Lianming Wu

Differentiation and quantitation of isomeric dipeptides by low-energy dissociation of copper(II)-bound complexes.

Application of the kinetic method based on the dissociation of transition metal centered cluster ions is extended from chiral analysis (Tao, W. A.; Zhang, D.; Nikolaev, E. N.; Cooks, R. G. J. Am. Chem. Soc.2000, 122, 10598) to quantitative analysis of isomeric mixtures, including those with Leu/Ile substitutions. Copper(II)-bound complexes of pairs of peptide isomers are generated by electrospray ionization mass spectrometry and the trimeric complex [CuII(ref)2(A) − H]+ (analyte A, a mixture of isomeric peptides; reference compound ref, usually a peptide) is caused to undergo collisional dissociation. Competitive loss of the neutral reference compound or the neutral analyte yields two ionic products and the ratio of rates of the two competitive dissociations, viz. the product ion branching ratio R is shown to depend strongly on the regiochemistry of the analyte in the precursor [CuII(A)(ref)2 − H]+ complex ion. Calibration curves are constructed by relating the branching ratio measured by the kinetic method, to the isomeric composition of the mixture to allow rapid quantitative isomer analysis.

Arginine clusters generated by electrospray ionization and identified by tandem mass spectrometry

Some α-amino acids, especially arginine, form protonated clusters when examined by electrospray ionization in an ion trap mass spectrometer. Singly-, doubly-, triply- and quadruply-protonated arginine clusters [(Arg) n + H]+, [(Arg) m + 2H]+2, [(Arg) l + 3H]+3 and [(Arg) k + 4H]+4, were further studied by collision-induced dissociation (CID). The singly-protonated cluster n = 4 displayed enhanced stability and CID of larger clusters (n > 4) showed fragmentation leading to the preferential formation of n = 4 product ions. The n = 4 stable cluster is proposed to bear a formal resemblance to the simple salt cluster [(NaCl)4 + Na]+, a 3 × 3 × 1 micro-crystallite. This leads to the suggestion that [(Arg)4 + H]+ is planar, with bonding primarily due to the electrostatic interactions between four zwitterionic arginine molecules. In the doubly-charged ion series, clusters of m = 12–15 have enhanced stability relative to those of immediately smaller size. Drawing on the analogous salt structures, the dication, [(Arg)12 + 2H]+2 might have a structure consisting of three layers of tetramers, two of which are protonated. This structure is analogous to that of the magic number doubly-charged ionic cluster [(NaCl)12 + 2Na]+2 which is a 3 × 3 × 3 micro-crystallite with an internal anion defect.

Recognition and quantification of binary and ternary mixtures of isomeric peptides by the kinetic method: metal ion and ligand effects on the dissociation of metal-bound complexes.

The kinetic method is applied to differentiate and quantify mixtures of isomeric tripeptides based on the competitive dissociations of divalent metal ion-bound clusters in an ion trap mass spectrometer. This methodology is extended further to determine compositions of ternary mixtures of the isomers Gly-Gly-Ala (GGA), Ala-Gly-Gly (AGG), and Gly-Ala-Gly (GAG). This procedure also allows to perform chiral quantification of a ternary mixture of optical isomers. The divalent metal ion CaII is particularly appropriate for isomeric distinction and quantification of the isobaric tripeptides Gly-Gly-Leu/Gly-Gly-Ile (GGL/GGI). Among the first-row transition metal ions, CuII yields remarkably effective isomeric differentiation for both the isobaric tripeptides, GGI/GGL using GAG as the reference ligand, and the positional isomers GAG/GGA using GGI as the reference ligand. This is probably due to agostic bonding: α-agostic bonding occurs between CuII and GAG and β-agostic bonding between CuII and GGI, each produces large but different steric effects on the stability of the CuII-bound dimeric clusters. These data form the basis for possible future quantitative analyses of mixtures of larger peptides such as are generated, for example, in combinatorial synthesis of peptides and peptide mimics.

Rapid enantiomeric determination of α-hydroxy acids by electrospray ionization tandem mass spectrometry

Direct chirality measurement of tartaric and other α-hydroxy acids at very low enantiomeric excess (ee) using a fast new mass spectrometric method.

Chiral and isomeric analysis by electrospray ionization and sonic spray ionization using the fixed-ligand kinetic method.

The fixed-ligand version of the kinetic method has been used for chiral and for isomeric analysis by studying the dissociation kinetics of transition metal-bound trimeric cluster ions ([(MII + Lfixed – H)(ref*)(An)]+, where MII is a transition metal, Lfixed is a fixed (non-dissociating) ligand, ref* is a reference ligand and An is the analyte. The trimeric cluster ions are readily generated by electrospray ionization (ESI) or sonic spray ionization (SSI). The size of the fixed ligand, L-Phe–Gly–L-Phe–Gly, is chosen based on previous results but with the inclusion of aromatic functionality to increase chiral recognition. Improved chiral/isomeric differentiation results from enhanced chiral/isomeric interactions (metal–ligand and ligand–ligand) due to the fixed ligand. As shown in the cases of chiral dipeptides (D-Ala–D-Ala/L-Ala–L-Ala), sugars (D/L-glucose, D/L-mannose) and isomeric tetrapeptides (L-Ala–Gly–Gly–Gly/Gly–Gly–Gly–L-Ala), improved chiral/isomeric discrimination by factors from three to six were obtained by the fixed ligand procedure. Chiral recognition is independent of the concentrations of the analyte, the reference ligand, the fixed ligand and the transition metal salt, a great advantage for practical applications. In addition to increased chiral distinction, the simplified dissociation kinetics also contribute to improved accuracy in chiral quantification, in comparison with data obtained by investigating the dissociation kinetics of simple trimeric cluster ions [MII(ref*)2(An) – H]+. Accurate determination of enantiomeric excess (ee) is demonstrated by enantiomeric quantification of D-Ala–D-Ala/L-Ala–L-Ala down to 2% ee. Both ESI and SSI allow chiral quantification with similar accuracies. The performance of chiral analysis experiments is not limited to ion trapping devices such as quadrupole ion trap mass spectrometers; a hybrid quadrupole-time of flight (Q-ToF) mass spectrometer is shown to provide an alternative choice. The fixed-ligand kinetic method is not restricted to any particular kinds of isomers and, hence, represents a general procedure for improving molecular recognition and chiral analysis in the gas phase.

Chiral quantification of D-, L-, and meso-tartaric acid mixtures using a mass spectrometric kinetic method

Accurate quantification of the optical isomers in a ternary mixture of D-, L-, and meso-tartaric acids is achieved using electrospray ionization tandem mass spectrometry for in-situ metal complex formation and a three-point calibration method to quantify the dissociation kinetics.

Alkali chloride cluster ion dissociation examined by the kinetic method: heterolytic bond dissociation energies, effective temperatures, and entropic effects.

Branching ratios have been measured as a function of collision energy for the dissociation of mass-selected chloride-bound salt cluster ions, [Rb-35Cl-Mi]+, where Mi = Na, K, Cs. The extended version of the kinetic method was used to determine the heterolytic bond dissociation energy (HBDE) of Rb-Cl. The measured value of 480.8 ± 8.5 kJ/mol, obtained under single collision conditions, agrees with the HBDE value (482.0 ± 8.0 kJ/mol), calculated from a thermochemical cycle. The observed effective temperature of the collisionally activated salt clusters increases with laboratory-frame collision energy under both single- and multiple-collision conditions. Remarkably, the effective temperatures under multiple collision conditions are lower than those recorded under single-collision conditions at the same collision energy, a consequence of the inability of the triatomic ions to store significant amounts of internal energy. Laboratory-frame kinetic energy to internal energy transfer (T→V) efficiencies range from 3.8 to 13.5%. For a given cluster ion, the T→V efficiency decreases with increasing collision energy. Many features of the experimental results are accounted for using MassKinetics modeling (Drahos and Vékey, J. Mass Spectrom. 2001, 36, 237).

Mass spectral methods of chiral analysis

Publisher Summary This chapter discusses mass spectrometry (MS) technique. It also emphasizes that mass spectrometry, especially tandem mass spectrometry, provides several unique analytical advantages for quantitative chiral analysis. These include intrinsically high sensitivity, molecular specificity, and tolerance to impurities, and the simplicity and speed of the mass spectrometric measurement. The limitation of MS chiral analysis is poor in applicability to samples <1% “ee”. This methodology has the potential to be applied into many fields. The three point interaction used for gas-phase chiral recognition is useful in the exploration of stereoselective ion channel-chiral drug interactions. Such interactions give rise to quantitative differences in stereoselectivity that can be utilized for the better understanding of different pharmacological effects of chiral drugs in different forms of enantiomers..