Tandem mass spectrometry of specific vs. nonspecific noncovalent complexes of vancomycin antibiotics and peptide ligands

The dianionic noncovalent complex between vancomycin and the bacterial cell-wall precursor peptide Ac-2-L-Lys-D-Ala-D-Ala (DD) has previously been shown to retain its structural specific interactions from solution into the gas phase [Int. J. Mass Spectrom. Ion Processes 188 (1999) 63]. We have now determined the destruction cross sections for this specific complex and for the nonspecific complex between vancomycin and Ac-2-D-Lys-L-Ala-L -Ala (LL). Although the gaseous nonspecific complex is only very loosely bound compared to the specific complex, their destruction cross sections are identical. This indicates that there is no difference in the compactness of their gas phase structures. Upon high-energy collisional activation (E-LAB = 100 keV) of the specific dianionic complex, electron detachment occurs and covalent bond cleavage with retainment of noncovalent bonds is observed; these reactions were not observed at low collision energy (E-LAB = 300 eV). Furthermore, we now report how small changes in the peptide binding pocket of vancomycin affect gas phase stability in a manner which parallels known solution binding affinity. The results show that Ac-2-L-Lys-D-Ala-D-Ala cannot form as strong electrostatic interactions with CDP-1 as with vancomycin (or dechlorovancomycin) in the gas phase.