The Raman spectroscopy team at Berkeley: (from left) postdoc Pradeep Perera, postdoc Martin Schmidt, Principal Investigator Paul Adams, and co-PI Jim Schuck.

Paul Adams:

Looking at the Plant Cell Wall in a Different Light

Prior to 2006, Berkeley Lab Senior Scientist and UC Berkeley bioengineer Paul Adams was focusing his research on structural biology, in particular in developing computer software and other tools to enable the characterization of proteins. Then two things happened. 

First, Adams met plant scientist Chris Somerville of Stanford University at a bioenergy forum assembled by then-LBNL Director Steve Chu. They talked about the problems associated with understanding the synthesis of the plant cell wall. 

Then Adams co-organized a bioimaging workshop on campus and learned about the potential for chemical spectroscopy at the nanometer scale.

“I found the problem (of the cell wall) interesting, even though I didn’t have a background in plants,” Adams recalled. “It is a complex mixture of chemical polymers, forming a matrix that isn’t amenable to many imaging techniques. I heard about Jim Schuck’s work on Raman imaging of materials, and I thought it might be possible to adapt it for biological samples.”

Schuck, a materials scientist at the lab, then joined with Adams to write a proposal for the EBI that became one of two programs dedicated to deciphering the architecture and chemical composition of the cell wall. The conversion of cell wall cellulose and hemicelluloses to sugars is central to biofuel production. Fellow LBNL microscopist Manfred Auer leads the other program, which is seeking to image the cell wall using electron microscopy. The programs are working closely together to combine results to develop a 3-D map that shows both structure and chemistry.

Raman imaging is a technique that relies on scattering of monochromatic light, like that from a laser, and the subsequent energy measurement of the shifting photons and vibrating molecules at the sample’s surface. Adams’ program performs micro-Raman spectroscopy – with image resolution of about 500 nanometers– using a new laser microscope in Calvin Lab on the Berkeley campus. Up the hill at Berkeley Lab, Schuck and other members of the team are using tools in the Molecular Foundry to develop Raman imaging at the nano scale – down to a resolution of 30-50 nanometers.

Why is such modeling so important to the biofuel development process?

“The biggest thing is to get a basic understanding of the cell wall and how it is made,” Adams said. “Then, if we can see how the cell wall changes (under different pretreatment conditions), and correlate this with the effectiveness of sugar release, we can help in the identification of better feedstocks and deconstruction methods.

In the future, we hope to look at the changes in real time, and maybe we can be predictive about the best conversion approaches.”

One of the next steps in the program, according to Adams, will be to use the Raman technique to study the effect that ionic liquids have on biomass during pretreatment. An understanding of how these agents solubilize the cellulose could help to define the best ionic liquid to use among the tens of thousands possible.

As with any new research approach, the Raman spectroscopy application to bioenergy presents its challenges. Nano-Raman imaging involves the making of tiny gold antennae, facing tip-to-tip, which scan across a surface and determine the chemical composition by measuring electromagnetic waves. Adams said progress is being made on producing functioning nanoantennae at the Molecular Foundry, and work is proceeding on the difficult process of ultra-thin sample preparation in collaboration with Auer’s program. Proof of principle in two dimensions could come in the next year or two, with a 3-D version to follow, he said.

Micro-Raman spectroscopy also has its challenges when studying plants, since fluorescence of the green samples tends to dominate the image and swamp the Raman signal. Adams said his team is working with the Somerville group to develop model Arabidopsis plants with lower levels of green pigments, one of the main causes of plant autofluorescence.

It is a classic pairing of physics and biology, itself a model system for the interdisciplinary effort required to enable the development of bioenergy products.