ON THE PHOTOIONIZATION OF LARGE MOLECULES

There is no apparent limit to the size of a molecule for which photoionization can occur. It is argued that it is difficult to obtain useful photoionization mass spectra of peptides (above similar to 2000 u), proteins, and oligonucleotides, because of the high internal energy of these polar molecules as a result of the desorption event and because vibrationally excited radical cations readily fragment. Evidence to support this hypothesis is presented from the 118-nm single-photon ionization (SPI) mass spectra of the cyclic decapeptide gramicidin S and of fullerenes, from null SPI results with the linear peptides substance P and gramicidin D and oligonucleotides, and from a variety of data found in the literature. The literature data include mass spectra from jet-cooled peptides, perfluorinated polyethers, collisional ionization of small neutral peptides, and the ultraviolet photoelectron spectroscopy of polymeric solids.