In a new experiment that combines ion mobility spectrometry with guided ion beam tandem mass spectrometry, we examine the collision-induced dissociation of the cis and trans conformers of protonated glycylprolylglycylglycine, H+GPGG. These two conformers differ in the orientation of the first peptide bond. As has been shown in the past, the initial distribution generated in the electrospray ionization source favors the trans conformer but collisional activation induces trans/cis isomerization such that the cis conformer is favored under many conditions. Because the energy needed for the trans/cis isomerization is much lower than that needed for fragmentation, both conformers dissociate identically, primarily by forming the [b2]+ ion. By measuring the thresholds for this decomposition from each conformer, the absolute energy difference between the two conformers can be directly measured. The results are then compared to comprehensive theoretical predictions already in the literature.

Measuring the absolute energy differences between conformers of protonated GlyProGlyGly

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