On the value of knowing a z* ion for what it is.

Computer simulation of database searches of electron transfer dissociation (ETD) spectra using both "bottom up" and "top down" approaches was performed to evaluate the utility of knowing a priori which product ions contain the C-terminus (i.e., the z* ions). In this work, knowledge of the identities of the z* ions was used to exclude putative identifications that are based solely on the mass matching of undifferentiated product ions derived from an experiment with those derived from in silico fragmentation. The benefit from knowing which ions are z* ions was found to be heavily dependent on the quality of the ETD spectra, in terms of sequence coverage afforded by the product ions, the amount of noise in the spectra (i.e., extraneous peaks that do not directly reflect primary structure), and mass measurement accuracy. Under conditions in which the likelihood for misidentifications are high without a priori knowledge of ion types (e.g., b-, y-, c-, or z-ions), a knowledge of which product ions are z* ions allows discrimination against false-positive identifications. Relatively little benefit from knowing which ions are z* ions was noted when product spectra reflected relatively high sequence coverage and when a low fraction of the products ions were due to extraneous peaks (i.e., spectra with relatively little noise). In all cases, specificity is higher with higher mass measurement accuracy with the consequent reduction in benefit from knowledge of which ions are z* ions.