Biological systems, complex interactions, modeling

Varying the side groups X in 27b affects both the stability and selectivity of the complexes (lateral discrimination), and allows the receptor-substrate interactions in biological systems to be modelled, for instance, the interaction between nicotinamide and tryptophan [2.109b]. One may attach to 27b amino acid residues (leading to parallel peptides [2.109] as in 27c), nucleic acid bases or nucleosides, saccharides, etc. The structural features of 27 and its remarkable binding properties make it an attractive unit for the construction of macropolycyclic multisite receptors, molecular catalysts, and carriers for membrane transport. Such extensions require sepa-... 

Tt is a complex and tedious task to understand completely the behavior and interaction of biological systems. Redundancies, interactions, and complexities are well beyond what might be imagined necessary for a physical or engineering system. Regardless of this, ideas are formed on the interaction of components, and theories are advanced to explain the behavior of biological systems. These ideas and theories constitute conceptual models which are evaluated and tested in experiments designed for that purpose. 

The theory of electron transfer in chemical and biological systems has been discussed by Marcus and many other workers 74 84). Recently, Larson 8l) has discussed the theory of electron transfer in protein and polymer-metal complex structures on the basis of a model first proposed by Marcus. In biological systems, electrons are mediated between redox centers over large distances (1.5 to 3.0 nm). Under non-adiabatic conditions, as the two energy surfaces have little interaction (Fig. 5), the electron transfer reaction does not occur. If there is weak interaction between the two surfaces, a, and a2, the system tends to split into two continuous energy surfaces, A3 and A2, with a small gap A which corresponds to the electronic coupling matrix element. Under such conditions, electron transfer from reductant to oxidant may occur, with the probability (x) given by Eq. (10),... 

Since the interpretation of NMR parameters in complex biological systems is considerably facilitated by the use of model compounds it is evident that a proper understanding of the metal-ligand interactions in the synthetic counterparts of vitamin B12, which are the subject of this review, can yield additional details and help clarify the role of corrinoids in Nature. 

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