Chiral Molecules To Identify New Methods Of Research

Chiral analysis has become increasing important in recent years because chirality is a central factor in biological phenomena, and chiral recognition is the basic aspect of the bio-system in nature. For example, the behavior of the enantiomers of a chiral drug may show striking differences in terms of biological activity, potency, toxicity, transport mechanisms, and routes of metabolism. Therefore the pervasiveness of chiral drug development and the rapid development of combinatorial synthesis demand simple, fast, reliable and sensitive methods for chiral analysis.In this dissertation, bovine serum albumin (BSA) was empolyed as chiral selector, and its recognition to chiral amino acids and chiral drugs was primarily investigated by electrochemical sensor and spectroscopic techniques, and the chiral discrimination of the metal complex on gold nanoparticles was also explored. The dissertation includes five chapters and they are separately introduced as the following: In chapter one, the importance of chiral recognition and the presented progress and development in chiral recognition have been reviewed. Chirality was first reported by Louis Pasteur in 1848, since that time, and especially in recent years, the phenomenon of chirality has become a central theme in scientific research. The field is driven by pharmaceutical industry and has extends further to studies about origin of life, and to the development of optically pure specialty materials and chemical products of pure enantiomer. Significant progress has been made during the past few years on chiral discrimination. The analytical methods used for chiral recognition can be classified into three types: (1) chromatographic methods, such as gas and liquid chromatography, as well as electrophoresis, which are characterized by the use of chiral stationary phases and chral additives, respectively; (2) Spectroscopic methods, such as circular dichroism, NMR, UV-Vis and NIR, etc; (3) chiral sensors, selective binding and catalytist selectivity are the two ways often used in chiral sensors. Althogth chiral HPLC, GC and CE have become the standard and effective methods for chiral separation and analysis, the physical separation of a pair of enantiomers isgenerally time-consuming and cannot be accomplished in some cases. Therefore, alternative simple, fast, precise and sensitive techniques such as spectroscopic methods and chiral sensors are demanded with the development of combinatorial synthesis and the pervasiveness of chiral drug development. On the other hand, chiral selectors are the most important part of the chiral recognition, and in this dissertation, the majority of currently known selectors were divided into the following categories: (1) proteins; (2) modified polysaccharides; (3) synthetic polymers; (4) chiral cage molecules; (5) metal ion complexes; (6) macrocyclic glycopeptides; (7) chiral surfactant; (8) chiral surfaces. The typical examples of the eight chiral selectors and their chiral recogniton mechanism were also illustrated in detail. At the end of this chapter, the significance and contents of the research work in this dissertation was introduced.In chapter two, a novel electrochemical sensor with capability of probing chiral amino acids with gold nanoparticle labels using bovine serum albumin (BSA) as a chiral selector and subsequent signal amplification step by silver enhancement is introduced. The assay relies on the stereoselectivity of BSA embedded in ultrathin γ-alumina sol-gel film coated on the surface of the glassy carbon electrode (GCE). The recognition to the gold nanoparticle-labeled L- or D-amino acids for BSA-GCE could be monitored by the differential pulse voltammetry (DPV), while the DPV signal was greatly amplified by the anchored silver atoms on the n-Au, leading to a new way of quantitatively analysis of chiral amino acids electrochemically at sub-picomolar level. With L-tryptophan as the probe solute, the linear concentration range was from 1.33 × 10¹² to 1.00×10^-9 Mand detection limit was 5.00 × 10^-13 M. For tryptophan enantiomers, the enantioselectivity coefficient 2.31 was obtained.In chapter three, a novel strategy was constructed to determine the enantiomeric composition of chiral substances by BSA based on the UV-Vis spectra of the receptor-ligand mixtures and the following partial least squares (PLS-1) analysis of the spectra data. Taking tryptophan enantiomer as an example, when 0.02 mM BSA was used, the enantiomeric composition was accurately determined with only 0.10 μM concentration and the corresponding enantiomeric excess as high as 98.00% (or -98.00%), which is relatively more sensitive than that reported. Furthermore, the BSA-based approach was also used to predict the enantiomeric composition of other chiral compounds: phenylalanine, tyrosine, alanine, cysteine, DOPA, and propranolol. The results fully demonstrate that BSA is effective in determination of enantiomeric composition of some chiral compounds.In chapter four, a new method based on fluorescence spectra was demonstrated to determine the enantiomeric composition of chiral substances with relatively higher sensitivity. In this method, the quantitative determination of the enantiomeric purity combines fluorescence spectra, BSA guest-host complexion, and chemometric modeling. The mol fractions of tryptophan, phenylalanine, tyrosine, alanine, cysteine, DOPA, and propranolol enantiomers in relatively lower concentration were separately determined by this method, and the prediction results show that the method is not only effective but also relatively sensitive in determination of enantiomeric compositions. Taking tryptophan enantiomer as an example, when 5.00 μM BSA was used, the enantiomeric composition was accurately determined with only 0.03 μM concentration and the corresponding enantiomeric excess as high as 98.00% (or -98.00%).In chapter five, chiral recognition by chiral metal complex was first achieved on gold nanoparticles surface. Our previous work has developed a new approach for electrochemical recognition of chiral amino acids using copper complexes on gold electrode based on the ligand-exchange mechanism. For further insight of the complicated electrochemical chiral recognition, the chiral metal complex of Cu²⁺ and N-acetyl-L-cysteine on the gold nanoparticle surface was used to discriminate phenylalanine enantiomers in this work, and circular dichroism and ¹H NMR were used to detect the recognition results. The experimental results quite validate the electrochemical results, and the method combining nanotechnology, chiral recognition and spectroscopy should be valuable for chiral analysis.