The reaction of hydroxyl radicals with Ser-Met and Thr-Met at slightly acidic to neutral pH results in the side chain fragmentation of the Ser and the Thr moiety into formaldehyde and acetaldehyde, respectively. The efficiency of this process depends on the concentration of the peptide and protons with maximum yields at low. peptide concentrations at near neutral pH. Significantly less aldehyde formation is observed for the reaction of hydn,xyl radicals with Ala-Met, Val-Met, Gly-Ser-Met, Met-Ser, Gly-Met-Ser, Ser-Leu, Gly-Thr-Met, and Gly-Met-Thr. These results indicate that the formation of aldehyde requires (i) an N-terminal Ser or Thr residue and (ii) the presence of Met in the sequence. The underlying mechanism involves an intramolecular proton transfer from the protonated N-terminal amino group to an initially formed hydroxy sulfuranyl radical at the Met residue. This process leads to the elimination of water and the simultaneous formation of a three-electron-bonded [>S therefore NH2](+)-peptide intermediate which absorbs at lambda(max) = 385 nm and has been identified by pulse radiolysis. This intermediate decays with t1/2 = 310 ns into aldehyde and an alpha-amino radical of the structure H2N-(CH)-H-.-C(=O)NH-peptide, which has been identified by ESR spectroscopy. Mechanistically, the latter process involves the formation of an intermediate nitrogen-centered radical cation which undergoes subsequent heterolytic scission of the C alpha-C beta bond of the Ser or Thr side chain, respectively. One-electron oxidation of Thr-Met by SO4.- at slightly acidic pH (approximate to 5.5-6) results in significantly lower yields of acetaldehyde as compared to the hydroxyl radical initiated process, indicating the importance of the intermediary formed hydroxy sulfuranyl radical. It is proposed, however, that generally sulfur-centered radical cations, derived through one-electron oxidation of the methionine sulfur, might convert into hydroxy sulfuranyl radicals which subsequently undergo the proton-transfer process with adjacent N-terminal Ser or Thr residues.