Gold nanoparticles monitor protein folding

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http://www.nanotechweb.org/articles/news/4/4/3/1

1 April 2005

Faulty protein folding plays a role in medical conditions such as

Alzheimer's disease, cystic fibrosis, and BSE (mad cow disease). Now,

scientists at Stanford University have found that they could monitor

changes in the folding of a yeast protein by attaching gold nanoparticles.

Conformational changes in the protein caused the nanoparticles to

aggregate to a greater or lesser extent, resulting in a colour change.

" What we've developed is a simple and inexpensive sensor for

determining when a protein changes its conformation, " said

Zare of Stanford University. " While we're not ready to mass-produce

this technology, we believe it will eventually be useful for testing

other, more complicated proteins. "

Zare and colleagues developed their technique for yeast

iso-1-cytochrome c (Cyt c). They covalently attached 19 nm-diameter

gold nanoparticles to the protein molecules before adding hydrochloric

acid to alter their folding.

As the pH of the solution decreased from 10.1 to 4.0, the protein

molecules unfolded. The sample changed colour from red to purple at

around pH 6. And by the time the solution had a pH of 4, it was light

blue in colour. The scientists measured these colour changes by UV-VIS

absorption spectroscopy.

The researchers believe that the unfolding of the proteins enabled the

coated gold nanoparticles to clump together. This changed the colour

of the solution.

The team also tested the effects of pH changes on a layer of Cyt c

bound to a gold film. They monitored changes in the refractive index

of the protein layer using a surface plasmon resonance sensor. The

refractive index decreased as the pH increased, mimicking the colour

change of the nanoparticle solutions. What's more, these refractive

index changes were fully reversible, indicating that the proteins were

able to unfold and refold.

The Cyt c-coated nanoparticles did not show such extensive

reversibility, on the other hand. Following three pH cycles the

solution retained a slight blue tinge at high pH, rather than

returning to its original red colour. The scientists believe this is

because of the formation of a bonded network of unfolded Cyt c-coated

gold nanoparticles.

The researchers reckon the technique may ultimately lead to fast,

affordable methods for detecting antibodies and other disease-related

proteins. They now plan to repeat their experiments using other

protein molecules.

The scientists reported their work in Chemistry and Biology.

About the author

Liz Kalaugher is editor of nanotechweb.org

take care

eric