convert it into H 2 and liquid fuels has also been achieved on a commercial scale. One of the
issues preventing these processes from being economically viable is the rapid poisoning of the
catalysts used in these processes.
The pyrolysis of biomass leading to the production of oil is not as well understood as
gasification. To make this oil suitable for commercial use, researchers need to overcome the
instability of the oil caused by chemically reactive impurities, phase separation, and acidity.
Standards relating to minimal stability requirements for the oil have not been developed, so the
required stability and how to achieve it are unknown at this time.
Natural Photosynthetic Systems
Elucidation of the molecular basis for photosynthesis is essential to optimizing the natural
process for biological solar fuels production. It is also essential that researchers provide both
proof of concept and inspiration for the construction of artificial photosynthetic devices to
produce solar fuels with higher efficiency and more convenience than is offered by existing
biomass approaches. Research on natural photosynthetic systems is an active area of study; the
goals are to define and understand the structure, composition, and physical principles of
photosynthetic energy conversion. In photosynthetic organisms, light is harvested by antenna
systems consisting of pigment-protein complexes that are tuned to the quality of light available
(Blankenship 2002). The captured excitation energy is transferred to reaction center (RC)
proteins, where it is converted by photoinduced electron transfer into electrochemical potential
energy. The resulting oxidizing and reducing equivalents are transported to catalytic sites, where
they are used to oxidize water and produce reduced fuels, such as carbohydrates.
Bacterial Photosynthesis. Purple bacteria are among the oldest photosynthetic organisms on
Earth and contain the most studied photosynthetic apparatus, which consists of two light-
harvesting pigment-protein complexes (LH1 and LH2) and a single type of RC. The structures of
the purple bacterial light-harvesting and RC proteins have been determined by means of x-ray
crystallography and reveal elegant symmetries that are intimately related to their functions. Both
LH1 and LH2 contain cyclic arrays of bacteriochlorophyll (BChl) molecules that capture
sunlight and circulate the captured energy within these arrays on approximately a 1-ps time scale
(1 trillionth of a second) (McDermott et al. 1995; Roszak et al. 2003; Yang et al. 2001). Energy
transfer from LH1 to the RC occurs about ten times more slowly. Carotenoids (molecules similar
in structure to beta-carotene, the orange pigment in carrots) present in LH1 and LH2 enhance the
light-harvesting capability in the blue-green region of the spectrum, while protecting the
complex from photo-oxidative damage (Polivka and Sundstrom 2004).
The structure of the bacterial RC has an intriguing two-fold symmetry and organizes the
pigments into two parallel electron transfer pathways, termed the A-side and B-side. However,
the RC only uses the A-side pigments for electron transfer. Excitation of a special pair of BChl
molecules that serve as the primary electron donor initiates charge separation on a picosecond
time scale, followed by subsequent thermal electron transfer steps. The rapidity of the initial