Formation and Stability of Peptide Enolates in Aqueous Solution
journal contributionposted on 20.06.2002, 00:00 by Ana Rios, John P. Richard, Tina L. Amyes
Second-order rate constants kDO (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 kHO (M-1 s-1) for proton transfer from these carbon acids to hydroxide ion in H2O. Values of the pKa for these carbon acids ranging from 23.9 to 30.8 were obtained by interpolation or extrapolation of good linear correlations between log kHO and carbon acid pKa 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 kHO for deprotonation of the N-terminal α-amino carbon of the glycylglycylglycine zwitterion (pKa = 25.1) is similar to that for deprotonation of the more acidic ketone acetone (pKa = 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 NHAc group, but the α-NH3+ and the α-NHAc substituents result in very similar decreases in the pKa of several α-carbonyl carbon acids.