XPS와 AFM에 의한 self-assembled peptide 막 분석

이 행자, 송 성훈, 조 홍식, 신 훈규*, Mark Lantz**

Takao Ishida**, Nakamura Chikashi***, Miyake Jun***, 장 상목

동아대학교 공과대학 화학공학과

동아대학교 공과대학 전기공학과

JRCAT** and NAIR***, Japan

XPS and AFM analysis of self-assembled peptide membrane

Lee Haeng-ja, Song Seong-Hun, Cho Hong-Sig, Shin Hoon-Kyu*, Mark Lantz** Takao Ishida**, Nakamura Chikashi***, Miyake Jun***, Chang Sang-Mok

Dept. of Chem. Eng, Dong-A Univ., Pusan, Korea

Dept. of Electric Eng*, Dong-A Univ., Pusan, Korea

JRCAT** and NAIR***, 1-1-4, Umezono, Tsukuba, Ibaraki 305-8562, Japan

INTRODUCTION

Recently, the self-assembly of thiols on noble metals has been studied intensively. Self-assembly has been established by a multitude of experimental methods that these organosulfur compounds form stable, well-ordered self assembled monolayer(SAM) on various metals. SAM has potential applications in many fields such as biosensors, corrosion inhibition, wetting control, optics, displays and biomolecular of electronic devices[1,2]. The SAM is also interesting as fundamental model systems of various interfaces phenomena studies such as electrode/electrolyte interfaces[3]. An approach comprising electrode modification with functionalized alkanes, resulting in well-defined two-dimensional SAM, has proved model systems for understanding fundamental physical, chemical, and biological interfacial processes.

For the construction of well designed SAM, it is essential to understand the fine structure and growth process of the SAM. The structures of the SAM have been studied using various techniques including diffraction methods, scanning tunneling microscopy (STM), atomic force microscopy(AFM) and X-ray photoelectron spectroscopy(XPS). Recently, STM measurement with high impedance gap provides real images of individual alkanethiol molecules on metal substrates.

The porphyrin derivatives, well-studied macrocyclic compounds, have a variety of applications, including catalytic, magnetic, sensing, and biological using their redox properties and photo-reactive pigments. The porphyrin rings in the myoglobin and cytochrome c are stabilized by docking into a hydrophobic pocket of proteins, which is often formed by aromatic residules[4]. Thus, the interaction between the porphyrin and proteins (peptide domains) is one of the key reactions in biochemistry. And therefore, it is important to know the characteristics of SAM of peptide for the application of peptide sensor using interaction between porphyrin and oligopeptide.

This paper forms part of a comprehensive investigation of the simplest peptide thiol, cystein, adsorbed from the liquid phases on Au. The focus of the present work is directed toward the surface spectroscopic characterization of PSP1(Tyr-Ala-Gly-Tyr-Cys) and PSP2(His-Ala-Ser-Tyr-Ser-Cys) peptides chemisorbed from the liquid phase on Au, with a view to elucidating the nature of the adsorption mechanism, adsorbate bonding, and molecular orientation.

EXPERIMENTAL

Peptide synthesis was performed using an Fmoc solid phase strategy and carried out with a PepPlus 9050 synthesizer (PerSeptive). Oligopeptides such as PSP1 and PSP2, which interact strongly with a typical cationic porphyrin derivative, are synthesized. The crude peptide was purified using a reversed phase HPLC system with a LibraKit RPC18 column (Shimadzu).

The SAM examination confirmed the existence of Au terraces on the gold surface. Gold electrode were prepared by thermal evaporation and annealing onto freshly cleaved mica.

SAM were produced by incubating the gold substrates for 12 hours in a solution containing 0.5 mM of the peptides dissolved in trifluorethanol and 10 mM MOPS buffer. Peptides adsorption from the liquid phase on Au has been studied using XPS spectroscopy and AFM image.

RESULTS and DISCUSSION

We have determined the binding energies by XPS. The results obtained are illustrated figure 1. These show an Al K□ induced XPS spectrum of the wide range of binding energy from a PSP1(a) and PSP2(b) self-assembled monolayer on the Au fitted using Gaussian/Lorentzial line shape.

By analysis of the S(2p) XPS peaks, we determined the structure of the SAM. Figure 2 shows an Al K□ induced XPS spectrum of the S(2p) core levels. In figure 2(a), the S(2p) core level spectrum from a liquid-phase self-assembled PSP1 monolayer film on Au reveals five-component peaks. A and B peaks are from MOPS buffer (SO3), C peak from unbinding PSP1 and D and E from binding PSP1. From these results, it is known that three PSP1 are bound on the Au, and one PSP1 is unbinding. On the other hands, PSP2 monolayer film on Au reveals six-component peaks. A and B peaks are from MOPS buffer(SO3), C peak from unbinding PSP1 and D, E, and F from binding PSP2 as followed figure 2(b). In case of PSP2, however, four PSP2 are bound on the Au, and one is unbinding.

We can also confirm these results by comparing the calculated steric energies of Au-Peptides, which is assumed that one thiol group is bind with three Au-atoms[5].

Attenuation of the Au(4f) signal upon self-assembly was used to estimate the thickness of the peptide overlayer using method of the inelastic mean free path(IMFP) of the Au(4f) photoelectron. The peak shape analysis can be used to obtain the in-depth concentration profile of electron emitters in XPS, ie. the nanostructure of the surface region can be determined. The thicknesses of the PSP1 and PSP2 layers on Au are below 2□ and 2∼3□, respectively. In addition, a relatively large gold peak was detected indicating that the SAM contain a relatively low density of peptides.

Figure 3 shows the AFM images of PSP1(a) and PSP2(b) that were self-assembled on the Au surface. The large domains of figure 3(a) mean unbinding peptide and small domains of figure 3(b) binding peptide. From these images, it is also known that PSP2 is well bind on the Au than PSP1.

ACKNOWLEDGEMENT

The authors are grateful for the financial supports of NEDO and Dong-A university.

REFERENCE

1.	David S. Lawrence, Tao Jiang, and Michael Levett, Chem.Rev., 95, 2229(1995)

2.	Ryo Yamada and Kohei Uosaki, Denki Kagaku, Vol.65, No.6, 440(1997)

3.	Caroline M.Whelan, Malcolm R.Smyth, and Colin J.Barnes, Langmuir, 15, 116

(1999)

4.	Naoki Sugimoto and Shuichi Nakano, Chemistry letters, 939(1997)

5.	Victor Chechik, Holger Schonherr, G.Julius Vancso, and Charkes J.M.Stirling,

Langmuir, 14, 3003(1998)

(a)PSP1

(b)PSP2

Fig. 1.	High-resolution Al K□ induced XPS spectra of the wide range of binding

energy from a PSP1(a) and PSP2(b) monolayer self-assembled on Au fitted

using Gaussian/Lorentzial line shape

(a)XPS spectra of the sulfur(2p) core levels from PSP1

(b)XPS spectra of the sulfur(2p) core levels from PSP2

Fig. 2.	High-resolution Al K□ induced XPS spectra of the sulfur(2p) core levels

from a PSP1(a) and PSP2(b) monolayer

(a)PSP1	(b)PSP2

Fig. 3.	AFM topographies of self-assembled PSP1(a) and PSP2(b) (Dim. 250□X250□)