"The structure of the pigment assemblies in chlorosomes is the subject of intense debate," Holten says, "and there are several competing models for it."
Given this uncertainty, the scientists wanted to study many variations of a pigment molecule to see what favored and what blocked assembly.
A chemist wishing to design pigments that mimic those found in photosynthetic organisms first builds one of three molecular frameworks. All three are macrocycles, or giant rings: porphyrin, chlorin and bacteriochlorin.
"One of the members of our team, Jon Lindsey can synthesize analogs of all three pigment types from scratch," says Holten. (Lindsey, PhD, is Glaxo Professor of Chemistry at North Carolina State University.)
In the past, chemists making photo pigments have usually started with porphyrins, which are the easiest of the three types of macrocycles to synthesize. But Lindsey also has developed the means to synthesize chlorins, the basis for the pigments found in the chlorosomes of green bacteria. The chlorins push the absorption to the red end of the visible spectrum, an area of the spectrum scientists would like to be able to harvest for energy.
Key to pigment self-assembly are the metal atoms and hydroxyl (OH) and carbonyl (C=O) groups in the pigment molecules.
Doctoral student Olga Mass and coworkers in Lindsey's lab synthesized 30 different chlorins, systematically adding or removing chemical groups thought to be important for self-assembly but also attaching peripheral chemical groups that take up space and might make it harder for the molecules to stack or that shift around the distributions of electrons so that the molecules might stack more easily.
Testing for aggregation
The powdered pigments were carefully
|Contact: Diana Lutz|
Washington University in St. Louis