We designed, expressed, and purified an ELP that we refer to as KI8, because the body of the polymer consists of the amino acid sequence [(VPGKG)(VPGIG)8]10 (Figure ). Lysine's eamino groups have a reference pKa value of 10.54, although this value may shift depending on the specific microenvironment. The ionizable Lys residues were incorporated to confer pH responsiveness to KI8, which has a predicted pI = 10.6. We also generated an ELP we called I90, which is the same length as KI8 but consists of the amino acid sequence (VPGIG)90 to serve as a comparison polymer that lacks pH-responsiveness and the charge/polarity of the lysine side chains. KI8 and I90 were purified to homogeneity by inverse temperature cycling and characterized for purity using SDS-PAGE (Figure ). Note that because both polymers have a single Cys residue near the N-terminus, formation of dimers via disulfide bridges is possible.