Elastin-Like Polypeptide Production and Purification

DNA sequences were designed to encode elastin like sequence [VPGXG]₄₀ (X is the guest residue) and purchased from GeneArt gene synthesis (Thermo Fisher Scientific Inc., Waltham, MA). The sequence used corresponds to ELP40, a 40-repeat ELP containing mainly VPGVG repeats (Supporting Information, Fig. S5). Cysteine residues were introduced either at position 4 (A4C) or 84 (A84C) to facilitate AuNP attachment. The sequences were cloned into a pET25b vector and expressed in E. coli BLR (DE3) cells. The expression strains were grown by inoculating cells in terrific broth (TB) medium. The purification was performed as described previously.^{23 (link),27 (link),28 (link)} For the cysteine variants, dithiothreitol (DTT) was added to keep the cysteine in reduced form during the purification process. In the final step, all the proteins were dialyzed into the fresh buffer containing 20 mM HEPES, 5 mM NaCl and 5 mM TCEP. Purified proteins were lyophilized for long-term storage after dialyzing the proteins in milli-Q water.