Supramolecular Adducts of Cucurbit[7]uril and Amino Acids in the Gas Phase

Abstract

The complexation of the macrocyclic cavitand cucurbit[7]uril (Q7) with a series of amino acids (AA) with different side chains (Asp, Asn, Gln, Ser, Ala, Val, and Ile) is investigated by ESI-MS techniques. The 1:1 [Q7 + AA + 2H](2+) adducts are observed as the base peak when equimolar Q7:AA solutions are electrosprayed, whereas the 1:2 [Q7 + 2AA + 2H](2+) dications are dominant when an excess of the amino acid is used. A combination of ion mobility mass spectrometry (IM-MS) and DFT calculations of the 1:1 [Q7 + AA + 2H](2+) (AA = Tyr, Val, and Ser) adducts is also reported and proven to be unsuccessful at discriminating between exclusion or inclusion-type conformations in the gas phase. Collision induced dissociation (CID) revealed that the preferred dissociation pathways of the 1:1 [Q7 + AA + 2H](2+) dications are strongly influenced by the identity of the amino acid side chain, whereas ion molecule reactions towards N-butylmethylamine displayed a common reactivity pattern comprising AA displacement. Special emphasis is given on the differences between the gas-phase behavior of the supramolecular adducts with amino acids (AA = Asp, Asn, Gln, Ser, Ala, Val, and Ile) and those featuring basic (Lys and Arg) and aromatic (Tyr and Phe) side chains.

Recent Progress in Gas Phase Cucurbit[n]uril Chemistry

“…On the other hand, larger CB[n]s predominantly form 1 : 1 complexes with doubly protonated a,w-n-alkyldiamines. The optimal a,w-n-alkyldiamine lengths for binding large CB[n]s (n > 5) under gas phase conditions were determined, and found to be 3-4 methylene units for CB [6], 4-5 methylene units for CB [7], and 5-6 methylene units for CB [8]. These results are consistent with threaded, rotaxane structures in the gas phase.…”

“…Noh et al studied the hydration of cucurbit [6]uril (CB [6]) in the gas phase using electrospray ionization traveling wave ion mobility mass spectrometry (ESI-TWIM-MS). [4] They ob-served a proton-bound water dimer complex with a Zundellike structure attached to CB [6].…”

“…[4] They ob-served a proton-bound water dimer complex with a Zundellike structure attached to CB [6]. When a large cation, such as alkylammonium or alkali metal cations with relatively large radii (such as K + or Cs + ) is bound to the CB [6] portal, hydration of the portal is not observed, presumably because the large ammonium or metal cations occupy a large area in the carbonyl portal and react with multiple carbonyl groups, hindering their hydrogen bonding with water molecules. They also found that the hydration pattern depends on the CB[n] portal size.…”

“…Evidence supporting this structure came from observed hydration patterns, the ion mobility spectrum, and DFT calculations. Only with small alkali metal cations, such as Li + and Na + , can a single water molecule interact with the CB [6] portal, forming hydrogen bonds with the carbonyl groups of CB [6]. When a large cation, such as alkylammonium or alkali metal cations with relatively large radii (such as K + or Cs + ) is bound to the CB [6] portal, hydration of the portal is not observed, presumably because the large ammonium or metal cations occupy a large area in the carbonyl portal and react with multiple carbonyl groups, hindering their hydrogen bonding with water molecules.…”

“…Yang et al investigated gas-phase complexes of a,w-nalkyldiammonium ions with a series of CB[n]s, including cucurbit [5]uril (CB [5]), decamethylcucurbit [5]uril (mc5), penta(cyclohexano)cucurbit [5]uril (CB* [5]), hexa(cyclohexano) cucurbit [6]uril (CB* [6]), cucurbit [7]uril (CB [7]) and cucurbit [8]uril (CB [8]) using ESI-FTICR-MS and sustained offresonance irradiation collision-induced dissociation (SORI-CID) techniques. [5] The results show that CB [5], mc5, and CB* [5] form singly-charged 1 : 1 and doubly-charged 2 : 1 diamine:CB[n] complexes.…”