Nucleophilicity and ionization of dihydrolipoamide

The reactivity of dihydrolipoamide has been studied at pHs 6 and 8 with a series of haloalkyl acids, iodoacetamide, and N-phenyl-α-bromoacetamide. The electron-withdrawing capability of the R group in these reagents (X—CH2—R) has the dominant influence on their reactivity with the thiolate. Thus, 3-bromopyruvic acid greatly exceeds N-phenyl-α-bromoacetamide, iodoacetamide, iodoacetic acid, and three bromoalkyl acids. A plot of the second-order rate constant of iodoacetamide with dihydrolipoamide supports a value of pK1, in the vicinity of 10.5. The second pKa of dihydrolipoamide cannot be deduced by this method, since reaction of the first thiol with iodoacetamide greatly alters the influences which the vicinal (normally unmodified) thiol has on the other. Potentiometric titration of dihydrolipoamide was not definitive, but the curve obtained was comparable to the titration curve of dihydrolipoamide (followed spectrophotometrically) which accommodated a pK1of 9.35 and pK2 of 10.65. As a control, a plot of...