Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry of dextran and dextrin derivatives

Mass Spectrometry of Esterified Cyclodextrins

Cyclodextrins are cyclic oligosaccharides that have received special attention due to their cavity-based structural architecture that imbues them with outstanding properties, primarily related to their capacity to host various guest molecules, from low-molecular-mass compounds to polymers. Cyclodextrin derivatization has been always accompanied by the development of characterization methods, able to unfold complicated structures with increasing precision. One of the important leaps forward is represented by mass spectrometry techniques with soft ionization, mainly matrix-assisted laser desorption/ionization (MALDI) and electrospray ionization (ESI). In this context, esterified cyclodextrins (ECDs) benefited also from the formidable input of structural knowledge, thus allowing the understanding of the structural impact of reaction parameters on the obtained products, especially for the ring-opening oligomerization of cyclic esters. The current review envisages the common mass spectrometry approaches such as direct MALDI MS or ESI MS analysis, hyphenated liquid chromatography-mass spectrometry, and tandem mass spectrometry, employed for unraveling the structural features and particular processes associated with ECDs. Thus, the accurate description of complex architectures, advances in the gas phase fragmentation processes, assessment of secondary reactions, and reaction kinetics are discussed in addition to typical molecular mass measurements.

First Steps to Rationalize Host-Guest Interaction between α-, β-, and γ-Cyclodextrin and Divalent First-Row Transition and Post-transition Metals (Subgroups VIIB, VIIIB, and IIB)

Cyclodextrins (CDs) are cyclic oligosaccharides mainly composed of six, seven, and eight glucose units, so-called α-, β-, and γ-CDs, respectively. They own a very particular molecular structure exhibiting hydrophilic features thanks to primary and secondary rims and delimiting a hydrophobic internal cavity. The latter can encapsulate organic compounds, but the former can form supramolecular complexes by hydrogen-bonding or electrostatic interactions. CDs have been used in catalytic processes to increase mass transfer in aqueous-organic two-phase systems or to prepare catalysts. In the last case, interaction between CDs and metal salts was considered to be a key point in obtaining highly active catalysts. Up to now, no work was reported on the investigation of factors affecting the binding of metal to CD. In the study herein, we present the favorable combination of electrospray ionization coupled to mass spectrometry [ESI-MS(/MS)] and density functional theory molecular modeling [B3LYP/Def2-SV(P)] to delineate some determinants governing the coordination of first-row divalent transition metals (Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺, and Fe²⁺) and one post-transition metal (Zn²⁺) with α-, β-, and γ-CDs. A large set of features concerning the metal itself (ionic radius, electron configuration, and spin state) as well as the complexes formed (the most stable conformer, relative abundance in MS, CE₅₀ value in MS/MS, binding energy, effective coordination number, average bond lengths, binding site localization, bond dissociation energies, and natural bond orbital distribution) were screened. Taking into account all of these properties, various selectivity rankings have been delineated, portraying differential association/dissociation behaviors. Nonetheless, unique 3D topologies for each CD-metal complex were emphasized. The combination of these approaches brings a stone for building a compendium of molecular features to serve as a suitable descriptor or predictor for a better first round rationalization of catalytic activities.

Probing the common alkali metal affinity of native and variously methylated β-cyclodextrins by combining electrospray-tandem mass spectrometry and molecular modeling

In the study herein, we investigated the solution and gas phase affinity of native and variously methylated β-cyclodextrins (CDs) as hosts towards three common alkali metals as guests namely lithium, sodium and potassium. For this purpose, two complementary approaches have been employed: electrospray-tandem mass spectrometry (ESI-MS/MS) with two energetic regimes: Collision Induced Dissociation (CID) and Higher Collision Dissociation (HCD), respectively, and DFT molecular modeling. These approaches have been achieved by taking into account the interaction of either one or two alkali metals with the host molecules. The results showed a good agreement between experimental and theoretical data. It was demonstrated that increasing the methylation degree strengthened the gas phase affinity towards all studied alkali metals. Furthermore, it was established that the cation selectivity was Na(+) > Li(+) > K(+) and Li(+) > Na(+) > K(+) for the solution and gas phase, respectively.

Influence of Single Skimmer Versus Dual Funnel Transfer on the Appearance of ESI-Generated LiCl Cluster/ß-Cyclodextrin Inclusion Complexes

Singly and doubly charged adducts of LiCl with β-cyclodextrin (βCD) of the type (βCD)(LiCl)(n)Li(+) and (βCD)2(LiCl)(p)Li2(2+) were studied using electrospray ionization mass spectrometry (ESI-MS). Insight into their structural composition was gained by analysis of their collision-induced dissociation (CID) mass spectra. The conditions the ions experience in the transfer region interfacing the ESI source and the mass analyzer were found to have a marked influence on the nature of the detected ions. In one instrument incorporating a single skimmer, individually attached LiCl ion pairs were observed, whereas the dual funnel ion guides of the second instrument allow the detection of previously unknown labile inclusion complexes of (LiCl)n clusters in βCD.

Characterization of low-molecular weight iodine-terminated polyethylenes by gas chromatography/mass spectrometry and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry with the use of derivatization

Gas chromatography/mass spectrometry (GC/MS) and matrix-assisted laser desorption/ionization time-of-flight (MALDI-ToF) mass spectrometry, in conjunction with various derivatization approaches, have been applied to structure determination of individual oligomers and molecular-mass distributions (MMD) in low-molecular mass polyethylene having an iodine terminus. Direct GC/MS analysis has shown that the samples under investigation composed of polyethyelene-iodides (major components) and n-alkanes. Exchange reaction with methanol in the presence of NaOH gave rise to methoxy-derivatives and n-alkenes. Electron ionization mass spectra have shown that the former contained terminal methoxy groups indicating the terminal position of the iodine atom in the initial oligomers. MMD parameters have been determined with the aid of MALDI mass spectrometry followed by preliminary derivatization-formation of covalently bonded charge through the reaction of iodides with triphenylphosphine, trialkylamines, pyridine or quinoline. The mass spectra revealed well-resolved peaks for cationic parts of derivatized oligomers allowing the determination of MMD. The latter values have been compared with those calculated from GC/MS data.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2003-2004

This review is the third update of the original review, published in 1999, on the application of matrix-assisted laser desorption/ionization (MALDI) mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings the topic to the end of 2004. Both fundamental studies and applications are covered. The main topics include methodological developments, matrices, fragmentation of carbohydrates and applications to large polymeric carbohydrates from plants, glycans from glycoproteins and those from various glycolipids. Other topics include the use of MALDI MS to study enzymes related to carbohydrate biosynthesis and degradation, its use in industrial processes, particularly biopharmaceuticals and its use to monitor products of chemical synthesis where glycodendrimers and carbohydrate-protein complexes are highlighted.

Study of Alkali Metal Cations Binding Selectivity of β-Cyclodextrin by ESI-MS

Cyclodextrins (CDs), cyclic oligosaccharides commonly composed of six, seven or eight (alpha, beta, and gamma respectively) D-glucopyranosyl units connected by alpha-(1,4)- glycosidic linkages, have the ability to form inclusion complexes with a wide range of substrates in aqueous solution. This property has led to their applications in different areas such as enzyme mimics, catalysis and the encapsulation. of drugs. ESI-MS has begun to be viewed as a useful tool for investigating the general area of molecular recognition thus providing a powerful mean for the analysis of a wide array of host-guest complexes and other non covalent complexes present in solution. The evaluation of the binding selectivity of beta-cyclodextrin towards the first group alkali cations is reported. The estimation of the affinity degrees has been achieved by competition ESI-MS experiments. In these experiments beta-CD was incubated at the presence of two different cations at the same time, and the ratio of the mass peaks corresponding to the two complexes was calculated. In general, it appears a much larger affinity of the beta-cyclodextrin molecule with sodium with respect to all the other alkaline cations, thus giving evidence that it is the size of the beta-cyclodextrin ring in relation with the cationic radius, which drives the formation of what, at this point, could be defined as an inclusion complex.

Characterization of a new methylated Beta-cyclodextrin with a low degree of substitution by electrospray ionization mass spectrometry and liquid chromatography/mass spectrometry

A new methylated beta-cyclodextrin (Me-beta-CD) with a low degree of substitution (DS) was characterized by electrospray ionization mass spectrometry (ESI-MS) and liquid chromatography coupled with ESI-MS (LC/ESI-MS). For ESI-MS analyses, the composition of the infused sample solution was optimized in order to obtain only singly charged ammoniated CDs without fragmentation. The DS value (i.e. the number of methyl groups per glucopyranose unit) was found to be 0.7, which was in accordance with the values previously obtained by other methods. The LC/ESI-MS analysis, derived from a method using evaporative light scattering detection, allowed the study of the substitution isomers of each derivative and appears to be an easy and rapid tool for the accurate characterization of Me-beta-CD mixtures.

Different behavior of dextrans in positive-ion and negative-ion mass spectrometry

A mass spectrometric (MS) comparative study of dextran samples using two different ionization techniques (matrix-assisted laser desorption/ionization (MALDI) and electrospray ionization (ESI)) in both positive- and negative-ion modes is reported. The experiments were carried out with two polydisperse dextrans (1000 and 8800 Da) and isomaltotriose. In the positive-ion mode, the expected alkali metal ion adducts of dextrans were observed in both techniques. In contrast, the expected preferential formation of deprotonated molecules [M - H](-) was not confirmed in negative mode MALDI time-of-flight (TOF) MS, where the series of ions [M(x)- H +42](-) or [M(x+1)- H - 120](-), coming either from some addition or fragmentation, were observed. In both ionization techniques, these ions formed the main distributions of dextrans in the negative-ion mode. It seems that the negative molecular ions formed from the alpha1 --> 6 linkage of polyglucose oligomers easily decompose, and the product ions [M - H - 120](-) markedly dominate. The fragmentation experiments and especially the investigation of the fundamental role of the nozzle-skimmer potential in ESI-MS supported our explanation of the observed behavior because its higher values caused higher fragmentation. The experiments with isomaltotriose excluded any addition of 42 Da during the MS procedures, which is not distinguishable from the loss of 120 Da in the case of polydisperse dextrans. MALDI-TOFMS was found to be more sensitive for the detection of higher oligosaccharides and ESI-MS more useful for structural studies.