Studies on the Initial Photochemical Event in Photosynthetic Systems

Michael C. Zerner

Quantum Theory Project

Departments of Chemistry and Physics

University of Florida

Gainesville, FL 32611

Abstract:

Photosynthesis is the process which converts atmospheric carbon dioxide

into sugars and starches and begins the process of biosynthesis. Nearly

all of the fixed carbon on earth is due to photosynthesis, and the

atmosphere of the earth has been transformed from one of reducing power

to one of oxidizing power.

Carbon dioxide is the product of combustion of hydrocarbons and carbohydrates.

Our sun supplies the energy to reverse the process of combustion and

drive the synthesis of complex organic molecules.

We report on quantum mechanical studies on the reaction center (RC) found in

photosynthetic bacteria, and study, in particular, the initial photochemical

events. The study begins with adressing the excited states of chlorophyll,

and then the special pair (dimer) of chlorophyll molecules that is the

energy trap in these systems. A summary is then made of the electron

transfer that shortly follows the excitation, and that supplies the energy

to fix atmospheric carbon dioxide, a very stable molecule, into sugars and

starches.

There are approximately 250 antenna chlorophyll molecules for every

reaction center. The photochemistry usually begins with excitation to

"light harvesting system 2" (LHII), and this energy is than transfered to

LH1 that surrounds the RC of bacterial photosynthetic systems. We examine

the excitastion energies of LHI and LHII, and speculate on the highly

efficient energy transfer through the antenna to the RC.