Abstract—5‐Bromouridine (BrUd) photochemically reacted withl‐tryptophan (trp) only in aqueous frozen solution giving rise to 2‐[1,2,3,4‐tetrahydro‐1‐(β‐d‐ribofuranosyl)‐2,4‐dioxo‐5‐pyrimidinyl]‐l‐tryptophan (1a). Under similar conditions irradiation of 5‐bromouracil (BrU) and trp gave the corresponding coupled product2.Addition of acetone, potassium carbonate or sodium chloride to the frozen system completely inhibited the photocoupling reaction. These photocoupling reactions are suggested to proceed via mixed aggregate formation between trp and BrUd (or BrU) in frozen aqueous solution. Stacking interactions in these aggregates are characterized by an efficient quenching of trp fluorescence by BrUd. Under the same conditions other amino acids such as phenylalanine, tyrosine, histidine and lysine did not react with BrUd, indicating that the photocoupling of B