THERMODYNAMICS AND KINETICS COMPUTATIONAL INVESTIGATION ON HYDROGEN ATOM TRANSFER MECHANISM OF NATURAL POLYPHENOLS TOWARD HYDROPEROXYL RADICAL BASED ON DENSITY FUNCTIONAL THEORY

• Main Author: SILFIA PULO BOLI (NIM. 23315302), LUSIA
• Format: Theses
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/22933
• Institution: Institut Teknologi Bandung

This research is aimed to understand thermodynamics and kinetics characteristics of three polyphenols molecules in attempting to prevent lipid peroxidation initiated by hydroperoxyl radical (HO2.). The three polyphenols proposed are Indonesian local plants extracts. They are gnetin c, gamma mangostin and tetrahydrocurcumin. HO2. is a free radical in biological system which is possible to interact with lipid membrane and initiate lipid peroxidation. This oxidation process lead to a cell destruction. We proposed hydrogen atom transfer mechanism from an antioxidant molecule toward this HO2. in order to scavenge this radical. <br /> <br /> We used density functional theory scheme in this research to study antioxidant activity of our proposing molecules. We analyzed antioxidant activity of the molecules, thermodynamically and kinetically, through their ability to transfer a hydrogen atom toward HO2.. We used B3LYP//6-31++G(d,p) level of theory to support our analysis. Geometry optimization and frequency calculation of all molecules were done in this level of theory. We calculated bond dissociation enthalpy to guide us to sites potentially in transferring a hydrogen atom. We also calculated Gibbs free energy to review sites of molecules with an exotermic reaction and where reaction occurs thermodynamically. Transition state calculation was performed in the same level of theory to get activation energy of reaction between three antioxidant candidates and HO2.. Finally, we used the activation energy to calculate the rate constant by using transition state theory in gas phase and water solvents environment. <br /> <br /> Our results suggested that there are three sites in gnetin c (site 1, 4 and 5), two sites in gamma mangostin (site 1 and 2) and one site of tetrahydrocurcumin (site 1) which are possible to transfer their hydrogen atom toward HO2.. Thermodynamically, it showed that a favoured reaction occurred through site 1 of each of the three antioxidant molecules. Kinetically, the value of rate constant of gnetin c at site 1 > tetrahydrocurcumin site 1> gamma mangostin at site 2. Antioxidant activity of these three molecules is independently correlated to the number of O-H bonds in each molecules.