A single chain insulin comprises a C domain of 6 to 11 amino acid residues comprising at least two acidic residues at the N terminal side of the C domain and at least two basic residues at the C terminal side of the C domain peptide a basic amino acid residue at the position corresponding to A8 of human insulin and an acidic amino acid residue at the position corresponding to A14 of human insulin. The C domain may contain a 2 to 4 amino acid joint region between the acidic and basic residues. Residues C1 and C2 may have a net negative charge of 1 or 2 and the remaining C domain segment may culminates with two basic residues. A pharmaceutical composition comprises the single chain insulin formulated at a pH within the range 7.0 to 8.0 and may be formulated at a concentration of 0.6 mM to 5.0 mM and/or at a strength of U 100 to U 1000.
