The controversial structural issue of a set of pyrazolo[4,3-e][1,2,4] triazine natural products with high antibiotic activity has been tackled with density functional theory calculations. The electronic structure of these nitrogen-rich drugs has been characterized by analyzing the C-13 NMR chemical shifts and UV-visible absorption spectra that are most sensitive to stereoelectronic factors. In particular, the remarkably changeable pattern of the pi -> pi(*) excitations accounts for the palette of bright colours exhibited by these compounds. (C) 2009 Elsevier B.V. All rights reserved.