Ubiquinones: stereochemistry and biological implications.

Proton NMR and 13C-NMR studies on the configuration of CoQn homologues show that the polyisoprenoid side-chain is in the all-trans configuration and confers a higher degree of rigidity to the quinones with respect to the acyl-chains of the phospholipids within the membrane bilayer. The quinonoid ring appears to be specifically involved in the redox function of the coenzyme while the side-chain length only affects the lipophilicity of the molecule. LIS data show that the ring strongly interacts with metals as a consequence of the high pi-electron density on the carbonyls that is somewhat larger on the carbonyl oxygen to alpha to the isoprenoid chain.