Multiply-charged negative clusters of adenosine-5′-monophosphate in the gas phase

Multiply-charged noncovalent cluster anions of adenosine-5′- monophosphate (AMP) were formed by electrospray ionization (ESI). Ions in higher charge states were observed when the ions were accumulated in an ion trap with helium buffer gas before detection. We determined the smallest size (n _a) or appearance size as a function of charge state (q), i.e., n _a = 4 for q = 2, n_a = 8 for q = 3, and n_a = 13 for q = 4. The relation between n_a and q can be described by a charged droplet model. When the size is larger than n_a for a given q, the fragmentation pathway of an anion cluster is dominated by loss of neutral fragments. In contrast, when the size approaches the appearance size, only charged fragments are formed.