Supramolecular modification of ion chemistry: modulation of peptide charge state and dissociation behavior through complexation with cucurbit[n]uril (n = 5, 6) or alpha-cyclodextrin

Abstract

Electrospray Fourier transform ion cyclotron resonance mass spectrometry, ion mobility spectrometry, and computational methods were utilized to characterize the complexes between lysine or pentalysine with three prototypical host molecules: alpha-cyclodextrin (alpha-CD), cucurbit[5]uril (CB[5]), and cucurbit[6]uril (CB[6]). Ion mobility measurements show lysine forms externally bound, singly charged complexes with either alpha-CD or CB[5], but a doubly charged complex with the lysine side chain threaded through the host cavity of CB[6]. These structural differences result in distinct dissociation behaviors in collision-induced dissociation (CID) experiments: the alpha-CD complex dissociates via the simple loss of intact lysine, whereas the CB[5] complex dissociates to yield [CB[5] + H(3)O](+), and the CB[6] complex loses neutral NH(3) and CO, the product ion remaining a doubly charged complex. These results are consistent with B3LYP/6-31G* binding energies (kJ mol(-1)) of D(Lys + H(+)-alpha-CD) = 281, D(Lys + H(+)-CB[5]) = 327, and D(Lys + 2H(2+)-CB[6]) = 600. B3LYP/6-31G* geometry optimizations show complexation with alpha-CD stabilizes the salt bridge form of protonated lysine, whereas complexation with CB[6] stabilizes doubly protonated lysine. Complexation of the larger polypeptide pentalysine with alpha-CD forms a nonspecific adduct: no modification of the pentalysine charge state distribution is observed, and dissociation occurs via the simple loss of alpha-CD. Complexation of pentalysine with the cucurbiturils is more specific: the observed charge state distribution shifts higher on complexation, and fragmentation patterns are significantly altered relative to uncomplexed pentalysine: C-terminal fragment ions appear that are consistent with charge stabilization by the cucurbiturils, and the cucurbiturils are retained on the fragment ions. Molecular mechanics calculations suggest CB[5] binds to two protonated sites on pentalysine without threading onto the peptide and that CB[6] binds two adjacent protonated sites via threading onto the peptide.