False positives and the detection of cyclodextrin inclusion complexes by electrospray mass spectrometry

Structural information of gas phase complexes of poly(ethylene glycol) (PEG) cationized by one or two different alkali metal ions is inferred from MS and MS/MS experiments performed with an electrospray quadrupole ion trap mass spectrometer. The rationale for selecting PEG was that its sites for cation binding are non-selective with respect to the repeating monomeric unit of the polymer, but there is selectivity with respect to the formation of an inner coordination sphere specific to each metal ion. The dissociation of [M 1 ϩ M 2 ϩ (EO23)], where EO23 ϭ linear polymer of ethylene oxide, 23 units in length, resulted in loss of one of the alkali metal ions, with preference for loss of the larger cation, with no fragmentation of the PEG backbone for Na, K, Rb, and Cs. Li was not examined in this portion of the study. The selectivity for loss of the larger alkali metal ion was [Na -20]. Factors involved in the solution phase molecular recognition are solvation enthalpy and entropy for both species and the number of atoms from the ligand that are involved with the binding of the metal ion, plus any conformational change of the ligand between its unbound and bound forms [21]. The intrinsic properties of the ion-dipole bonds in these non-covalent complexes are being deduced using a variety of gas phase techniques, including the equilibrium method [22], threshold CID [23][24][25][26], and ion chromatography [27,28].The long chain podands have received comparatively less attention by mass spectrometry [29,30], yet their industrial usage is immense, ranging from the fabrication of materials with varied properties to their use as phase transfer catalysts in industrial processes [31]. Long chain podands are also used as agents for improving bio-compatibility in immunological applications [32][33][34] and as ion channel models [35]. Improved knowledge of the structures formed when long chain podands coordinate with alkali metal ions, and particularly with the ubiquitous Na ϩ and K ϩ , can be of use in improving the applications of these materials.Here we report the uni-molecular dissociation of modest length podands (PEG, M.W. avg ϭ 1000) that were doubly cationized by different alkali metal ions. These complexes are abbreviated as [M 1 ϩ M 2 ϩ (EOx)], where M 1 ϩ and M 2 ϩ are different alkali metal ions, EO ϭ ethylene oxide unit, and x ϭ degree of polymerization. This study of the doubly cationized PEG has permitted an in situ measure of the relative binding strength of the cumulative ion-dipole bonds when a single PEG molecule coordinates to the s-orbital of two different alkali metal ions.The dissociation of these complexes inside a quadrupole ion trap is a form of the kinetic method [36,37]. In this variation, rather than sandwiching a single cation between two ligands, a single ligand that was large enough to bind two different metal ions was

Section: Methodsmentioning

confidence: 99%

Poly(ethylene glycol) doubly and singly cationized by different alkali metal ions: Relative cation affinities and cation-dependent resolution in a quadrupole ion trap mass spectrometer

Bogan

Agnes

2002

“…In all these studies, the aromatic guest molecules were bearing at least one polar group. However, Cunniff and Vouros [18] have reported that ES-MS on complexes with non-aromatic amines gave false positives. Conversely, ES-MS on ␤-CD complexes with aromatic non-polar molecules resulted in false negatives.…”

mentioning

confidence: 99%

On the specificity of cyclodextrin complexes detected by electrospray mass spectrometry

Gabelica

Galić

Pauw

2002

122

116

␣-cyclodextrin complexes with linear ␣, -dicarboxylic acids were investigated by electrospray mass spectrometry. These hydrophobic complexes are known to have an equilibrium binding constant that increases with the diacid chain length. However, the electrospray mass spectrometry (ES-MS) spectra showed that the relative intensity of the complex did not vary significantly with chain length. This contradiction is caused by a contribution of nonspecific adducts to the signal of the complex in ES-MS. In order to estimate the contribution of nonspecific adducts to the total intensity of the complexes with ␣-cyclodextrin, the comparison was made between ␣-cyclodextrin and maltohexaose, the latter being incapable of making inclusion complexes in solution. The signal observed for complexes between diacids and maltohexaose can only result from nonspecific electrostatic aggregation, and is found to be more favorable with the shorter diacids. This is also supported by MS/MS experiments. A procedure is described which allows estimation of the contribution of the nonspecific complex in the spectra of the complexes with ␣-cyclodextrin by using the relative intensity of the complex with maltohexaose. The contribution of the specific complex to the total signal intensity is found to increase with the diacid chain length, which is in agreement with solution behavior. (J Am Soc Mass Spectrom 2002, 13, 946 -953)

“…However, extensive dissociation during ESI is also observed for some electrostatically stabilized complexes, even under the most gentle ionization conditions [34]. ESI-MS can also produce false-positive results, i.e., some analytes tend to form noncovalent gas-phase assemblies during ESI, despite the nonexistence of the corresponding complexes in solution [35,36]. Hu et al have demonstrated that the nonspecific binding of metal ions to proteins can be a severe problem when the ionization conditions are not chosen properly [37].…”

mentioning

confidence: 99%

Diffusion measurements by electrospray mass spectrometry for studying solution-phase noncovalent interactions

Clark

Konermann

2003

This study describes a novel approach for monitoring noncovalent interactions in solution by electrospray mass spectrometry (ESI-MS). The technique is based on measurements of analyte diffusion in solution. Diffusion coefficients of a target macromolecule and a potential low molecular weight binding partner are determined by measuring the spread of an initially sharp boundary between two solutions of different concentration in a laminar flow tube (Taylor dispersion), as described in Rapid Commun. Mass Spectrom. 2002Spectrom. , 16, 1454Spectrom. -1462. In the absence of noncovalent interactions, the measured ESI-MS dispersion profiles are expected to show a gradual transition for the macromolecule and a steep transition for the low molecular weight compound. However, if the two analytes form a noncovalent complex in solution the dispersion profiles of the two species will be very similar, since the translational diffusion of the small compound is determined by the slow Brownian motion of the macromolecule. In contrast to conventional ESI-MS-based techniques for studying noncovalent complexes, this approach does not rely on the preservation of solution-phase interactions in the gas phase. On the contrary, "harsh" conditions at the ion source are required to disrupt any potential gasphase interactions between the two species, such that their dispersion profiles can be monitored separately. The viability of this technique is demonstrated in studies on noncovalent heme-protein interactions in myoglobin. Tight noncovalent binding is observed in solutions of pH 10, both in the absence and in the presence of 30% acetonitrile. In contrast, a significant disruption of the noncovalent interactions is seen at an acetonitrile content of 50%. Under these conditions, the diffusion coefficient of heme in the presence of myoglobin is only slightly lower than that of heme in a protein-free solution. A breakdown of the noncovalent interactions is also observed in aqueous solution of pH 2.4, where myoglobin is known to adopt an acid-unfolded conformation. (J Am Soc Mass Spectrom 2003, 14, 430 -441)