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 containing as many as 52 serine amino acid units (and one to four protons) are 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 distorted block geometries that arise from molecular modeling studies of octamers comprising zwitterionic amino acid units. Abundant peaks are observed for [8(Ser)+2H] 2+, [16(Ser)+2H] 2+, and [24(Ser)+3H] 3+ clusters, indicating that the octameric unit is a building block in assembly of larger serine clusters.