Understanding Biological Systems at the Quantum Level Using 2D Electronic Spectroscopy

Spectroscopy recently spoke to Greg S. Engel, who is an Associate Professor in the Department of Chemistry at the University of Chicago in Chicago, Illinois, and one of the winners of the 2013 FACSS Innovation Awards. Here, Engel discusses his research examining quantum dynamics in biology, chirality in dynamics, and the use of two-dimensional electronic spectroscopy.

How did you become interested in studying quantum transport in photosynthetic systems?Engel: As with much of science, I came to this field by accident while trying to study energy transfer in photosynthesis as a prelude to looking at energy transfer in other biological systems. My doctoral work with Jim Anderson in atmospheric chemistry led me to want to examine energy transfer in biology; I wanted to think about the microscopic physical basis of human health consequences from increased UV dosage because of the thinning ozone. Working with Graham Fleming to study energy transfer in photosynthesis, we noticed unusual quantum beating signals in our spectra. It was the analysis of these signals that launched my interest in quantum dynamics in biology.

Can you briefly describe the two-dimensional electronic spectroscopy approach and how it is used to probe the chiral response of the light-harvesting complex 2  of purple bacteria? What type of apparatus is used to achieve these measurements?Engel: To study ultrafast dynamics, we use a series of femtosecond laser pulses to excite the molecules, then probe them mere femtoseconds later to learn if the system has changed its state. Two-dimensional spectroscopy resolves the frequencies of both the excitation energy and the energy of the states populated at a later time. The two dimensional spectrum therefore maps energy transfer. In chiral systems, small differences exist between absorption of left- and right-circularly polarized light. Our new chiral 2D spectroscopy extends 2D spectroscopy to specifically follow these differences. The challenge is that the differences in these signals are 10^-3 to 10^-5 times smaller than the normal 2D nonlinear signal (which itself is already weak). To acquire these signals, we adapted a new spectrometer developed by Professor Elad Harel at Northwestern while he was a postdoctoral scholar in my group. This spectrometer exploits spatiotemporal gradients in analogy to magnetic resonance imaging (MRI). By mapping our time delays onto spatial coordinates, we improved our signal-to-noise dramatically. This advance enabled us to acquire these new measurements of chiral nonlinear response.

What are the advantages of the chiral two-dimensional spectroscopy approach over typical circular dichroism (CD) experiments for probing changes in chiral structure? Engel: Chiral two-dimensional spectroscopy is both time- and frequency-resolved. Linear CD spectroscopy is a powerful tool, but it cannot probe femtosecond dynamics of molecular motion in a state specific way.

What kind of challenges have you had to overcome in this research?Engel: My research team led by Andrew Fidler, PhD, and Ved Singh accomplished this groundbreaking new experiment through extremely careful work. Chiral 2D signals are buried underneath achiral background that is orders of magnitude stronger than the signals we sought to obtain. Isolating and separating the chiral signal required extremely careful calibration of polarizations of our beams, sample preparation with minimal scatter, and extreme laboratory stability.

What impact will this work have in the field?Engel: Chemists have an excellent understanding of chirality in structure. This work opens the door to understand chirality in dynamics.

What are the next steps in your research?Engel: We are excited to understand how ideas inspired by energy transfer in photosynthesis can be exported to synthetic systems. We are currently exploring both organic and inorganic model systems to mimic the dynamics we observe in nature.