Aperiodicity Effects in Polymers

Impurity and Aperiodicity Effects in Polymers.—The presence of various impurity centres (cations and water in DNA, halogens in polyacetylenes, etc.) contributes basically to the physics of polymeric materials. Many polymers (like proteins or DNA) are, however, by their very nature aperiodic. The inclusion of these effects considerably complicates the electronic structure investigations both from the conceptual and computational points of view. We briefly mentioned earlier the theoretical possibilities of accounting for such effects. Apart from the simplest ones, periodic cluster calculations, virtual crystal approximation, and Dean s method in its simplest form, the application of these theoretical methods [the coherent potential approximation (CPA),103 Dean s method in its SCF form,51 the Hartree-Fock Green s matrix (resolvent) method, etc.] is a tedious work, usually necessitating more computational effort than the periodic calculations 

Mely and A. Pullman, Theor. Chim. Acta Berlin), 1969, 13, 278. 

Polyglycine main chain Polyglycine H-bondedperpendicular chain 

Periodic Cluster Calculations. In this method the elementary cell(s) of the host crystal and the impurity atom(s) are treated together as a cluster. This cluster is periodically repeated and if it is large enough compared with the size of the impurity itself the method provides a fairly realistic description of impurity levels, changes in the electron density around the impurities, etc. We present here two applications of this procedure to DNA and protein chains. 

Effect of Water and Divalent Metal Ions on the Electronic Structure of DNA. As a first step in investigating the effect of impurities on the electronic structure of DNA, the band structure of poly(G-C) was computed in the PPP CO approximation assuming that one or two water molecules are bound by hydrogen bonds to the NH2 group of each C molecule or/and to the 0=0 group of each G molecule.94 According to the results obtained for these systems the additional 7r-orbital in the H20 molecules produced an extra 71-band between the lowest filled bands, while the other bands remained practically unchanged. 

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