Hydration and self-association of adenosine triphosphate, adenosine diphosphate, and their 1:1 complexes with magnesium(II) at various pH values: infrared investigations

Abstract

The dependences on pH of the hydrogen bonding of the K⁺ salts and the 1 : 1 complexes of ATP and ADP with Mg²⁺ have been studied for aqueous solutions by i.r. spectroscopy. Water of hydration is strongly bound to the phosphate chains. Proton attachments cause rearrangements of electrons in phosphate groups and adenine residues changing mesomeric bond resonance. With 50% protonated ATP (POH ⋯ O–P)⇌(P–O ⋯ HOP) and with 100% protonated ATP and ADP (POH ⋯ N)⇌(P–O ⋯ H⁺N) polarizable bonds are formed. In weakly acidic media. (N⁺H ⋯ O–P)⇌(N ⋯ HOP) polarizable bonds are formed between two molecules of ATP and ADP. We assume that all these effects contribute to changes in free energy occurring during ATP hydrolysis. In more acidic solutions, the protonated phosphate chains form POH ⋯ OH₂⇌ P–O ⋯ H + OH₂ and POH ⋯ OP ⇌ P–O ⋯ H⁺OP bonds. Magnesium(II) ions interact only with the phosphate groups. Due to the decrease of the pK_a values of the terminal phosphate groups upon interactions by Mg²⁺ ions, polarizable hydrogen bonds both between the terminal phosphate groups and between protonated adenine residues and phosphate chains are formed simultaneously.