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Magnetic properties RMn2Ge2 compounds where R is La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er, and Th have been investigated from 4.2 to 500 K. LaMn2Ge2, CeMn2Ge2, PrMn2Ge2, and NdMn2Ge2 are ferromagnetic with Curie temperatures of 306, 316, 334, and 334 K, respectively. The heavy rare-earth compounds do not order ferromagnetically until low temperatures. GdMn2Ge2 shows a sharp magnetic transition at 97 K both in the magnetization-vs-temperature and resistivity-vs-temperature data. For this compound, it is proposed that above 97 K the Mn moments order antiferromagnetically among themselves while the Gd moments are disordered. Below 97 K the Mn moments couple ferromagnetically with each other but are opposed to the ordered Gd sublattice. Magnetic moment obtained at 4.2 K can be explained for all the compounds on the basis of an antiferromagnetic coupling between Mn and rare-earth spins.

The immobilization of enzymes In a manner that promotes direct electron transfer between the enzyme cofactor and an electrode surface is of considerable Interest for the develop ment of biosensors having high substrate specificity. Direct electron transfer between immobilized glucose oxidase (GO) and (aminopheny1)boronic acid modified glassy carbon (GC) electrodes was observed in the present study. Surface carboxylic acid groups were generated on gtassy carbon rods by chemical oxidation. (3-Aminophenyi)boronlc acid (APBA) was coupled to the carboxylic acid functions after actlvatlon with water-soluble carbodiknkle to give GC-APBA electrodes. Interaction with GO furnished GC-APBA-GO electrodes with enzyme immobillred presumably by complex formation of the sugar portlon of the enzyme with the boronic acid groupings on the GC surface. This is the first report of the lmmoblllzation of an enzyme to an electrode surface through boronate groups. Direct electron transfer between the enyme cofactor and the electrode was demonstrated by cyck and differential pulse voltammetry. No significant electron transfer was observed with GO in solution either alone or with added APBA. Possible explanatlons are examlned for the direct electron transfer.The direct electron transfer (without the use of mediators) between the cofactor portion of a flavoenzyme and an electrode surface is of interest for development of novel enzyme biosensors or enzyme electrochemical reactors. The possibilities of achieving direct electron transfer in principle should be enhanced by bringing the flavoenzyme close to the electrode surface, such as by immobilization. Although many studies have been reported wherein the flavoenzyme glucose oxidase or L-amino acid oxidase have been immobilized on electrode surfaces, very few of these studies have included an investigation of the possibilities for direct electron transfer in the absence of mediators. With glucose oxidase immobilized by adsorption on graphite electrodes, no evidence for direct electron transfer has been obtained by using cyclic voltammetry ( I , 2). However, a reduction peak has been observed with glucose oxidase adsorbed on graphite using the more sensitive differential pulse voltammetry (2,3). Direct electron transfer has been claimed for glucose oxidase covalently attached via a cyanuric chloride bridge to graphik and measured using differential pulse voltammetry (3). The voltammograms that showed direct electron transfer were attributed to oxidation-reduction of the flavin cofactor while still complexed with the apoenzyme and not to flavin molecules that had become dissociated from the apoenzyme and adsorbed onto the graphite surface.We have been interested for over 8 years in the immobilization and characterization of glucose oxidase on electrode surfaces (4-7), especially for the development of an in vivo glucose sensor. Much of our recent work has dealt with methodology for the attachment of the enzyme to the electrode surface in such a way as to achieve rapid and direct electron transfe...

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