ESI-MS method for in vitro investigation of skin penetration by copper-amino acid complexes: from an emulsion through a model membrane. 
Copper can be found in many cosmetic formulations, mainly as complexes with peptides, hydroxyacids or amino acids.  The main reason that the usage of this element in this context is still increasing is its beneficial biochemical activity, although the mechanism that enables its complexes to permeate through skin barriers is largely unknown.  The ability of copper complexes with amino acids to penetrate through the stratum corneum and participate in copper ion transport processes is key to their cosmetic and pharmaceutical activities.  The penetration process was studied in vitro in a model system, a Franz diffusion cell with a liposome membrane, where a liquid crystalline system with physicochemical properties similar to those of the intercellular cement of stratum corneum was used to model the skin barrier.  The influences of various ligands on the model membrane migration rate of copper ions was studied, and the results highlighted the crucial roles of metal ion complex structure and stability in this process.