Experimental and Computational Investigation of The Adsorption of Pyocyanin on Clay Minerals

Abstract

Pyocyanin is one of the most important virulence factors in the resistance of Pseudomonas aeruginosa to antibiotics. The biotoxin facilitates the establishment of biofilms, suppression of the immune system, and cytotoxicity of hosts’ cells. In view of the importance of pyocyanin in the pathogenicity of Pseudomonas aeruginosa, the inactivation and degradation of pyocyanin has been proposed as a novel approach of disarming the pathogen. The present study focuses on the inactivation (by adsorption and degradation) of pyocyanin by two groups of clay minerals ― palygorskite and sepiolite group and smectite group (e.g., montmorillonite, hectorite, etc.). A combination of adsorption experiments, in-situ variable-temperature x-ray diffraction, variable-temperature and moisture infrared spectroscopy, energy dispersive x-ray spectrometry, UV spectroscopy, and molecular dynamics (MD) simulation were employed to investigate: (1) the potential of clay minerals as pyocyanin binder and degraders, (2) the adsorption mechanism and the binding sites for pyocyanin adsorption on the clay minerals, (3) the role of the structural properties of clay minerals on pyocyanin adsorption, (4) the bonding mechanism in pyocyanin‒clay complexes, and (5) the structure and dynamics of the pyocyanin‒clay complex. Sepiolite, palygorskite and montmorillonite demonstrated high adsorption capacity for pyocyanin. The adsorption site was dominantly in the tunnel (sepiolite), external surfaces (palygorskite), and interlayer (montmorillonite). The adsorbed pyocyanin was slightly transformed on the surfaces of the clay minerals except on the surface of palygorskite. Adsorption of pyocyanin increased the thermal stability of the layers (montmorillonite) and tunnels (sepiolite) of the clay minerals. On montmorillonite, two types of adsorption mechanisms were observed: (1) protonation of the neutral pyocyanin in the interlayer followed by ion-exchange (e.g., Na+‒montmorillonite) and (2) adsorption of pyocyanin without protonation (e.g., Cu2+‒montmorillonite). The bonding mechanism in pyocyanin‒montmorillonite is either outer-sphere coordination (e.g., Na+‒montmorillonite) or inner-sphere coordination (e.g., Cu2+‒montmorillonite). In the tunnels of sepiolite, pyocyanin out-competes zeolitic water for adsorption sites on the bound water (OH2). Classical molecular dynamics simulation agreed with experimental results in that the bonding in pyocyanin‒montmorillonite complex was dependent on the moisture regime ― dry or moist and speciation of pyocyanin ― neutral or protonated.