230 REDUCTION OF CARBON DIOXIDE CHAP. 9 



boxyl group in the complex {CO2}. However, the reason why no 

 auxUiary oxidation-reduction systems more positive than the pyridinium 

 nucleotides occur in respiration may be that none are required, and 

 there is no reason why HX (or its transformation products) could not 

 be much stronger reductants, able to reduce carboxyl groups even with- 

 out the assistance of phosphates. It was found above that no quino- 

 noid-benzenoid or ferri-ferro system is likely to have the required high 

 potential. Systems of the type of hexadiene-benzene, while sufficiently 

 positive (c/. page 221), are unlikely to be reversible (in the kinetic sense). 

 A possibility worth considering is that HX may be a free radical, since 

 the oxidation-reduction potentials of free radicals vary over a much wider 

 range than those of the saturated systems considered on pages 217-222. 

 According to the standard bond strengths, the energies of free radicals 

 should be so high that two of them could not be formed by the action 

 of a single quantum of red light. For example, the transfer of a hydrogen 



atom from a HC CH group to a C=^=C double bond, leading to 



two HC C radicals (the arrow representing a free valency) should 



i 



require 62 kcal, and a transfer of a hydrogen atom from a .C — OH 



group to a /C=0 group, forming two /C — OH radicals, should 



consume as much as 75 kcal, while only 40-45 kcal are available in a 

 quantum of red light. This energy should be liberated again in the dis- 



i I I 



mutation of two — C — C — or — C — OH radicals to saturated molecules. 



H i i 



Free radicals should be violent oxidants and strong reductants at the 



I I 

 same time. The standard energy of hydrogenation of a — C — C — or 



H i 



I 

 — C — OH is —46 kcal, and the corresponding oxidation-reduction poten- 



i 

 tial can be estimated to ^0 ~ — 1-5 volt or E^ (pH 7) ;^ — 1.1 volt. 



The standard energy of dehydrogenation of a — C — C — radical is — 16 



kcal, corresponding to a potential of about -f- 0.7 volt at pH 0, or + 1-1 



I 

 volt at pH 7; while the dehydrogenation of an — C — OH radical should 



