Molecular Orientation in the Peptide Self-Assembled Monolayers
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MOLECULAR ORIENTATION IN THE PEPTIDE SELF-ASSEMBLED MONOLAYERS Katsuhiko Fujita, Ryuichi Yokoyama and Tetsuo Tsutsui Department of Applied Science for Electronics and Materials, Graduate School of Engineering Sciences, Kyushu University, Kasuga, Fukuoka 816-8580, Japan ABSTRACT Self-assembled monolayer (SAM) of peptides is investigated in terms of the correlation of their molecular orientation and the dipolar interactions. The used helical peptides containing a disulfide group on the terminal, which can form SAMs on a gold substrate, were synthesized. The molecular orientation was estimated from the reflection absorption IR intensity ratio of amide I and amide II bands which have transition moment almost parallel and perpendicular to the molecular axis, respectively. The adsorption behavior and layer thickness were determined by surface plasmon resonance. The molecular orientation depended on the dipolar interaction between the peptide molecules and between the peptide dipole and its image. The molecular direction against the substrate affects the orientation probably due to the interaction between the peptide dipole and Au-S polarization. This speculation is supported by the fact that the orientation difference by the molecular direction became smaller when the spacer from the helix body to the disulfide group is longer. The results indicate that the careful molecular design and optimal preparation condition of SAMs allow formation of well-organized and predetermined structure of nano-architecture even by use of large and complex molecules. INTRODUCTION Thiol or disulfide derivatives spontaneously form a SAM on a gold substrate by the immersion of the substrate into the solution.[1] Today SAMs of functional thiols are widely utilized for surface modification and nano-fabrication on metals due to their well-ordered and stable structure.[2] It is expected that more sophisticated and functionalized systems will be realized by specially designed molecules having a larger component than a simple alkyl chain.
SSA16B
(SA16B)2
-SCH2CH2CO-(Ala-Aib)8-OCH2 -SCH2CH2CO-(Ala-Aib)8-OCH2 2
SCH2CH3 BA16SS
(BA16S)2
(CH3)3COCO-(Ala-Aib)8-OCH2CH2S-
(CH3)3COCO-(Ala-Aib)8-OCH2CH2S2
BA16C11SS
CH3CH2S-
(CH3)3COCO-(Ala-Aib)8-OC11H22SC10H21SFigure 1. Molecular structure of the helical peptides used in this study. Ala: L-alanine, Aib: α-aminoisobutyric acid. P6.24.1
This strategy would force us to manage more complicated interactions in the SAM systems. In the present study the SAM formation of helical peptides, which have a pillar-shape, a large dipole moment along the molecular axis and many side chains potentially functionalized by various groups, is investigated to reveal the effects of an electrostatic or a geometric interaction to the SAM structure of such a large molecule. The five kinds of peptides (Figure 1) were prepared for this SAM formation study. All of these peptides have the same helix part and a disulfide group, which is a binding site for a gold substrate. A peptide containing α-aminoisobutyric acid (Aib) tends
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