Photochemical transformation and dephosphorylation of 5′-AMP in water: roles of reactive intermediates and matrix effects

Phosphate monoesters constitute an important photoreactive class of dissolved organic phosphorus (DOP) in surface waters, yet the linkage between their initial photochemical transformation and subsequent dephosphorylation remains unclear. In this study, UV photolysis experiments were conducted for adenosine monophosphate (5′-AMP), a model nucleotide phosphate monoester, to examine this linkage in a defined molecular system, together with density functional theory (DFT) calculations of its excited-state behavior, energy/electron transfer, and C-O-P cleavage pathways. 5′-AMP absorbed mainly within 200–290 nm, and short-wavelength UV markedly enhanced degradation. DOM accelerated photolysis in a concentration-dependent manner. Under irradiation with fulvic acid, the pseudo-first-order rate contribution of 3 DOM * was higher than that of 1 O 2 and ·OH. NO 3 - , Cl - , and SO 4 2- promoted degradation, whereas HCO 3 - and Br - inhibited it. DFT calculations using a protonated 5′-AMP reference model indicated that direct hydrolytic dephosphorylation involved a high energy barrier (~151 kJ mol − 1 ), whereas ·OH- and 1 O 2 -associated pre-oxidation generally lowered the barriers for subsequent water attack, with several ·OH-associated candidate routes exhibiting the lowest barriers within this model. Both natural waters exhibited higher apparent photolysis rates than ultrapure water, with a higher rate in freshwater than seawater. These differences reflected combined matrix effects involving DOM, inorganic ions, and reactive-intermediate processes, rather than any single component or reaction pathway. Overall, this work integrates experiments and theory to reveal the structure-dependence and environmental sensitivity of 5′-AMP phototransformation under the tested aqueous-matrix conditions, offering insights into light-driven abiotic dephosphorylation in aquatic P cycling and eutrophication dynamics.