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ANALYTICAL CURRENTS An ultrafast stopwatch Femtosecond laser pulses have made ultrafast spectroscopy possible, and they are making ultrafast electron diffraction (UED) on a picosecond timescale possible as well. Ahmed H. Zewail and his colleagues at the California Institute of Technology have developed instrumentation that measures transient chemical changes with UED. The apparatus includes a femtosecond laser, an ultrafast electron gun, a free-jet expansion source, and a twodimensional single-electron detector. The clocking technique they developed uses the crossed-beam geometry of the diffraction experiment. Part of each laser pulse is used to initiate die chemical reaction; the remainder focuses on a photocathode to generate the electron pulse. The initiation laser pulse created a coulombic field that deflected the unscattered electron beam only during and after the time the laser pulse passed through. This "lensing" occurs because
New approach to protein electrochemistry
the coulombic field focuses die electron obtained at each time point and used to beam and is at a maximum (used to deter- calculate the radial distribution curves. The mine t = 0) when the laser and electron amplitudes of the dominating C-I and I-I pulses are tempopeaks decreased, rally overlapped. corresponding to The method the loss of an marked t = 0 to iodine atom within ~ 2 ps. as would be expected. They used the dissociation of diFuture work iodomethane as a will focus on the model system, besuppression of cause the loss of an background iodine atom after scattering from dissociation is a unreactive spemajor structural cies. The auchange that occurs thors say that on a faster timediffraction techscale than the rotaniques should tional period. They permit the study recorded diffraction of more complex images at t = -20 pp molecular sys0ps and up to tems such as Measuring transient chemical changes. (Top) +70 ps The modibiomolecules. The ultrafast electron diffraction apparatus. fied molecular scat- (Bottom) The detection scheme. (Adapted (Nature 1997 tering intensity was with permission from Macmillan Magazines.) 386 159-62)
of protein to study redox behavior. Previous studies using these cells have relied on either direct charge transfer with die electrode or spectroscopic methods to determine quantities of oxidized and reduced proteins. The Utah State researchers showed that rapid homogeneous electron transfer between the analyte protein and a mediator provided a response that closely mimicked what would be expected between proteins and solid electrodes The technique is demonstrated with measurements on cytochrome c ferro-
Despite the many advances in determining protein redox potentials by electrochemical techniques,tiiereare still lots of proteins for which it is not possible to make direct electrochemical measurements. Vernon D. Parker and Lance C. Seefeldt of Utah State University attacked mis problem by developing a tiiinlayer voltammetry technique protein redox behaviortiirougha soluble redox mediadoxin and the iron tor with an appropriate Thin-layer voltammogram for a nrotei'n of nirroffeelectrode potential solution of ferrodoxin with the Thin-layer electro- mediator 1, f-dibenzyl-4,4hem 19977 9d7 chemical cells need bipyridylium. .Adapted with only small quantities permission from Academii Press.)
Tiny mass spectrometer array Like other federal agencies, NASA has taken up the call to be faster, better, and cheaper in its work. What this means practically is that the analytical instrumentation the agency flies into space will have to be smaller and use less power. 0. J. Orient, A. Chutjian, and V. Garkanian of the Jet Propulsion Laboratory took the
Schematic of miniature mass spectrometer components. (Adapted with permission of the American Institute of Physics.)
Analytical Chemistry News & Features, June 1, 1997 3 3 5 A
