Self-Assembled Protein–Surfactant Nanoaggregates for Tunable Peptide Bond Hydrolysis by Polyoxometalate Nanoclusters

The development of robust artificial proteases is of crucial importance for the study of proteins since natural proteases only retain their proteolytic activity under specific conditions. The presence of surfactants, which aid in solubilizing proteins and in probing their structure, is particularly detrimental to natural proteases. Therefore, artificial proteases that can function in the presence of surfactants are needed. Here, we report the hydrolysis of horse heart myoglobin (Mb) in the presence of a Zr(IV)-substituted Keggin polyoxometalate cluster (Et2NH2)8[{α-PW11O39Zr-(μ-OH)(H2O)}2]·7H2O (Zr–K 2:2) as an artificial protease and different surfactants: sodium dodecyl sulfate (SDS), N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (Zw3-12), and 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS). The formation of nanoaggregates consisting of micellar structures containing the protein, the surfactant, and Zr–K 2:2 was detected by dynamic light scattering and conductivity measurements. Hydrolytic reactions were monitored by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), and the hydrolytic efficiency was observed to increase in the presence of all three surfactants, but the fragmentation pattern was different depending on the nature of the surfactant used. Furthermore, a multitechnique approach combining cyclic voltammetry, 31P nuclear magnetic resonance, fluorescence, circular dichroism, and UV–vis spectroscopy was used to gain a better understanding of the protease activity of Zr–K 2:2 in the presence of surfactants. Based on this approach, a general model for the interactions typically observed in protein/surfactant/POM ternary nanoassemblies has been proposed. The hydrolytic efficiency of a POM nanocluster toward a protein in the presence of surfactants was found to depend on (i) the structure of the protein and accessibility of the cleavage sites, (ii) the structure of the surfactants, (iii) the ease of exchange between the POM and the surfactants on the surface of the protein, and (iv) the influence of surfactants on the speciation of the POM catalyst.