Images of DNA produced during gel electrophoresis A new electrophoretic effect and associated technique— pulsed oriented electrophoresis (POE)—have been intro duced by two scientists at Carnegie Institution of Washington's department of embryology in Baltimore. At the same time, the two scientists—Carnegie staff associate David Schwartz and Johns Hopkins University graduate student Michael Koval—have produced what they say are the first clear images of individual, fluorescently stained molecules of DNA as they migrate during gel electrophoresis [Nature, 338, 520 (1989)]. Traditional gel electrophoresis—most successful with small DNA molecules, Schwartz and Koval note—separates molecules of DNA according to their sizes by running them through a gel-like matrix using a steady electric field. An electrophoretic system developed several years ago by Schwartz and called pulsed field electrophore sis took advantage of the distortion of large molecules that interact more frequently with the matrix by continuously reorienting the direction of an applied electrical field, as determined by molecule length. It thus became possible to resolve even chromosome-sized DNA molecules. POE, devel oped recently by Schwartz, is similar to pulsed field electro phoresis but uses pulses that are much shorter in duration—
the Cp*Ir fragment is bound. The net result of the reaction is the for mation of a C-C and two C-N bonds, with the complete cleavage of the Ir-N multiple bond. Synthesis of the ruthenium benzyne complex also followed from studies of C-H activation chemis try. The initial goal was to produce a ruthenium complex containing four trimethylphosphine ligands by the e l i m i n a t i o n of two p h e n y l groups from Ru[P(CH3)3]4(C6H5)2. The idea was that such a complex might insert ruthenium into carbonhydrogen bonds. Instead, the chem ists found that Ru[P(CH3)3]4(C6H5)2 undergoes a cyclometallation type of reaction in which one phenyl group and a hydrogen on the other
three to five seconds rather than one to two minutes. POE allows the separation of still larger DNA molecules. Images shown above are of DNA molecules from a bacteriophage, 700 kilobases long, imaged at different times as they proceeded through a gel matrix under POE condi tions. Samples were subjected to a series of short electri cal pulses with the field switched 90° in frame c, and reversed 180° in frame g. Frame a shows a trapped molecule (1). Another mole cule (2) is coming into view. In frame b, newly arrived molecules, bottom, display hook-shaped conformations (which can temporarily hang up on obstacles in the gel matrix) and condensed coils. In frame c, after a field switch, the hookshaped molecules resolve into single strands with dense regions at their ends. The molecule at (2) has changed from a thick rod to a stubby-shaped structure to a round, dense mass. The two trapped coils in frames d, e, and f show a staircase effect characteristic of POE. Frames g through j show that, with the field reversed 180°, both molecules disentangle and then collapse so that another cycle can begin. The bar in j is 10 μηη.
phenyl group eliminate to produce the benzyne complex. Bergman says that other benzyne complexes have been synthesized, but they do not react similarly to the new compound. "The unique thing about this complex is that it reacts with almost everything we throw at it," he says. Perhaps the most unusual reac tion of the r u t h e n i u m complex, Bergman says, is the one that ap pears to be the most simple: wa ter, which is not particularly acid ic, adds across the carbon-ruthe nium bond to produce a hydroxy compound. The complex also re acts with the N-H bond of an arylamine to produce an azametallacycle, a process that presumably pro
James Krieger
ceeds via initial cleavage of the arylamine N-H bond to give an unusual metal (aryl)(amido) com plex. The complex reacts with benzaldehyde to produce single insertion product, and with acetophenone to give a cyclometallated enolate. Re markably, Bergman says, it also reacts with acetone to give the same enolate produced in the reaction with acetophenone. Not all of the chemistry involved in these reac tions is as yet completely under stood. "I think the ruthenium benzyne system is going to be a useful pre cursor for complexes with metaloxygen and metal-nitrogen bonds," Bergman says. D April 17, 1989 C&EN
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