thermodynamic description of simple peptoid sequences at the aqueous/SiO2 interface without extrinsic probes or signal convolution from solution-phase peptoids is not available. Given the promising biomedical and environmental applications of peptoid coatings, quantifying the surface concentrations and binding affinities of peptoids is an important priority.


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H N

O OH n

R Peptide

R N

O OH n

Peptoid

Figure 1. Peptoid (right) versus peptide (left) bonds. In the current study, nonlinear optical surface second harmonic generation (SHG) enables direct measurement of adsorption and desorption of water-soluble peptoids at the SiO2/water interface. An understanding of the solid/aqueous liquid interface is particularly relevant for development of peptoid coatings with potential utility in medical devices. Moreover, this work describes adsorption of water-soluble peptoids that lack tethers or polymer additives. Because previous work has shown that oligomer length influences surface effects,12,13,15 we chose to study water-soluble 6-residue (1) and 15-residue (2) sequences (Figure 2). Both peptoids comprise the same repeating pattern of three residues: 1) (S)-N-(1-carboxyethyl)glycine (Nsce), a chiral substituent that is anionic near neutral pH, 2) (S)-N-1-(naphthylethyl)glycine (Ns1npe), a chiral, bulky aromatic substituent, and 3) N-(2-aminoethylglycine) (Nae), an achiral, substituent that is cationic near neutral pH. Importantly, the Ns1npe residue provides a UV-active chromophore, which allows SHG detection at low aqueous concentrations without the incorporation of an exogenous probe. The solution structures of peptoids 1 and 2 in aqueous buffers are hypothesized to be amphiphilic helices with a net charge of +1 at pH 7.4. However, the spectroscopic features of 1 and 2 vary with oligomer length, solution ionic strength, temperature and solvent,16 suggesting that they may have different surface interactions as well.


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Figure 2. Structures of 6- (1) and 15-residue (2) peptoids. Here, we combine SHG with complementary fluorescence and circular dichroism (CD) spectroscopies to determine that in the absence of peptide- or polymer linkers, a water-soluble peptoid adsorbs to a SiO2 surface. We demonstrate that sequence length impacts binding affinity and surface concentration. By varying temperature, we determined enthalpies of adsorption at the liquid/solid interface and observed that peptoid adsorption to SiO2 is an endothermic process. Changes in conformation and interfacial peptoid concentrations in the presence of SiO2 were probed with CD spectroscopy. Together, these studies provide the first detailed thermodynamic description of simple water-soluble peptoids binding to SiO2, providing a foundation for further studies to correlate peptoid sequences with their surface properties. II. MATERIALS AND METHODS a.

Buffer and peptoid sample preparation: Peptoids used in these studies were synthesized

and purified according to reported procedures.16 Phosphate buffered saline (PBS buffer) at pH 6.2, 7.4 and 8.2 were prepared with 50 mM sodium phosphate. Acetate buffer (50 mM) at pH 5.0 was prepared by combining sodium acetate and acetic acid. 2-(cyclohexylamino)ethanesulphonic


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acid (CHES) buffer at pH 9.0 was prepared at 50 mM concentration. Sodium chloride was added to all buffers to ensure the total ionic strength was equal to 215 mM. All salts were dissolved in ultrapure 18.2 MΩ water (ThermoScientific Barnstead Micropure). The pH of all buffers were adjusted to the desired pH by addition of 1 M sodium hydroxide or 4 M hydrochloric acid and stored at 4 °C. Concentrated stock solutions of peptoids (ranging from 1x10-3 M to 4x10-3 M) were prepared by dissolving lyophilized peptoid in methanol (Alfa Aesar, HPLC grade 99.8%), and dilutions were made by addition of buffer so that final methanol percentage remained below 4%. b.

Brief description of SHG setup and sample cell: Adsorption of peptoids dissolved in PBS

buffer at pH 7.4 to SiO2 surfaces was monitored using counter-propagating SHG.17,18 SHG experimental procedures and a detailed description of the sample cell are available in the Supporting Information. Briefly, SHG experiments were performed using the 532 nm output of a Q-switched Nd:YAG laser with a 7 ns pulse width at a repetition rate of 10 Hz (Surelite III-EX, Continuum). The incident laser light (~15 mJ/pulse at the sample) was directed onto the surface of a fused silica prism at an angle of 67° under total internal reflection. The reflected beam was steered back onto the surface using a high energy Nd:YAG mirror (0° and 45° AOI, #KB1-K12, Thorlabs), allowing only s-polarized SHG light through at 532 nm. The reflected beam was spatially overlapped with the incident laser light to result in a SHG signal generated perpendicular to the SiO2 surface (Figure 3). Elevated temperatures used in this study (27, 38 and 44 °C) were achieved with a Lauda immersion thermostat compact water bath (Type B), which provided a method of indirect heating of the custom-built flow cell described in the Supporting Information. Temperature within the flow cell was monitored continuously using a Type K thermocouple (beaded probe, VWR#61161-372).


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2ω ω

ω

SiO2 prism

aqueous peptoid in flow cell

Figure 3. Schematic of counter-propagating SHG used to interrogate aqueous peptoids at the SiO2 interface

c.

Cleaning procedures: All components of the sample cell (SiO2 prism, flow cell body,

injection ports and o-rings) were cleaned in piranha etch, a 70:30 v:v solution of 18 M sulfuric acid (VWR) and 30% hydrogen peroxide (VWR) for a minimum of 4 hours. (CAUTION: This solution is a strong oxidant and reacts violently with organic solvents. Extreme caution must be taken when handling this solution). Prior to each experiment, SiO2 substrate and flow cell components were removed from piranha etch and rinsed with copious amounts of ultrapure 18.2 MΩ water. SiO2 prisms were plasma cleaned (Harrick Scientific) for 3 minutes immediately before they were mounted to the flow cell. d.

Isotherm collection procedure: Aqueous peptoid solutions used in adsorption isotherms

were dissolved in PBS buffer. Dilute aqueous solutions of peptoids 1 or 2 ranging in concentration from 1x10-7 to 4 x10-5 M were prepared freshly each day at the beginning of each experiment before introduction into the flow cell. To ensure that peptoid concentration in the bulk phase above the SiO2 surface was not depleted by adsorption, at least six 3 mL injections of


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the same concentration of peptoid was introduced into the flow cell at the lowest concentrations (ranging from 1x10-7 M to 1x10-6 M), and were equilibrated with the SiO2 surface for a minimum of 90 minutes. For intermediate concentrations (ranging from 2x10-6 M to 1x10-5 M), at least three 3 mL injections of the same concentration of peptoid were equilibrated for a minimum of 45 minutes. At high concentrations (at or above 1.5x10-5 M), at least two 2 mL injections of the same concentration of peptoid were equilibrated for a minimum of 30 minutes. We determined experimentally that these equilibration times were sufficient to detect a steadystate SHG response. SHG intensities were corrected for changes in collection efficiency and dayto-day laser fluctuations following an established normalization procedure,19 which is described in the Supporting Information. e.

SHG Theory: Although SHG theory is well established,20-23 we present a brief summary.

A second harmonic photon with frequency 2ω is generated when two photons of frequency ω are spatially and temporally overlapped at an interface. SHG intensity (ISHG) is proportional to the (2) macroscopic second-order susceptibility tensor χ ijk , squared, which can be expressed as the sum

(2) (2) of nonresonant and resonant contributions, χ NR and χ R , respectively.

2

(2) (2) I SHG ∝ χ ijk ∝ χ NR + χ R(2)

2

(1)

(2) is related to N, the number of molecules adsorbed to the interface and the orientational χ ijk

(2) average of the molecules’ corresponding molecular hyperpolarizability tensors, βijk .

(2) (2) χ ijk = N β ijk

(2)


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In the case of resonantly-enhanced SHG, utilized here, the second order susceptibility of the resonant contribution is significantly larger than the nonresonant contribution, and can be considered the dominant contributor to χ

(2)

. Resonance enhancement occurs when the frequency

of the incident ( ω ) or second harmonic ( 2ω ) light approaches the frequency of an inherent electronic transition in an adsorbed peptoid molecule.21

( )

(2) χ ijk ∝N ∑

R

a,b ,c

a µi c a µ j b b µk c

(2hω − E

)(

− iΓ ca hω − Eba − iΓ bc ca

)

(3)

In Equation 3, h is Planck’s constant, µ is the Cartesian coordinate dipole operator, Γ is the transition line width, and the indices i,j, and k represent the output (i) and input (j,k) fields which can assume any of the three Cartesian coordinates (x,y,z).19 Subscripts a, b, and c are the initial, (2) intermediate and final states, respectively. Within the electric dipole approximation,24,25 χ

equals zero when molecules are isotropically distributed within bulk media. However, at the SiO2 interface, where symmetry is necessarily broken, oriented peptoid molecules contribute to

χ f.

(2)

which may result in increased SHG intensities.50 Fluorescence spectroscopy: Fluorescence experiments were conducted using a Horiba

Fluorolog 3 spectrofluorometer equipped with FluorEssence software. Emission spectra were collected from 300-500 nm in 1 nm increments. Excitation wavelength was set to 270 nm. Excitation and emission slit widths were set to 6 nm and 3 nm, respectively. Peptoid solutions used in fluorescence studies were prepared in PBS buffer at 2x10-6 M to 4x10-5 M, and spectra were collected at room temperature. Fluorescence calibration curves were generated by plotting the fluorescence intensities at 337 nm (1) and 392 nm (2) vs. peptoid concentration. Calibration


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curves were generated from the fluorescence spectra collected from three dilute solutions prepared from three separate stock solutions to account for small differences in solution concentrations (Figure S4). Peptoid samples with 500 nm SiO2 microspheres (Polysciences #24323) used to determine surface concentrations of peptoid contained final concentrations of 5 g/L microspheres and 4x10-5 M peptoid with a final volume of 1.000 mL. Samples were prepared in duplicate and shaken at room temperature for 24 hours using a standard laboratory shaker (Thermolyne RotoMix Type 50800). SiO2 microspheres, which contained adsorbed peptoids, were separated from supernatant containing unbound, aqueous peptoids by centrifugation using an Eppendorf 5417C microcentrifuge at 15000 rpm for 20 min (relative centrifugal force = 23900 g). Fluorescence spectra of the supernatant were collected using the parameters above (Figure S5), and supernatant peptoid concentrations were interpolated from the calibration curve. The reduction in peptoid concentration after equilibration with microspheres was determined to be the concentration adsorbed to the microspheres. A control sample was shaken and centrifuged in the absence of microspheres, and fluorescence spectra of this sample confirmed that the experimental procedure did not deplete peptoid from the supernatant. g.

CD spectroscopy: CD spectra were acquired using an OLIS Rapid Scanning

Monochromator (RSM) equipped with a Quantum TC 125 temperature controller. All spectra were collected at room temperature from 200 to 280 nm in 1 nm increments. Data were averaged for 10 seconds at each wavelength. Peptoid solutions (final concentration = 4 x10-5 M) were prepared in 50 mM phosphate buffer (pH 7.4) in the absence of sodium chloride because chloride ions are known to absorb strongly at low UV wavelengths. A circular quartz cell with a pathlength of 1.0 mm was used (Hellma Analytics, #121-1-40). Data were collected in millidegrees. A spectrum of the phosphate buffer solvent (in the absence of peptoid) was


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subtracted from each CD spectrum. CD spectra shown are the average of three different solutions prepared from separate stock solutions to account for differences in solution concentrations. To minimize colloidal scattering of light in the near-UV range, smaller microspheres and lower particle loading were used in CD experiments compared to fluorescence experiments (Table S2). Suspensions containing final concentrations of 2.7 g/L 50 nm SiO2 microspheres (Polysciences #24040) and 4x10-5 M peptoid were prepared by mixing equal volumes (0.500 mL each) of SiO2 microsphere stock solution (5.4 g/L) and aqueous peptoid (8x10-5 M) in plastic microcentrifuge tubes, and peptoid-microsphere suspensions were equilibrated for a minimum of one hour before CD spectra were acquired. Following centrifugation at 15000 rpm for 20 min (relative centrifugal force = 23900 g) and removal of microspheres, CD spectra of the supernatants were collected. h. UV-Vis spectra: Spectra of 6-residue (1) and 15-residue (2) peptoids were collected with a CARY UV-Vis spectrophotometer (Model: 50 BIO). Samples were prepared in PBS buffer pH 7.4 and monitored at room temperature using peptoid concentrations of 4x10-5 M. III. RESULTS AND DISCUSSION a.

UV-Vis spectra: In the UV-Vis spectra of 1 and 2 (Figure 4), multiple bands between 260

and 300 nm are attributed to the Ns1npe substituent. In this investigation, the SHG frequency is resonant with UV transitions of 1 and 2 at 266 nm, which have molar extinction coefficients (ε) in PBS buffer at pH 7.4 of 3200±400 M-1 cm-1 and 13300±600 M-1 cm-1, respectively. This fourfold difference in absorbance at 266 nm is attributed to three additional naphthalene units in 2, which are not found in 1.


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Figure 4. UV-Vis spectra of peptoid 1 (red) and peptoid 2 (blue) in PBS buffer pH 7.4. b.

SHG data fit with Langmuir model: A comparison of the SHG intensities as a function of

aqueous peptoid concentration indicates that SHG intensities from 2 are higher than those measured for 1 at the same aqueous peptoid concentrations (Figure 5a). This difference may be related to the higher intensity of 2 at 266 nm. Following SHG theory described in section II.e., SHG intensity is a function of the number of molecules at the interface squared (N2). In the Langmuir model26 (equation 4), fractional surface coverage (θ) is related to the bulk concentration of analyte (c) at a fixed temperature via an equilibrium binding constant, Ka.

θ=

K ac 1+ K ac

(4)

Following Conboy and coworkers,19 we utilized a simplified form of the Langmuir model (equation 5) to fit the SHG adsorption data (Figure 5a) and obtained the fit coefficients Ka and maxISHG (Table 1).


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(maxI ) K [peptoid] ∝ (1+ K [peptoid]) SHG

2 a

2

2

(5)

a

In equation 5, [peptoid] represents the aqueous concentration of the peptoid analyte, and maxISHG is a fit parameter associated the maximum SHG signal intensity at saturation, which is related to Nmax. Derivation of equation 5 is described in the Supporting Information and assumes the nonresonant SHG signal intensity is negligible compared to the resonant contribution.23 Our assumption is a reasonable one because 1 and 2 exhibit high molar absorptivities at the SHG wavelength used in this study (266 nm).17,19

Table 1. Langmuir fit coefficients for 1 and 2 at varying temperature at pH 7.4 temp max. surface excess Ka (M-1) max ISHG (x1013 molecules/cm2) (°C) 5 18 6.5±1.2 x10 0.108±0.006 2.0±0.9a 6 27 4.4±1.7 x10 0.111±0.009 1 38 5.1±3.5 x105 0.116±0.027 44 1.0±0.3 x107 0.088±0.005 18 3.4±0.8 x106 0.104±0.007 0.5±0.1a 6 27 3.2±1.8 x10 0.098±0.016 2 6 38 3.7±1.8 x10 0.110±0.015 44 1.8±1.3 x107 0.085±0.012 a Fluorescence experiments used to determine max. surface excess were conducted at room temperature. peptoid


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(a)

(b)

Figure 5. Adsorption data for 1 and 2 interacting with SiO2. (a) Adsorption isotherms for 1 (red) and 2 (blue) fit to the simplified Langmuir model. Data were collected at 18 °C. Each data point is the average of 4 separate adsorption experiments. (b) Adsorption data calibrated to maximum surface excess determined in fluorescence experiments.


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Constraints of the Langmuir model: The Langmuir model generally describes single layer

and sub-monolayer coverage and assumes reversible adsorption.27-29 The Langmuir fit for 2 shown in Figure 5a includes only low aqueous concentrations (at or below 1x10-5 M) because at higher concentrations (empty circles), adsorption of 2 deviates from simple Langmuir behavior (Figure S2). At aqueous concentrations above 1x10-5 M, self-association of 2 in aqueous buffer has been shown,16 and we posit that aggregates of 2 adsorb to SiO2 surfaces via a more complex, non-Langmuir adsorption mechanism. At or below concentrations 1x10-5 M at pH 7.4, 2 does not show evidence of self-association and appears to interact with SiO2 in a manner that adheres to a simple Langmuir model. Fitting the SHG data in Figure 5a to a multi-layer model was considered but determined to be unwarranted. First, our data support that peptoids in aqueous concentrations (at or below 4x10-5 M for 1 and at or below 1x10-5 M for 2) do not adsorb to SiO2 in multiple layers. If a second layer of peptoid forms on SiO2 in an opposing orientation, we would expect a decrease in SHG intensities with higher peptoid concentrations,30,31 which was not observed. Second, when peptides adsorbed to SiO2 have been fit using multi-layer models,32,33 concentrations were at least 10-fold higher than those used here. Adsorption of 1 and 2 from aqueous buffer was determined to be partially reversible when more than fifteen rinse cycles of PBS buffer were used to remove peptoids from the SiO2 surface (Figure S3). The Langmuir model has been previously used to describe special cases of partially reversible adsorption, such as the adsorption of the globular protein, lysozyme, to SiO2.34 In this study, and the thermodynamic investigations described below, we assume a quasiequilibrium state exists during the initial adsorption event at low surface coverage.


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d.

Maximum surface excess: Fluorescence spectroscopy was used to evaluate the

concentration of peptoid adsorbed to SiO2 (maximum surface excess, Table 1). Following protocols used previously to indirectly quantify adsorption of bovine proteins to SiO2,35 the concentration of peptoid remaining in the supernatant after sedimentation and removal of SiO2 spheres was determined by comparison of the fluorescence spectrum of the supernatant to the spectra of peptoid solutions at known concentrations. Surface concentrations of 1 and 2 on SiO2 are 2.0±0.9x1013 and 0.5±0.1x1013 molecules/cm2, respectively, and these values were used to generate calibrated SHG adsorption isotherms (Figure 5b). These surface excess values are equivalent to 0.2 peptoid molecules/nm2 for 1 and 0.05 peptoid molecules/nm2 for 2. The surface concentration of a small cationic protein, cytochrome c (1.4x1013 protein molecules/cm2), adsorbed to SiO2 was comparable to the values determined here.36 Because 2 has a molecular weight that is 150% larger than that of 1, more molecules of 1 can fit in the same surface area. Previous nonlinear optical studies of polymer-linked peptoids also indicated that a larger number of hydrophobic substituents contributed to a lower surface concentrations of adsorbed peptoids.15 This trend is consistent here as well; the longer 2 is more hydrophobic than 1,16 despite that both peptoids contain one-third hydrophobic Ns1npe residues. e.

Comparisons to small molecules and proteins: The Langmuir model was previously

applied to describe adsorption to SiO2 surfaces from aqueous solutions of small molecules such as oxytetracycline,37 morantel,38 and 1,1’-bi-2-naphthol.39 This model was also used to describe adsorption of proteins cytochrome c,36 lysozyme,34 fibrinogen,35 and serum albumin40 to SiO2. Equilibrium binding constants (Ka values) determined from adsorption of small molecules and proteins were compared to those determined for 1 and 2 (Table S1). Ka values do not scale with molecular weight or protein size, but vary with charge at pH 7.4. Morantel citrate, cytochrome c,


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2, lysozyme, and 1 exhibit the five highest Ka values (in descending order) shown in Table S1. These five species possess a positive charge under neutral pH conditions while others possess a negative or neutral charge. These data suggest that electrostatic interactions between a +1 charged adsorbate and negatively-charged SiO2 are important for a higher binding affinity. High binding affinities (determined from the Freundlich model) were also observed for hydrophobic, cationic 7-residue peptides adsorbed to SiO2 nanoparticles.32,33 The affinities of proteins adsorbed to SiO2 surfaces in capillary tubes exhibited a similar dependence on isoelectric point.41 Table 2. Free energy (∆

G) and enthalpy (∆

H) of peptoids adsorbed to SiO2.

Ga (kJ/mol) peptoid ∆adsH (kJ/mol) 1 -43± 6 76±31 2 -47±14 43± 6 a ∆ G was determined at 18°C and referenced to molarity of water under standard conditions (55.5 M)

∆


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peptoid 1 (a)

peptoid 2 (b)

Figure 6. Adsorption isotherms for 1 (a) and 2 (b) fit to the Langmuir model at 27 °C (empty grey triangles, dashed line), 38 °C (black squares, solid line) and 44 °C (filled black circles, bold line).


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Free energies and enthalpies of adsorption: SHG studies provide insight not only into

binding affinity and relative surface coverage, but can also provide a thermodynamic description of the binding process. There is an increase in the binding affinities of 1 and 2 to SiO2 with temperature (Figure 6). The Ka values determined at three different temperatures (18 °C, 27 °C, 38 °C, and 44 °C) vary by about 20-fold for 1 and 5-fold for 2 (Table 1). We calculated Gibbs free energies of adsorption (∆adsG0) at standard state from equation 6.

∆ adsG 0 = −RT lnK eq

(6)

In equation 6, R is the ideal gas constant (8.3145 J mol-1 K-1) and T is the temperature (K). To evaluate free energy values under experimental conditions ( ∆ ads G ) reported in Table 2, we

referenced Keq to the molarity of water under standard conditions (55.5 M). We determined

∆ G at 18 °C for adsorption of 1 and 2 are equal to -43±6 and -47±14 kJ/mol, respectively. ads These ∆ ads G values are comparable to free energies of adsorption reported for the small molecule

oxytetracycline (-41 kJ/mol)37 and cytochrome c protein (-49 kJ/mol)36 adsorbed to SiO2. Generally, ∆ ads G values of this magnitude are attributed to a combination of hydrogen-bonding

interactions (10-20 kJ/mol) and hydrophobic interactions (2-8 kJ/mol per methylene or methyl M group).42,43 From ∆ ads G , we determined molar equilibrium constants ( K eq ). The slope of the

van’t Hoff plot ( lnK eqM versus 1/T, Figure 7) was used to obtain the enthalpy of adsorption (∆adsH)

(Table 2) following the Gibbs-Helmholtz relationship.

∂ln K eqM

()

∂

1 T

=−

∆ ads H R

(7)

P


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The enthalpic contributions to free energy are positive values (76±31 kJ/mol and 43±6 kJ/mol for 1 and 2, respectively). These ∆adsH values indicate that adsorption to SiO2 from aqueous peptoid solutions is an endothermic process. Our molecular description of the adsorption of peptoids to SiO2 builds upon extensive studies of protein-SiO2 interactions, which suggest three processes that might individually or collectively contribute to adsorption of a water-soluble peptoid to hydrophilic SiO2:44,45 I.

structural rearrangement of the peptoid upon adsorption,46,47

II.

release of water from the SiO2 surface,48

III.

re-distribution of charged groups at the liquid/solid interface.

Like our peptoids, adsorption of some globular proteins to SiO2 is also an endothermic process.48,49 To determine the origin of entropic effects on adsorption—either from conformational changes or from the release of water from the SiO2 surface—we used CD spectroscopy.


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Figure 7. Van’t Hoff plot for 1 (red) and 2 (blue) binding to SiO2. Lines are the linear fits.


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peptoid 1 (a)

(b)

peptoid 2

Figure 8. CD spectra of peptoid 1 (a) in the absence (red solid) and presence (black dotted) of SiO2 microspheres and 2 (b) in the absence (blue solid) and presence (black dashed) of SiO2 microspheres. CD spectra of supernatant solutions after removal of the particles (grey solid lines). All peptoids were 4x10-5 M solutions in 50 mM phosphate buffer pH 7.4.


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CD spectra suggest no conformational change upon adsorption: Using CD spectroscopy,

we monitored for changes to peptoid conformation upon adsorption to the SiO2 surface. CD spectra of 1 and 2 acquired in the absence of SiO2 microspheres (Figure 8) match previously published CD spectra of these peptoids dissolved in 5 mM Tris buffer pH 7.5.16 In the current studies, CD spectra of 1 and 2 in the presence and absence of nonporous SiO2 microspheres indicate that major spectral features are preserved for each of the two sequences. A small decrease in the absolute intensity of the CD spectral maximum at 231 nm for peptoid 2 was observed (Figure 8b). However, the intensity and spectral features of the CD spectrum of 2 in the presence of 50 nm microspheres (black dashed) were the same as those observed for the supernatant solution (grey solid). Thus, this change in magnitude can be attributed to a decrease in solution-phase peptoid concentration upon adsorption to SiO2. Based on the similarities of peptoid CD spectra in the presence and absence of SiO2 microspheres, we suggest that adsorption to SiO2 does not effect significant changes in secondary structures. In addition to quantifying relative surface coverage and binding affinities, SHG data reveal details about molecular ordering at the aqueous peptoid/SiO2 interface. Due to the coherent nature of SHG, only peptoids adsorbed at the interface with an oriented net dipole will contribute to SHG signal.50 Increases in SHG intensities may be attributed not only to increased number of adsorbed peptoids, but also to changes in conformation as surface concentration increases.20,30,51 However, because CD spectra indicate modest changes in conformation upon adsorption to SiO2, changes in SHG intensity are attributed only to increased number of adsorbed peptoids.


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(a)

(b)

peptoid 1

peptoid 2

Figure 9. Adsorption isotherm data for 1 (a) and 2 (b) at pH 9.0 (black open squares), pH 8.2 (tan open circles), pH 6.2 (green filled triangles), and pH 5.0 (red open triangles) with fits to the Langmuir model shown as solid lines.


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h.

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Impact of pH on adsorption: To confirm whether adsorption is affected by electrostatic

interactions, we varied the pH of the PBS buffer while maintaining total ionic strength equal to 215 mM. We plotted SHG intensity versus bulk peptoid concentration for 1 (Figure 9a) and 2 (Figure 9b) at pH 5.0, 6.2, 8.2 and 9.0. These data were fit to the Langmuir model and the resulting fit coefficients (Ka values and maxISHG) are shown in Table 3. Within this pH range, the SiO2 surface maintains an overall negative charge,52 but more deprotonated surface hydroxyl sites are present at pH 8.2 and pH 9.0. For this reason, it is not surprising that the relative surface coverage, indicated by maxISHG, increases with pH for both peptoids. Binding affinities and maxISHG values increased with pH for both 1 and 2. Notably, the Langmuir model provides an excellent fit for all data for 2 at or below 4x10-5 M at all pH conditions shown in Figure 9b. These results suggest that pH may alter the mechanism by which 2 adsorbs to SiO2 at high concentrations. The pH-dependence of the Ka values reported for 1 and 2 supports the hypothesis that electrostatic interactions contribute to the adsorption of aqueous peptoid to the SiO2 interface. Future studies will build upon the current work to study adsorption at a wider range of pH and ionic strength conditions. Table 3. Langmuir fit coefficients for 1 and 2 at varying pH. All experiments were conducted at room temperature. peptoid 1

2

pH 5.0 6.2 8.2 9.0 5.0 6.2 8.2 9.0

Ka (M-1) 0.8±0.2 0.9±0.1 1.0±0.1 1.7±0.6 1.8±0.9 3.2±1.6 7.4±0.9 5.1±0.7

x106 x106 x106 x106 x105 x105 x105 x105

max ISHG 0.015±0.001 0.047±0.004 0.128±0.007 0.117±0.010 0.032±0.008 0.118±0.025 0.195±0.007 0.246±0.011


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IV. CONCLUSIONS To summarize, we have determined Ka values and maximum surface excess for two peptoid sequences interacting with SiO2 surfaces under steady-state quasi-equilibrium conditions. The data and analyses described in these studies are a first step towards fully describing features of peptoids at the aqueous liquid/ SiO2 interface. Sequence length and overall charge modulate surface concentrations and binding affinities of water-soluble peptoids. Both 1 and 2 adsorb to SiO2 with Ka values comparable to those observed for cationic small molecules and globular proteins. Evidence of significant SiO2-induced peptoid structural re-arrangement in CD spectra is not observed. The current studies provide the first thermodynamic description of how a simple water-soluble peptoid binds to SiO2. These data will inform future studies of peptoids with varied sequence features to investigate the details of this interaction, including the release of water from the SiO2 surface upon peptoid adsorption. ASSOCIATED CONTENT Detailed SHG experimental procedures, a table comparing binding constants of proteins, small molecules and peptoids, fluorescence spectra, simplified Langmuir equation derivation, desorption data, and Dynamic Light Scattering (DLS) characterization of the 50 nm and 500 nm nonporous SiO2 microspheres are found in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org/. AUTHOR INFORMATION Corresponding Author *Email: [email protected]


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources GYS gratefully acknowledges startup funds from the College of Arts and Sciences at Santa Clara University. AAF acknowledges support from a CAREER Award (NSF CHE1056520). GYS and AAF acknowledge funds from Clare Boothe Luce (CBL) professor awards. ALC and JY were supported by CBL research scholar awards administered by Santa Clara University. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT G.Y.S. thanks Gary Sloan for construction of the flow cells and optical mounts used in the SHG setup and Dr. Linda Brunauer for the donation of the Lauda immersion thermostat used in this study. The authors also thank Amanda D. Fearon for assistance with buffer preparation and occasional data collection. REFERENCES (1) Brubaker, C. E.; Messersmith, P. B. The Present and Future of Biologically Inspired Adhesive Interfaces and Materials. Langmuir 2012, 28, 2200-2205. (2) Statz, A. R.; Barron, A. E.; Messersmith, P. B. Protein, cell and bacterial fouling resistance of polypeptoid-modified surfaces: effect of side-chain chemistry. Soft Matter 2008, 4, 131-139. (3) van Zoelen, W.; Buss, H. G.; Ellebracht, N. C.; Lynd, N. A.; Fischer, D. A.; Finlay, J.; Hill, S.; Callow, M. E.; Callow, J. A.; Kramer, E. J.; Zuckermann, R. N.; Segalman, R. A. Sequence of Hydrophobic and Hydrophilic Residues in Amphiphilic Polymer Coatings Affects Surface Structure and Marine Antifouling/Fouling Release Properties. ACS Macro Letters 2014, 3, 364368. (4) Lau, K. H. A. Peptoids for biomaterials science. Biomaterials Science 2014, 2, 627-633.


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TOC Graphic


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A Thermodynamic Description of the Adsorption of Simple Water

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