Mononucleotide gas-phase proton affinities as determined by the kinetic method

The goal of this work is to determine the proton affinities of (deoxy)nucle oside 5'- and 3'-monophosphates (mononucleotides) using the kinetic method with fast atom bombardment mass spectrometry. The proton affinities of the (deoxy)nucleoside 5'- and 3'-monophosphates yielded the following trend: (d eoxy)adenosine monophosphates > (deoxy)guanosine monophosphates > (deoxy)cy tidine monophosphates >> deoxythymidine/uridine monophosphates. Ln all case s the proton affinity decreases or remains the same with the addition of th e phosphate group from those values reported for nucleosides. The proton af finity is dependent on the location of the phosphate backbone (5'- vs. S'-p hosphates): the 3'-monophosphates have lower proton affinities than the 5'- monophosphates except for the thymidine/uridine monophosphates where the tr end is reversed, Molecular modeling was utilized to determine if multiple p rotonation sites and intramolecular hydrogen bond formation would influence the proton affinity measurements. Semiempirical calculations of the proton affinities at various locations on each mononucleotide were performed and compared to the experimental results. The possible influence of intramolecu lar hydrogen bonding between the nucleobases and the phosphate group on the measured and calculated proton affinities is discussed. (J Am Soc Mass Spe ctrom 2000, 11, 24-32) (C) 2000 American Society for Mass Spectrometry.