Proton diffusion at phospholipid assemblies.

A new scanning electrochemical microscopy proton feedback method has been developed for investigating lateral proton diffusion at phospholipid assemblies: specifically Langmuir monolayers at the water/air interface. In this approach, a base is electrogenerated by the reduction of a weak acid (producing hydrogen) at a "submarine" ultramicroelectrode (UME) placed in the aqueous subphase of a Langmuir trough close to a monolayer. The electrogenerated base diffuses to and titrates monolayer-bound protons and is converted back to its initial form, so enhancing the current response at the UME. Local deprotonation of the monolayer creates a concentration gradient for lateral proton diffusion. A numerical model has been developed, taking into account the potential-dependent association/dissociation constant of the interfacial acid groups. A comparison is made of monolayers comprising either acidic DL-alpha-phosphatidyl-L-serine, dipalmitoyl (DPPS) or zwitterionic L-alpha-phosphatidylcholine, dipalmitoyl (DPPC) monolayers at a range of surface pressures. It is demonstrated that lateral proton fluxes at DPPS are significant, but the lateral proton diffusion coefficient is lower than in bulk solution. In contrast, lateral proton diffusion cannot be detected at DPPC, suggesting that the acid/base character of the phospholipid is important in determining the magnitude of interfacial proton fluxes.