Specific Noncovalent Association of Chiral Large-Ring Hexaimines: Ion Mobility Mass Spectrometry and PM7 Study.

Ion mobility mass spectrometry and PM7 semiempirical calculations are effective complementary methods to study gas phase formation of noncovalent complexes from vaselike macrocycles. The specific association of large-ring chiral hexaimines, derived from enantiomerically pure trans-1,2-diaminocyclohexane and various isophthaldehydes, is driven mostly by CH-π and π-π stacking interactions. The isotrianglimine macrocycles are prone to form two types of aggregates: tail-to-tail and head-to-head (capsule) dimers. The stability of the tail-to-tail dimers is affected by the size and electronic properties of the substituents at the C-5 position of the aromatic ring. Electron-withdrawing groups stabilize the aggregate, whereas bulky or electron-donating groups destabilize the complexes.

Dynamic Formation of Noncovalent Calixsalen Aggregates.

A procedure for studying "dynamic structural behavior" of large chiral macrocycles is presented. Ion mobility MS, diffusion-ordered NMR spectroscopy (DOSY NMR), and optical rotation (OR) measurements, supported by calculations, are used together as effective complementary methods to study dynamic formation of noncovalent aggregates. It is shown that the monomer-dimer equilibrium is driven by π-π or CH-π interactions and controlled largely by the substitution pattern of the calixsalen skeleton.

Structural elucidation of specific noncovalent association of folic acid with native cyclodextrins using an ion mobility mass spectrometry and theoretical approach.

The combination of ion mobility mass spectrometry studies and theoretical calculations including docking studies permitted a detailed structural description of noncovalent complexes of folic acid (FA) and native cyclodextrins (α-CD, ß-CD, and γ-CD). The mode of noncovalent association depended on the cavity size of the cyclodextrin. The structure of FA/α-CD represented the exclusion complex in which the aminobenzoic moiety and the aromatic pteridine ring of folic acid remain outside the cyclodextrin cavity, while the glutamate residue is anchored in the interior of the α-cyclodextrin. A rotaxane-type structure was proposed for the FA/ß-CD complex with the aminobenzoic part of FA being trapped in the central cavity of ß-CD. The glutamate residue and the aromatic pteridine ring interact with the primary and secondary rim hydroxyl residues, respectively, enhancing complex stability. Two possible structures of FA/γ-CD were suggested, the first one being analogous to the FA/ß-CD complex and the second one being more stable-in which the aromatic pteridine ring penetrates into the CD cavity while the glutamate residue with the aminobenzoic part of FA is exposed to the cone exterior of CD at its wider edge. Further insight into the association behavior of the folic acid toward cyclodextrins evaluated by thermodynamic calculations indicates that the process is highly exothermic. The complex stability increased in the order FA/α-CD < FA/ß-CD < FA/γ-CD. This order is consistent with the previously determined relative gas-phase stability established based on the dissociation efficiency curves of the FA/CD complexes.

Novel antimicrobial peptide dendrimers with amphiphilic surface and their interactions with phospholipids--insights from mass spectrometry.

A series of new peptide dendrimers with amphiphilic surface, designed around a dendronized ornithine (Orn) core were synthesized and characterized by ESI-MS, ¹H-, ¹³C- NMR, and CD spectrometry. An improved antimicrobial potency against S. aureus and E. coli was detected as a result of an increased charge, higher branching and variable lipophilicity of the residues located at the C-terminus. Minimal inhibitory concentration (MIC) values indicated that the selected dendrimers were not sensitive to the physiological concentration of Na⁺ and K⁺ ions (100 mM), but expressed reduced potency at 10 mM concentration of Mg²âº and Ca²âº ions. Circular dichroism (CD) curves measured under various conditions revealed structure and solvent-dependent curve evolution. ESI-MS studies of gas-phase interactions between selected dendrimers and both anionic (DMPG) and neutral (DMPC) phospholipids revealed the presence of variously charged dendrimer/phospholipid aggregates with 1:1 to 1:5 stoichiometry. The collision-induced fragmentation (CID) of the most abundant [dendrimer/phospholipid]²âº ions of the 1:1 stoichiometry demonstrated that the studied dendrimers formed stronger complexes with anionic DMPG. Both phospholipids have higher affinity towards dendrimers with a more compact structure. Higher differences in CID energy necessary for dissociation of 50% of the complex formed by dendrimers with DMPG vs. DMPC (ΔCID50) correlate with a lower hemotoxicity. Mass spectrometry results suggest that for a particular group of compounds the ΔCID50 might be one of the important factors explaining selectivity of antimicrobial peptides and their branched analogs targeting the bacterial membrane. Both circular dichroism and mass spectrometry studies demonstrated that dendrimers of N(α)- and N(ε)-series possess a different conformation in solution and different affinity to model phospholipids, what might influence their specific microbicidal mechanism.

Structural diversity in native cyclodextrins/folic acid complexes--from [2]-rotaxane to exclusion compound.

The formation of different complexes of folic acid depending on the size of the host cyclodextrin resulting in either an exclusion compound (with the smallest α-cyclodextrin) or 2-rotaxane, where cyclodextrin is threaded over folic acid (with ß- and γ-cyclodextrins), is presented. The formation is carried out in water which allows both possible application in pharmaceutical sciences and usage of environmentally friendly "green chemistry". The obtained compounds are thoroughly characterized using one and two dimensional NMR, mass spectrometry, differential scanning calorimetry and thermogravimetric analysis.

Acid-based approach for separation of peptide epimers using IM-MS.

Chiral molecules frequently remain undistinguishable using ion mobility mass spectrometry (IM-MS), due to insufficient differences of their collision cross sections at the available mobility resolution of the ion mobility drift tubes. The influence of the complexation with organic acids on the ion mobility separation of peptide epimers is evaluated using traveling-wave ion mobility (TWIMS). The examined epimeric tripeptides containing Arg residue with the sequence: Ac-Phe-Arg-Trp-NH2 formed stable complexes in the gas phase, and under the increased pressure in ion mobility drift tube, noncovalent associates formed with carboxylic or sulfonic monoacids and diacids with chiral variation of certain acids. Overall, the complexation with an acid leads to the improvement in stereodifferentiation among epimeric peptides, in comparison to the analysis of pure epimers. Detailed characterization of peptide epimer-acid associates obtained for dibenzoyl-D-tartaric acid by theoretical calculations and collisional dissociation studies revealed that the presence of multiple hydrogen bonding interactions between carboxylate anions and hydrogens from N-H of both the guanidinium group of arginine and the indole of tryptophan, as well as the amide backbone hydrogens in the peptide, is responsible for stability of acid-peptide complexes and for their differentiation in the ion mobility drift tube. The specificity of complex formation toward Arg was determined in terms of complex stability. Based on the reported results, we present general conclusions regarding the utility of the acid-based complexation in the separation of peptide isomers.

Structural Elucidation of ß-Lactam Diastereoisomers through Ion Mobility Mass Spectrometry Studies and Theoretical Calculations.

The ion mobility combined with mass spectrometry and theoretical calculations were used to characterize and separate six diastereoisomeric ß-lactams. The influence of traveling wave height and wave velocity, size of the alkali metal ion (Li(+), Na(+) and K(+)) and drift gases with varying masses and polarizabilities (N2 and CO2) on separation efficacy was additionally examined. The best separation of diastereoisomers of ß-lactams was observed for adducts with Na(+) and Li(+) ions, whereas other parameters had little impact on separation process. The isomeric ß-lactams were characterized by both experimental and theoretical collision cross sections. The theoretically calculated values of collision cross sections obtained from extensive molecular dynamics and density functional theory calculations for model structures agreed well with those established experimentally. The relationship between separation efficacy and the configuration at the carbon atoms C5 and C6 of ß-lactam ring was defined.

Separation of catechin epimers by complexation using ion mobility mass spectrometry.

Ion mobility coupled with mass spectrometry provides a fast and repeatable method to separate catechin epimers by previous complexation with selected chiral modifiers and transition metals. Several combinations with chiral ligands such as D- and L-amino acids and/or additional metal cations, chiral crown ethers, tartaric acid and heptakis(2,6-di-O-methyl)-ß-cyclodextrin were screened for their ability to affect the separation efficiency. The clusters having the form of [2M + D-amino acid + Cu(2+) - 3H](-) (M stands for (-)-epicatechin or (+)-catechin) showed improvement in stereodifferentiation between two epimeric catechins in comparison to the analysis of pure epimers, where no separation was observed or the separation was hampered by the formation of mixed dimer complexes. Among various examined D-amino acids only those possessing hydrophobic side chains induced the improvement of separation efficiency. The best peak-to-peak resolution (Rp-p) was determined to be 0.71 for [2M + D-Leucine + Cu(2+) - 3H](-) clusters.