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.
