Functionalized Antibodies as Biosensing Materials and Catalysts Hiroyasu Yamaguchi and Akira Harada  (Received August 14, 2008; CL-088009) Abstract Monoclonal antibodies have been prepared against water-solu- ble porphyrins, viologen derivatives, and transition-metal com- plexes, respectively. These monoclonal antibodies were utilized to devise biosensing and catalytic systems. Introduction The immune system has an ability to generate antibodies against virtually any molecule of interest. Recently, much atten- tion has been directed toward antibodies not only in the field of biology but also in the field of chemistry because of their unique structures and functions. Antibodies, immunoglobulins, have been studied as sensors, 1 diagnostics, 2 DDS, 3 catalysts (catalytic antibodies), 4 and components for nanotechnology. 5 With the advent of cell technology, 6 it has become possible to prepare individual immunoglobulins that are called ‘‘mono- clonal antibodies’’ in large amounts and in homogeneous form. We have focused our attention on the special behavior of anti- bodies, especially monoclonal antibodies, because they can recognize a larger and more complex compound with higher specificity than enzymes can. We have prepared monoclonal antibodies for porphyrins, 7 viologens, 8 and transition-metal complexes 9 to construct supramolecular materials 10 and to use these complexes as biosensing materials or specific catalysts (Scheme 1). The first topic in this review is the construction of antibody supramolecules and their application for biosensing systems. Antibodies have been widely used as efficient reagents to detect target molecules. Based on the principle that an antibody reacts with an antigen specifically and by non-covalent bonds, several procedures have been developed in the immunosorbent assay. 11 Labeled antibodies or antigens are used for the detection, local- ization, and quantification of biological constituents. More re- cently, an optical technique based on surface plasmon resonance (SPR) 12 or a microgravimetric quartz-crystal-microbalance (QCM) 13 technique has been found to be useful for measuring and characterizing macromolecular interactions in the increas- ingly expanding area of biosensor technology. SPR in particular has great potential for macromolecular interaction analysis in terms of sensitivity and signal translation. The use of biosensors based on SPR has made it possible to determine kinetic param- eters in real time and without any labeling of biomacromolecules for detection. However, the SPR response reflects a change in mass concentration at the detector surface as molecules bind or dissociate; the specific sensing of substrates with low molecu- lar weight is difficult. In such a case, functional molecules with a high molecular weight such as antibodies have a great potential for amplification of the response signals. Here, we designed lin- ear and dendritic antibody supramolecular complexes. These supramolecular formations are utilized for the amplification of detection signals for biosensor techniques. The second topic is concerned with functionalized catalysts prepared by the combination of monoclonal antibodies with co- factors. The development of general strategies for introducing catalytic activity into antibody combining sites could lead to a new class of enzyme-like catalysts with tailored specificities. Es- pecially, strategies that allow incorporation of cofactors into an- tibody combining sites should expand the scope of antibody cat- alysis. We used porphyrins or transition-metal complexes as a cofactor. Peroxidase activity of iron porphyrin–antibody com- plexes has been investigated. The electron-transfer reaction from porphyrin molecules to electron acceptors could be controlled by the binding of the antibodies to porphyrin molecules. We have constructed a hydrogen evolution system using porphyrin–anti- body complexes. Transition metals such as Rh, Pd, Pt, and Ru have been ex- tensively used as heterogeneous catalysts 14 for various transfor- mations of molecules. A great number of transition-metal com- plexes have been prepared and used as homogeneous catalysts, 15 because they are considered as an intermediate of metal-cata- lyzed reactions. However, complexes of transition metals such as Rh, Pd, Pt, and Ru have not been found in enzymes. If these complexes can be used as if they are cofactors of enzymes, the scope of the catalysts will be revolutionarily broadened. Asym- metric catalyses 16,17 have attracted much attention because of the importance of chirality for living systems. In recent years, water- soluble complexes of transition metals with proteins or DNAs have played an important role in synthetic chemistry as environ- mentally benign catalysts. 18 In all cases, the metal complexes were incorporated into biomolecules by non-direct methods, for example the utilization of avidin–biotin interactions during the complex formation of avidin with a biotinylated metal com- plex. 19 The most important method to directly incorporate tran- sition-metal complexes into proteins is thought to be the prepa- ration of monoclonal antibodies 20 against transition-metal com- plexes. Now we have succeeded in preparing monoclonal anti- bodies for a transition-metal complex. Highlight Review Dr. Hiroyasu Yamaguchi, Prof. Akira Harada Department of Macromolecular Science, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043 E-mail: harada@chem.sci.osaka-u.ac.jp 1184 Chemistry Letters Vol.37, No.12 (2008) Copyright Ó 2008 The Chemical Society of Japan