The electronic absorption spectra of some derivatives of pterin and N5
-deazapterin are analysed using the CNDO/S-CI method, including allowa
nce for solvent shifts. These calculations give good agreement with th
e spectra, which may be assigned to a group of pi<--pi transitions. T
here are some differences between calculated values for gas-phase and
solution models but their general level of conformity with experiment
is similar. Band shifts caused by methyl substitution, protonation and
replacement of ring nitrogens are investigated, and a number of struc
tural and spectroscopic problems are addressed. The spectral predictio
ns agree well with experimental assignments for tautomeric forms and p
rotonation sites for known compounds, and predictions are made for the
spectra of N8-deazapterin and N5,N8-dideazapterin which have not been
reported.