Solubilization of hydrophobic peptides by reversible cysteine PEGylation

'PEG-a-Cys' reagent, synthesized by the esterification of monomethoxy-poly(ethylene glycol) to Ellman's reagent [5,5'-dithiobis (2-nitrobenzoic acid)], is demonstrated to react to form a disulfide bond with the cysteine residue of ALC, a 25-residue synthetic peptide (H2 N-REAAALAAAAALAAWAALCPARRRR-CO2H) designed to model a fusion protein of a membrane protein transmembrane segment. Enzymatic digestion of the four charged residues at the carboxyl terminus of the peptide---which mimics the enzymatic cleavage of a TM segment from the fusion protein---releases a hydrophobic peptide which precipitates from aqueous solution within 24 hours. The PEG-a-Cys-modified peptide remains soluble for at least 7 days following identical enzymatic digestion, as measured by amino acid analysis of ultracentrifuged samples. Circular dichroism spectra reveal that neither PEG-a-Cys-modification nor digestion of ALC cause major changes in its primarily alpha-helical secondary structure. Routine solubilization of hydrophobic peptides is expected to simplify the purification and structural characterization of membrane protein TM segments.