The Conformational Flexibility of Humic Supramolecular Structures

The energetic implications behind the supramolecular structure of SOM are well depicted by molecular simulations through conformational softwares.

A minimization of conformational energy was conducted using HyperchemT 4.0 software to describe the interactions of humic supramolecular associations with an organic acid. Eleven different molecular structures of compounds identified as components of HS (Stevenson 1994) were grouped together in the simulation to form a supramolecular association. The structures represented molecules such as saturated and unsaturated fatty acids, carbohydrates, peptides, lignin derivatives, etc., with molecular weights varying from 116 Da for a dihydroxybenzene to 504 Da for a triglucose. The molecular weight sum of the 11 molecules was 3,065 Da.

The geometry of the association was automatically adjusted and its conforma-tional energy was minimized in the vacuum (Fig. 1.2a). Ten molecules of acetic acid were added first to surround the hypothetical supramolecular association (Fig. 1.2b) and then placed within the conformation of the association (Fig. 1.2c). The resulting association energies were calculated by the software to be 114, 91.2, and 84.0 Kcal mol-1, respectively.

The association of the different molecules also varied its physical appearance with the approach of acetic acid molecules which caused a loosening of intermolecular attractions until some spaces among the molecules were formed.

Fig. 1.2 Computer simulation of the optimum conformational energy (in vacuo) for an association of 11 different humic precursors with a total molecular weight of 3,065 Da. (a) Upper picture: molecular association with an energy of 114 Kcal moP1; (b) middle picture: molecular association surrounded by ten molecules of acetic acid with an energy of 91.2 Kcal moP1; (c) lower picture: molecular association containing ten molecules of acetic acid with an energy of 84.0 Kcal moP1

The computer simulation (Fig. 1.2) pictorially shows that the addition of organic acids to humic molecules is capable of reducing the solvation energy and, concom-itantly, causing a partial disruption of their association. These results are in line with the different experiments describing the behavior of organic matter dissolved in solution reported by Piccolo (2001, 2002).

Conversely, when a covalently linked structure of HS, based on the traditional macro polymeric model, was placed in the same exercise of molecular simulation, no significant changes in energy content and physical association were noted with the addition of acetic acid. Figure 1.3a shows that a covalently bonded polymeric structure with a molecular weight of 6,326 Da is not significantly altered (Fig. 1.3b) by the same number of acetic acid molecules used for the simulation of the weakly bound supramolecular association shown in Fig. 1.2. Moreover, the gain in conformational energy was only of 10 Kcal mol_1, passing from 627.40 Kcal moP1 for the polymer to 617.26 Kcal moP1 for the same polymer added with acetic acid. Thus, it would be hardly possible, using this hypothetical polymeric model, that the simple addition of acetic acid molecules to such a high molecular weight polymer would provide a rearrangement of molecular associations leading to conformational disruptions as that described by the experiments funding the supramolecular structure of SOM (Piccolo 2001, 2002).

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