Adsorption of Amino Acids and Dipeptides to the Hydrophobic

Article pubs.acs.org/JPCC

Adsorption of Amino Acids and Dipeptides to the Hydrophobic Polystyrene Interface Studied by SFG and QCM: The Special Case of Phenylalanine Robert M. Onorato, Alfred P. Yoon, James T. Lin, and Gabor A. Somorjai* Department of Chemistry, University of California, Berkeley, Berkeley, California 94720, United States Materials Science Division, Lawrence Berkeley National Laboratories, Berkeley, California 94720, United States S Supporting Information *

ABSTRACT: Aqueous solutions of the amino acids Lphenylalanine, L-lysine, and L-glycine and the homo- and heterodipeptides comprising these amino acids were studied by vibrational sum frequency generation (SFG) and quartz crystal microbalance (QCM) at the hydrophobic polystyrene interface. The phenyl ring of the phenylalanine side chain was determined to adsorb preferentially in a nearly flat geometry relative to the hydrophobic surface based on the concentration dependence of the SFG spectra and symmetry arguments. The amount of adsorbed dipeptide follows a hydrophobic series at concentrations well below monolayer formation, as determined by QCM. However, at higher concentrations, adsorbate− adsorbate interactions play a significant role in the adsorption, and adsorption no longer follows a hydrophobic series. These changes in the quantitative adsorption from QCM correlate with changes in the SFG spectra for phenylalanine, lysylphenylalanine, and glycyl-phenylalanine, but not for lysyl-lysine, which shows the most striking adsorbate interaction effect.

1. INTRODUCTION Understanding the behavior of biomolecules at solid-aqueous interfaces is one of the important problems in biointerface science. These interfacial systems include protein−protein interactions and interactions at lipid bilayers. Practically, the field of biointerfaces is integral to many medical technologies, such as synthetic implants, drug delivery, and biosensor development.1−6 Sum frequency generation (SFG) is well-suited to study molecules at the solid−liquid interface. It is a vibrational spectroscopy technique that is inherently surface-specific due to symmetry considerations.7 SFG is sensitive to both the average orientation and the amount of adsorbed species. For this reason, it is useful to pair SFG analysis with quartz crystal microbalance (QCM) experiments, which provide a quantitative measure of the amount of adsorbed species. Still, the vibrational spectra of even small proteins are so complex that a bottom-up approach, starting with amino acids and small peptides, is a necessary step for understanding biomolecule adsorption. SFG of the CH vibrational region was acquired for several amino acid monolayers formed at the liquid−liquid interface between water and hydrophobic CCl4.8 SFG has been used to probe amino acids at the solid−aqueous interface with both hydrophilic (silica) and hydrophobic (deuterated polystyrene (PS)) surfaces.9 Another study looked at amino acid adsorption to the hydrophilic TiO2 interface.10 In general, SFG spectra of the CH region show direct evidence of adsorbed amino acids at © 2012 American Chemical Society

the hydrophobic interface. At the hydrophilic interface, there is only indirect evidence, a change in the water OH spectrum, except in the case of the hydrophilic amino acids aspartic acid and glutamic acid. Detailed SFG studies considering the orientation of both phenylalanine and leucine adsorbed on PS have concluded that these hydrophobic side chains dominate the interactions with PS.11,12 One model peptide that has been extensively studied is the LK14 peptide, comprising leucine and lysine units in a sequence that yields an alpha helical structure with lysine side chains oriented on one side of the rod-like structure and leucine side chains on the other. SFG and other techniques have confirmed that the hydrophobic leucine side chains orient toward hydrophobic surfaces, whereas the hydrophilic lysine side chains orient toward hydrophilic surfaces.13−15 Further studies have focused on the effects of changing the ionic strength,16 changing the identity of the hydrophilic amino acid used and shortening the chain length,17 the orientation of individual lysine side chains through isotopic labeling,18 and side-chain dynamics.19 There remains much to uncover in the emerging picture of small biomolecule adsorption. In this Article, we begin to bridge the gap between single amino acid studies and the highly structured LK14 peptide by studying both the homo- and Received: November 11, 2011 Revised: April 19, 2012 Published: April 23, 2012 9947

dx.doi.org/10.1021/jp210879p | J. Phys. Chem. C 2012, 116, 9947−9954

The Journal of Physical Chemistry C

Article

orientation average of each molecular vibrational mode. The infrared laser frequency is ωIR, and Aq, ωq, and Γq represent the oscillator strength, frequency, and damping constant of the qth vibrational mode. Because of symmetry conditions of secondorder nonlinear processes, SFG is only generated where inversion symmetry is broken on both a macroscopic and microscopic level. The requirement for noncentrosymmetric symmetry on the macroscopic level results in SFG being a surface-specific spectroscopy. The microscopic symmetry requirement dictates that strongly oriented systems will have a larger SFG response than disordered systems. SFG experiments were performed with a Continuum (Santa Clara, CA) Leopard D20 Nd:YAG laser (1064 nm output, 20 ps pulsewidth, 20 Hz repetition rate, ∼20 mJ pulse energy) pumping a Laservision (Bellevue, WA) optical parametric generation (OPG)/optical parametric amplification (OPA) system. The OPG/OPA system provides a visible beam at 532 nm and a tunable infrared beam that was scanned from 2800 to 3200 cm−1 in this study. The pulse energies of each beam are approximately 150 μJ before entering the prism. The visible and infrared beams are incident on the sample at angles of 65 and 42°, respectively, relative to the surface normal. These angles were chosen to maximize the SFG signal while minimizing variations in the signal due to the frequencydependent nature of the refractive index of water.20 The SFG signal was detected by a photomultiplier tube. All SFG spectra presented represent the average of 1000 to 2000 laser shots per 5 cm−1 frequency step, unless otherwise noted. In all spectra each point in the spectrum is boxcar averaged with its adjacent points to decrease noise. The SFG signal generated in the CH vibrational region by amino acids and small peptides at the aqueous-solid interface is generally weak, and also weak in comparison with the OH features of water and the CH features of the PS surface. To generate and detect the desired adsorbate SFG spectrum near total internal reflection (TIR), we must first combine SFG geometry with the use of deuterated water as the solvent and deuterated PS as the interface. The near-TIR geometry, using the angles for the visible and IR beams mentioned above, enhances the overall signal generation and is discussed in detail elsewhere.20 Deuteration of both the solvent and the polymer surface serve to shift the respective vibrational features out of the CH vibrational region (2800−3100 cm−1). 2.2. Quartz Crystal Microbalance. A detailed description of methods used for QCM experiments has previously been provided,21 and a brief description is given here. A Q-Sense D300 instrument was used with gold-coated quartz sensor crystals, which were spin-coated with PS as described in the sample preparation section. The Sauerbrey equation, Δm = − C(Δf/n), was used to convert the frequency change, Δf, to the mass adsorbed, Δm. This linear relationship between the frequency change and adsorbed mass assumes a rigid film. The mass sensitivity constant, C, for the sensor crystals used is 17.7 ng cm−2 Hz−1, and the third harmonic, n, of the resonance frequency was used in the Sauerbrey analysis. Because there is a wide range of molecular weights for the species studied, it is instructive to consider the molar distribution ratio

heterodipeptides of phenylalanine, glycine, and lysine (see Figure 1) at the hydrophobic PS interface. These three amino

Figure 1. Structures and abbreviations of the amino acids and dipeptides studied.

acids were selected due to the varied hydrophobic ranks of their side chains: phenylalanine is one of the more hydrophobic amino acids, glycine is relatively neutral, and lysine is at the hydrophilic end of the scale. Phenylalanine was selected instead of other hydrophobic options, such as leucine, because the aromatic CH vibration is clearly distinct from the methylene vibrational modes of the glycine and lysine side chains.

2. EXPERIMENTAL SECTION 2.1. Sum Frequency Generation. A thorough explanation of SFG and our experimental setup in particular can be found elsewhere.9,13,20 In brief, visible light and infrared light are mixed at the sample, generating the SFG signal at the sum of the frequencies of the visible and infrared beams. When the infrared frequency is equal to that of a vibrational mode of the sample, the SFG signal is resonantly enhanced according to eq 1 ISFG ∝ |χijk(2) |2 2 (2) = χNR +

∑ q

Aq ωIR − ωq − i Γq

orientation

(1)

D=

χ(2) ijk

Here is the second-order nonlinear susceptibility, and i, j, and k represent the polarization of the SFG, visible, and infrared beams, respectively. χ(2) NR is the nonresonant SFG response, and the resonant response is the ensemble

adsorbed moles (mol/cm 2) bulk concentration (mol/mL)

(2)

which quantifies the distribution of species between the bulk and the surface. This is defined in a similar manner to a 9948



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distribution ratio between two phases or between two immiscible liquids. A change in D with concentration is due to intermolecular interactions between adsorbates for the systems studied herein. However, at concentrations higher than those studied here, a change in the solvation of the solute due to high solute to solvent ratio could also cause a change in D. 2.3. Sample Preparation. The sample substrates for SFG experiments were prepared on fused silica equilateral prisms, which were cleaned by soaking in a solution of Nochromix (Godax Laboratories) in concentrated sulfuric acid for at least 1 h. After being washed with distilled deionized water, the prisms were cleaned using oxygen plasma (Herrick Plasma, 18 W RF, 150 mtorr O2) for 1 min to remove any residual organic content and to maximize the oxidation of the SiO2 surface. Deuterated PS (DPS) (Polymer Source, MW = 300 000) and D2O (Aldrich 99.9% D) were used to shift the vibrational modes of the substrate and solvent to lower frequencies so that there would be no interference with the sample vibrational modes. A thin DPS film was prepared by spin coating a 3% weight solution of DPS in toluene (3000 rpm, 40 s) and annealing at 120 °C for 12 h. An SFG study of the surface morphology of the DPS/water interface with a similar DPS film preparation indicates that the phenyl ring lies at an angle near 70° relative to the surface normal.22 The sample substrates for QCM experiments were prepared on gold-coated QCM sensors. The QCM sensors were exposed to oxygen plasma for 30 s, and the PS film was prepared by spin coating a 2% weight solution of PS in toluene (3000 rpm, 40 s) and annealing at 120 °C for 12 h. The solvent used in QCM experiments is 18.2 MΩ water (Millipore Milli-Q). All amino acids and dipeptides are the L-enantiomer and were purchased from Sigma-Aldrich and used without further purification.

3. RESULTS AND DISCUSSION 3.1. SFG Results. In general, the amino acid and dipeptide SFG spectra comprise three aliphatic CH modes (2880, 2940, and 2980 cm−1) and an aromatic CH feature (3065 cm−1). The aliphatic modes are the CH2 symmetric stretch, the CH stretch, or a CH2 Fermi resonance and the CH2 asymmetric stretch in order of increasing frequency.9,11,23 The aromatic feature, which is only present in SFG spectra of the phenylalanine containing species, is composed of five vibrational modes that have frequencies ranging from 3024 to 3084 cm−1.24 The main peak at 3065 cm−1 is the symmetric v2 stretching mode. Figure 2 compares the SFG spectra of the phenylalanine amino acid for the ssp, ppp, and sps polarization combinations. Figure 3 contains the ssp SFG spectra for the lysine and glycine amino acids. Figures 4 and 5 are the ssp SFG spectra of the homo- and heterodipeptides, respectively. The SFG spectra of the phenylalanine-containing species studied here indicate that the phenyl ring of the phenylalanine side chain preferentially adsorbs flat or nearly flat in the plane of the hydrophobic DPS surface. This conclusion is based on the concentration dependence of the phenylalanine SFG spectra and a symmetry argument. An ensemble of phenyl rings in the plane of the surface exhibits inversion symmetry, and because the aromatic stretching vibrational modes occur in the plane of the ring, they will also possess inversion symmetry. In this idealized case there would be no SFG detected for these vibrational modes because they would not break inversion symmetry. Furthermore, the ssp polarization combination probes vibrational modes that are perpendicular to the interface. Therefore if the aromatic ring is parallel with the

Figure 2. SFG spectra showing the concentration dependence of phenylalanine for the (a) ssp, (b) ppp, and (c) sps polarization combinations. The peaks at 2880 and 2940 cm−1 are aliphatic CH peaks, whereas the peak at 3065 cm−1 is an aromatic CH peak. The sps spectra are not boxcar-averaged.

interface, these vibrational modes will not be SFG active under ssp conditions. A comparison of the SFG spectra of F1 (Figure 9949



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Figure 3. SFG spectra showing the concentration dependence of (a) lysine and (b) glycine in D2O at the DPS interface. The peaks at 2880 and 2940 cm−1 are aliphatic CH peaks.

2), F2 (Figure 4a), KF (Figure 5a), and GF (Figure 5b) shows that the phenylalanine side chain preferentially adsorbs nearly in the plane of the interface. The F1 SFG spectra (Figure 2) show that the phenyl ring is in the plane of the surface at low concentrations, but at higher concentrations the phenyl ring adsorbs with a different geometry. The ssp polarization probes χ(2) yyz with an IR transition moment oriented along surface normal (z axis). The ssp aromatic CH feature exhibits a positive peak at 3065 cm−1, the v2 symmetric stretching mode, and a mode that destructively interferes with the nonresonant background at 3035 cm−1 (see Figure 2a). Focusing on the intensity of the v2 mode, there is a relatively slow increase in intensity from a barely identifiable peak at 0.5 to 1 to 2.5 mg/mL, followed by a dramatic increase at 5 mg/mL. The relatively weak SFG intensity at concentrations of 2.5 mg/mL and less is indicative of the phenyl ring adsorbing nearly in the plane of the surface. This orientation is only weakly SFG active because the ssp polarization combination probes vibrations oriented perpen-

Figure 4. SFG spectra showing the concentration dependence of (a) phenylalanyl-phenylalanine, (b) lysyl-lysine, and (c) glycyl-glycine in D2O at the DPS interface. The peaks at 2880 and 2940 cm−1 are aliphatic CH peaks, whereas the peak at 3065 cm−1 in panel a is an aromatic CH peak.

dicular to the surface. The sudden increase in the aromatic CH feature at 5 mg/mL, however, is attributed to either a second 9950



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The ppp spectra show a similar trend to the ssp spectra but with some differences (Figure 2b). The ppp SFG spectrum (2) (2) (2) represents a linear combination of χ(2) zzz, χyyz , χzyy , and χyzy . The v2 mode intensity is greater in the ppp spectra than in the ssp spectra at concentrations of 0.5 and 1 mg/mL. However, there is still a large increase in intensity of this feature at 5 mg/mL, indicative of an orientation change. The aliphatic peak intensities are relatively weak at 2.5 mg/mL as in the ssp spectrum, indicating a weak net orientation of these groups at this concentration. The asymmetric CH2 feature at 2980 cm−1, however, is generally more prominent in the ppp spectra than the ssp spectra. The sps spectra are characterized by a weak SFG response, and it is difficult to assign any peaks (Figure 2c). The correlation of the concentration dependence of the phenylalanine QCM data with the SFG data lends further support to an orientation change at higher concentrations. There is an increase in the distribution ratio (see Figure 6a) for phenylalanine from 2.5 to 5 mg/mL, which indicates interactions between adsorbed phenylalanine molecules playing a significant role in the adsorption at the higher concentration. The argument against the interpretation that the phenyl ring is in the plane of the interface is that the phenyl ring could simply be disordered at the surface. The F2 SFG spectra

Figure 5. SFG spectra showing the concentration dependence of (a) lysyl-phenylalanine and (b) glycyl-phenylalanine in D2O at the DPS interface. The peaks at 2880 and 2940 cm−1 are aliphatic CH peaks, whereas the peak at 3065 cm−1 in panel a is an aromatic CH peak.

adsorption site where the phenyl ring does not lie flat or a reorientation of the adsorbed surface species. It is also notable that the aliphatic CH features increase from 0.5 to 1 mg/mL, decrease from 1 to 2.5 mg/mL, and increase again from 2.5 to 5 mg/mL. (The asymmetric CH stretch is weak at all concentrations.) The initial increase is an effect of increased adsorption. At 2.5 mg/mL, the nature of adsorption has begun to change, and the weaker aliphatic CH signal can be explained by a weaker net orientation or destructive interference that occurs between aliphatic CH groups that generate SFG with opposing phases. At 5 mg/mL, the aliphatic CH groups again exhibit a strong ensemble average orientation.

Figure 6. Concentration dependence of the molar distribution ratio for (a) the amino acids and (b) the homo- and heterodipetides at the PS interface as determined by QCM. 9951



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have found to be a necessary step to attain repeatable results. However, they similarly concluded that the weakness of the aromatic modes resulted from the phenyl ring being close to parallel with the surface and speculated that it may be intercalated with the PS phenyl rings. The SFG spectra of the other amino acids and dipeptides provide less information about the nature of adsorption. The SFG spectra for both K1 (Figure 3a) and K2 (Figure 4b) show little concentration dependence and are quite similar to one another. On the basis of the weak SFG response in the G2 spectra (Figure 4c), G2 is either not strongly oriented or the aliphatic groups are oriented in opposite directions on the hydrophobic surface. At both 0.8 and 2.5 mg/mL concentrations, it is even difficult to discern distinct aliphatic CH peaks. In contrast, G1 does clearly show distinct aliphatic CH peaks (see Figure 3c), although the intensity of these peaks is quite weak. The effect of adding a sodium chloride was also investigated for F1, K1, and G1 (see Supporting Information). In the case of F1, there was no change in the SFG spectrum; however, the spectra of K1 and G1 changed upon the addition of the salt. 3.2. QCM Results. Dipeptide adsorption appears to follow a hydrophobic series at the lowest concentration studied by QCM, 0.8 mg/mL. According to both the adsorbed mass and the molar distribution ratio (see Table 1 and Figure 6b), the most hydrophobic dipeptide F2 (phenylalyl-phenylalanine) adsorbs the most, followed by GF (glycyl-phenylalanine), KF (lysyl-phenylalanine), G2 (glycyl-glycine), and the most hydrophilic dipeptide K2 (lysyl-lysine). However, at higher concentrations, adsorption becomes more complicated due to intermolecular interactions between adsorbates. Despite being the most hydrophilic dipeptide, K2 has the highest adsorbed mass at the intermediate concentration, 2.5 mg/mL. The adsorbed masses of GF, KF, and G2 all increase proportionally with the bulk concentration from 0.8 to 2.5 mg/ mL, but K2 adsorption increases by an order of magnitude (F2 is not soluble at this concentration). The molar distribution ratio provides a clear visualization of this in Figure 6b. In the cases of GF, KF, and G2, the change in D from 0.8 to 2.5 mg/ mL is less than the uncertainty of the measurements, indicating that the nature of adsorption is primarily unchanged and that there are few interactions between the adsorbed dipeptides. In contrast, K2 appears to have intermolecular interactions that stabilize surface adsorption, as evidenced by a significant increase in the molar distribution ratio. There is no clear signature of this change in the K2 SFG spectra. In fact the SFG spectra at all three concentrations are quite similar (see Figure 4b). Considering the spectra in light of the QCM data, it appears there is a change in net orientation as surface concentration increases. Assuming no net orientation change, the SFG peak intensity is expected to increase with adsorbed mass. However, it is difficult to make any further correlations between these spectra and the apparently significant change in the nature of surface adsorption indicated by the QCM data. At the highest concentration, 5 mg/mL, the other dipeptides GF, KF, and G2 exhibit signs of stabilizing intermolecular interactions. The molar distribution ratio increases by a similar amount for all three dipeptides, but the increase is much less than that of K2 between 0.8 and 2.5 mg/mL. The onset of intermolecular interactions at these surface concentrations is expected. Table 2 shows the average area per adsorbed molecule for all species and concentrations studied, which at

demonstrate that the disordered interpretation is unlikely. (See Figure 4a.) Even at the low concentration of 0.2 mg/mL there is a fairly strong aromatic CH signal in the F2 SFG spectrum. This is the only phenylalanine-containing species studied herein with a strong aromatic peak at low concentration. It appears that for conformational reasons both F2 phenyl rings do not adsorb in the plane of the interface simultaneously. The strong signal even at low concentrations indicates that the phenyl ring that is not in the plane of the surface on average has a nonzero net orientation. The SFG spectra of KF and GF further support the conclusion that the phenylalanine ring adsorbs in the plane of the surface due its their weak aromatic CH features. In the case of KF (see Figure 5a), there is a weak aromatic CH feature at all concentrations. Meanwhile, the aliphatic CH modes double in peak height from 0.8 to 2.5 mg/mL and at 5 mg/mL are the most intense SFG features in this study. A new peak also appears at 2915 cm−1; this peak is likely a methylene mode that arises due to a change in the lysine side chain geometry due to adsorbate−adsorbate interactions. The intensity of the aliphatic CH features and the appearance of a new peak indicate that KF has a strong ensemble orientation, but the relatively weak aromatic feature illustrates the nearly flat orientation of the phenyl ring. The molar distribution ratio for KF also increases between 2.5 and 5 mg/mL, suggesting that adsorbate− adsorbate interactions are present at the 5 mg/mL concentration. At the lowest concentration, GF does not appear to have an aromatic CH feature. (See Figure 5b.) On the basis of the QCM results (see Table 1), GF does clearly adsorb at this Table 1. Adsorbed Mass Determined by QCM and the Sauerbrey Equation concentration

material F1 G1 K1 F2 GF KF G2 K2

0.8 mg/mL

2.5 mg/mL

5 mg/mL

adsorbed mass

adsorbed mass

adsorbed mass

ng/cm2

19.2 7.3 7.0 5.7 2.3

±

0.6 0.5 1.2 1.8 0.8

ng/cm2

±

ng/cm2

±

21 16 19

2 5 3

83 31 41

3 4 8

21 22 15 33

1 3 3 3

58 65 45 64

3 6 4 4

concentration, and there are weak features for the aliphatic CH modes. The relative weakness of the aromatic feature serves as further evidence that its preferential orientation is in the plane of the DPS interface. At 2.5 mg/mL, there is a weak aromatic CH peak, and at 5 mg/mL there is a relatively strong peak. This is likely due to intermolecular interactions, which are predicted from the distribution ratio. The SFG spectra of F1, F2, KF, and GF provide a compelling argument that the phenyl ring of the phenylalanine side chain adsorbs to the hydrophobic interface in the plane of the surface. Hore et al. also studied phenylalanine via SFG under similar experimental conditions (SSP polarization, DPS interface, 6.28 mg/mL in D2O) but did not observe the aromatic CH peak with nearly the same intensity, as seen here at 5 mg/mL.11 We believe that their different result stems from the lack of an annealing step in the DPS spin-coating procedure, which we 9952



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the PS interface follows a hydrophobic series. However, at higher concentrations interactions between amino acids and between dipeptides have a large effect on the quantity of adsorbed species. In the cases of F1, KF, and GF, the onset of intermolecular interactions in the QCM data corresponds to changes in the SFG spectra. While not corroborated by the SFG spectra, there appear to be strong adsorbate−adsorbate interactions for the most hydrophilic species studied, K2, which may agglomerate at the water/PS interface.

Table 2. Average Area Per Molecule Adsorbed concentration

0.8 mg/mL

2.5 mg/mL

5 mg/mL

material

Å2/molecule

Å2/molecule

Å2/molecule

130 80 130

33 41 59

170 220 150 140

64 75 48 72

F1 G1 K1 F2 GF KF G2 K2

270 510 700 380 2000

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ASSOCIATED CONTENT

S Supporting Information *

Brief discussion, including SFG spectra, on the effect of salt on amino acid adsorption to the polystyrene interface for phenylalanine, lysine, and glycine. This material is available free of charge via the Internet at http://pubs.acs.org.

5 mg/mL is similar in magnitude to the molecular area of a single dipeptide. It is interesting that D increases in each case at higher concentrations because typical Langmuir adsorption would appear as a decrease in D. It is also not clear why the onset of intermolecular interactions occurs at a lower concentration for K2 than for the other dipeptides or why the effect is much stronger. It is possible that this is an indication of K2 agglomeration. Interestingly, the amino acids G1 and K1 do not show any intermolecular interaction effect. The molar distribution ratio for these two species does not change between the two concentrations studied. F1 does show an apparent stabilization of adsorbed species at the higher concentration. This concentration dependence for F1 observed with QCM correlates with the SFG concentration dependence. It is an interesting question whether the relatively strong adsorption of phenylalanine-containing species to the PS interface (most notably F2 0.8 mg/mL and F1 5 mg/mL) is driven by the hydrophobicity or a more specific interaction such as π-stacking of the aromatic rings of the surface and adsorbate. It is undoubtedly a combination of both hydrophobic effects and intermolecular interactions, but the relative weight of each contribution is difficult to determine. The ordering of dipeptide absorption at low concentrations according to the hydrophobic series suggests that in general hydrophobicity is very important. It is interesting to note that the tilt angle of the PS phenyl ring at the DPS−water interface is ∼70° relative to the surface normal.22 Considering this and the orientation of the phenylalanine phenyl ring described here, it is likely that there is a π-stacking geometry between the phenylalanine side chain and PS.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, of the U.S. Department of Energy under contract no. DE-AC02-05CH11231.

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REFERENCES

(1) Yoshida, M.; Langer, R.; Lendlein, A.; Lahann, J. Polym. Rev. 2006, 46, 347. (2) Adhikari, B.; Majumdar, S. Prog. Polym. Sci. 2004, 29, 699. (3) Van Butsele, K.; Jérôme, R.; Jérôme, C. Polymer 2007, 48, 7431. (4) Rusmini, F.; Zhong, Z.; Feijen, J. Biomacromolecules 2007, 8, 1775. (5) Ratner, B. D.; Bryant, S. J. Annu. Rev. Biomed. Eng. 2004, 6, 41. (6) Healy, K. E. Curr. Opin. Solid State Mater. Sci. 1999, 4, 381. (7) Shen, Y. R. The Principles of Nonlinear Optics; Wiley-Interscience: New York, 1984. (8) Watry, M. R.; Richmond, G. L. J. Phys. Chem. B 2002, 106, 12517. (9) Holinga, G. J.; York, R. L.; Onorato, R. M.; Thompson, C. M.; Webb, N. E.; Yoon, A. P.; Somorjai, G. A. J. Am. Chem. Soc. 2011, 133, 6243. (10) Paszti, Z.; Guczi, L. Vib. Spectrosc. 2009, 50, 48. (11) Hall, S. A.; Hickey, A. D.; Hore, D. K. J. Phys. Chem. C 2010, 114, 9748. (12) Hall, S. A.; Jena, K. C.; Trudeau, T. G.; Hore, D. K. J. Phys. Chem. C 2011, 115, 11216. (13) Mermut, O.; Phillips, D. C.; York, R. L.; McCrea, K. R.; Ward, R. S.; Somorjai, G. A. J. Am. Chem. Soc. 2006, 128, 3598. (14) Weidner, T.; Apte, J. S.; Gamble, L. J.; Castner, D. G. Langmuir 2009, 26, 3433. (15) Long, J. R.; Oyler, N.; Drobny, G. P.; Stayton, P. S. J. Am. Chem. Soc. 2002, 124, 6297. (16) York, R. L.; Mermut, O.; Phillips, D. C.; McCrea, K. R.; Ward, R. S.; Somorjai, G. A. J. Phys. Chem. C 2007, 111, 8866. (17) Phillips, D. C.; York, R. L.; Mermut, O.; McCrea, K. R.; Ward, R. S.; Somorjai, G. A. J. Phys. Chem. C 2006, 111, 255. (18) Weidner, T.; Breen, N. F.; Drobny, G. P.; Castner, D. G. J. Phys. Chem. B 2009, 113, 15423. (19) Weidner, T.; Breen, N. F.; Li, K.; Drobny, G. P.; Castner, D. G. Proc. Natl. Acad. Sci. U. S. A. 2010, 107, 13288. (20) York, R. L.; Li, Y.; Holinga, G. J.; Somorjai, G. A. J. Phys. Chem. A 2009, 113, 2768.

4. CONCLUSIONS We have studied the adsorption of aqueous amino acids, homodipeptides, and heterodipeptides of phenylalanine, lysine, and glycine to the hydrophobic DPS interface using SFG and QCM. On the basis of the strength and concentration dependence of the aromatic CH feature of phenylalanine species, we conclude that the phenyl ring adsorbs nearly flat with respect to the hydrophobic surface when not constrained by other adsorbates. Hydrophobic effects are undoubtedly an important factor in adsorption of amino acids and dipeptides; however, it is not clear from this work whether the adsorption of species with phenyl ring containing side chains to PS is dominated by the hydrophobic nature of this group or a particular interaction of the side chain with PS. We hope to address this question in future works. Using QCM and the molar distribution ratio, we have determined that at concentrations well below monolayer formation the adsorption of the dipeptides studied here to 9953



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(21) York, R. L.; Holinga, G. J.; Somorjai, G. A. Langmuir 2009, 25, 9369. (22) Yang, C. S. C.; Wilson, P. T.; Richter, L. J. Macromolecules 2004, 37, 7742. (23) Lu, R.; Gan, W.; Wu, B.-h.; Chen, H.; Wang, H.-f. J. Phys. Chem. B 2004, 108, 7297. (24) Gautam, K. S.; Schwab, A. D.; Dhinojwala, A.; Zhang, D.; Dougal, S. M.; Yeganeh, M. S. Phys. Rev. Lett. 2000, 85, 3854.

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Adsorption of Amino Acids and Dipeptides to the Hydrophobic

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