THERMODYNAMICS OF FREE RADICALS 229 



be achieved if phosphorylation is the primary photochemical process, as 

 postulated by Umbreit and coworkers. 



The phosphate storage hypothesis appears somewhat less improbable 

 when applied to chemosynthesis. One may assume that the oxidation of 

 hydrogen, sulfur, ferrous iron or other substrates by oxygen proceeds in 

 easy steps (as in the oxidation of glucose in the muscle), each step being 

 coupled with the production of a high-energy phosphate, and that the 

 energy of these phosphates is utilized later to transfer hydrogen, by 

 similar easy steps, from water to carbon dioxide. In the case of reduc- 

 tants as mild as ferrous ions, the free energy of oxidation of one gram 

 atom is just about sufficient to produce one mole of high-energy phos- 

 phate, so that, in this case, the "phosphate storage" would involve no 

 dissipation of the oxidation energy. 



However, the metabolism of "iron bacteria" is not well known, and 

 in the better investigated cases of hydrogen, sulfur or thiosulfate bacteria, 

 the reductants have comparatively high potentials, and the intermediate 

 dissipation of their oxidation energy in the form of "phosphate quanta" 

 of 10 kcal each appears implausible. 



The experiments of Vogler (1942) on the "delayed" carbon dioxide up- 

 take by Thiohacillus thiooxidans (cf. page 114) provide the only experi- 

 mental argument favoring the phosphate storage theory of chemosynthe- 

 sis. However, it was mentioned on page 114 that the carbon dioxide 

 uptake in Vogler's experiments may well be a reversible fixation of this gas 

 (e. g., by carboxylation) rather than a reduction to a carbohydrate. 



To sum up: we think it unlikely that the bulk of the light energy 

 utilized in photosynthesis (or of the oxidation energy utilized in chemo- 

 synthesis) is first converted into phosphate energy. Furthermore, if 

 phosphorylation does play an auxiliary role in photosynthesis (e. g., in 

 the way envisaged by Ruben) — which is by no means certain — we think 

 it much more probable that the required high-energy phosphates are 

 supplied by nonphotochemical oxidation processes than that light quanta 

 are diverted for their synthesis. 



6. The Thermodynamics of Free Radicals 



In the preceding section, we reached the conclusion that the primary 

 photochemical reduction products, HX, probably serve directly for the 

 reduction of the {CO2} complex, and that the chemosynthetic reductants 

 (hydrogen, sulfur, hydrogen sulfide, thiosulfate, etc.) can be assumed to 

 produce, in the course of their oxidation, reducing agents similar to, or 

 identical with, HX. The example of the strongest catalytic reductants 

 in the respiration mechanism — pyridinium nucleotides — caused Ruben 

 to suggest that HX has a reduction potential < + 0.3 volt, and that, 

 therefore, a phosphorylation is required to enable it to reduce the car- 



