PH-induced structural changes of surface immobilized poly(l-lysine) by two-dimensional (2D) infrared correlation study

Article abstract of DOI:10.1016/j.cclet.2014.12.012

This paper reports the pH-induced structural changes in the surface immobilized poly(l-lysine) (PLL) film. Two-dimensional (2D) correlation analysis was applied to the Fourier transform infrared (FTIR) spectra of the surface-immobilized PLL film to examin

Full text of DOI:10.1016/j.cclet.2014.12.012

G Model
CCLET 3181 1–4
Chinese Chemical Letters xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
1
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Original article
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4
pH-induced structural changes of surface immobilized poly( -lysine)
by two-dimensional (2D) infrared correlation study
L
a,
Q1 Eun Joo Yoo a,1, Boknam Chae b,1, Young Mee Jung c, , Seung Woo Lee
*
*
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a School of Chemical Engineering, Yeungnam University, Gyeongsan 712-749, Republic of Korea
b Pohang Accelerator Laboratory, Pohang 790-784, Republic of Korea
c Department of Chemistry, Kangwon National University, Chunchon 200-701, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
This paper reports the pH-induced structural changes in the surface immobilized poly(
Article history:
L-lysine) (PLL)
Received 4 November 2014
Received in revised form 5 December 2014
Accepted 15 December 2014
Available online xxx
film. Two-dimensional (2D) correlation analysis was applied to the Fourier transform infrared (FTIR)
spectra of the surface-immobilized PLL film to examine the spectral changes induced by the alternations
of the protonation state of the amino group in the side chain. Significant spectral changes in the FTIR
spectra of the PLL film were observed between pH 7 and 8. The decrease in the protonation state of the
amino group in the side chain induced spectral changes in the amino group as well as conformational
changes in the alkyl group in the side chain. From pH 1–8, the spectral changes in the amino and alkyl
groups in the side chain occurred before those of the amide group in the main chain of the surface
immobilized PLL film.
Keywords:
Poly(L-lysine) (PLL)
Surface-immobilized polypeptide
FT-IR spectroscopy
ß 2014 Young Mee Jung. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights
reserved.
2D correlation spectroscopy
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1. Introduction
Among the surface grafted polymers, polypeptides-grafted 28
films have attracted attention because of their potential applica- 29
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Q2
Surface grafted polymer films on various substrates have
attracted considerable research attention because of their unique
surface properties, such as friction, biocompatibility, wettability,
and corrosion resistance [1–6]. Surface grafted polymer films can
be prepared by attaching one chain end group of polymers to a
variety of substrates through a covalent bond [7–9]. To obtain
surface-grafted polymer films, in situ polymerization processes are
conducted on the initial modified surface of substrates. A range of
polymerization methods, such as atom transfer radical polymeri-
zation (ATRP) [6–10], reversible addition fragmentation chain
transfer (RAFT) polymerization [11,12] and ring opening metathe-
sis polymerization (ROMP) [13,14], using vinyl derivatives as the
monomers have been studied to obtain versatile, reliable and
controllable vinyl polymer film surfaces. The ring opening
polymerization (ROP) of N-carboxy anhydride (NCA) derivatives
as monomers to prepare surface-grafted polypeptides has been
examined [15–18].
tions, such as biomedical, anti-fouling and stimuli responsive 30
materials [15,19–21]. The ordered secondary structures ( -helix 31
and -sheet) and random coil conformation provide extraordinary 32
a
b
thin film properties. In addition, the conformational transition can 33
be obtained easily changing the external stimuli, such as pH, 34
temperature and solvent [16–18]. Many research groups have 35
reported the basic properties of surface-grafted polypeptide films 36
and applications, such as chiral separating membranes, optical 37
switches and biosensors. Despite the extraordinary features of 38
surface grafted polypeptide films, the structural changes in grafted 39
peptide films by external stimulation are not completely under- 40
stood. Therefore, it is important to reveal the chemical behaviors of 41
the secondary structure and random coil conformational transition 42
upon changes in external stimulation for surface grafted polypep- 43
tide films.
In this study, a grafted film composed of poly(
44
L
-Lysine) (PLL), 45
which has a peptide main chain and amino side chains, was 46
fabricated using a ring opening surface-initiated polymerization 47
method with a NCA of N-carbobenzyloxyl-L-lysine (CBL) (Fig. 1) 48
[18]. This study examined the pH-induced structural changes in 49
surface immobilized PLL by Fourier transform infrared (FTIR) 50
spectroscopy. FTIR spectroscopy is a sensitive technique that can 51
be used to identify the spectral changes in the proteins including 52
*
Corresponding author.
E-mail addresses: yumjung@kangwon.ac.kr (Y.M. Jung),
leesw1212@yun.ac.kr (S.W. Lee).
1
These authors contributed equally to this work.
http://dx.doi.org/10.1016/j.cclet.2014.12.012
1001-8417/ß 2014 Young Mee Jung. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Please cite this article in press as: E.J. Yoo, et al., pH-induced structural changes of surface immobilized poly(L-lysine) by two-
dimensional (2D) infrared correlation study, Chin. Chem. Lett. (2015), http://dx.doi.org/10.1016/j.cclet.2014.12.012

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CCLET 3181 1–4
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E.J. Yoo et al. / Chinese Chemical Letters xxx (2014) xxx–xxx
white crystals. The product was identified by proton nuclear
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magnetic resonance (1H NMR) spectroscopy (model AM300,
Bruker) from a solution in chloroform-d1 (CDCl3). 1H NMR
(300 MHz, CDCl3):
d
7.1–7.3 (m, 5H, Ph–H), 6.8 (s, 1H, NH–), 5.0
98
(s, 2H, CH2-benzylic), 4.8 (t, 1H, O–C–H), 4.1 (t, 1H, C–H), 3.1 (d, 2H,
99
CH2–NH), 1.9 (m, 1H,
–CH2CH2).
Preparation of surface immobilized poly(
immobilization of poly(N6-carbobenzyloxy-
conducted using CBLNCA and APS silanized Si wafer in anhydrous
DMF. The APS-modified Si wafer was immersed in a 100 mM
solution of CBL at ambient temperature under a N2 atmosphere for
12 h. After polymerization, the substrates were rinsed with
copious amounts of DMF. The substrates were sonicated several
times in DMF to completely remove the physisorbed polymer and
unreacted CBL from the surface, rinsed with ethanol, and then
b
-CH), 1.6 (m, 2H, –RCH2), 1.3 (m, 4H,
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L-lysine): The surface
L-lysine) (PCBL) was
dried with N2. Surface immobilized poly(L-lysin) (PLL) was
prepared by a deprotecting reaction of N6-carbobenzyloxy groups
from immobilized PCBL on a Si wafer. The PCBL-immobilized Si
wafer was immersed in a HBr/benzene solution for 2 h [18]. After
the reaction, the substrates were rinsed with acetone and distilled
water, and dried with N2.
Fig. 1. Synthetic scheme of surface immobilized poly(L-lysine) (PLL).
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the amide group in the main chain and the alkyl and amino groups
in the side chain of PLL [23–25]. Moreover, 2D correlation
spectroscopy was conducted to closely investigate the pH-induced
structural changes in the surface immobilized PLL film. 2D
correlation spectroscopy can be used to analyze the pH-induced
spectral changes in the amide group in the main chain and the alkyl
and amino groups in the side chain as well as to enhance the
spectral resolution in the N–H stretching vibrations related to the
amide group in the main chain and the amino group in the side
chain [26–28]. Therefore, 2D correlation analysis of the PLL FTIR
spectra can be used to examine closely the pH-induced spectral
changes in the characteristic bands of the peptide unit in the main
chain and the alkyl and amino groups in the side chain of the
surface immobilized PLL film after different pH treatments.
2.2. Measurements
117
The 1H NMR (Bruker AM 300) spectrum was obtained at room
temperature. FTIR spectroscopy was conducted on a Bruker Vertex
80/v FTIR spectrometer equipped with an ATR accessory (PIKE) at
the Pohang Accelerator Laboratory (PAL). The FTIR spectra were
recorded at a 4 cm–1 resolution using a liquid-nitrogen-cooled
mercury cadmium telluride (MCT) detector. The 2D correlation
spectra were obtained using an algorithm based on the numerical
method reported by Noda. 2D correlation analysis was performed
after a baseline correction. A subroutine called KG2D, which was
written in Array Basic language (GRAMS/386; Galactic Inc., NH),
was used in 2D correlation analysis [29].
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2. Experimental
3. Results and discussion
129
2.1. Materials and synthesis
Fig. 2 shows the pH-dependent FTIR spectra of the surface
immobilized PLL in the region of 3500–2650 cmꢁ1 and 1800–
1400 cmꢁ1. The intensities of the band at 3030 cmꢁ1 decreased
with increasing pH (Fig. 2(a)). In particular, the intensities of the
band at 3030 cmꢁ1 decreased abruptly between pH 7 and 8. In
contrast, the intensities of the band at 3270 cmꢁ1 increased with
increasing pH from 1 to 8. The bands at 3030 and 3270 cmꢁ1 were
assigned to the N–H stretching vibration of the protonated amino
group (NH3+) and the deprotonated amino group (NH2) in the side
chain, respectively [26,30]. Therefore, these spectral changes
suggest an alteration of the protonation state of the amino group
in the PLL side chain depending on the pH. The amino group in the
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Materials and preparation of substrate: N6-carbobenzyloxy-
-lysine (CBL, Aldrich), triphosgene (Aldrich), n-hexane (Samchun),
toluene (Aldrich), and 3-(aminopropyl)triethoxysilane (APS,
Aldrich) were used as received. The solvents for synthesis, i.e.,
dimethylformamide (DMF) and ethyl acetate (EA), were purchased
from Aldrich, purified by distillation over calcium hydride and
L
˚
stored over 4 A molecular sieves. All buffer solutions were
purchased from Duksan Chemicals. An oxidized silicon (Si) wafer
was cut into 2 cm ꢀ 2 cm. After sonication with ethanol for 10 min
and rinsing with de-ionized (DI) water, the Si substrates were
immersed in a ‘‘piranha solution’’ (H2SO4/30% H2O2 = 7/3 (v/v)),
(Caution: Piranha solutions are extremely dangerous and should
be used with extreme caution) at 80 8C for 10 min. The cleaned
substrates were rinsed with DI water and dried with N2.
Silanization of the Si wafer was conducted by immersing the Si
substrates into a 2 wt% solution of an APS solution in toluene. The
silanized substrates were rinsed with ethanol and distilled water
and dried with N2.
PLL side chain was protonated at acidic and neutral pH. The amino Q3 142
group in the PLL side chain was deprotonated at basic pH, as
described in previously [18,26,30,31].
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In addition, the bands due to the asymmetric and symmetric
C–H stretching vibrations of the CH2 group in the PLL side chain
showed spectral changes with increasing pH. The asymmetric and
symmetric C–H stretching vibrations of CH2 groups were observed
at 2940 and 2870 cmꢁ1 at acidic pH. On the other hand, the
stretching vibration of the CH2 group were detected at 2922 and
2854 cmꢁ1 at basic pH (pH 8). The band positions of the CH2
stretching vibration are conformation sensitive, where lower
wavenumbers indicate ordered conformations [32]. This suggests
that the alkyl chain in the surface immobilized PLL side chain at
basic pH exhibits a more ordered conformation than at acidic and
neutral pH. The spectral changes in the alkyl group occurred
Synthesis of N6-carbobenzyloxy-
dride: N6-carbobenzyloxy-
-lysine N-carboxylic anhydride
L-lysine N-carboxylic anhy-
L
(CBLNCA) was synthesized from a reaction of CBL and triphosgene.
A mixture of CBL and triphosgene in EA was heated under reflux for
12 h under a N2 atmosphere. The resulting pale yellow solution
was cooled to room temperature, and washed with cold deionized
water. The organic layer was dried with MgSO4 and concentrated.
The crude product was recrystallized from n-hexane to obtain
Please cite this article in press as: E.J. Yoo, et al., pH-induced structural changes of surface immobilized poly(
L-lysine) by two-
dimensional (2D) infrared correlation study, Chin. Chem. Lett. (2015), http://dx.doi.org/10.1016/j.cclet.2014.12.012

G Model
CCLET 3181 1–4
E.J. Yoo et al. / Chinese Chemical Letters xxx (2014) xxx–xxx
3
Fig. 2. FTIR spectra of the surface immobilized poly(L
-lysine) in the region of (a) 3500–2650 cmꢁ1 and (b) 1800–1400 cmꢁ1 at various pH.
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abruptly between pH 7 and 8, as shown in the spectral changes in
the amino group in the side chain.
In Fig. 2(b), the characteristic bands assigned to the stretching
vibration of the C55O group (amide I) and the in-plane bending
vibration of the NH group (amide II) in the peptide unit of the PLL
main chain were detected at 1654 and 1547 cmꢁ1, respectively
measured over the range, 3500–2650 cmꢁ1. According to the 187
synchronous 2D correlation spectrum (Fig. 3(a)), the change in 188
intensity at 3030 cmꢁ1 suggests that the protonation state of the 189
amino group in the side chain is strongly affected by changes in 190
pH. Moreover, the change in the intensity at 2870 cmꢁ1 showed 191
that the alkyl group in the side chain is also strongly affected by 192
the pH. The decrease in protonation of the surface-immobilized 193
PLL film with increasing pH induced spectral changes in the amino 194
group as well as conformational changes in the alkyl group in the 195
side chain. From the asynchronous 2D correlation spectrum 196
(Fig. 3(b)), the FTIR band associated with the stretching N– H 197
vibration of the amide group in the main chain of the surface 198
immobilized PLL film was differentiated. The band assigned to the 199
stretching vibration of the amide group was observed at 200
3200 cmꢁ1 [26]. The signs of the cross peaks at (3270 and 201
3200), (3270 and 3030) and (3200 and 3030) cmꢁ1 suggests that 202
the decrease in protonation induced sequential spectral changes 203
in the amino group in the side chain and the amide group in the 204
main chain. The spectral changes in the peptide linkage in the 205
main chain occur after the decrease in protonation. The signs of 206
the cross peaks at (3200 and 2870) and (3030 and 2870) cm ꢁ1 207
suggest that the spectral changes in the alkyl group in the side 208
chain occur after the decrease in protonation. Furthermore, the 209
spectral changes in the peptide linkage in the main chain occur 210
after those of the alkyl group in the side chain. Overall, these 211
observations suggest that the decrease in protonation induces 212
sequential spectral changes in the alkyl group of the side chain and 213
the peptide linkage of the main chain. The spectral changes in the 214
amino group and the alkyl group in the side chain occur before 215
[18,24,25,33]. The amide
conformational transition of the peptide linkage. The
structure is located at 1610–1640 cmꢁ1 and 1680–1690 cmꢁ1 [33].
I
band is quite sensitive to the
b
-sheet
The random coil and
a-helix structure were observed at 1650–
1660 cmꢁ1 and 1640–1650 cmꢁ1, respectively [18,33]. The con-
formational transition of the secondary structure of PLL among the
a
-helix,
[18,31,33]. At low pH, PLL consists of the random coil structure
and PLL can exist as the -helix and/or -sheet at high pH
b-sheet and random coil were induced by the pH
a
b
depending on the external environment [18,33]. Therefore, the
band at 1654 cmꢁ1 can be assigned to the random coil structure at
acidic pH [18]. On the other hand, the random coil and
structure could not be resolved in the present study due to overlap
of the band caused by the random coil with -helix structure. In
-sheet structure could not be
a-helix
a
addition, the bands due to the
observed between pH 1 and 8.
b
2D correlation spectroscopic analysis of the FTIR spectra was
applied to further understand the pH-induced spectral changes in
the surface immobilized PLL films in the range. The 2D correlation
spectra were constructed from the FTIR spectra measured at pH
1–8. 2D correlation analysis results in the amide I and II region did
not provide new insights. Here, we only focused on the 2D
correlation spectra in the region of 3500–2650 cmꢁ1. Fig. 3 shows
the synchronous and asynchronous 2D IR correlation spectra
those of the peptide linkage in the main chain.
216
Fig. 3. (a) Synchronous and (b) asynchronous 2D correlation spectra the region, 3500–2650 cmꢁ1, generated from the FTIR spectra of the surface-immobilized PLL film. The
open and filled regions indicate positive and negative cross peaks, respectively.
Please cite this article in press as: E.J. Yoo, et al., pH-induced structural changes of surface immobilized poly(
L-lysine) by two-
dimensional (2D) infrared correlation study, Chin. Chem. Lett. (2015), http://dx.doi.org/10.1016/j.cclet.2014.12.012

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E.J. Yoo et al. / Chinese Chemical Letters xxx (2014) xxx–xxx
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4. Conclusion
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The surface immobilized PLL film was examined in detail to
provide pH-induced structural changes by FTIR spectroscopy. The
decrease in the level of protonation of the side chain induced
spectral changes in the amino group in the side chain and the
peptide linkage in the main chain. In particular, the alkyl side
group in the surface immobilized PLL film is strongly affected by
pH changes. The alkyl side chain exhibited a more ordered
conformation as the pH was increased. The stretching N–H
vibration of the amide group was resolved by 2D FTIR correlation
analysis. The 2D FTIR correlation spectra of the surface immobi-
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Acknowledgments
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This study was supported by Yeungnam University Research
Grants in 2013 and a Human Resources Development Program of
Korea Institute of Energy Technology Evaluation and Planning
(KETEP) grant (No. 20104010100580) funded by the Korean
Ministry of Knowledge Economy.
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Please cite this article in press as: E.J. Yoo, et al., pH-induced structural changes of surface immobilized poly(
L-lysine) by two-
dimensional (2D) infrared correlation study, Chin. Chem. Lett. (2015), http://dx.doi.org/10.1016/j.cclet.2014.12.012