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Thermodynamic description of linoleic and α-linolenic acid biohydrogenation in the rumen: A computational approach

Julián Andrés Castillo Vargas  ; Edgar E. D. Caicedo ; Claudio V.D.M Ribeiro
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Several studies have been conducted to estimate the transfer rates for the main biohydrogenation (BH) steps of linoleic acid (LA; c9,c12-18:2), ?-linolenic acid (LN; c9,c12,c15-18:3), and oleic acid (OL; c9-18:1), because their BH can produce fatty acids with potential biological activity, enhancing the nutritional value of meat, milk and derived products. Despite this, the free energy (?G; which define if a chemical reaction will occur or not) change of each step of LA and LN BH have not been addressed yet. Thus, the aim of this study was to estimate the ?G for the principal steps of LA, LN, and OL BH, and calculate the net ?G for the BH of LA, LN, and OL, using molecular modelling by quantum computational chemistry. The study was conducted in two steps. In the first step, ?G of formation (?Gf) at 39?C of LA, c9,t11-18:2 (CLA), t11-18:1 (VA), stearic acid (SA), LN, c9,t11,c15-18:3 (CTC), t11,c15-18:2 (TC), and OL were calculated using the parameter optimization method PM6 in Molecular Orbital Package Software (MOPAC; Version 8.289L). In the second step, the ?G for principal BH steps of LA (i.e., LA->CLA; CLA->VA; VA->SA), LN (i.e., LN->CTC; CTC->TC; TC->VA; VA->SA), OL (i.e., OL->SA), and the net ?G for LA->18:0, LN->SA, and OL->SA conversions were calculated according to Hess' law: ?G = (?Gf product) - (?Gf reagent), where: reagent->product. The ?Gf for LA, CLA, VA, SA, LN, CTC, TC, and OL were -144.6, -163.9, -166.6, -150.6, -143.9, -163.6, -166.3, and -147.7 kcal/mol. For LA BH, the ?G of LA->CLA, CLA->VA, and VA->SA were -19.36, -2.65, and +15.96 kJ/mol respectively. In the case of LN BH, the ?G of LN->CTC, CTC->TC, TC->VA, and VA->SA were -19.69, -2.70, -0.29, and +15.96 kJ/mol respectively. The net ?G for the LA->SA, LN->SA, and OL->SA steps were 6.05, -6.72, and -2.90 kJ/mol, respectively. Thus, the results of our study indicated the LA->CLA and LN->CTC conversions were the most thermodynamically favorable steps for the BH of LA and LN, respectively. In addition, the BH of LN was slightly more spontaneous than LA BH, and OL BH was less spontaneous than LA and LN, suggesting that LA and LN could be biohydrogenated more spontaneously than OL in the rumen.