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.