Interação de naftoquinonas biologicamente ativas em superfícies de quitosana: mecanismo, cinética de adsorção e liberação controlada

Abstract

Quinones are natural or synthetic organic compounds and play important biological and physiological roles. Among the naphthoquinones, two of them appears as promising compounds: lapachol, found in large quantities in plants of the Bignoniaceae family, and its synthetic isomer, isolapachol. Lapachol and its derivative show several activities against the snail Biomphalaria glabrata, the intermediate host of the helminth Schistosoma mansoni, causative agent of schistosomiasis; against Trypanosoma cruzi, Chagas Disease causative agent and others. Isolapachol shows toxicity against fishes (Tilapia nilótica) and Artemia salina. A fact that can directly interfere in the activity of these substances is their lower solubility in aqueous media. It is necessary to employ of more water soluble derivatives, like their salts. Another substance in study is chitosan, a polisacaride from chitin found in great amounts at crustaceous, insects and fungi. It possesses a several of usefull things, like in treatment of wastewater, and in drug delivery systems. The interaction of lapachol and isolapachol sodium salts (LPS and IPS respectively) with chitosan has not found in literature. The aim of this work was to perform a detailed analysis of the interaction of chitosan, LPS and IPS, to obtain the physical-chemical parameters which influence the of adsorption processes of these substances. The first studies has shown that LPS and IPS undergo a chemical denaturation for a long period of storage time. Another results showed that the LPS and IPS solutions are also chemically instable. It was found that chitosan is fully protonated after 15 minutes of contact time, which was confirmed by the pH increasing to 7,0 after acid contact. The adsorption studies of LPS and IPS showed that both adsorb on chitosan, and those processes are more effective at lower pH solutions. The kinetic studies showed that the adsorptions are fast and the equilibria adsorption are reached after 60 minutes of contact time. Adsorption kinetics fit follows well the pseudo-second-order model. The adsorption isotherms of LPS and IPS exhibit higher amounts of IPS than LPS and both fit preferentially the model described by Langmuir, indicating adsorptions in monolayers on energetically homogeneous surfaces. LPS and IPS adsorptions were also influenced by the increase of ionic strength in solution indicating that the main interaction is of electrostatic nature. The thermodynamic parameters showed that the adsorption processes are spontaneous for the two quinones. In LPS the process is endothermic with positive entropy. In IPS, the adsorption is exothermic with negative entropy. These results shown the importance of the resonance of the naphtoquinonic group with the double bond at the side chain, in the adsorption of LPS and IPS. When the desorption was studied, the presence of a phosphate buffer media influence this process significantly, while the temperature does not affect the process and that the IPS is released in lower amounts that LPS.