Removal of the functional groups of guanine, i.e. ketone and amino, one by one produces model molecules of hypoxanthine, 2-aminopurine and unsubstituted purine. The impact of the ketone and amino moieties on guanine is revealed using their atomic-site-based inner-shell electronic properties and spectra. A density functional theory based model has been employed to study the model molecules. Electronic properties, such as Hirshfeld charges and inner-shell chemical shift, are found to be both site-dependent and moiety-dependent. The site-based inner-shell chemical shift of the species exhibits a simple linear correlation, although certain similarities among the model molecules regroup the species into two pairs of purine and 2-aminopurine, as well as hypoxanthine and guanine.