Scientists shoot molecular motion on film

Unlike inorganic salts, biological molecules typically change as they move through water, and it is very important for researchers to understand this movement in order to predict accurately they will react during different biological and chemical processes.

According to one of the researchers, this will be the first time that scientists have been able to verify and correct their simulations of molecular movements using real physical data.

David Tiede, a senior chemist at the Argonne National Laboratory who worked on the project said: " For the first time, we are able to test the accuracy of the simulation and change it to fit data better.

Without it, we had no way of knowing how accurate the models were ."

The transition from crude, uncorrected computer simulations of the molecule's behaviour to accurate movies involved a number of different stages.

The first step was to use computer algorithms to convert the simulation of the molecule's movements into a moving simulation of the way the molecules X-ray scattering pattern would change over time due to this movement.

The team then captured still X-ray scattering images from the real molecule moving in a solution.

Although the movement of the molecules means that these images are very blurred, the way it looks is still very dependent on the exact way the molecule is moving.

By comparing these images to stills from the X-ray scattering simulations, they can find ways to tweak the simulation to produce more realistic results.

Ultimately, these corrections are also translated to the original molecular simulation to represent more accurately the molecule's movement.

" We average all the frames from the x-ray scattering movie to create a "still" picture and compare it to x-ray scattering experiment ," Tiede explained to LabTechnologist.com.

" Depending upon the agreement in this comparison, we can go back and 'edit' the molecular movie by changing the atomic force field parameters that underlie the molecular dynamics simulations to make the molecular movie still and experimental x-ray experiment match. "

He says that scientists at the National Institutes of Health have already used this approach to help determine structures of important biological molecules.

The group plans to use the technique to examine how molecules react to outside laser pulses, and how they return to a stable state afterwards.

They also hope to make use of pulsed X-ray light sources, like Argonne's Advanced Photon Source, to create shorter individual X-ray scattering frames that could be used to build even more accurate molecular movies.

" Currently, the time structure of pulsed X-rays at APS allows snapshots to be taken with approximately 100 picosecond time-resolution, and we are using these short X-ray pulses record what happens to when molecules are excited with short laser pulses ," said Tiede. "

In addition, APS is developing plans to provide x-ray pulses in the one picosecond range that will allow 100-fold greater time resolution of the molecular movie. "