Problems of in vitro SPF measurements brought about by viscous fingering generated during sunscreen applications.

Up to date, no worldwide standard in vitro method has been established for the determination of the sun protection factor (SPF), since there are many problems in terms of its repeatability and reliability. Here, we have studied the problems on the in vitro SPF measurements brought about by the phenomenon called viscous fingering. A spatially periodic stripe pattern is usually formed spontaneously when a viscous fluid is applied onto a solid substrate. For the in vitro SPF measurements, the recommended amount of sunscreen is applied onto a substrate, and the intensity of the transmitted UV light through the sunscreen layer is evaluated. Our theoretical analysis indicated that the nonuniformity of the thickness of the sunscreen layer varied the net UV absorbance. Pseudo-sunscreen composites having no phase separation structures were prepared and applied on a quartz plate for the measurements of the UV absorbance. Two types of applicators, a block applicator and a 4-sided applicator were used. The flat surface was always obtained when the 4-sided applicator was used, while the spatially periodic stripe pattern was always generated spontaneously when the block applicator was used. The net UV absorbance of the layer on which the stripe pattern was formed was found to be lower than that of the flat layer having the same average thickness. Theoretical simulations quantitatively reproduced the variation of the net UV absorbance led by the change of the geometry of the layer. The results of this study propose the definite necessity of strict regulations on the coating method of sunscreens for the establishment of the in vitro SPF test method.

Kinetic study of the polymerization of alpha-amino acid N-carboxyanhydrides in aqueous solution using capillary electrophoresis.

N-Carboxyanhydrides of amino acids (NCAs) are very reactive monomers able to polymerize into oligopeptides. They are assumed to be prebiotic precursors of the first polypeptides. Few reports have been published on the study of NCA polymerization in aqueous solution. In this work, a kinetic study focused on the hydrolysis of NCA and its coupling with amino acids and homopeptides (up to tripeptide) was carried out, taking L-valine derivatives as model compounds. For that purpose, capillary electrophoresis appeared to be an effective and reliable technique for the measurement of the kinetic constants. The electrophoretic separation conditions, the procedure for stopping NCA reactivity, as well as the conditions of reaction are discussed in detail. We report the variation of the kinetic constant of the coupling reaction of the NCA of valine with an oligovaline as a function of its degree of polymerization. Finally, a temperature study also allowed us to estimate the activation energies associated with the NCA of valine hydrolysis and its coupling reaction with valine.