CHEMICAL PROPERTIES 7 



is oxidized and in which a high-energy phosphate is incorporated. Much 

 work since that time has confirmed the sensitivity of phosphoglyceraldehyde 

 dehydrogenase to iodoacetate, but has also produced evidence that the in- 

 hibition is perhaps not as selective as originally believed unless great care 

 is taken in choosing the experimental conditions, especially the concentra- 

 tion of iodoacetate. 



CHEMICAL PROPERTIES 



lodoacetic acid is a rather reactive and unstable substance, due primarily 

 to the relative weakness of the C — I bond, so that it slowly decomposes to 

 release I~ and Ig, as is evident in most commercial samples. These reac- 

 tions are accelerated in aqueous media, and by light or increase in tem- 

 perature. It has long been known that heating aqueous solutions of the 

 halogen acids leads to a hydrolysis: 



XCH2COO- + H2O i^ HOCH2COO- -f H+ + X- 



forming glycolate and the halide anion. Under physiological conditions of 

 pH and temperature these reactions are slow and not important in experi- 

 ments of several hours' duration; however, they are partly responsible for 

 the decomposition of stored solutions of the halogen acetates, and may be 

 of some significance in long-term experiments. The activation energy for 

 the hydrolyses is quite high — 23.1 kcal/mole for bromoacetate and 22.6 

 kcal/mole for iodoacetate — so the reactions are very temperature-depend- 

 ent (Kunze and Merkader, 1940). At 70° and 100 mM, the half-reaction 

 times are 10 hr for bromoacetate, 25 hr for chloroacetate, and 61 hr for 

 iodoacetate (Drushel and Simpson, 1917). In 1 hr the reactions would pro- 

 ceed 7%, 3%, and 2%, respectively, The kinetics of hydrolysis are complex 

 and various salts exert catalytic effects (Dawson and Dyson, 1933). Hy- 

 drolysis is usually somewhat faster for the halogen acetates than for the 

 undissociated acids (Dawson, 1935). Exchange reactions of iodoacetate with 

 other halides may occur: 



ICH^COO- -i- CI- ^ CICH^COO- + I" 



and the rates are quite appreciable at physiological temperatures (Wagner, 

 1925). Such exchange reactions, indicating the instability of the iodine 

 atom, have recently been studied by van der Straaten and Aten (1954) 

 using I^^^: 



