Analysing the complexation of amino acids and peptides with β‐cyciodextrin using electrospray ionization mass spectrometry

Abstract

The formation of non-covalently bound inclusion complexes between p-cyclodextrin and peptides containing aromatic acid residues has been confirmed by electrospray ionization mass spectrometry. This preliminary study has indicated that phenylalanine and tryptophan residues can form 1 : 1 complexes with fi-cyclodextrin whereas no evidence for such a complex could be found for tyrosine residues. Bradykinin, which contains two phenylalanine residues, formed both 1 : 1 and 1 : 2 complexes.6-Cyclodextrin (p-CD) is a cyclic oligosaccharide consisting of seven glucopyranose units. The 'torus'-shaped structure of this molecule has seven primary and fourteen secondary hydroxyl groups located at the narrower entrance and the wider entrance, respectively. The presence of these hydroxy groups makes the outer surface of 0-CD hydrophilic, whereas the hydrocarbon structure of the interior of the 'torus' renders the cavity lipophilic in nature.The hydrophobic interaction between p-CD and a number of aromatic moieties in aqueous solution leads to the formation of inclusion complexes. 1-3 Besides its hydrophobicity, the size of the 'guest' molecule relative to that of the p-CD cavity is an important factor in determining the extent of the non-covalent interaction. A number of molecules containing phenyl, naphthyl, or other aromatic groups of comparable size, form, in the majority of cases, 1 : 1 inclusion complexes. Studies on these 'host-guest' complexes in solution have utilized physico-chemical techni ues such as nuclear magnetic resonance spectroscopyp4 spectrofluorimetry? circular dichroism5 and mass spectrometry.6 In the solid form, the ability of j3-CD to include aromatic moieties has been confirmed by X-ray crystallography.' p-CD and related cyclic oligosaccharides have been reported to interact with peptides and proteins through the formation of inclusion complexes with aromatic amino acids, in particular, phenylalanine, tyrosine and tryptophan.', The occurrence of these complexes has been inferred from observations of significant improvement in the solubility and stabilization of these proteins in the presence of cyclodextrins. In this Communication we report the use of electrospray ionization mass spectrometry (ESI-MS) to study the interaction of p-CD with phenylalanine, tyrosine, tryptophan and some peptides containing one or more of these amino acid residues. The utility of this relatively 'soft' ionization technique for the study of the noncovalently bound complexes has been demonstrated recently by several ESI-MS analysis was performed on a Vestec 201 mass spectrometer (Vestec Corp., Texas, USA). Compounds were dissolved in a solvent mixture made up of water saturated with p-CD (about 0.185g/L,

Structural relationships in small molecule interactions governing gas-phase enantioselectivity and zwitterionic formation

“…The net reaction is the replacement of the amino acid by the alkylamine in the cavity of ␤-CD* resulting in a gas-phase guestexchange reaction. The interactions of the guest molecule with the hydrophobic cavity and the hydrophilic rim of CD are known to play important roles in stabilizing the charge on the protonated amino acid [11,17]. The intermolecular hydrogen bonding interaction between the hydroxyl groups in the CD ring and the positive charge on the amino acid result in very stable complexes.…”

On the specificity of cyclodextrin complexes detected by electrospray mass spectrometry

“…Various studies of noncovalent complexes between ␤-cyclodextrins and aromatic molecules have shown that the complexes that were known to be present in solution could be detected by ES-MS [11][12][13][14][15][16][17]. In all these studies, the aromatic guest molecules were bearing at least one polar group.…”

What Happens to Hydrophobic Interactions during Transfer from the Solution to the Gas Phase? The Case of Electrospray-Based Soft Ionization Methods

“…However, the signal intensity was too low to perform MS/MS experiments, which, together with the lack of isotopic resolution, prevented us from unambiguously assigning these peaks. Given that the concentration of analytes in this control experiment was quite high (~100 μM), the presence of these small peaks in the mass spectrum is more consistent with the non-specific association of RuCD2 and cationized CDs in the ESI plume rather than with non-specific complexation between the CDs and ligand 2 in solution [31,43].…”

“…A number of studies report on more or less successful analyses of noncovalent complexes where hydrophobic forces are considered to play a dominant role for the stability [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33]. Liu et al [19,20] showed the preservation of hydrophobically stabilized complex between bovine β-lactoglobulin and its natural ligands, fatty acids, for the fraction of ESI-generated ions.…”

“…CDs allow for the formation of inclusion complexes with various nonpolar molecules in polar solvents [34]. Several studies have reported on the detection of inclusion complexes between CDs and aromatic or aliphatic guests, amino acids, peptides, and drugs by ESI-MS [23][24][25][26][27][28][29][30][31][32][33]. However, in these studies, guest molecules bore at least one polar group that pointed out of the cavity and enabled hydrogen bonding or dipole-dipole interactions with hydroxyls of the CD rim.…”