X-ray absorption near edge structure (XANES) spectra for adenine-containing nucleotides, adenosine 5'-monophosphate (AMP) and adenosine 5'-triphosphate (ATP) in aqueous solutions at the nitrogen K-edge region were measured. The two intense peaks in XANES spectra are assigned to transitions of 1s electrons to the pi* orbitals of different types of N atoms with particular bonding characteristics. The difference between their spectra is ascribed to protonation of a particular N atom. Similarity observed in XANES spectra of guanosine 5'-monophosphate (GMP) and ATP is also interpreted as similar bonding characters of the N atoms in the nucleobase moiety. (C) 2013 Elsevier B.V. All rights reserved.