Amino acids, conformational properties

The behavior of immobilized enzymes differs from that of dissolved enzymes because of the effects of the support material, or matrix, as well as conformational changes in the enzyme that result from interactions with the support and covalent modification of amino acid residues. Properties observed to change significantly upon immobilization include specific activity, pH optimum, Km, selectivity, and stability.23 Physical immobilization methods, especially entrapment and encapsulation, yield less dramatic changes in an enzyme s catalytic behavior than chemical immobilization methods or adsorption. The reason is that entrapment and encapsulation result in the enzyme remaining essentially in its native conformation, in a hydrophilic environment, with no covalent modification. 

The transition dipole moment, and therefore the oscillator strengths, are sensitive to the inclusion of the amino acid, conformation, method, and basis sets. This makes accurate calculations involving the oscillator strengths difficult to obtain. The oscillator strengths of the amino acids qualitatively show their order of absorptivity and lifetimes, but do not agree well with the experimental lifetimes [4, 5, 35]. The sensitivity of these properties on the conformers and the method contributes to the poor agreement with experiment. 

In the gas phase, amino acids are neutral and the conformational properties of the backbone are determined by the torsions Tj(H-N-C-C), t2(N-C-C=0), and t3(0=C-0-H). In the planar structures of glycine, two meaningful arrangements can be expected for each of these torsions, in the vicinity of 60° or 120° for tp and 0° or 180° for t2 and t3. The possible combinations yield eight different conformers. The number is enhanced when planar or nonplanar geometries are considered in some cases. A summary of the possible conformations of glycine is given in Fig. 7.2. 

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