Correlation between initial biodegradability determined by docking studies and structure of alkylbenzene sulfonates: A new tool for intelligent design of environmentally friendly anionic surfactants

Abstract

Gray water constitutes an important fraction of total wastewater. Some of the most problematic compounds in gray water are the anionic surfactants used as an ingredient for domestic and industrial soaps and detergents. The alkylbenzene sulfonates used in commercially available formula are highly complex mixtures of linear (LAS) and branched (BAS) molecules. LAS are classified generally as biodegradable, although their widespread use generates accumulation in the environment. Docking tools, widely used in recent years in the bioremediation field, allow molecular modeling of the ligand-enzyme interaction, which is key to understanding and evaluating the possibility of biodegradation. In this work, molecular details that allow us to establish a biodegradation pattern for some alkylbenzene sulfonates were elucidated. Two hydrogen bonds, key for the anchorage of surfactants to the monooxygenase active site involved in the initial biodegradation, were found. These bonds determine the way surfactants locate in the hydrophobic pocket of the enzyme affecting the biodegradation rate in a structurally dependent manner. For C10 to C12 linear isomers, the degradation rate increased together with the length of the hydrocarbon chain. For C13 and C14 isomers, steric difficulties to accommodate the surfactant molecule in the catalytic site were observed. For branched chain isomers, little or no biodegradation was found. In addition, biodegradation was lower in mixtures than for the pure isomers. These results will allow an intelligent design of this family of anionic surfactants to attenuate their contaminating effects in waters and soils. This study constitutes, to the best of our knowledge, a novel contribution towards the design of environmentally friendly surfactants with higher probabilities of being biodegraded to complete mineralization.