NEWS OF THE WEEK SCIENCE
TIGHT SQUEEZE Dissociated DNA hairpin (composed of yellow loop and white and red stems), now a single strand, passes through a nanopore in a lipid bi layer.
NAN0P0RES LET DNA SLIP THROUGH Ion channels in membranes aid oligonucleotide analysis
A
T I N Y PORE CAN T R A N S -
form a simple membrane into a device that within milliseconds distinguishes between D N A strands that differ by a single nucleotide. The work, led by University of California, Santa Cruz, chemistry professor David Deamer and researcher Mark Akeson, joins a growing body of research on the uses for nanopores drilled into membranes by barrelshaped assemblies of toxic proteins.
In nature, the proteins cause damage by creating channels through a normally insulating cell membrane, allowing ions to flow through. But scientists have realized that a molecule moving through a pore temporarily blocks the channel, causing a change in ionic current. T h e profile of current change over time serves as a unique fingerprint that can identify and distinguish between individual molecules. The Santa Cruz researchers demonstrate their technique with a solution of short D N A hairpin molecules. They apply a voltage to an artificial lipid
E N V I R O N M E N T
ARSENIC STANDARD IN MUDDY WATERS EPA plans to withdraw rule tightening level in drinking water
E
NVIRONMENTAL
PROTEO
tion Agency Administrator Christine Todd W h i t m a n says a regulation issued in the closing days of the Clinton Administration that sets a 10-ppb limit on arsenic levels in drinking water needs further scientific review "EPA will examine what may have been a rushed decision," she explained last week. The review and planned withdrawal are part of the Bush Administration's program to scrutinize and, in some cases, rescind regulations issued at the
20
C&EN
/
MARCH
26,
2001
end of Clinton's tenure. Whitman's move means the current arsenic standard of 50 ppb, set in 1942, will remain in effect. The National Academy of Sciences in 1999 said the 50-ppb arsenic benchmark needed to be lowered to protect heath. Arsenic in drinking water can cause cancer of the bladder, lung, and skin, and is linked to kidney and liver cancer, NAS says. Both the World Health Organization and the European Union have a drinking water standard for arsenic of 10 ppb.
bilayer equipped with a pore p u n c t u r e d by the protein ahemolysin. A D N A hairpin enters the pore and gets momentarily stuck; but then the hairpin dissociates into a single D N A strand, slipping through the pore [Nat. BiotechnoL, 19, 248 (2001)]. Graduate student Stephen Winters-Hilt has designed a learning algorithm that translates parameters such as current amplitude, modulation, and duration into strand concentration or strand length. The group says an ultimate goal—though years away—is to refine the technique to yield fingerprints unique and sensitive enough to sequence DNA. Hagan P. Bayley, medical biochemistry and genetics professor at Texas A&M University, finds it remarkable that "they're able to watch just one DNAhairpin bind to the pore entrance, define the moment when the single strand is formed, and then track its transit to the opposite side of the bilayer."—ELIZABETH WILSON
W h i t m a n said, "Certainly the standards should be less than 50 ppb, but the scientific indicators are unclear as to whether the standard needs to go as low as 10 ppb." Mining industry representatives, many small water supply companies, and some congressional Republicans praised Whitman's action, saying the 10-ppb standard was not based on good science and will cost too much to implement. Environmental activists decried EPA's action, calling it a political payback from President Bush to mining companies that contributed to his campaign. Arsenic is a common byproduct of mining, and the 10ppb standard likely would raise the costs of pollution control at mines. Many D e m o c r a t s on Capitol Hill said W h i t m a n ' s move endangers public health. —CHERYLH0GUE
HTTP://PUBS.ACS.ORG/CEN
