An Alternative Approach to Quantify Partition Processes in Confined Environments: The Electrochemical Behavior of PRODAN in Unilamellar Vesicles

Abstract

Herein, we investigate the behavior of the electroactive molecular probe 6-propionyl-2-dimethyl amino naphthalene (PRODAN) in large unilamellar vesicles (LUV) formed with the phospholipid 1,2-di-oleoyl-sn-glycero-3-phosphatidylcholine (DOPC) by using cyclic voltammetry (CV). The CV studies in pure water confirm our previous spectroscopic results that PRODAN self-aggregates due to its low water solubility. Moreover, the electrochemical results also reveal that the PRODAN aggregated species are non-electroactive within the studied electrochemical potential region. In DOPC LUV media, the redox behavior of PRODAN shows how the LUV bilayer interacts with PRODAN aggregated species to form PRODAN monomer species. Moreover, the electrochemical response of PRODAN allows us to propose a model for explaining the electrochemical experimental results and--in conjunction with our measurements--for calculating the value of the partition constant (K(p)) of PRODAN between the water and LUV bilayer pseudophases. This value coincides with that obtained through an independent technique. Moreover, our electrochemical model allows us to calculate the diffusion coefficient (D) for the DOPC LUV, which coincides with the D value obtained through dynamic light scattering (DLS). Thus, our data clearly show that electrochemical measurements could be a powerful alternative approach to investigate the behavior of nonionic electroactive molecules embed in a confined environment such as the LUV bilayer. Moreover, we believe that this approach can be used to investigate the behavior of non-optical molecular drugs embedded in bilayer media.