Formation of peptide aggregates during ESI: Size, charge, composition, andcontributions 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 cond itions employed, there is evidence that simple binary and quaternary mixtur es of these dipeptides produce clusters containing as many as 16 to 75 pept ide units (and 1 to 7 charges). In some systems, cluster compositions appea r to come about largely from statistical association of peptide units; othe r dipeptide mixtures (and generally for small clusters) show evidence for n onstatistical behavior which could arise from some differences in gas-phase or solution thermochemistry. The minimum aggregate size appears to be larg ely determined by the charge state. Average thresholds for aggregate format ion in the z = 2, 3, and 4 charge state families occur at m/z similar to 50 0, 660, and 875, respectively. We briefly consider the idea that aggregates formed during electrospray ionization (ESI) may contribute to the backgrou nd signal observed in the analysis of complex peptide mixtures. (C) 2001 Am erican Society for Mass Spectrometry.