Photo-induced electron detachment of protein polyanions in the VUV range

Biomolecular polyanions mainly relax by electron emission after UV excitation. Here, we study photodetachment of protein polyanions in the 6-16 eV VUV range by coupling a linear quadrupole ion trap with a synchrotron beamline. Gas-phase VUV action spectra of electrospray-produced multiply deprotonated insulin (5.6 kDa) and myoglobin (16.7 kDa) proteins are reported, which significantly increases the amount of data available on the optical response of proteins in the VUV. The influence of the protein charge and oxidation state upon the electron detachment efficiency is discussed. For small protein such as insulin, it appears that higher charge states produce higher detachment yields. Investigations on oxidized species show that the nature of the groups bearing the negative charges has an influence on the yields. For larger proteins, comparison of two forms of myoglobin clearly indicate that the three-dimensional structure does not impact much on the shape and the magnitude of the photodetachment spectra, in spite of a slight shift for the first electronic excited states.