Magic number clusters of serine in the gas phase

Ion mobility measurements and molecular modeling simulations have been used to examine the conformations of electrosprayed serine clusters. Clusters c ontaining as many as 52 serine amino acid units (and one to four protons) a re observed. The distribution of singly charged ions is dominated by a peak corresponding to [8(Ser)+H](+). The measured cross section for this ion is in close agreement with a value that is calculated for tightly packed dist orted block geometries that arise from molecular modeling studies of octame rs comprising zwitterionic amino acid units. Abundant peaks are observed fo r [8(Ser)+2H](2+), [16(Ser)+2H](2+), and [24(Ser)+3H](3+) clusters, indicat ing that the octameric unit is a building block in assembly of larger serin e clusters.