Formation and stability of peptide enolates in aqueous solution

Second-order rate constants k_DO (M^-1 s^-1) were determined in D₂O for deprotonation of the N-terminal α-amino carbon of glycylglycine and glycylglycylglycine zwitterions, the internal α-amino carbon of the glycylglycylglycine anion, and the acetyl methyl group and the α-amino carbon of the N-acetylglycine anion and N-acetylglycinamide by deuterioxide ion. The data were used to estimate values of k_HO (M^-1 s^-1) for proton transfer from these carbon acids to hydroxide ion in H20. Values of the pK_a for these carbon acids ranging from 23.9 to 30.8 were obtained by interpolation or extrapolation of good linear correlations between log k_HO and carbon acid pK_a established in earlier work for deprotonation of related neutral and cationic α-carbonyl carbon acids. The α-amino carbon at a N-protonated N-terminus of a peptide or protein is estimated to undergo deprotonation about 130-fold faster than the α-amino carbon at the corresponding internal amino acid residue. The value of k_HO for deprotonation of the N-terminal α-amino carbon of the glycylglycylglycine zwitterion (pK_a = 25.1) is similar to that for deprotonation of the more acidic ketone acetone (pK_a = 19.3), as a result of a lower Marcus intrinsic barrier to deprotonation of cationic α-carbonyl carbon acids, The cationic NH3+ group is generally more strongly electron-withdrawing than the neutral NHA_c group, but the NH₃⁺ and the α-NHAc substituents result in very similar decreases in the pK_a of several α-carbonyl carbon acids.