Structural characterization of naturally occurring pteridines by laser desorption Fourier transform mass spectrometry

Detailed structural information can be obtained without derivatization for a variety of naturally occurring pteridines by using laser desorption Fourier transform mass spectrometry. Laser desorption provides a soft ionization technique for these non‐volatile compounds and generates characteristic positive and negative ions. The positive‐ion spectra usually consist of [M + Na]⁺ and [M + K]⁺ ions, whereas the negative‐ion spectra reveal [M – H]⁻ ions and fragment ions. Collison‐induced dissociation experiments were used to probe the fragmentation pathways of these negative ions. The fragmentation observed for negative ions supports known resonance structures and provides detailed structural information which can be used to differentiate isomers, such as L‐sepiapterin, L‐biopterin, and L‐primapterin (sometimes referred to as 7‐biopterin). However, optical isomers, such as monapterin and neopterin, cannot be resolved by differences in fragmentation alone. Twenty‐three compounds that are pterins or related structures were characterized.