New and Improved 2D Electronic Spectroscopy

Something new under the sun… a laser spectroscopy talk that even biologists understand!

Today’s post is the final installment for the Midwest Photosynthesis Meeting highlight reel. Peter Dahlberg, a graduate student in the Engel lab at the University of Chicago, won the award for best graduate student talk. Of course the work he is doing is cool and providing the field with interesting new information, but let me just set up what a testament his presentation was for science communication.

The Engel lab studies systems at the intersection of biology and quantum mechanics. Great examples of this include energy transfer within various antenna complexes in photosynthetic systems. These events occur on the fastest of timescales. The types of information this lab group is after requires new methodologies for measurement. So, what does this translate into? They use lasers and build their own fancy optics systems from scratch. They use complicated algorithms and terms like ‘time domain’ and ‘frequency domain.’ Needless to say, when these kinds of spectroscopists start talking, those of us on the biochemical and molecular biology side of photosynthesis research start to tune out and doodle in our abstract books because the speaker has lost us after the title slide. I think that studies have even been done on the snooze-inducing properties of femtosecond resolution spectroscopy talks on biologists. It should also be noted that his talk was scheduled to begin at the same time as the LSU vs. Alabama game.* However, it was the beginning of the evening session and I was determined to listen with fresh ears after a beautiful afternoon of walking through the surrounding fall forest. Before I knew it he was already on his fourth slide and I was still keeping up with what he was talking about.**

The key to explaining the general type of spectroscopy they use was a 1.5 min video animation of photons moving through their system from the first beam split, through the sample, removal of scattering interference and onto the detector. Yes, there were more steps along the way, but that gives you the gist. If I had watched that video a few more times, even I could probably explain it to you. I highly recommend checking out the Engel lab website. I couldn’t find a link to the video animation (but it would require some voiceover work to be a stand-alone piece), but there are lots of other descriptions of their novel spectroscopies.

Peter’s talk was all about overcoming some technical limitations in the fundamental data collection system. In order to capture the information they needed, it would normally require sampling a couple of dimensions worth of time domain space. Translation: their measurements took ~10 hrs. So for reconstituted systems like the antenna systems from purple bacteria in lipid micelles, that’s OK albeit a pain in the *ss. But, the Engel lab wants to study real systems in vivo. ~10 hours per measurement just isn’t going to cut it and the data would still be crappy because real biological systems have a considerable amount of inherent light scattering that is going to add tons of noise to their precious 2D spectra. Peter presented a modification to their 2D spectroscopy that allowed them to sample the same time domain space as the 10 hour measurement, but get collect the data in a single laser shot. Of course it has a cool acronym ‘GRAPES’.

“Gradient Assisted Photon Echo Spectroscopy (GRAPES) uses tilted wavefronts to create an array of delay times spatially within the sample and then images the signal with a CCD. An entire 2D spectrum can be acquired in a single laser shot.”

So now the same data from an experiment that used to take 10 hours can now be collected in 3.5 seconds! That is sure to shave some time off of Peter’s time-to-PhD.

Light harvesting complex (LH2) of purple bacteria. Top view. Credit: aegon via Wikimedia Commons

He also presented a modification that allowed them to eliminate the scattering noise associated with the in vivo samples so that clean data could be obtained for whole cells of the purple bacterium Rhodobacter sphaeroides. These samples were used as a test case, and the new in vivo data is very similar to that previously collected for micelle-reconstituted light-harvesting complex 2 samples. This provides an experimental proof that the new technique works. There were some subtle differences in the in vitro vs. in vivo spectra, but these hint at biological differences in energetic coupling that must be further explored.

The moral of the story: Complicated laser spectroscopy can be presented to a general scientific audience in a way that is understandable. Biophysicists take note because the Engel lab is raising the bar for science and communication.


*We will not speak of the final score of that game. That is all.

**It really was his presentation skills and not any recently-acquired Dr. PhD superpowers.


P.D. Dahlberg, A.F. Fidler, J.R. Caram, P.D. Long, and G.S. Engel, “Energy Transfer Observed In Live Cells Using Two-Dimensional Electronic Spectroscopy” J. Phys. Chem Lett. 4, 3636-3640 2013.

A.F. Fidler, V.P. Singh, P.D. Long, P.D. Dahlberg, and G.S. Engel, “Probing Energy Transfer Events in the Light Harvesting Complex 2 (LH2) of Rhodobacter sphaeroides with Two-Dimensional Spectroscopy” J. Chem. Phys. 139, 155101 2013.

E. Harel, A.F. Fidler, G.S. Engel, “Real-time Mapping of Electronic Structure with Single-shot Two-dimensional Electronic Spectroscopy” PNAS 107 16444-16447, 2010.


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s