Fig. S1. ESI-MS spectra of insulin, C-peptide and insulin + C-peptide at pH 5, HCl, pH 4, acetic acid, and pH 5–8 in ammonium acetate.

Fig. S2. A500 of insulin, C-peptide and insulin + C-peptide in acetic acid, pH 4, and ammonium acetate, pH 5–8.

Fig. S3. Electron microscopy of insulin (top panel), C-peptide (middle panel) and the insulin–C-peptide precipitate (lower panel).

Fig. S4. A280 of 100 μL of a 100 μm insulin standard (blue) and 100 μL each of the pellet (red) and supernatant (green) fractions.

Fig. S5. Chemically inducible dimerization of heteromers between insulin and E3,11,27A C-peptide.

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