The conformational dynamics of humic polyanions in model organic and organo-mineral aggregates

Molecular mechanics calculations and simulated annealing were applied to mo del humic polyanions originating from lignin, The dynamic behavior of such oxidized lignins in model soil organic complexes, such as an oxidized ligni n-carbohydrate complex (LCC) and humic (oxidized LCC)-clay aggregates, was analyzed. Neither ionization nor hydrogen bonding bring significant changes in the conformational properties of oxidized lignin and LCC, Oxidized lign in and LCC oligomers (humic substances in soil) bind to the mineral surface s, a process that was exemplified in computational experiments on complexes with muscovite. Upon ionization, a lignin-derived oligomer develops strong attractive organo-mineral interactions through cation bridges. Without met al cations, electrostatic repulsion between negatively charged anions and t he oxygen-mineral surface prevails, and the two parts of the organo-mineral complex drift apart. This tendency is typical of an oxidized lignin oligom er but not of a topological oxidized LCC. (C) 1999 Elsevier Science B.V. Al l rights reserved.