ACID-BASE PROPERTIES OF ADENOSINE 5'-O-THIOMONOPHOSPHATE IN AQUEOUS-SOLUTION

The acidity constants of H-2-(AMPS)(+/-) were determined by potentiome tric pH titrations in aqueous solution at 25 degrees C and I = 0.1 M ( NaNO3). Titrations with a combined single-junction glass electrode wer e hampered in the presence of AMPS by a ''poisoning'' effect; the prob lem could be avoided by use of two separated electrodes. The values of the acidity constants pK(H2(AMPS))(H) = 3.72 +/- 0.03 and pK(H(AMPS)) (H) = 4.83 +/- 0.02 are relatively close to each other; the buffer reg ions of the two equilibria overlap, and therefore a micro acidity cons tant scheme was developed and the constants for the various sites calc ulated. It is concluded that the thiophosphate-protonated species (AMP S.H)(-) dominates at about 75% occurrence, while the form (H.AMPS)(-), with the proton at the N1 site of the adenine residue, occurs at abou t 25%. Semiempirical AM1 and PM 3 calculations including water as a so lvent locate the proton in (AMPS.H)(-) mainly on the terminal oxygen a toms rather than the sulfur. The acid-base properties of H-2(AMPS)(+/- ) are considerably more complicated than those of the parent nucleotid e, H-2(AMP)(+/-); for the latter the two (intrinsic) acidity constants are well separated and consequently practically all protons have left the N1 site before deprotonation at the monoprotonated phosphate grou p occurs. Finally, an estimate for the acidity constants of H-2(ATP ga mma S)(2-) is given.