Gel polymerization in detergents: Conversion efficiency of methylene blue vs. persulfate catalysis, as investigated by capillary zone electrophoresis

Four types of detergents are commonly used in biochemical analysis of proteins and polypeptides: neutral, e.g., Triton X‐100, Nonidet P‐40; anionic, typically sodium dodecyl sulfate (SDS); cationic, e.g., cetyltrimethylammonium bromide CTAB), and zwitterionic, e.g., sulfobetaine 3–12 and 3 [(3‐cholamido‐propyl) dimethyl‐ammonio]‐1‐propane sulfonate (CHAPS). These detergents are utilized not only in the protein solubilization step, but also in the polyacrylamide gel matrix in which subsequent electrophoretic separation is carried out. The conversion efficiency of monomers into the growing polymer, in the presence of the four types of detergents, was assessed by capillary zone electrophoresis (CZE) in a micellar system comprising 100 mM SDS, by extracting unreacted monomers from the gel phase and quantifying the peaks separated by CZE. Two different catalyst systems were evaluated: the standard persulfate‐N,N,N′,N′‐tetramethylethylenediamine (TEMED) couple and a mixture comprising methylene blue in presence of the redox couple sodium toluene sulfinate and diphenyliodonium chloride. In the chemically initiated system (persulfate), there was a strong inhibition of polymerization, decreasing in the following order: CTAB > sulfobetaine 3–12 > SDS > CHAPS > Triton X‐100 (e.g., in 10 mM CTAB 100% inhibition was experienced). On the contrary, in photopolymerization (as driven by methylene blue) good conversion of monomers into the growing polymer (> 95 %) was always obtained, independent of the nature of the detergent present in the polymerization step. It is suggested that acrylamide and N,N′‐methylenebisacrylamide (Bis) partition in the detergent micelles, thus being subtracted to equilibrium during polymerization: the order of inhibition in persulfate gels could then be proportional to the monomer partition coefficients. Conversely, the total lack of inhibition in methylene blue gels could be due to the extremely high performance of photocatalysis and/or to its partitioning also inside the detergent micelle.