Xiaoliang Xie

Picosecond time‐resolved circular dichroism spectroscopy: experimental details and applications

An experimental apparatus for measuring time‐resolved circular dichroism (CD) with picosecond resolution is described. The time resolution of the device is determined by the laser pulse width, not the modulation frequency of an electro‐optic or piezo‐optic crystal as is the case for commercial CD spectrometers. The time‐dependent CD signal can be monitored over the wavelength range from 280 to 850 nm. The data from single wavelength studies can be pieced together to generate time dependent spectra. The experimental approach is compared to previous time‐resolved CD techniques which, for technical reasons, have been limited to, at most, nanosecond resolution. The picosecond apparatus is shown to be free of many of the polarization artifacts present in these previous devices. The utility of this technique is demonstrated by examining result on the time dependent near‐UV CD of myoglobin following the photoelimination of coordinated CO from carbonmonoxymyoglobin.

Picosecond circular dichroism spectroscopy: a Jones matrix analysis

Measurement of time-resolved circular dichroism signals can be complicated by transient linear dichroism and birefringence effects. These signals arise from using a polarized light beam for excitation and from imperfect modulation of the circularly polarized probe beam. In a polarization-modulation experiment designed to measure the time dependence of circular dichroism, a small amount of pump-induced linear dichroism coupled with birefringence of optics would dominate the observed signals, rendering an accurate measurement virtually impossible. We examine these effects within the Jones matrix formalism. These calculations demonstrate how one can experimentally eliminate transient linear polarization signals, permitting accurate measurement of the transient circular dichoism of a sample with picosecond resolution. Theory and experiment are compared for time-dependent data on myoglobin following the photoelimination of CO from carbonmonoxy myoglobin.

A picosecond circular dichroism study of photosynthetic reaction centers from rhodobacter sphaeroides

Picosecond transient circular dichroism spectra are reported for the primary intermediates in the photocycle of reaction centers isolated from Rhodobacter sphaeroides. The time-resolved circular dichroism spectra of the two electron transfer intermediates (BChl2) +BPh-LQA and (BChl2) +BPhLQ-A reveal a large, nonconservative, and fairly stationary CD band at 800 nm. These results suggests that mechanisms other than exciton interactions need to be included in order to explain the optical activity of this biological system.

High energy and tunable picosecond laser pulses at 1 kHz: synchronously pumping a dye laser with a mode-locked, Q-switched and cavity dumped Nd:YAG laser system

Abstract Efficient operation of a mode-locked, Q-switched and cavity dumped YAG laser and a synchronously pumped dye laser is described. Stable operation is observed up to repetition rates of 1.3 kHz. Introduction of the cavity dumping optics into the YAG oscillator results in only a modest decrease in the energy of the output pulse train, the operation of the dye laser is unaffected. At a 1.0 kHz repitition rate, cavity dumping the YAG oscillator immediately following cavity dumping the dye laser at the maximum of its gain results in a single pulse energies in excess of 0.6 mJ. Efficient second harmonic generation using CD ∗ A produces pulse energies of 0.25 mJ (±5% stability) with a pulse width of 80 ps. In addition, the second harmonic of the cavity dumped YAG can be used to amplify the output of the dye laser. Amplification to 20 μJ/pulse is observed for input energies ranging from 0.1 μJ/ pulse to 10 μJ/pulse.

Real-time spectroscopic techniques for probing conformational dynamics of heme proteins.

Publisher Summary One of the most thoroughly studied classes of proteins is those that contain heme groups. Heme groups are important in a variety of electron and molecular transport processes. In proteins involved in electron transport reactions, the metal ion complexed by the porphyrin is generally coordinated to two amino acids. In molecular transport proteins, the metal is usually coordinated by one amino acid; the remaining coordination site is where the molecule being transported by the protein binds. Over the past several decades, there has been an intense effort to understand the factors that control the binding to and subsequent releasing of small molecules from heme proteins. The vast majority of this work presents complexes involving myoglobin and hemoglobin. This chapter discusses new experimental techniques for studying the protein conformational dynamics that occur on breaking the heine-molecule bond. Because of the immense amount of time-resolved spectroscopic studies that have been reported on heine proteins, the scope of this chapter is limited to dynamics that occur on the picosecond time scale.

Photodissociation of Cr(CO)6 in solution: Solvation dynamics of Cr(CO)5

Picosecond absorption spectroscopy is used to examine the dynamics of formation of Cr(CO)5(solvent) generated by photolysis of Cr(CO)6 in neat alcohol and hydrocarbon solvents. In several solvents, the time resolved absorption data show that the photodissociation results in the formation of a distribution of solvated Cr(CO)5 intermediates.