Non-linear hydrophobic-induced pKa shifts: Implications for efficiency of conversion to chemical energy

By using one Asp or one Glu per thirty residues in a polytricosapeptide capable of exhibiting a hydrophobic folding and assembly transition and stepwise converting a set of the five Val residues (most proximal to the Asp or Glu residue) to more-hydrophobic Phe residues, a non-linear hydrophobic-induced pKa shift was observed with a ΔpKa of 0.4 (Asp) and 0.3 (Glu) on addition to 2 Phe residues per 30mer but with a ΔpKa of 4.7 (Asp) and 2.7 (Glu) on going from 4 Phe/30mer to 5 Phe/30mer. As a shift in pKa can be equivalent to the conversion to chemical energy from whatever energy input — mechanical, chemical, electrochemical, pressure or light — which effects a change in hydrophobicity, the non-linear hydrophobic-induced pKa shift means increased efficiency of energy conversion with increased hydrophobicity of the protein-based polymer.