Effect of pH on the stability and molecular structure of nitrosyl hemochromogen

• Nitrosyl hemochromogen (NO-Heme) is extremely unstable under weakly acidic conditions.
• Strong acidic and weak basic conditions favor the red stability of NO-Heme.
• The color of NO-Heme changed from pink to yellow under strong basic conditions.
• Spectroscopic data shows how the conjugate structure of NO-Heme changes with pH.

This study explored the effect of pH on the stability and molecular structure of nitrosyl hemochromogen (NO-Heme) using ultraviolet (UV), electron paramagnetic resonance (EPR) and Fourier transform infrared (FT-IR) spectroscopy. The NO-Heme was extracted from pre-cooked cured beef and was dissolved in an 80% acetone solution pre-adjusted to different pH values with 2 mol/L HCl and 2 mol/L NaOH. The results show that pH significantly influences the optical properties and stability of NO-Heme. Strong acidic and weak basic conditions favor the red stability of NO-Heme. The conjugate structure of NO-Heme changed under strong basic conditions, as the NO− group was shown to be still associated with the iron porphyrin, but the color of NO-Heme changed from pink to yellow. NO-Heme is extremely unstable at weakly acidic pH, and this may be the why pH affects the apparent color of cured meat products during storage.