Characterization of photo‐induced pyrimidine cyclobutane dimers by laser desorption fourier transform mass spectrometry

Laser desorption Fourier transform mass spectrometry was used to characterize the cis–syn cyclobutane photodimers of uracil–uracil, uracil–thymine and thymine–thymine. This soft ionization technique generated [M  H]⁻ ions as well as some fragment ions. Investigation of the laser desorption process indicated that gas‐phase dimerization reactions do not occur for pyrimidine monomers and dimers under these experimental conditions. Collisional dissociation of the [M  H]⁻ ions provided structural information for the pyrimidine rings of the dimers. The fragment ions observed in the collisional dissociation spectra of these cyclobutane dimers suggested rearrangement of the [M  H]⁻ parent ions to a macrocycle prior to dissociation.