Gas-Phase Binding of Noncovalent Complexes Between α-cyclodextrin and Amino Acids Investigated by Mass Spectrometry

Ternary complexes of cyclodextrins with alkali earth cations and amino acids in gas phase investigated by mass spectrometry

Noncovalent ternary complexes between cyclodextrins (CDs), small molecules and alkali earth cations drew growing attention due to their potential application in many chemical and pharmaceutical fields. To date, the main factors affect the formation mechanism of noncovalent ternary complexes in gas phase have not been fully investigated. In this study, ternary complexes of CDs, divalent metal cations and amino acids (AAs) were investigated by electrospray ionization mass spectrometry (ESI-MS), demonstrating the formation of 1:1:1 stoichiometric noncovalent ternary complex of [CD + cation(II)+AA]²⁺ in gas phase. The results revealed that only +2 valence cations can form stable ternary complexes in ESI-MS. The ratio of peak intensities for [β-CD + Mg(II)+AA]²⁺ to those for [β-CD + Mg(II)]²⁺ hydrophobicity of AAs was also determined to discuss the effect of hydrophobicity of AAs. Exceptions exist for Pro, Gly, and Val indicated that other factors such as side-chain structure and rigidity of AAs can also influence the binding strength for ternary complexes. Collision induced dissociations (CID) were performed to further confirm the formation of the β-CD ternary complexes, revealing the binding strength of [CD + Mg(II)+Phe]²⁺ decreased in the order of γ-CD, β-CD, and α-CD. Although Leu and Ile are isomers, the ESI-MS demonstrated the peak intensity for ternary complexe of [β-CD + Mg(II)+Ile]²⁺ exhibited stronger than that of [β-CD + Mg(II)+Leu]²⁺, DFT theoretical calculations were conducted to explain the phenomenon. The calculation indicated when Mg²⁺ existing, the conformations of the two ternary complexes could be affected due to the electrostatic force. In the complexes, the Leu and Ile turn a way round, inserting to the cavity with their carboxylic acid side into the large rim side of β-CD and interacting with Mg²⁺. This work not only clearly explained the factors influencing the formation of [CD + cation(II)+AA]²⁺ in gas phase, but it also provides an insight in designing ternary complexes for areas such as drug design and chiral discrimination.

Molecular-level enhanced clusterization-triggered emission of nonconventional luminophores in dilute aqueous solution

Nonconjugated and nonaromatic luminophores based on clustering-triggered emission derived from through-space conjugation have drawn emerging attention in recent years. The reported nonconventional luminophores are emissive in concentrated solution and/or in the solid state, but they tend to be nonluminescent in dilute solution, which greatly limits their sensing and imaging applications. Herein, we design unique clusteroluminogens through modification of cyclodextrin (CD) with amino acids to enable the intermolecular and intramolecular clusterization of chromophores in CD-based confined space. The resulted through-space interactions along with conformation rigidification originated from hydrogen bond interaction and complexation interaction generate blue to cyan fluorescence even in the dilute solution (0.035 wt.%, quantum yield of 40.70%). Moreover, the prepared histidine-modified CD (CDHis) is demonstrated for fluorescent detection of chlortetracycline with high sensitivity and selectivity. This work provides a new and universal strategy to synthesize nonconventional luminophores with bright fluorescence in dilute aqueous solution through molecular-level enhanced clusterization-triggered emission.

Gas-Phase Fragmentation of Cyclic Oligosaccharides in Tandem Mass Spectrometry

Modern mass spectrometry, including electrospray and MALDI, is applied for analysis and structure elucidation of carbohydrates. Cyclic oligosaccharides isolated from different sources (bacteria and plants) have been known for decades and some of them (cyclodextrins and their derivatives) are widely used in drug design, as food additives, in the construction of nanomaterials, etc. The peculiarities of the first- and second-order mass spectra of cyclic oligosaccharides (natural, synthetic and their derivatives and modifications: cyclodextrins, cycloglucans, cyclofructans, cyclooligoglucosamines, etc.) are discussed in this minireview.

Development of QSAR model for predicting the inclusion constants of organic chemicals with α-cyclodextrin

Solubility is a crucial limiting factor in pharmaceutical research and contaminated site remediation. Cyclodextrin, with its structure of hydrophilic exterior and hydrophobic cavity, has a potential ability to enhance the hydrophobic chemical's solubility through the formation of host-guest complex. The stability of host-guest complex is often quantified by the inclusion constant. In this study, the logarithm of 1:1 α-cyclodextrin inclusion constants (log K_α) for 195 organic chemicals was collected. With this parameter as the endpoint, a quantitative structure-activity relationship (QSAR) model was developed using DRAGON descriptors and stepwise multiple linear regression analysis. The model statistics parameters indicated that the established model had a good determination coefficient of 0.857, a high cross-validation coefficient of 0.835, a low root mean square error of 0.380, together with the acceptable results of external validation, which indicate a satisfactory goodness-of-fit, robustness, and predictive ability of the model. Based on the screened eight descriptors, we propose an appropriate mechanism interpretation for the inclusion interaction. Additionally, the applicability domain of the current model was characterized by the Euclidean distance-based method and Williams plot, and results indicated that the model covered a large number of structurally diverse chemicals belonging to 13 different classes. Comparing with the previous reported models, this model has obvious advantages with a larger dataset, a higher value of correlation coefficient, and a wider application domain.

Quantifying non-covalent binding affinity using mass spectrometry: a systematic study on complexes of cyclodextrins with alkali metal cations

RATIONALE

To date, the quantification of binding affinities for non-covalent complexes between cyclodextrin (CD) and alkali cations including Li(+) , Na(+) , K(+) , Rb(+) , and Cs(+) has not been investigated in detail by electrospray ionization mass spectrometry (ESI-MS) due to the unknown ionization efficiencies of the different species. In this study, the binding constants of CD-Cs(+) complexes were determined by an improved mass spectrometric titration methodology, which was based only on the peak intensities of equilibrium CD. Hence, the discrepancy of ionization efficiencies of CD, alkaline cation and their complex would not affect the measurement. Then the obtained lgKa values were provided as references for competitive ESI-MS. The binding constants for complexes of α-, β- or γ-CD with Li(+) , Na(+) , K(+) and Rb(+) could be derived directly and quickly.

METHODS

The lgKa values between α-, β- or γ-CD and Cs(+) data were processed by curve fitting. These lgKa values were provided as references for competitive ESI-MS. In addition, linear fit equations for complexes of α-, β- or γ-CD with Cs(+) were derived. Through the linear fit equations of competitive ESI-MS, the binding constants for complexes of Li(+) , Na(+) , K(+) and Rb(+) with α-, β- or γ-CD were acquired.

RESULTS

Results showed that the binding constant (lgKa ) values for the complexes of Cs(+) with α-, β- and γ-cyclodextrins were 3.94, 3.88 and 3.80, respectively, revealing that the binding strength decreased with the increase in diameter of cyclodextrins. The competitive ESI-MS results showed a clear trend of decreasing affinity for complexes of cyclodextrins in the order of Li(+) , Na(+) , K(+) , Rb(+) .

CONCLUSIONS

The binding constants of non-covalent cyclodextrin-alkali cation complexes have been systematically studied by an improved mass spectrometric titration and competitive ESI-MS. Also, the structural features of the complexes were discussed. Our results are valuable for better understanding of mechanisms driving inclusion chemistry under well-defined conditions.