Siqi Du

Graduation Semester and Year




Document Type


Degree Name

Doctor of Philosophy in Chemistry


Chemistry and Biochemistry

First Advisor

Daniel Armstrong


Amino acids are essential building blocks in all living organisms. Initially it was believed that only L-amino acids were relevant in higher organisms, and D-amino acids were laboratory artifacts. Today, various D-amino acids have been detected in different organisms, including humans, and some even play essential roles in biological processes. In addition, abnormal D-amino acid levels have been reported in patients with different diseases. The variations in D-amino acid levels might be of some diagnostic value. However, our knowledge of D-amino acids remains limited and most of the D-amino acids are not well investigated due to the lack of a comprehensive analytical platform. Analysis of D-amino acids in biological samples is challenging, because of the interference of large amounts of L-amino acids and a plethora of other indigenous compounds. The complete analysis of all D-amino acids requires a long period of time. Therefore, the goals of this dissertation are to 1) enhance current analytical methods for analyzing D-amino acids in biological samples, and 2) provide information that further studies can reference in finding functions related to pathology and potentially other unexplored D-amino acids processes. Novel methodologies, based on high performance liquid chromatography-paired ion electrospray ionization mass spectrometry (HPLC-PIESI-MS), two dimension-HPLC (2D-HPLC), and HPLC-tandem mass spectrometry (HPLC-MS/MS), were developed and evaluated for the analysis of free L- and D-amino acids and peptides-containing isomeric amino acid residues from complex matrices. For the achiral analysis of amino acids, an ultrasensitive detection method using PIESI-MS was developed for the analysis of Fmoc-amino acids in urine samples. This method showed improved detection sensitivity down to sub-pg level. A 2D-HPLC chiral separation method with high sensitivity and selectivity was developed for the comprehensive baseline study of L- and D-amino acids in mouse brain and blood. A simple, rapid and sensitive chiral analysis method was developed using HPLC-MS/MS for the simultaneous analysis of 20 common amino acids and their enantiomeric compositions in wild-type mice and mutant mice lacking D-amino acid oxidase activity. Also, the intracellular and extracellular profiles of L- and D-amino acids in human breast cancer cells (MCF-7) and non-tumorigenic epithelial breast cells (MCF-10A) were reported for the first time using this HPLC-MS/MS method. We have also developed the first comprehensive analytical platform that can separate all 20 possible β-amyloid peptides containing Asp, isoAsp, and Ser isomers. Using this simple and high-throughput separation method, we will be able to identify and quantify Asp, isoAsp, and Ser isomers at every position of the β-amyloid peptides from Alzheimer’s patients simultaneously.


D-amino acids, Beta-amyloid peptides


Chemistry | Physical Sciences and Mathematics


Degree granted by The University of Texas at Arlington

Included in

Chemistry Commons