In each of N-(6-amino-3,4-dihydro-3-methyl-5-nitroso-4-oxopyrimidin-2-yl)valine, C₁₀H₁₅N₅O₄ (3) (orthorhombic, P2₁2₁2₁), N-(6-amino-3,4-dihydro-3-methyl-5-nitroso-4-oxopyrimidin-2-yl)serine monohydrate, C₈H₁₁N₅O₅·H₂O (4) (orthorhombic, P2₁2₁2₁), and N-(6-amino-3,4-dihydro-3-methyl-5-nitroso-4-oxopyrimidin-2-yl)threonine, C₉H₁₃N₅O₅(5) (monoclinic, P2₁), the C-nitroso fragments exhibit almost equal C—N and N—O bond lengths: the C—N range is 1.315 (3)–1.329 (3) Å and the N—O range is 1.293 (3)–1.326 (3) Å. In each compound there are also very short intermolecular O—H...O hydrogen bonds, in which carboxyl groups act as hydrogen-bond donors to the nitrosyl O atoms: the O...O distances range from 2.440 (2) to 2.504 (4) Å and the O—H...O angles lie between 161 and 163°. An interpretation of the relationship between the unusual intramolecular bond lengths and the very short intermolecular hydrogen bonds has been developed based on database analysis and computational modelling. In each of (3)–(5) there is an extensive network of intermolecular hydrogen bonds, generating three-dimensional frameworks in (3) and (5), and two-dimensional sheets in (4).