Quantum computational study on the release of natural antimicrobial compounds from biopolymer into water

The release of active compounds in aqueous medium from polymers is a fundamental problem. The release of antimicrobial compounds can solve several problems as fouling due to the proliferation of microorganisms on membrane surface. An accurate description of hydrogen bonds and van Der Waals interactions used by the antimicrobials to bond with polymers is desirable. The aim of this work is to describe, in the frame of Density Functional Theory, the non-covalent bonds used by p-coumaric-acid(pC), Pinobanksin(Pb), Pinobanksin-5-di-methylether(Pb5ME) and Chrisyn(C) to bind with polylactic acid polymer (PLA). At equilibrium, the concentration of each substance depend on the thermodynamical parameter K. All experimental K values are positive which means that the phenol concentration on the PLA is greater than in water. In addition this trend of K was observed: C> Pb> Pb5ME > pC. Thus, the pC shows less affinity to PLA interface with respect to the other phenols, whereas C has the highest affinity towards the PLA. As first approximation, K depends on the non-covalent interactions between the antimicrobial molecules and the PLA functional groups. The electronic energy contribution to the non-covalent binding energy was evaluated as the difference between the energy of the PLA-phenol adduct and the energies of the single separated components. This difference includes the interactions due to molecular orbital overlap and hydrogen bonding. The contribution, due to intermolecular van Der Waals forces, was then subsequently added to the non-covalent binding energy according to an empiric long-range contribution. The van Der Waals term takes into account the interaction due to the electron fluctuation. The difference between phenol hydration energy and the binding energy of the PLA-phenol adduct ( ) must be positive in order to fit the experimental K values. The quantum calculations show that the Pb gives positive when it is linked to PLA by means of a specific hydrogen bond. Instead, the values of , referred to pC are never positive although they are all slightly negative, around -0.39 (kcal/mol). These small differences may be due to the evaluation of the weak intermolecular van Der Waals energies. However, the accurate evaluation of the dispersion energies (van Der Waals forces) is a fundamental, current and open problem in the framework of DFT, that goes beyond the scope of this work. Using a hydrolyzed PLA model, the most probable adduct, PLA(cooh)-pC, gives a positive and bigger than the value referred to the PLA(cooh)-Pb complex. This is not in agreement with the aforementioned experimental data. Thus, the adducts formed with the non hydrolyzed PLA should be considered the best structures to model the bonds between the antimicrobials and PLA. The accurate modeling, at molecular level, of non covalent interactions between polyphenols (flavonols) showing antimicrobial proprieties and functional groups of other bio-polymers (i.e. chitosan) is important because it permits modulating the release of these active substances.