Adenine-Uridine Base Pairing at the Water-Solid-Interface Michael Weisser, Josua Ka 1 shammer, Bernhard Menges, Jin Matsumoto, ² Fumio Nakamura, ² Kuniharu Ijiro, ² Masatsugu Shimomura, ² and Silvia Mittler* Contribution from the Max-Planck-Institut fu ¨ r Polymerforschung, Ackermann Weg 10, 55128 Mainz, Germany, and Research Institute for Electronic Science, Hokkaido UniVersity, N12W6, Sapporo 060, Japan ReceiVed February 11, 1999. ReVised Manuscript ReceiVed August 24, 1999 Abstract: The formation of the base pair adenine-uracil at a water-solid interface, at an immobilized monolayer of adenine disulfide with adenine groups exposed to the very surface, respectively, is shown here. To overcome the steric hindrance of tightly packed adenine groups in a pure adenine thiolate monolayer on gold, the formation of self-assembled monolayers out of a binary mixture of the adenine disulfide and CH 3 - or OH-terminated thiols are investigated. Electro-chemical investigations, surface plasmon spectroscopy (PSP, plasmon surface polariton), multimode waveguide-PSP-coupling spectroscopy, contact angle measurements, and spontaneous desorption time-of-flight mass spectrometry were used to characterize the monolayers. The specific base pairing was investigated for a variety of monolayer compositions. A specific base pairing was successful for an optimized mixed adenine/OH-terminated thiol monolayer. Nevertheless unspecific binding is a problem. Introduction To tailor molecular organization is one of the final goals of supra molecular chemistry 1,2 and is essential for the design of molecular devices. 3,4 Weak intermolecular interactions such as hydrogen bonds, 5 as well as the interactions in typical guest- host systems such as in the biotin-streptavidin-system 6,7 or in the family of the cyclic molecules such as cylodextrins 8-12 or calixarenes 13,14 are well-known architectural tools for the assembling of molecular organization. Nature has used these tools in a wide variety to create functionality. 15 Cell-cell recognition or communication via guest-host reactions of proteins or the most versatile DNA double helix are typical examples where molecular organization based on specific intermolecular interactions delivers a very particular biological functionality and therefore contains well-defined information. 15 Hydrogen bonding was studied intensely for the past decades within three-dimensional geometry, for example, DNA in solution. 15 Two-dimensional arrangements of molecules being able to develop hydrogen bonds with a fitting partner were studied for the first time by Kurihara et al. in 1991. 16 The two- dimensional geometry was achieved by an air-water interface on a Langmuir-Blodgett (LB) trough, where the hydrogen bonding took place in the water phase. An amphiphilic diami- notriazine was able to selectively bind nucleosides and nucleic acid bases. In 1997 the base pairing of cytosine with guanosine at the air-water interface was found as the first two-dimensional system being biologically relevant. 17 In the same year Matsuura et al. 18 have demonstrated the two-dimensional hydrogen bonding on a solid-air interface via a self-assembled monolayer. Recently an artificial hydrogen bonding molecular pair was demonstrated at a solid-hydrophilic organic solvent interface. 19 Here we like to demonstrate the possibility of using hydrogen bonding in a two-dimensional array at the solid-water interface. Hydrogen bonding in water is especially significant due to its relevance to biological molecular recognition. The base pair adenine-uracil was chosen. Therefore a disulfide with two spacers and an adenine headgroup at each end (Figure 2) was synthesized for forming self-assembled adenine thiolate mono- layers exposing adenine groups to the very surface of a solid metal substrate. 20 Uridine as the water soluble derivative of uracil was investigated for the binding processes. For unspecific binding tests cytidine was used. Figure 1 demonstrates the specific recognition and binding via hydrogen bonds of the base pairs adenine/uracil and guanine/cytosine. The chemical struc- tures of the water soluble uridine and cytidine used in this study are shown as well (Figure 1b). * Corresponding author: e-mail: mittler@mpip-mainz.mpg.de. ² Hokkaido University. (1) Lehn, J. M. Angew. Chem., Int. Ed. Engl. 1989, 27, 89. (2) Lehn, J. M. Angew. Chem., Int. Ed. Engl. 1990, 29, 1304. (3) Ringsdorf, H.; Schlarb, B.; Venzmer, J. Angew. Chem., Int. Ed. Engl. 1989, 27, 113. (4) Kunitake, T. Angew. Chem., Int. Ed. Engl. 1992, 31, 706. (5) Boschke, F. 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