Organic Acid Specificity for Acid-Induced ATP Synthesis by Isolated Chloroplasts

A study has been made of the structural requirements for organic acids that increase the yield of ATP formed by spinach chloroplasts in the dark due to an acid-base transition. Dicarboxylic acids of 4 and 5 carbon atoms with 1 pKa in the range of 4.2 to 4.4 and the second at 5.3 to 5.5 are most effective. A fairly good correlation was found between the fraction of the acid completely undissociated at the test pH and the final yield. Other considerations are also important, however, since introducing hydroxyl groups or excessive chain length decreased yields in spite of pKa considerations. Aromatic acids, except for the dicarboxylic phthalic acids, were inhibitory. The diversity of organic acids having some effect makes it unlikely that they serve as substrate for a chloroplast enzyme system.