Formation of peptide aggregates during ESI: Size, charge, composition, and contributions to noise

Ion mobility/time-of-flight techniques have been used to examine the onset of aggregation in model systems of Gly-Xxx (where Xxxx = Ala, Asn, Asp, Gln, Glu, His, Leu, Ser, Thr, and Trp) dipeptides. Under the experimental conditions employed, there is evidence that simple binary and quaternary mixtures of these dipeptides produce clusters containing as many as 16 to 75 peptide units (and 1 to 7 charges). In some systems, cluster compositions appear to come about largely from statistical association of peptide units; other dipeptide mixtures (and generally for small clusters) show evidence for nonstatistical behavior which could arise from some differences in gas-phase or solution thermochemistry. The minimum aggregate size appears to be largely determined by the charge state. Average thresholds for aggregate formation in the z = 2, 3, and 4 charge state families occur at m/z ∼500, 660, and 875, respectively. We briefly consider the idea that aggregates formed during electrospray ionization (ESI) may contribute to the background signal observed in the analysis of complex peptide mixtures.