Direct Conversion of Chemical Energy to Mechanical Work Using Phosphate-Charged Peptides

Muscle expands and contracts based on the sliding filament model of the actin-myosin complex. We have built a system with less components based on one highly phosphorylated protein to attempt to increase the thermodynamic efficiency of muscle and demonstrate direct conversion of chemical to mechanical energy without any external stimulus using enzymes. Casein, a phosphoprotein, was cross-linked with 10% w/w divinyl sulfone (DVS) using a nucleophilic addition reaction known as the Michael addition. The Michael addition allows us to cross-link and modify the protein at standard conditions to greatly reduce the possibility of damaging the protein. The resulting cross-linked protein can be hydrated to 10x its weight with H2O and behaves like a polymeric acid, i.e., it will expand or contract in basic or acidic solution because of negatively charged phosphate ions (Pi) at high pH. Phosphate has 3 pKa's (2.1, 7.2, 12.7) because of the 3- valency and the protein has pI~4.6. We have swelled phosphorylated and dephosphorylated cross-linked caseins over a large pH range and found a significant contribution to the swelling from Pi. We have demonstrated that a dephosphorylation reaction using bovine phosphatase yields a net contraction of the cross-linked casein of 13% and a highly acidic solution when Pi is released. The reaction kinetics were monitored using FT-IR spectroscopy by monitoring peaks from serine and threonine residues (the phosphorylatable groups) at around 1200 cm-1.