Redox noncovalent interactions

In contrast to the HOMO and LUMO, the singly occupied molecular orbital (SOMO) can be correlated both qualitatively and quantitatively with experimentally measurable EPR hyperfine couplings (hfcs). As a result, host-guest systems that have redox-active guests that are stable as radicals provide excellent tools for studying the effects of noncovalent interactions on redox properties. 

Interactions with a protein may profoundly change the E° value of an associated redox group from the solution value. The chemical nature of the ligands, the detailed coordination geometry, and noncovalent interactions between the protein and redox group all contribute to the observed E° value. 

Similar to quinones and NAD+ the flavins are one- and two-electron redox systems (Fig. 7.2.10a). Upon half-reduction a quinhydrone-type dimer appears, producing a charge transfer band at 820 nm (Fig. 7.2.10b). The noncovalent interactions of flavins arise from electrostatic attraction between electron-rich donor atoms, in particular the basic oxygen atoms of amide groups, and electron-deficient aromatic systems, the inner conjugation system of oxidized flavins (Breinlinger et al., 1998). This is reminescent of the interactions between porphyrin macrocycles (see Fig. 6.2.15). 

Rotaxanes are molecules that consist of one or more macrocyclic rings (wheels) threaded by a linear component (axle) with bulky stopper groups at both ends. The sliding, back-and-forth motion of the macrocyclic rings is spatially limited by the stopper groups, which prevent axle-wheel dissociation. A catenane is composed of two or more interlocked macrocycles. The relative positions of the interlocked components in these fascinating molecules can be determined to some extent by noncovalent interactions, and thus, the possibility to exert interactive control by redox conversions quickly became a goal in the development of molecular machines. A review of the extensive work in this area is available.Here, we will discuss some key examples. 

Destabilization of complexes upon redox stimulation tends to occur for a very practical reason. The redox center is usually involved in the specific noncovalent interaction that holds the complex together. Therefore, a fundamental change in its redox state removes some or all of the attractive interaction, making the complex unstable with respect to complexation that is, a net attraction is transformed into a net repulsion. At an intellectual level, this behavior has led to the creation of some exciting MIMs for use as artificial molecular machines (see Molecular Devices Molecular Machinery, Supramolecular Devices and Pho-tochemically Driven Moiecular Devices and Machines, Nanotechnology). 

Conditions for achieving efficient DET via enzyme immobilization are dictated partly by materials architecture. Enzyme immobilization techniques may include nonspecific adsorption, covalent linkage, entrapment in conductive polymeric films, association with metal colloids, and encapsulation within porous matrices (see Chapter 11). The simplest method is nonspecific adsorption, but control is limited various noncovalent interactions will yield different orientations of the redox center with respect to the electrode interface and, as a result, inefficient DET. 

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