Figure 2.

Loss of volatile counter-ions Protein-COO⁻NH₄⁺ → Protein-COOH + NH₃ (g) (1)  Protein-NH₃⁺HCOO⁻ → Protein-NH₂ + HOOCH (g) (2) Exchanges with partly volatile salt  Protein-COOH + Na⁺HCOO⁻ → Protein-COO⁻Na⁺  + HCOOH (g) (3)  Protein-NH₂ + NH₄⁺H₂PO₄⁻ →  Protein-NH₃⁺H₂PO₄⁻  + NH₃ (g) (4)  Fig. 2. Reactions affecting protein ionization state in our experiments. NH₃ boils at −34°C whereas HCOOH has a vapor pressure of 10 mmHg (1 mmHg = 133 Pa) at 2°C (19). Their salt NH₄OOCH is readily sublimed under vacuum (20).