Biosciences Graduate Program

M. Lei L. Geng, BS, PhD

Portrait

Professor of Chemistry

Contact Information

Office: 330 IATL
Iowa City, IA 52242
Office Phone: 319-335-167

Email: lei-geng@uiowa.edu
Web: More About Dr. Geng - Related Websites and Resources

Education

BS, University of Science Tecnhology China
PhD, Duke University

Post Doctoral, University of Wisconsin-Madison

Education/Training Program Affiliations

Biosciences Graduate Program

Research Summary

Our research program is directed towards the development and application of laser spectroscopic methods for the studies of biological, medical, and chemical systems. Current effort in our group is focused on cancer diagnosis with laser spectroscopy, probing molecular distributions in chemical separation, and study of protein folding mechanisms. A challenge in fluorescence studies of complex multicomponent systems, such as biomolecular, environmental, and clinical samples, is to dissect spectra of individual components. The fluorescence spectrum of the sample is the sum of heavily overlapped vibronic spectra of individual species. We have developed a new technique, two-dimensional fluorescence correlation spectroscopy (2D FCS), to enhance spectral resolution in these systems. In 2D FCS, a perturbation is introduced into the molecular system. Temporal responses of the system to the perturbation are monitored at many wavelengths and the time correlation function between wavelengths is evaluated. Based on the differences in relaxation rates of different fluorescent species, 2D FCS significantly enhances the spectral resolution and can facilitate structural and kinetic studies of individual species. The relaxation rates could be rates of translational or rotational diffusion, fluorescence decay rates, or rates of chemical reactions, depending on the nature of the external perturbation. With the enhanced spectral resolution, 2D FCS provides a method for simultaneous characterization of multiple species in environmental and biomedical samples. Early diagnosis is the key to improved treatment of cancers. If the cancer is diagnosed at an early stage without spread, the five-year survival rate of the patient is over 90%. If the cancer has spread to remote organs at the time of diagnosis, however, the survival rate decreases to below 10%. Optical spectroscopy provides a promising alternative to histopathology, the standard method of cancer diagnosis in clinics. Optical methods are noninvasive, inexpensive, and the diagnosis can be in situ, without time delay. To be a successful technique for cancer diagnosis, an optical method is required to provide high contrast between healthy and adenomatous tissue and short measurement time. By combining both spectral and temporal information of the tissue fluorescence, 2D FCS provides a high contrast between samples, forming the basis for cancer diagnosis. Fast collection of two-dimensional fluorescence spectra of tissue is facilitated by frequency domain measurements. Tissue phantoms are employed to elucidate the influences of photobleaching, light scattering, and matrix absorption on the in situ spectroscopy of tissue. Fluorescence imaging is used to probe the molecular and morphological changes in tissue upon cancerous transformation. The distribution of solute molecules across column, and between stationary phase and mobile phase is an important factor for chemical separation in capillary electrophoresis (CE) and capillary electrochromatography (CEC). We have used laser scanning confocal microscopy to probe the separation process in CE and CEC. By cutting thin optical slices, confocal imaging allows direct observation of molecular distributions across the CE column and into the bonded stationary phase in CEC. Spectrally and time-resolved fluorescence spectroscopy generates direct information about the heterogeneous microenvironments in the stationary phase and the molecular distribution in these environments. The influences of high voltage, stationary phase, pH, and solvent composition on molecular distribution are investigated. To fully understand the protein structure-function relationship, it is essential to understand the folding mechanism: how a protein molecule attains its native structure from an unfolded state. We are currently studying the folding mechanism with fluorescence spectroscopy of intrinsic tryptophan residues and extrinsic probes. The changes in the protein structure upon unfolding are revealed by solvent exposure of the probe molecules, and the distance between a donor-acceptor pair through fluorescence resonance energy transfer. Conformational fluctuation is probed by fluorescence lifetime distributions. Time-resolved fluorescence emission spectra reveal the reorientation kinetics of protein structure. We are pursuing methods of single molecule spectroscopy to monitor the conformational fluctuation and to dissect folding pathways. Structures of single protein molecules at different folding stages will be probed spectroscopically.

Date Last Modified: 06/07/2014 - 21:56:23