Structure of ascorbic acid and its biological function. VI. Its importance for the Na+/K+-transport.

Ascorbic acid/isoascorbic acid are present as radicals at physiological pH with the unpaired electron located in the C(4) region. Since a distinction can be made between both types of radicals, the electron spin resonance technique can be used for discrimination between the epimers of vitamin C. The radical has a cyclic side-chain structure which is formed by the hydrogen bond C(3)-O-... HO-C(6) (approximately equal to 2.7 kJ) and which engulfs Na+ or K+ in the case of the ascorbyl or the isoascorbyl radical, respectively. The radicals Na-ASC and K-Iso-ASC are electroneutral. Red. glutathione affects both types of radicals by restoring the original electronic configuration at C(4) without changing the electroneutral bicyclic structure. In this way, the mobile carriers Na-ASC and K-Iso-ASC can transport Na+ and K+ across membranes. Its highest efficiency is around 37 degrees C and pH approximately equal to 7, that is, at physiological values. The biological importance of the side chain of vitamin C is outlined and a possible transport mechanism proposed.