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ANALYTICAL CURRENTS

N O V E M B E R 1 , 2 0 0 4 / A N A LY T I C A L C H E M I S T R Y 3 8 3 A

Lewis Rothberg and Huixiang Li at theUniversity of Rochester have discovereda new phenomenon that allows them todetect specific DNA sequences and singlenucleotide polymorphisms (SNPs) within10 min simply by looking at the color ofa DNA–gold-nanoparticle colloid.

Rothberg and Li observed a previous-ly unknown attraction between single-stranded DNA (ssDNA) and colloidalgold nanoparticles. Short DNA strandsand high temperatures were found tospeed up this adsorption process.

Gold nanoparticles aggregate whenthey are exposed to salt, which causesthem to change color from pink to blue-gray. But when Rothberg and Li addedenough ssDNA and then the salt, thenanoparticles did not aggregate and thecolor change did not occur.

The researchers used the color changeas an indication that PCR-amplified testDNA contained specific sequences. Apink colloid indicated that hybridizationbetween short ssDNA probes and testDNA did not occur and that aggregationwas prevented. A blue-gray colloid indi-cated that the sequences were comple-mentary; hybridization occurred, and theprobes were no longer free to preventaggregation.

To detect the single base-pair changescharacteristic of SNPs, the researchersused four probes that would dehybridizefrom test DNA at the same temperature ifthe sequences were perfectly complemen-tary. Two control probes bound to se-quences outside the SNP region, but theother two probes overlapped the SNP re-gion. Because of a mismatch, probes that

bound to a SNP region melted away fromthe test DNA at a lower temperature thandid control probes. The color change wastested at various temperatures.

The assay was successfully used toidentify SNPs associated with a heredi-tary cardiac arrhythmia, and it could beused to detect SNPs implicated in otherdiseases and conditions without expen-sive instruments or labor-intensive DNAmodifications. (J. Am. Chem. Soc. 2004,126, 10,958–10,961)

A colorful way to detect SNPs

© 2 0 0 4 A M E R I C A N C H E M I C A L S O C I E T Y

(a) A blue-gray color in-dicates that a controlprobe is bound to DNA.(b) A pink color indicatesthat a mismatched probehas melted off the SNP-containing DNA.

(a) (b)

James Gimzewski and colleagues at the

University of California, Los Angeles, have

measured the motions of the cell wall in

baker’s yeast, Saccharomyces cerevisiae.

The investigators showed, for the first time,

that cooperative action between motor

proteins inside the yeast cell propagates

through the cell wall. The work demon-

strates that local motion observed in the

yeast cell wall can be related to processes

occurring inside the cell.

Gimzewski and colleagues trapped sin-

gle, live yeast cells in ~5 µm pores of a poly-

carbonate membrane under physiological

conditions. Using a cantilever in an atomic

force microscope, they probed the cells and

measured the spring constant of the yeast

cell wall as ~0.06 N/m. The spring constant

of the cantilever was 0.05 N/m, which al-

lowed them to study the coupling of the

cantilever and cell wall movements as two

springs in a series.

The motion of the cell wall was temper-

ature-dependent between 22 and 30 °C,

which suggests that the movements were

due to either active metabolic processes

occurring inside the cell or Brownian mo-

tion. Gimzewski and colleagues treated the

yeast cells with sodium azide to inhibit ATP

production and confirmed that the cell wall

motions were due to ATP-dependent, ac-

tive metabolic processes.

The investigators calculated an activa-

tion energy of 58.15 kJ/mol, which is con-

sistent with the activation energy known

for motor proteins such as myosin, kinesin,

and dynein. Because a force of 10 nN was

measured at the cell wall, which is too

large for a single motor protein to exert,

they suggested that the motor proteins

acted together in a cooperative fashion.

The cooperative action of the motor pro-

teins inside the cell propagated through

the cell wall.

The researchers hope to next probe the

mechanical properties of mammalian cell

membranes. Because the membranes have

a very low spring constant (~0.002 N/m),

special cantilevers will be needed to carry

out the experiments. (Science 2004, 305,

1147–1150)

Probing the yeast cell wall