Robust Surface Plasmon Resonance Chips for Repetitive and

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Article Cite This: ACS Omega 2018, 3, 7483−7493

Robust Surface Plasmon Resonance Chips for Repetitive and Accurate Analysis of Lignin−Peptide Interactions Katsuhiro Isozaki,*,†,∥ Takafumi Shimoaka,‡ Satoshi Oshiro,§ Asako Yamaguchi,§,# Francesca Pincella,†,⊥ Ryo Ueno,†,∥ Takeshi Hasegawa,‡ Takashi Watanabe,§,⊥ Hikaru Takaya,*,†,∥ and Masaharu Nakamura*,†,∥

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†

International Research Center for Elements Science, Institute for Chemical Research, ‡Division of Environmental Chemistry, Institute for Chemical Research, and §Research Institute for Sustainable Humanosphere, Kyoto University, Gokasho, Uji 611-0011, Kyoto, Japan ∥ Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan ⊥ CREST, Japan Science and Technology Agency (JST), 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan S Supporting Information *

ABSTRACT: We have developed novel surface plasmon resonance (SPR) sensor chips whose surfaces bear newly synthesized functional self-assembled monolayer (SAM) anchoring lignin through covalent chemical bonds. The SPR sensor chips are remarkably robust and suitable for repetitive and accurate measurement of noncovalent lignin−peptide interactions, which is of significant interest in the chemical or biochemical conversion of renewable woody biomass to valuable chemical feedstocks. The lignin-anchored SAMs were prepared for the first time by click chemistry based on an azide−alkyne Huisgen cycloaddition: mixed SAMs are fabricated on gold thin film using a mixture of alkynyl and methyl thioalkyloligo(ethylene oxide) disulfides and then reacted with azidated milled wood lignins to furnish the functional SAMs anchoring lignins covalently. The resulting SAMs were characterized using infrared reflection−absorption, Raman, and X-ray photoelectron spectroscopies to confirm covalent immobilization of the lignins to the SAMs via triazole linkages and also to reveal that the SAM formation induces a helical conformation of the ethylene oxide chains. Further, SPR measurements of the noncovalent lignin−peptide interactions using lignin-binding peptides have demonstrated high reproducibility and durability of the prepared lignin-anchored sensor chips. acid,15 and fulvic acid,16 of significant botanical and ecological importance, have remained virtually unexamined due to the lack of the appropriate method of anchoring such class of biopolymers. We report herein development of novel SPR chips whose surfaces bear newly synthesized functional SAM covalently anchoring lignin. The SPR sensor chips showed remarkable durability for accurate and repetitive measurement of noncovalent lignin−peptide interactions. Lignin, cellulose, and hemicelluloses are primary components of lignocellulosic biomass and have gained considerable attention for their efficient and practical use as a renewable energy source and sustainable chemical feedstock.13,14 In particular, enzymatic conversion of cellulose and hemicelluloses to ethanol has been recognized as a crucial technology to produce liquid fuels (bioethanol) free from the excessive use of fossil resources.17 On the other hand, the

1. INTRODUCTION Surface plasmon resonance (SPR) is one of the most powerful analytical methods to evaluate interactions of various compounds with biopolymers, such as proteins, peptides, (oligo)sugars, and DNAs.1−5 The biopolymers are conventionally loaded onto the surface of the sensor chips by physical adsorption through dip or spin coating, often resulting in a significant error and poor reproducibility due to the detachment of the biopolymers.1 Immobilization of biopolymers on the sensor chip surface via covalent chemical bonds can solve this problem and produce highly robust SPR sensor chips with sufficient durability and reproducibility.1−5 A mixed selfassembled monolayer (SAM) has been adopted for this purpose because the number or concentration of immobilized molecules is easily controlled to achieve high surface density and to suppress their self-aggregation, simultaneously.6,7 Thus, the mixed SAMs with covalent immobilization of structurally well-defined proteins, peptides, (oligo)sugars, and DNAs have been well established.8−12 However, mixed SAMs bearing structurally undefined biopolymers, such as lignin,13,14 humic © 2018 American Chemical Society

Received: May 28, 2018 Accepted: June 22, 2018 Published: July 9, 2018 7483

DOI: 10.1021/acsomega.8b01161 ACS Omega 2018, 3, 7483−7493

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ACS Omega Scheme 1. Schematic Representation of MWL−SAM Fabrication

terminated thioalkyloligo(ethylene oxide) (MeO(EO)n(CH2)mSH) SAM was found to possess high resistivity to nonspecific protein adsorption in a manner similar to SAMs with oligo(ethylene oxide)-terminated thiolates.39 Further, terminal methoxy groups, which are chemically more inert than hydroxy groups, make methyl-terminated oligo(ethylene oxide) thiolates better SAM interfaces for biosensor applications. Two types of disulfides (i.e., methoxy-terminated disulfide 4 and alkynyl-terminated disulfide 10) were synthesized to fabricate the mixed-SAM. Disulfide 4 was prepared from triethylene glycol methyl ether (1) by a three-step reaction involving alkylation, thioesterification, followed by deprotection with concomitant oxidation to form the disulfide (Scheme 2). Alkynyl-terminated disulfide 10 was prepared via an

coexistence of lignin with cellulose and hemicelluloses is known to limit this conversion process severely by adsorbing or trapping the cellulase enzymes.17−19 Considerable efforts have thus far been paid to study the interactions of lignin with proteins or peptides by highly sensitive SPR20,21 and quartz crystal microbalance22−24 sensor techniques in addition to conventional less sensitive methods, such as enzymatic reactions,25−27 adsorption isotherms,28−30 and potentiometric titrations.31 However, lignins have usually been attached to sensor chips via physical adsorption;32 consequently, undesired detachment of lignin from the sensor chip surface has impeded accurate analysis. We therefore have engaged in the fabrication of covalently lignin-anchored SPR sensor chips through a SAM interface. Covalent immobilization of milled wood lignins (MWLs) was achieved by Huisgen cycloaddition-based click chemistry8,9,33,34 between an azide-functionalized MWL (N3-MWL) and a mixed SAM composed of alkynyl- and methoxyterminated oligo(ethylene oxide) thiolates (mixed-SAM, Scheme 1). A dense SAM and hydrophilic interface was thus formed by the hexyl and tri(ethylene oxide) units. The dissociation constant of the recently discovered 12-residual lignin-binding peptide (i.e., C416) to the immobilized MWL was measured accurately with notable robustness and reproducibility.20 In addition, spectroscopic investigation by means of infrared reflection−absorption (IR RA) and X-ray photoelectron spectroscopy (XPS) elucidates that the fabricated SAMs adopt a helical conformation of the ethylene oxide (EO) chains, resulting in dense packing of the SAM molecules and contributing to their observed high accuracy.

Scheme 2. Synthesis of Methoxy-Terminated Disulfide 4a

a Reaction conditions: (a) 1 (29.2 mmol), 1,6-dibromohexane (5.2 equiv), NaH (2.3 equiv), dimethylformamide (290 mL), room temperature (rt), 12 h, 23%; (b) 2 (6.8 mmol), KSAc (3.0 equiv), acetone (68 mL), reflux, 4 h, 80%; (c) 3 (3.1 mmol), NaOH (5.0 equiv), H2O−tetrahydrofuran (1:1, 31 mL), rt, 12 h, 97%.

essentially similar route (Scheme 3). Initially, selective monoprotection of pentaethylene glycol (5) was performed using a tert-butyldimethylsilyl (TBS) group. The obtained protected glycol 6 was then converted to S-thioacetate 8 by a similar synthetic protocol shown in Scheme 2. The TBS and acetyl protecting groups were sequentially deprotected to give hydroxy-terminated disulfide 9. Finally, the hydroxy groups were alkynylated by the reaction of propargyl bromide to give the desired alkynyl-terminated disulfide 10. 2.2. Fabrication of Alkyne-Terminated Mixed Thiolate SAMs. The alkyne-terminated mixed thiolate SAM (mixed-SAM) was prepared by immersing a gold thin-filmcoated quartz substrate into a mixture of 4 and 10 with a molar ratio of 3:1 in EtOH and left to stand at rt for 12 h (Scheme 4). Additionally, SAMs of 4 and 10 (i.e., MeO-SAM and

2. RESULTS AND DISCUSSION 2.1. Synthesis of Methoxy- and Alkynyl-Terminated Disulfides. The click immobilization of the biomolecules was mostly achieved using mixed SAMs formed by mixing azideterminated alkanethiols (N3(EO)4−7(CH2)11−12SH) and oligo(ethylene oxide)-terminated alkanethiols (HO(EO)3−4(CH2)11SH) via the methods established by Whitesides35 using various molar ratios. Note that few reports on mixed SAMs with alkyne-terminated alkanethiolates are available, even though azide groups are ideal for biomolecule functionalization because of their higher tolerance than alkynyl groups in diverse reaction conditions.36−38 Recently, methyl7484


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ACS Omega Scheme 3. Synthesis of Alkynyl-Terminated Disulfide 10a

ence in the IR spectra of MeO-terminated SAM (MeO(EO)nC3SH), and similar intensity changes of the νCO band were observed in the spectra of SAMs with n > 3 due to the cooperative formation of an energetically stable helical structure.40,42,43 For this reason, we consider that helical structures are also formed in alkyne-SAM and mixed-SAM. Indeed, the spectral bands at 1242 cm−1 of the CH2 twisting vibrational mode and 1349 cm−1 of the CH2 wagging vibration mode are characteristic of helical structures. The observed formation of a methoxy-terminated (EO)3 thiolate helical structure is substantially induced by the introduction of a small amount of alkyne-terminated (EO)5 thiolate, since the intensity of the characteristic bands at 1349 and 1242 cm−1 of alkyne-SAM and mixed-SAM is almost identical, even though the molar ratio of 4 and 10 was 3:1. The creation of such helical structures indicates the formation of dense SAMs amenable for SPR applications. Figure 1c shows magnified spectra of the νCH region of the SAM methylene groups (2930−2850 cm−1). Although the bands derived from the alkyl chain (CH2)6 and (EO)3 are largely overlapped, similar band shapes were reported previously to be generated from such moieties.40,42,43 Note that the weak peaks at 2981 and 2817 cm−1 are assigned to the CH3 asymmetric (νaCH3) and symmetric (νsCH3) stretching vibration bands of the terminal methyl group. Raman spectra of both alkyne-SAM and mixed-SAM showed a characteristic band of the CC stretching vibration (νCC) mode at 2328 cm−1, as depicted in Figure 1d. In addition, in the spectrum of alkyne-SAM (blue, Figure 1a), the obvious νCH band of the alkynyl C−H bond newly appears at 3450 cm−1, whereas the three peaks resulting from the methyl group disappear. Overall, these spectral features clearly indicate the formation of mixedSAM. It should also be noted that the alkynyl νCH band at 3450 cm−1 of mixed-SAM is broadened compared with the spectrum of alkyne-SAM, suggesting that the alkynyl C−H bond interacts with an oxygen atom of EO in a neighboring molecule(s).45 As described above, AA-SAM was prepared by click chemistry via a copper-catalyzed Huisgen cycloaddition reaction.8,9 Immersing the mixed-SAM into a solution of the azide-functionalized protected amino acid Boc-L-Ala(N3)-OMe 11 and shaking the flask at rt afforded AA-SAM. The IR RA

a

Reaction conditions: (a) 5 (1.2 equiv), TBS-C1 (12.8 mmol), 4dimethylaminopyridine (1.0 equiv), pyridine (2.6 mL), rt, 12 h, 36%; (b) 6 (5.9 mmol), 1,6-dibromohexane (3.0 equiv), tetra-nbutylammonium bromide (0.2 equiv), NaOH (4.9 equiv), H2O (1.1 mL), rt, 48 h, 82%; (c) 7 (1.9 mmol), KSAc (3.0 equiv), acetone (68 mL), reflux, 4 h, 98%; (d) 8 (1.6 mmol), tetra-n-butylammonium fluoride (1.2 equiv), THF (16 mL), rt, 12 h; (e) NaOH (5.0 equiv), THF (7.9 mL), H2O (7.9 mL), rt, 12 h, 93%; (f) 9 (0.6 mmol), NaH (2.6 equiv), THF (4.6 mL), rt, 1 h, then 1-bromopropyne (3.0 equiv), rt, 12 h, 52%.

alkyne-SAM, respectively) were also prepared as reference samples using the same method. An amino acid-tethered SAM (AA-SAM) was prepared from mixed-SAM by click chemistry as described below. The self-assembled structure and terminal functional groups of the SAMs were analyzed by means of infrared reflection-absorption (IR RA) and Raman spectroscopic measurements. Stacked IR RA spectra of MeO-SAM, alkyne-SAM, mixed-SAM, and AA-SAM are shown in Figure 1a−c. Figure 1a shows the entire spectral range (900−4000 cm−1), in which characteristic bands of the C−O stretching (νCO) and C−H stretching (νCH) vibration modes are observed at approximately 1130 and 2900 cm−1, respectively, as discussed later in detail. Magnified spectra of the νCO region at 900−1400 cm−1 (Figure 1b) show a characteristic strong band with two peaks at 1146 and 1120 cm−1, which, on account of similarity to the pr ev io usly re porte d IR s pe ctr um of the MeO(CH2CH2O)3(CH2)3SH (MeO(EO)3C3SH) SAM,40−44 are assigned to the νCO mode of the tri(ethylene oxide) (EO)3 chain with a less-ordered conformation. The minute peak at 1201 cm−1 is assigned to the methyl group. It is noteworthy that the νCO band becomes stronger as the structure is changed, that is, the wavenumber component at 1130 cm−1 changes considerably in alkyne-SAM and mixed-SAM. Vanderah et al. investigated the (EO)n chain length dependScheme 4. Fabrication of SAM Substrates

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Figure 1. (a) IR RA spectra of MeO-SAM (black line), alkyne-SAM (blue line), mixed-SAM (red line), and AA-SAM (green line). Magnified spectra of (b) fingerprint and (c) C−H stretching regions. (d) Raman spectra of alkyne-SAM (blue line) and mixed-SAM (red line).

immobilization of lignin, azide functional groups were introduced to MWLs32 derived both from a softwood, Japanese cedar, Cryptomeria japonica (CMWL), and a hardwood, Eucalyptus globulus (EMWL). Softwood lignin is known to be mostly composed of guaiacyl phenylpropanoid (G) units, whereas hardwood lignin principally consists of G and syringyl phenylpropanoid (S) units.50,51 Although lignins have structural diversity based on the type and area of the source woods, analytical efforts using NMR spectroscopy revealed a substantial content of aliphatic hydroxy groups in various types of lignins (1.0−6.0 mmol/g).52−56 In the present study, azide groups readily replaced the aliphatic hydroxy groups by a mesylation reaction using methanesulfonyl chloride followed by an SN2 reaction with sodium azide to give the desired azidefunctionalized MWLs (N3-CMWL and N3-EMWL, Scheme 5). Infrared attenuated total reflection (IR ATR) measurements confirmed the introduction of the azide groups by the appearance of N3 stretching vibration bands at 2102 and 2104 cm−1 for N3-CMWL and N3-EMWL, respectively, together with a diminishing O−H stretching vibration band observed around 3500 cm−1 (Figure 3, black and red lines) and negligible spectral changes in the MWLs themselves. The reactivity of the incorporated azide groups was examined by the Huisgen cycloaddition reaction8,9 and the Staudinger reaction57,58 (Scheme 5). The copper-catalyzed Huisgen cycloaddition reaction of N3-CMWL and N3-EMWL with Boc-L-propargyl-Gly-OH 12 proceeded to give amino acid-linked MWLs through triazole rings (AA-CMWL and AAEMWL). The IR ATR spectra of the obtained AA-CMWL and

spectra of the obtained SAMs are indicated by the green lines in Figure 1a−c. The newly appeared CO stretching at 1749 cm−1 and the disappearance of the terminal alkynyl νCH band around 3250 cm−1 clearly demonstrate formation of the desired AA-SAM product (Figure 1a, subtraction spectrum in Figure S1). In addition, the N3 stretching at 2102 cm−1 observed in the starting compound 11 disappeared in AASAM, with almost complete consumption of the azide group. The similarity of the AA-SAM and MeO-SAM spectra indicates that the EO helical structure in mixed-SAM unfolds following the introduction of the amino acid. X-ray photoelectron spectroscopy (XPS) measurements were also performed to confirm the formation of the triazole linkage resulting from the click reaction of mixed-SAM with 11 (Figure 2). In accordance with the IR RA analysis, the XPS spectrum of AA-SAM shows the incorporation of nitrogen, as evidenced by the broadened N 1s peak at 400.1 eV with a very weak azide peak at 404 eV (Figure 2c).46 The observed broad N 1s peak can be assigned to the carbamate group and the triazole ring.47−49 The XPS spectrum in the C 1s region exhibits two peaks arising from C−C and C−O bonds at 284.5 and 285.8 eV, respectively, together with the O 1s and S 2p3/2 peaks at 532.4 and 161.7 eV, respectively, which reflect the oxygen and sulfur contents of the SAM layer (Figure 2b,d,e). These results indicate formation of the triazole linkage in AASAM and further suggest the applicability of mixed-SAM for the covalent immobilization of lignin via click chemistry. 2.3. Synthesis and Reactivity of Azide-Functionalized MWLs. To apply the click reactions8,9 to the covalent 7486


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presence of water. The IR ATR spectra of the reaction products show complete disappearance of azide stretching around 2100 cm−1 and broad N−H stretching at 3332 and 3324 cm−1 for N3-CMWL and N3-EMWL, respectively, which is indicative of the hydrolytic conversion of azide (−N3) to amine (−NH2) (Figure 3, green lines). Overall, these results support the introduction of azide groups to MWLs, suggesting that the obtained N3-CMWL and N3-EMWL possess suitable reactivity for the click immobilization of the MWLs onto the SAMs. 2.4. Click Immobilization of Azide-Functionalized MWLs to SAM. The azide-functionalized MWLs N3-CMWL and N3-EMWL were covalently immobilized onto the alkynylterminated mixed-SAM substrate by click reactions (Scheme 1). Efficient click reactions of N3-CMWL and N3-EMWL to the mixed-SAM substrate were achieved by simply shaking the reaction solution. After click immobilization, unbound MWLs were washed away from the substrate by sonication in dimethyl sulfoxide (DMSO) for 3 min. Observation of the CMWLanchored SAM CMWL−SAM by optical microscopy showed many bright spots corresponding to the covalently immobilized MWL particles (Figure 4a). In contrast, the reference sample prepared without treatment with click reaction reagents, such as CuSO4·5H2O and ascorbic acid, showed negligible bright spots because of irreversible adsorption of N3-CMWL to the SAM surface (Figure 4b). Interestingly, before sonication in DMSO, a much larger amount of adsorbed MWLs was observed for the reference sample than that of the clickimmobilized sample, though almost all of the adsorbed MWLs were removed by sonication (Figure S2). Evidently, this result indicates the durability of the covalently immobilized MWLs. The immobilization of the MWLs to SAM was confirmed by their corresponding IR RA spectra (Figure 5). Both CMWL− SAM and EMWL−SAM exhibit complicated vibrational bands derived from the MWL at 900−1800 cm−1 and characteristic bands of the unreacted azide groups on the MWLs at 2100 cm−1. The XPS spectra of CMWL−SAM show a broadened N 1s peak at 400.7 eV, indicating the formation of the triazole ring from the click reaction (Figure 2c). In the N 1s region, no azide peak was observed at 404 eV but a new peak at 399.3 eV was present, which corresponds to the reported photoinduced side reactions of azide during the long measurement time required to collect high-resolution data.59,60 In the XPS

Figure 2. (a) XPS spectra of AA-SAM (black line) and CMWL−SAM (red line). Magnified XPS spectra of (b) O 1s, (c) N 1s, (d) C 1s, and (e) S 2p3/2.

AA-EMWL show almost complete disappearance of the characteristic azide band at 2100 cm−1, together with strong CO stretching at 1710 and 1712 cm−1 and broad O−H/N− H stretchings at 3356 and 3358 cm−1 for AA-CMWL and AAEMWL, respectively (Figure 3, blue lines). The reactivity of the azide groups of N3-CMWL and N3-EMWL was assessed by the Staudinger reaction with triphenylphosphine in the

Scheme 5. Synthesis and Further Transformation of Azide-Functionalized MWLs

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Figure 3. IR ATR spectra of (a) CMWL and (b) EMWL derivatives. MWL (black lines), N3-MWL (red lines), N3-MWL after the Huisgen reaction with 12 (blue lines), and the Staudinger reaction with PPh3 (green lines).

Figure 4. Optical micrographs of (a) click-immobilized and (b) adsorbed N3-CMWL on mixed-SAM. Scale bars are 250 μm.

Figure 5. IR RA spectra of MWL−SAMs (red lines), mixed-SAMs (black lines), and IR ATR spectra of N3-MWLs (blue lines) for (a) CMWL and (b) EMWL.

Figure 6. (a) SPR sensorgram of click-immobilized N3-CMWL toward C416 peptide at various concentrations (6.25, 12.5, 25, 50, 100, 200, and 300 μM; PBS, pH 7.0). (b) SPR response vs C416 concentration of click-immobilized CMWL. Inset shows the corresponding Scatchard plot. 7488


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Figure 7. (a) Recycling SPR measurements of click-immobilized (red line) and physisorbed CMWL (black line) toward a 100 μM C416 peptide solution in PBS pH 7.0, regenerated by DMSO. (b) Magnified SPR response of the click-immobilized CMWL with C416.

identical to that of the click-immobilized N3-CMWL, indicating that click immobilization of MWL does not affect the binding nature between CMWL and this lignin-binding peptide. A similar tendency was observed for the clickimmobilized N3-EMWL and physisorbed N3-EMWL, whose dissociation constants toward C416 were calculated to be 1.9 × 10−5 and 3.2 × 10−5 M, respectively, which are of the same order of magnitude but slightly lower values than that obtained from the physisorbed native EMWL, 7.0 × 10−5 M (Figure S2).20 2.6. Repetitive SPR Analysis under Harsh Regeneration Condition. In general, weak intermolecular interactions between sensor materials and analytes are suitable for SPR analysis because the detachment of analytes easily proceed. In contrast, strongly interacting analytes need harsh condition for their complete detachment but such harsh condition detach the sensor materials also with the conventional SPR sensor chips, which are prepared by physisorption methods. The covalently anchored sensor chips offer the applicability of such strong interacting analytes to SPR analysis. To demonstrate the durability of the click-immobilized N3-CMWL sensor chip, repetitive SPR measurements were carried out under harsh conditions. Recycling binding isotherms were measured by flowing a C416 solution (100 μM) in PBS (pH 7.0) followed by regeneration with DMSO (Figure 7a). The small content of residual peptide after the binding and rinsing steps was completely removed by the DMSO, and the baseline is recovered (Figure 7b). During 30 cycles of C416 binding, detaching, and regeneration of the sensor chip, negligible changes in the binding isotherms were observed. In contrast, the simply physisorbed CMWLs were significantly detached by DMSO, resulting in a decrease of both the baseline and the SPR binding signal (Figure 7a). These results demonstrate the high durability of the click-immobilized CMWLs toward repetitive analysis under harsh conditions for SPR measurements, which originates from covalent bonding of the MWLs to the sensor chip.

spectra, the intensities of the O 1s and C 1s peaks are observed to be higher than those of the 11-click-immobilized SAM AASAM due to the high O and C contents with immobilized MWL (Figure 2b,d). These results confirm the successful immobilization of MWLs to the SAM through covalent linking with the formation of triazole rings by click reactions. 2.5. SPR Measurements Using MWL-Anchored SAMs. The noncovalent binding behaviors of MWL-anchored SAMs toward lignin-binding peptide C41620 were investigated using SPR measurements. SPR measurements were performed with a two-channel instrument (OPTOQUEST SPR02). The flow cell divides the sensor chip into two equally sized channels, namely, a reference channel (SAM-functionalized Au film) and a sample channel (lignin-immobilized SAM-functionalized Au film). Both are connected to a syringe pump that was operated at a flow rate of 20 μL/min. SPR signals (as the intensity of the reflected light at a constant angle) were recorded over time and displayed as mdeg units. The click chemistry-based immobilization of N3-CMWL and N3-EMWL onto SAM was efficiently performed to fabricate sensor chips for SPR. SPR measurements were performed using a range of C416 concentrations (i.e., 6.25, 12.5, 25, 50, 100, 200, and 300 μM) in a phosphate buffered saline (PBS) (pH 7.0) solution (Figure 6a). With the flow of C416 solution, SPR signal increased to reach the saturation around 225 s due to the noncovalent interaction between the immobilized lignins and C416 peptides. Such noncovalent interaction is weak as reported in the previous paper,20 thus enabling the bound C416 peptides to detach by rinsing with PBS. But, a small content of irreversibly adsorbed peptides was not completely detached only by rinsing and gives rise to the increase of baseline and decrease of SPR signals, thus the residual C416 peptides were removed by regeneration step using glycine·HCl solution for the accurate and repetitive SPR measurements. The observed binding isotherms clearly show the binding of the C416 peptide to the click-immobilized N3-CMWL, in a manner similar to that observed for the physisorbed native CMWL.20 The dissociation constant of the click-immobilized N3-CMWL toward C416 was calculated by the Scatchard plot to be 3.0 × 10−5 M, which is similar to the value obtained for the native CMWL physisorbed on the sensor chip surface, 10.3 × 10−5 M (Figure 6b).20 In addition, the dissociation constant of N3-CMWL was obtained by SPR measurements of physisorbed N3-CMWL to be 3.4 × 10−5 M, which is almost

3. CONCLUSIONS A novel SAM-functionalized SPR sensor chip was successively fabricated by covalently anchoring lignin at the terminal ends of the SAM. Two types of oligo(ethylene oxide)-tethered disulfides were synthesized for the fabrication of mixed SAMs with alkynyl and methoxy terminal groups. IR RA, Raman, and 7489


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MWL particles have a size distribution ranging from 3 to 30 μm diameter. 4.4. Azidation of MWLs. A 50.0 mg powder of MWL was mixed with CH3CN (50 mL), methanesulfonyl chloride (7.4 g, 64.6 mmol), and triethylamine (3.7 g, 36.1 mmol) in the ice bath, and the heterogeneous solution was stirred in the ice bath for 1 h. The solvent was removed in vacuo, and the residue was washed with Et2O (50 mL) and water (100 mL). The resulting solid was mixed with DMSO (50 mL) and NaN3 (0.80 g, 12.3 mmol), and the mixture solution was stirred at 80 °C for 12 h. The homogeneous solution was cooled to room temperature (rt), and the solvent was removed in vacuo. The residue was mixed with water (100 mL) to form precipitate. The precipitate was collected by centrifugation to give azidefunctionalized MWLs N3-CMWL and N3-EMWL. 4.5. Preparation of Alkyne-Terminated Mixed-SAM Substrate. Gold film-coated substrates (quartz or silicon) were precleaned by immersing into a piranha solution for 5 min and then washed with ultrapure water. Gold substrates were immersed into a mixture solution of methoxy-terminated disulfide 4 (1.5 mM) and alkyne-terminated disulfide 10 (0.5 mM) in EtOH and stored at rt for 12 h. The substrates were washed with ultrapure water to afford alkyne-terminated mixed-SAM substrates. 4.6. Procedure of Click Immobilization of AzideFunctionalized MWL to Gold Substrates. A 1.0 mg of azide-functionalized MWL was mixed with water−DMSO mixture solvent (1:2 v/v, 1.0 mL), CuSO4·5H2O (0.25 mg, 1.0 μmol), and ascorbic acid (1.8 mg, 10 μmol). The alkyneterminated mixed SAM-functionalized gold substrates were immersed into the solution and shaken by HiPep Laboratories PetiSizer (1000 rpm) at rt for 12 h. The substrates were washed by sonication for 3 min in DMSO to afford the MWLimmobilized SAM substrates. 4.7. Procedure of Click Immobilization of Boc-LAla(N3)-OMe to Gold Substrates. The click immobilization was carried out with the same procedure of azide-functionalized MWL. 4.8. Procedure of Amine-Functionalized MWLs. A 1.0 mg of azide-functionalized MWL was mixed with mixture solvent of water−THF (1:1 v/v, 13.0 mL) and triphenylphosphine (341.0 mg, 1.30 mmol). The heterogeneous solution was refluxed at 90 °C for 12 h. The solid precipitate of MWL was isolated by centrifugation, washed with THF, and dried in vacuo to afford amine-functionalized MWLs. 4.9. Preparation of MWL-Immobilized Sensor Chips for SPR Measurement. At first, the surface of gold films (3.5 mm × 1.0 mm, two films for sample and reference sides) on the sensor chip (SPR02-SDS01A, OPTOQUEST) was cleaned by piranha solution (7:3 H2SO4/30% H2O2) and washed by H2O. The cleaned sensor chip was immersed into the mixture solution of 4 and 10 in a 3:1 molar ratio in EtOH at rt for 12 h. Then, the sensor chip was rinsed by EtOH and H2O. Only the mixed-SAM-functionalized gold film of the sample side was immersed into the mixture solution for click reaction with N3CMWL or N3-EMWL and shaken at rt for 3 d. After click reaction, the sensor chip was rinsed by DMSO and then sonicated in DMSO at rt for 3 min to remove adsorbed MWLs. The cleaned sensor chip was docked into a OPTOQUEST prism-coupling SPR sensor SPR02, and the adsorption of C416 peptides onto the sensor surface was monitored using SPR02. 4.10. Baseline Correction of XPS Spectra. Two kinds of baseline correction methods were applied to XPS spectra of O

XPS spectroscopic measurements were performed to characterize the fabricated SAM structures. Azide-functionalized MWLs were synthesized and used for click reactions with the mixed SAMs to afford the first covalently lignin-anchored SPR sensor chips. SPR measurements of the fabricated lignin-anchored SAMs showed the substantially same binding affinity for ligninbinding peptide C416 to those of native MWLs. Repetitive SPR measurements and regeneration under harsh conditions demonstrated the high durability and reproducibility of the covalently immobilized MWLs. The robustness of these newly developed lignin-anchored SPR sensor chips offers the availability of a wide range of solvent systems to the SPR measurements. In addition to the analysis of weak intermolecular interaction described herein, the present analytical method is potentially applicable to the measurement of interactions between lignins and large biomacromolecules, such as cellulases and hemicellulases, which is a fundamental concern in enzymatic lignocellulosic biomass conversion albeit being hampered by a lack of suitable sensor chip. We believe that the covalent anchoring of structurally undefined but botanically essential biopolymers will allow the elucidation of fundamental interactions of those with a variety of natural and artificial molecules to spur efficient and practical use of the renewable woody biomass.

4. EXPERIMENTAL SECTION 4.1. General. IR attenuated total reflection (ATR) spectra were recorded on JASCO FT/IR-460 Plus and PerkinElmer Spectrum One Fourier-transform infrared (FT-IR) spectrometers. IR reflection−absorption (RA) measurements61 were performed on a Thermo Fischer Scientific (Madison, WI) Magna 550 FT-IR spectrometer equipped with a Harrick (Pleasantville, NY) VR1-NIC variable angle reflection accessory mounted with a Harrick PWG-UIR wire-grid polarizer for passing the p-polarization only. 1H, 13C, COSY, and NOESY spectra were recorded on JEOL JNM-ECS400 spectrometers. Melting points were recorded on a Yanaco MP500D. The high-precision isotope peak intensities ratios were determined by Fourier transformation ion cyclotron resonance mass spectrometry (FT-ICR-MS) coupled with electron spray ionization technique using a solariX FT-ICR-MS spectrometer (Bruker Daltonics). Elemental analyses were carried out at the Microanalytical Laboratory of the Institute for Chemical Research, Kyoto University. Flash column chromatography was performed on Wakogel 60N (Wako, 38−100 μm) and auto column chromatography system Biotage SP1 (Biotage Corp., Sweden) or YFLC-AI-580 (Yamazen Corp., Osaka, Japan). Centrifugation was carried out on MCF-1350 (Laboratory and Medical Supplies). SPR measurements were performed on an OPTOQUEST SPR02 instrument. 4.2. Materials. Solvents were purchased from Nacalai Tesque or Wako Pure Chemical Industries and used without further purification. Boc-L-Ala(N3)-OMe (methyl (S)-3-azide2-tert-butoxycarbonylaminopropanoate) was synthesized according to the literature procedure.62 Boc-L-propargyl-Gly-OH ((S)-2-tert-butoxycarbonylamino-4-pentynoic acid) was purchased from Sigma-Aldrich and used without further purification. 4.3. Preparation of MWLs. MWLs were prepared by extraction with aqueous dioxane from finely ball-milled softwood Japanese cedar (CMWLs) and hardwood Eucalyptus globulus (EMWLs) as previously described.63 The prepared 7490


ACS Omega 1s, N 1s, C 1s, and S 2p3/2. Linear baseline subtraction was performed to N 1s, C 1s, and S 2p3/2. Shirley baseline subtraction was applied to O 1s.64 4.11. SPR Measurement. All SPR binding studies were performed in PBS (pH 7.0), which was filtered (0.22 μm) and degassed before use. After the sensor chip was docked into the instrument, PBS was injected over the chip at a flow rate of 20 μL/min for 20−30 min and a baseline was established for the sensor surface. For determination of equilibrium binding, analyses were performed at real time and at a flow rate of 20 μL/min. Solutions containing C416 peptide dissolved in PBS were used as the samples. The C416 peptide solutions were injected for 240 s, after which the bound peptides were dissociated by washing with PBS buffer. The adsorption of the peptides onto the MWL-immobilized sensor surface was then monitored. The binding of peptides to the sensor surface was marked by corresponding SPR angle changes. The sensor chip surfaces were regenerated using 10 mM aqueous glycine·HCl (pH 2.0) or DMSO. Binding affinities were calculated from a Scatchard analysis65 of equilibrium binding, based on the assumption of a 1:1 binding model, using the equation

where [peptide], Rmax, and KD are the peptide concentration, maximum resonance, and dissociation constant, respectively. The resonances in the association phase at 225 s were used as the equilibrium resonance units (Req). The Req values were plotted in the form of Req/[peptide] versus Req, and KD and Rmax were calculated from the slope and the intercept of the linear part of the plot, respectively. The KD values were determined over the peptide concentration range of 12.5−300 μM.

ACKNOWLEDGMENTS

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REFERENCES

(1) Handbook of Surface Plasmon Resonance; Schasfoort, R. B. M., Tudos, A. J., Eds.; RSC Publishing: Cambridge, 2008. (2) Masson, J.-F. Surface Plasmon Resonance Clinical Biosensors for Medical Diognostics. ACS Sensors 2017, 2, 16−30. (3) Abadian, P. N.; Kelly, C. P.; Goluch, E. D. Cellular Analysis and Detection Using Surface Plasmon Resonance Techniques. Anal. Chem. 2014, 86, 2799−2812. (4) Liu, L.; Xia, N.; Wang, J. Potential applications of SPR in early diagnosis and progression of Alzheimer’s disease. RSC Adv. 2012, 2, 2200−2204. (5) Konradi, R.; Textor, M.; Reimhult, E. Using Complementary Acoustic and Optical Techniques for Quantitative Monitoring of Biomolecular Adsorption at Interfaces. Biosensors 2012, 2, 341−376. (6) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Self-Assembled Monolayers of Thiolates on Metals as a Form of Nanotechnology. Chem. Rev. 2005, 105, 1103−1169. (7) Mixed SAMs, containing two or more terminal functional groups, have been used frequently for controlled covalent immobilization of biomolecules in order to suppress surface selfaggregation. In this case, oligo(ethylene glycol)-terminated alkanethiolates have been widely used due to their high wettability toward aqueous media and resistivity to non-specific protein adsorption Prime, K. L.; Whitesides, G. M. Self-Assembled Organic Monolayers: Model Systems for Studying Adsorption of Proteins at Surfaces. Science 1991, 252, 1164−1167. (8) Mandal, J.; Ramakrishnan, S. Azide-Alkyne Click Reaction in Polymer Science. In Click Reactions in Organic Synthesis; Chandrasekaran, S., Ed.; Wiley-VCH: Weinheim, 2016; pp 203−254. (9) Haensch, C.; Hoeppener, S.; Schubert, U. S. Chemical modification of self-assembled silane based monolayers by surface reactions. Chem. Soc. Rev. 2010, 39, 2323−2334. (10) Dugger, J. W.; Webb, L. J. Preparation and Characterization of Biofunctionalized Inorganic Substrates. Langmuir 2015, 31, 10331− 10340. (11) Gooding, J. J.; Parker, S. G.; Lu, Y.; Gaus, K. Molecularly Engineered Surfaces for Cell Biology: From Static to Dynamic Surfaces. Langmuir 2014, 30, 3290−3302. (12) Gooding, J. J.; Ciampi, S. The molecular level modification of surfaces: from self-assembled monolayers to complex molecular assemblies. Chem. Soc. Rev. 2011, 40, 2704−2718. (13) Biomass Conversion; Baskar, C., Baskar, S., Dhillon, R. S., Eds.; Springer: Heidelberg, 2012.

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b01161. Experimental details on organic syntheses, subtracted IR spectrum between AA-SAM and mixed-SAM, optical microscope images of MWL-immobilized SAMs before and after sonication, NMR and MS spectra (PDF)

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This research was supported by the Japan Science and Technology Agency (JST), the Core Research for Evolutional Science and Technology Program (CREST 11103784), the Japan Society for the Promotion of Science (JSPS) through the “Funding Program for Next Generation World-Leading Researchers (Next Program)” initiated by the Council for Science and Technology Policy (CTSP), and in part by JSPS KAKENHI Grant Numbers JP15H01085 and JP26410118, the Collaborative Research Program of the Institute for Chemical Research (Grant Numbers 2015−16 and 2014−19), and a research grant for Exploratory Research on Sustainable Humanosphere Science from the Research Institute for Sustainable Humanosphere (RISH), Kyoto University. FTICR-MS analyses were supported by the JURC at ICR, Kyoto University. The authors thank Yui Uno and Yutaka Sonobayashi (Faculty of Engineering, Kyoto University) for performing the XPS measurements, Toshiko Hirano (ICR, Kyoto University) for performing the elemental analysis, and Dr. Koichi Yoshioka (RISH, Kyoto University) for preparing the MWLs.

R eq /[peptide] = R max /KD−R eq /KD

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (K.I.). *E-mail: [email protected] (H.T.). *E-mail: [email protected] (M.N.). ORCID

Katsuhiro Isozaki: 0000-0002-0990-1708 Takeshi Hasegawa: 0000-0001-5574-9869 Takashi Watanabe: 0000-0003-0220-4157 Hikaru Takaya: 0000-0003-4688-7842 Masaharu Nakamura: 0000-0002-1419-2117 Present Address #

Department of Biological Science, Graduate School of Science, Osaka Prefecture University, 1-1 Gakuen-cho, Nakaku, Sakai, Osaka 599-8531, Japan (A.Y.).

Notes

The authors declare no competing financial interest. 7491


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