### Abstract

The sulfur-containing amino acid, methionine, has a role in the physiological environment because of its strong interactions with metals. To understand these interactions of metals with methionine, one needs reliable dissociation constants for the protonated methionine species (NH _{3}^{+}CH(CH_{2}CH_{2}SCH_{3})COOH; H_{2}B^{+}). The values of stoichiometric dissociation constants, pK_{i}*, for protonated methionine species (H _{2}B^{+}⇔H^{+}+HB, K_{1}; HB⇔H ^{+}+B^{-}, K_{2}) were determined from potentiometric measurements in NaCl solutions as a function of ionic strength, 0.25-6.0 mol (kg H_{2}O)^{-1} and temperature (5-45 °C). The results were extrapolated to pure water using the Pitzer equations to estimate the activity of H^{+}, H_{2}B^{+}, HB and B^{-} as a function of ionic strength and temperature. The resulting thermodynamic values of K_{1} and K_{2} were fit to the equations (T/K): ln K_{1}=69.0013-3496.58/(T/K)-10.9153 ln (T/K); ln K _{2}=116.4162-10638.02/(T/K)-18.0553 ln (T/K) with standard errors of 0.003 and 0.033, respectively, for ln K_{1}* and ln K _{2}*. Pitzer interaction parameters (λ_{HB-Na} and ζ_{HB-Na-Cl}) for the neutral HB were determined from literature data. The Pitzer parameters (β_{H2BCl}^{0}, β_{H2BCl}^{1} and C_{H2BCl}^{φ}) for the interactions of H_{2}B^{+} with Cl^{-} and Na ^{+} with and B^{2-} (β_{NaB}^{0}, β_{NaB}^{1} and C_{NaB}^{φ}) were also determined. These coefficients can be used to make reasonable estimates of the activity coefficients of methionine species and the pK_{i}* for the dissociation of methionine in physiological solutions, composed mostly of NaCl over a wide range of temperature and ionic strength.

Original language | English (US) |
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Pages (from-to) | 79-87 |

Number of pages | 9 |

Journal | Biophysical Chemistry |

Volume | 105 |

Issue number | 1 |

DOIs | |

State | Published - Aug 1 2003 |

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### Keywords

- Dissociation constants
- Methionine species
- NaCl
- Protonated

### ASJC Scopus subject areas

- Biophysics
- Biochemistry
- Organic Chemistry

### Cite this

*Biophysical Chemistry*,

*105*(1), 79-87. https://doi.org/10.1016/S0301-4622(03)00137-6