Despite significant affinity to carbonyl oxygens, thermal hydrogen atoms attach to unmodified polypeptides at a very low rate, while the hydrogen- hydrogen exchange rate is high. Here, using the novel omnitrap setup, we found that attachment to polypeptides is much more facile when radical site is already present, but the rate decreases for larger radical ions. The likely explanation is the intramolecular hydrogen atom rearrangement in hydrogen-deficient radicals to a more stable or less accessible site.