Chirality and chiral discrimination are essential for living systems and are of fundamental importance in life sciences. Nearly all important biological molecules are chiral. In fact, the chemistry of life is built almost exclusively on L-enantiomers of amino acids and D-sugars, a pattern known as the "homochirality of life". Our research is focused on the spectroscopic and theoretical studies of this class of molecules.
The main objective of the research is to obtain a detailed molecular level description of the interaction between chiral molecules. Specifically, we are interested in quantifying how the intermolecular forces come together in a stereo-specific way to enable a chiral molecule to recognize and to discriminate one enantiomer from the other. We use a combination of the molecular beam method with the cavity ring down spectroscopic technique. The molecular beam serves to synthesize and to stabilize the weakly bound homochiral (R·R' or S·S') and heterochiral (R·S' or S·R') diastereomers. The cavity ring down technique provides the ultrahigh sensitivity necessary to detect the infrared spectra of the chiral diastereomers. The state specific quantitative information from the spectra is used to derive structural and dynamical information about the system. These experimental analyses are complemented with high level ab initio calculations to construct the chiral recognition interaction surface and to provide insight into the specific roles of hydrogen-bonded networks and dispersion interactions in chiral discrimination.
Another research direction is concerned with the measurements of vibrational circular dichroism spectra of chiral molecules. The experimental work is centered around developing new instruments that are capable of measuring optical properties of chiral molecules in the gas phase. The goal is to explore the link between molecular structure and optical activity and to assess the effects of solvation on optical properties of chiral molecules.
N. Borho and Y. Xu, “Lock-and-key principle on a microscopic scale: the case of the propylene oxide···ethanol complex”, Angew. Chem. Int. Ed., 2007, 46, 2276-2279. (cover of the March 19 issue, VIP paper, Published Online: 17 Nov 2006, DOI: 10.1002/anie.200603809).
Z. Su, N. Borho, and Y. Xu, “Chiral self-recognition: direct spectroscopic detection of the homochiral and heterochiral dimers of propylene oxide in the gas phase”, J. Am. Chem. Soc., 2006, 128, 17126-17131. (ASAP Web Release Date: 06-Dec-2006; (Article) DOI: 10.1021/ja066128j).
W. Tam, I. Leonov, and Y. Xu. "Pulsed slit jet cavity ring-down spectroscopy with a mid-infrared lead salt diode laser." Rev. Sci. Instrum. 2006, 77, 063117.
Z . Su, W. Tam, and Y. Xu. "High resolution infrared spectroscopy and ab intio studies of the cyclopropane-carbon dioxide weak interaction." J. Chem. Phys. 2006, 124, 024311.
Z. Su and Y. Xu. "Ab initio study of chiral recognition in the propylene imine - hydrogen peroxide complex." Phys. Chem. Chem. Phys. 2005, 7, 2554-2560.