EDITORS
EDITORS' CHOICE: HIGHLIGHTS OF THE RECENT LITERATUREJuly 7 2006, 313 (5783)CHEMISTRY: Prearranged LuminescenceJake YestonConjugated organic polymers such as poly(p-phenylenevinylene) (PPV) haveattracted interest for use in sensors, light-emitting diodes, and other
electronic applications. One advantage of these materials is their faci
le processability; for example, films can be fabricated under mild condi
tions by spin-casting from solution.Satrijo et al. show that a judicious choice of solvent for the spin-cast
ing process can influence the structural and electronic properties of th
e resultant films. They studied the absorption and luminescence spectra
of films prepared from a PPV derivative modified with chiral alkyl-subst
ituted hydroquinone side chains. Films cast from chloroform, in which th
e polymer was highly soluble, were disordered but adopted an M-configure
d helical arrangement on annealing, as revealed from the circular dichro
ism spectrum and detection of right circularly polarized luminescence. W
hen 1,2-dichloroethane (DCE) was used as the solvent, unannealed films e
mitted left circularly polarized luminescence, indicative of a kinetical
ly stable, oppositely configured P helical structure. Spectra of the DCE
solution before spin-casting supported self-assembly of helical aggrega
tes that conserved their configuration during casting. By adding acetoni
trile to the original chlo! roform solutions, the authors could induce c
omplementary solution-phase aggregation, leading to right circularly pol
arized luminescence on irradiation. -- JSYJ. Am. Chem. Soc. 128, 10.1021/ja063027c (2006).MATERIALS SCIENCE: Idling Away LossesIan S. OsborneThe development of metamaterials, in which electromagnetic behavior can
be tuned by design, has already yielded such interesting effects as nega
tive refraction and superlensing (subwavelength focusing). Progress in t
he fabrication of subwavelength-featured structures has carried these ef
fects from the microwave regime through the terahertz and into the infra
red region. However, one severe problem in the materials has been high s
ignal losses, particularly due to absorption. Among the ideas floated to
counter these losses is incorporating a gain medium into the design, th
ough a major drawback to this approach is that it would indiscriminately
amplify noise in addition to the desired signal.Popov and Shalaev propose an alternative: moving into the nonlinear regi
me and using optical parametric amplification. Their calculations show t
hat mixing the signal field with an optimally tuned auxiliary electromag
netic pump field should produce an idler field at the difference frequen
cy; this idler would then induce amplification of the signal field throu
gh three-wave mixing with the pump, thus compensating for absorption los
ses precisely at the signal frequency. The challenge now will be to impl
ement these theoretical ideas and thereby achieve the promising applicat
ions of the materials. -- ISOOpt. Lett. 31, 2169 (2006).BIOCHEMISTRY: DNA Unzipper FoundGuy RiddihoughOne of the critical steps an organism must perform in order to duplicate
its genome is to unzip the two base-paired strands of DNA so that DNA p
olymerase can read and copy the information. Minichromosome maintenance
(Mcm) proteins 2-7 are thought to be involved in unwinding double helica
l DNA in eukaryotes, but this six-subunit complex does not display helic
ase activity in vitro. Other proteins--Cdc45 and the four-subunit GINS (
Go, Ichi, Nii, and San) complex--have also been implicated, but the prec
ise composition of the active assembly has remained elusive.Moyer et al. have purified factors that interact with the Drosophila Cdc
45 protein and find that they include both the Mcm hexamer and the GINS
tetramer. The 11-membered CMG (Cdc45/Mcm2-7/GINS) complex is highly stab
le and, satisfyingly, exhibits adenosine triphosphate-dependent DNA heli
case activity, which is lost or reduced upon immunodepletion of any of i
ts components. Whether the Mcm proteins, which have all the structural f
eatures of a bona fide helicase, provide the unzipping activity in the C
MG complex is not yet known. Intriguingly, GINS, like Mcm2-7, forms a ri
ng, and the two might stack on top of each other, with DNA passing throu
gh the center. -- GRProc. Natl. Acad. Sci. U.S.A. 103, 10236 (2006).
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