Recollections.

About 1939, Sam Ruben and Martin Kamen introduced me to the emergent application of artificial radio-isotopes in the study of photosynthesis. While my own experiments on CO2 fixation by isolated chloroplasts turned out to be negative, their laboratory provided me with an informative and exciting experience. Also, there were many stimulating contacts with Cornelis van Niel, Robert Emerson, Don DeVault and many other outstanding scientists. Efforts on my part to obtain a better understanding of intermediary metabolism, eventually led me to Fritz Lipmann's laboratory. There I was encouraged to study the metabolic activities of cell-free preparations of photosynthetic purple bacteria. Investigations of oxidative phosphorylation by isolated bacterial chromatophores in the dark raised questions about the possible effects of light on the phosphorylation activities of such preparations. Surprisingly, high rates of phosphorylation were observed in the light in the absence of molecular oxygen ('light-induced phosphorylation'). In this process, adenosine diphosphate (ADP) and inorganic phosphate (Pi) could be converted quantitatively into adenosine triphosphate (ATP). It was postulated that this process was 'cyclic' in nature, as only catalytic concentrations of added electron donors were required. Later, at Minnesota, it could be shown that similar chromatophore preparations, in the presence of suitable electron donors, could reduce nicotinamide-adenine dinucleotide (NAD(+)) to NADH in the light. It was then demonstrated that the chromatophores of Rhodospirilum rubrum, as well as the smaller membrane components derived from them, must contain the active metabolic components for these photosynthetic reactions.These observations, and studies on the kinetics of the formation and decay of light-induced free radicals, appeared to demonstrate the usefulness of bacterial chromatophores and of their membrane fragments in the study of partial reactions of bacterial photosynthesis. Since that time, numerous investigators elsewhere have carried out remarkable research on the purification and eventual crystallization of distinct bacterial membrane components, capable of carrying out well characterized photochemical and electron transport reactions.