dence of the rate of electron transfer (called β) through a specially modified DNA system designed by Kazuyoshi Ta-naka and Keijiro Fukui of the department of molecular engineering at Kyoto Uni­versity, Japan.

The researchers inserted an electron acceptor—the fluorescing dye 9-amino-5-chloro-2-methoxymeridine—into a spe­cific site on a small piece of DNA. Gua­nine, an easily oxidized purine base in DNA that serves as an electron donor,

Attached dye replaces base pair in DNA helix

Source: Angewandte Chemie

was placed at varying distances from the dye. Tanaka and Fukui irradiated the sys­tems and measured the resulting fluores­cence spectra. They calculated the rates of electron transfer from the fluores­cence quantum yields, plotted them as a function of donor-acceptor distance, and obtained a relatively large β value of 1.4, comparable to that of proteins, which hover around 1.0 [Angew. Chem. Int. ^.,37,158(1998)].

"This signifies that DNA-base stacking does not form a special mediator for the fast electron transfer process," writes Tanaka.

"It's very exciting," says University of Pittsburgh chemistry professor David Beratan, whose theoretical studies pre­dict that electron transfer in DNA should be similar to electron transfer in pro­teins. "In fact, their drop-off in current is embarrassingly close to what theory predicts."

Paul F. Barbara, a chemistry professor at the University of Minnesota, Minneap­

olis, who has modeled DNA electron transfer, also concludes that "the results significantly add to a growing body of ev­idence against the existence of ultrafast long-range transfer in DNA."

Researchers have been wrangling over electron transfer in DNA for years, but a definitive answer has been difficult to reach. The problem has been in devel­oping methods of synthesizing DNA snippets with electron donors and ac­ceptors tethered at precisely known lo­

cations on the double helix. As - ~ the synthesis of such systems be­

comes easier, values for β are be­ing reported—but with little agreement.

Chemistry professor Jacque­line K. Barton at California Insti­tute of Technology, for instance, has conducted many DNA elec­tron-transfer experiments using metallocomplex intercalators. Her lab has reported β values be­tween 0.1 and 0.4, which would indicate fast-electron transfer with little dependence on distance.

In contrast, Northwestern Uni­versity chemistry professor Fred­erick D. Lewis and colleagues used a stilbene-bridged DNA hair­pin system to obtain a β value of 0.6—lower than most proteins, but higher than Barton's (C&EN, Aug. 4, 1997, page 29).

I think they're all correct," Barton says. "What I think is re­markable is that we've got β val­

ues all over the place." She believes a sec­ond parameter may be at work, one that involves stacking of the DNA π orbitals. A system whose bases are well stacked may exhibit low β values, while values may be higher for systems with poorly stacked bases, she says.

Elizabeth Wilson

Endocrine testing scheme proposed for most chemicals An Environmental Protection Agency ad­visory committee last week proposed a scheme for screening nearly all chemi­cals in commerce—about 87,000 chemi­cals and mixtures—for effects on the en­docrine system.

The draft report, released at a public meeting, reflects many areas of agreement reached since the committee first met in

December 1996. For instance, the panel— which has 40 members representing in­dustry, academia, state and federal agen­cies, and environmental groups—has decid­ed to test chemicals for interference with three types of hormones—thyroid, estro­gen, and androgen—in both humans and wildlife. Also, it has agreed that nearly all synthetic chemicals except drugs should be tested.

And at the meeting last week, the com­mittee decided on a testing regime for var­ious types of chemicals. The regime— which has three basic parts—would sub­ject each chemical to high-throughput prescreening (HTPS), an automated and relatively inexpensive screen that tests the ability of a chemical to attach to estrogen, androgen, or thyroid receptors. At the same time, the chemicals would be used in short-term assays on live animals, such as fish, frogs, and rats. If positive or equiv­ocal results were obtained in any of these screens, then the chemical would under­go more detailed tests, including a two-year, two-generation study on live rodents. If one or more of the short-term assays and the two-year study found adverse effects, the chemical would be considered an en­docrine disrupter and probably taken off the market.

For testing purposes, chemicals are placed in five groups. In the first group are about 50 chemicals, such as DDT, whose hormonal effects are so well known that further testing is probably not needed. Six commonly encountered mixtures, such as the synthetic chemi­cals usually found in breast milk, make up a second group.

The third group is made up of about 30,000 polymers. Those polymers with a molecular weight greater than 1,000 would not be tested, because their large size makes them biologically unavailable. But all other polymers—and the mono­mers, oligomers, and additives in the polymers—would be tested.

Another 15,000 chemicals—most of which fall under the Toxic Substances Control Act—are found in the fourth group. Since there are little toxicological data for most of these, they would be subjected to the entire testing regime.

The fifth group includes pesticides and other chemicals for which there al­ready are quite a bit of data and that man­ufacturers consider important to keep on the market. These would be subjected to HTPS and to the two-year bioassay.

There was considerable discussion at the meeting about whether this last cate­gory should have to go through the short-

FEBRUARY 9, 1998 C&EN 7

n e w s of t h e w e e k

term bioassays. Eventually, it was decided that only the detailed tests would be done, but that the tests would have to be modi­fied to provide all the data generated in the short-term bioassays.

John F. McCarthy, director of regula­tory affairs at the American Crop Protec­tion Association, agrees with this conclu­sion. "Enough information would be gleaned from HTPS and the detailed tests to classify the pesticide as an endocrine disrupter or not," he said.

"I continue to feel very positively about EPA's role in this," says Jon C. Holtzman, vice-president for communi­cations at the Chemical Manufacturers Association. "They've taken a very com­plex subject and brought together peo­ple with differing points of view on it and managed their way through a pret­ty sticky kind of process, probably successfully."

The report will be submitted to EPA's Science Advisory Board in April for re­view and comment, and is expected to be finalized in June. In August 1999, as required by law, EPA will be reporting to Congress on progress made in imple­menting the testing regime.

Bette Hileman

Photoluminescent films brighten liquid-crystal displays Researchers in Switzerland have shown that photoluminescent layers can im­prove the brightness of flat-panel liquid-crystal displays (LCDs) of the type used in digital watches, cellular phones, and car dashboards.

The group at the Swiss Federal Insti­tute of Technology (ΕΤΗ), Zurich, has unveiled a design for LCD devices that incorporates one or more thin, highly lin- ~•———^"^~ early polarized pho­toluminescent layers [Science, 279, 835 (1998)]. The layers, consisting of a blend of polyethylene and 1% or 2% by weight of a semiconducting conjugated polymer, emit polarized col­ored light when irra­diated with ultravio­let light.

The design was

developed by senior research associates Christoph Weder and Cees Bastiaansen; Paul Smith, head of the ΕΤΗ polymer technology group; and graduate students Andrea Montali and Christian Sarwa.

"This is a significant development," comments Martin Schadt, coinventor of the twisted-nematic liquid-crystal cell and chief executive officer of Rolic Ltd., an R&D firm in Basel, Switzerland. "It is the first time that photoluminescent polariz­ers have been built into LCD devices."

According to LCD expert Jos van Haaren, research scientist at Philips Re­search Laboratories, Eindhoven, the Neth­erlands, the layers have the potential to change the relatively dim LCDs in calcula­tors or watches, which are difficult to read in dim light, into bright-colored displays. "The ΕΤΗ group has achieved polarized photoluminescence, which requires both a good design of the molecules and a good control of their orientation in the polymer layer," he explains.

The polarization results from the ori­entation of the luminescent polymer molecules in one direction. The group tested the design in the LCD of a com­mercial calculator using a thin film of a highly uniaxially oriented blend of a yel­low-green light-emitting derivative of poly(2,5-dialkoxy-p-phenyleneethy-nylene) and ultrahigh molecular weight polyethylene.

"We used the twisted-nematic liquid-crystal cell of a standard calculator, ex­changed one polarizer with our new photoluminescent polarizer, and then fit­ted the display with a UV backlight source," Weder tells C&EN.

The researchers also successfully tested polarized photoluminescent films based on blends of polyethylene and a polyphe-nylenevinylene polymer that emits orange-red light. "Other colors, such as blue, can also be readily produced, because suitable conjugated polymers covering the full visi-

Poly(2,5-dialkoxy-p-phenyleneethynylene) derivative

A single-color display device developed by ΕΤΗ researchers is based on a conventional LCD in which the inefficient elements (polarizer and color filter) have been replaced with a photoluminescent polymer film; a UV lamp is used for backlighting.

ble range are available," the ΕΤΗ group points out.

The new design not only improves the brightness of LCDs but also improves their energy efficiency, enabling batter­ies to last longer. Conventional displays generally use white light that must pass through a polarizer, explains Weder. About 60% of the initial light is absorbed and thus wasted in that step. The remain­ing light (now polarized) is rotated by the twisted-nematic liquid-crystal cell and transmitted through a second polarizer, the analyzer. Finally, a color filter converts the transmitted white light into the de­sired color by absorbing, and again wast­ing, about 80% of the remaining light.

"The new system uses UV light which is polarized by the novel photolumines-cent polymer film, and converted into a brilliant color in one single step with, in principle, an ultimate efficiency of 100%," Weder states. "The light detected by the analyzer is already of bright color and thus the new system operates without highly inefficient color filters."

The new photoluminescent polymer films also promise to overcome other dis­advantages of currendy available commer­cial LCD devices. Images on laptop com­puter LCDs vanish at oblique angles, for example. Photoluminescent films, in spe­cific configurations, can improve viewing angle, according to the ΕΤΗ group.

Work is currently under way at ΕΤΗ to pattern photoluminescent films and devel­op devices of even higher brightness and efficiency. "Although much work still needs to be done, especially in terms of device design and fabrication, it is highly encouraging that this powerful new con­cept already has yielded examples of working displays," notes Smith.

Michael Freemantle

8 FEBRUARY 9, 1998 C&EN

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