Investigation on Non-Covalent Complexes of Cyclodextrins with GGG and GFF Tripeptides in Gas Phase by Mass Spectrometry

To investigate the non-covalent interaction between alpha-, beta-, gamma-cyclodextrins and peptides, a stoichiometry of alpha-, beta-, gamma-cyclodextrins (CD) with GGG (Gly-Gly-Gly) or GFF (Gly-Phe-Phe) was mixed respectively, and then incubated at room temperature for 12 h to reach the equilibrium. In positive mode, the electrospray ionization mass spectrometry (ESI-MS) results indicated that alpha-, beta-, gamma-CD with GGG or GFF could form non-covalent complexes, respectively. The binding of cyclodextrins with GGG or GFF was further confirmed by collision induced dissociation (CID) in a tandem mass spectrometer. The formation constants of six complexes (GGG+CD and GFF+CD) were determined by mass spectrometric titration. The results showed the formation constants for both GGG's and GFF's complexes increased according to the order gamma-CD, beta-CD, alpha-CD. The formation constants K-st, values for GGG complexes with alpha-CD, beta-CD or gamma-CD are 2799.96, 2528.73, 1697.11 L.mol(-1), respectively. While the formation constants K-st values for GFF complexes with alpha-CD, beta-CD or gamma-CD are 2773.94, 2134.03, 1330.68 L.mol(-1) respectively. For alpha-CD, beta-CD or gamma-CD, the K-st values of GFF complexes containing aromatic group are smaller than those of GGG complexes only containing aliphatic group. The main reason is that in gas phase, with the weakening of hydrophobic force, Van der Waals force plays an important role in the conjugation process of OFF with CD, the coordinating group of OFF is still phenyl group. While in GGG's complexes, the hydrogen bond dominates in the conjugation process. Our convincing results from the formation constants provides a new evidence, indicating that although the conformations for GFF+CD complexes change slightly when the analysts transfer from solution to gas phase, the phenyl group still takes part in coordinating.