Methyl 2-azido-2-deoxy-alpha-D-lyxofuranoside (1a) and methyl 2-azido-2-deoxy-beta-D-ribofuranoside (2) were prepared from D-xylose or D-arabinose, respectively. Employing ESR and DFT/B3LYP/6-31G* calculations, we investigated (i) aminyl radical (RNH.) formation and (ii) reaction pathways of RNH.. Prehydrated electron attachment to 1a and 2 at 77 K produced transient azide anion radical (RN3.(-)) which reacts via rapid N-2 loss at 77 K, forming nitrene anion radical (RN.(-)). Rapid protonation of RN.(-) at 77 K formed RNH. and -OH. N-15-labeled-1a confirmed this mechanism. Investigations employing in-house synthesized site-specifically deuterated derivatives of 1a (e.g., CH3 (1b), C4 (1c), and C5 (1d)) established that (a) a facile intramolecular H atom transfer from C5 to RNH. generated C5. and RNH2. C5. formation had a small deuterium kinetic isotope effect suggesting that this reaction does not occur via direct H atom abstraction. (b) Subsequently, C5. underwent a facile unimolecular conversion to ring-opened C4.. Identification of ring-opened C4. intermediate confirms the mechanism of C5'.mediated unaltered base release associated with DNA-strand break. However, for 2, ESR studies established thermally activated intermolecular H atom abstraction by RNH. from the methyl group at C1. Thus, sugar ring configuration strongly influences the site and pathway of RNH. mediated reactions in pentofuranoses.