For Thr-(X)(n)-Met peptides, we have correlated the efficiency of proton transfer and an intramolecular radical reaction between a methionyl hydroxy sulfuranyl radical and the N-terminus with the flexibility of the spacer sequence -(X)(n)-, X = Gly and Pro and n = 0-4. Hydroxy sulfuranyl radicals accept a proton from the N-terminus to yield a cyclic (S thereforeN)(+) three-electron-bonded intermediate which ultimately expels acetaldehyde. Acetaldehyde formation was used to quantify the efficiency of proton transfer and cyclization, whereas peptide flexibility was calculated by molecular modeling methods. For X = Gly, proton transfer and cyclization proceeds in a concerted manner, and peptide flexibility mainly controls the proton transfer step. For X = Pro, no low-energy conformation was located, which allows concerted proton transfer and cyclization, indicating that proton transfer occurs over longer distances, likely via bridging water molecules. These sequence-dependent differences are reflected in different beta -values for the proton-transfer step, beta = 0.66 +/- 0.02 Angstrom (-1) for X = Gly and beta approximate to 0. 12 +/- 0.1 Angstrom (-1) for X = Pro.