B.S., Beijing University 1994-1998, Chemistry

Ph.D. Universtiy of Chicago 1998-2004, Optics and Biophysics

1) Instrumentation and Optics

This part of my research had to do with the development of laser based optical instruments that has immediate applications in materials, biology and biophysics. Highlights include a multi-photon microscope with 800nm pulsed laser source, and a one-photon confocal microscope for single molecule FRET and correlation spectroscopy.

Details of my home-built instruments:

* Development of an ultrafast pump-probe apparatus with interferometric capability.
End result: A 20fs Ti-sapphire laser based pump-probe system with lock-in data acquisition and nonlinear interferometer for pulse characterization.

* Development of a two-photon microscope for biological applications. End result: A two-photon microscope based on 800nm pulsed laser with 300nm spatial resolution and 3D scanning capability.

* Development of a confocal one-photon microscope for added biophysical and biomedical applications. End result: A two-color microscopy system that is ideal for single molecule FRET and correlation spectroscopy with wavelength tunability.

2) Confocal fluorescence microscopy and and Single molecule spectroscopy

The core of my project is to use single molecule FRET (fluorescence resonance energy transfer) to study biomolecular (RNA in my case) folding. Single-molecule measurements look beyond ensemble averages to obtain novel and complementary information. FRET measurements have been used to track the conformational changes of single RNA or protein molecules, identify intermediates in RNA folding, and examine RNA–protein interactions.

Links to some single molecule groups:

a) Xiaowei Zhuang from Harvard

b) T. Ha from UIUC

c) Steven Chu from Stanford

d) Sunney Xie from Harvard

e) Eaton group from NIH

Results overview:

Images above show my target RNA molecule that is labeled with Fluoroscein (FL, donor) and Cy3 (acceptor) FRET pair. Panel B is a cartoon that illustrates how the motions of three helix arms are going to correlate to the distance change between the FRET pair, which is our observable. Then, from the FRET signal change, we can then back out the information about conformational dynamics of the host RNA molecules, on a one on one basis.

This study contributed to a recent paper in PNAS (Xie et al, 101 (2), 534). Below are more results from a ongoing effort to study a slightly different mutant RNA sequence.

As we titraite more Mg2+ into the system, more RNA molecules are folded therefore structurally more compact, resulting in more acceptor fluorescence due to FRET. Above panel shows a high Mg2+ (left) and a low Mg2+(right) condition. Color coded images clearly indicate a total reverse in the donor/acceptor emissions.

Things are getting even more exciting when we look at each individual molecules for a while, which is called a fluorescent emission trajectory, recording conformational fluctuations in real time. Like shown below:

These anti-correlated fluctuations directly reveal: 1) intermediate molecular states and their structural compactness; 2) Rate constants at which these states inter-convert. We are writing up another manuscript to summarize all these results.