(1) Siderophores are low-molecular weight organic compounds produced by some microorganisms to facilitate iron transport. Pseudoverdin, a novel siderophore, produced by a PAO1 mutant strain of Pseudomonas aeruginosa, has been characterized as 3-formylamino-6,7-dihydroxycoumarin on the basis of its chemical and spectral properties. This structural assignment was confirmed by the successful synthesis of its dimethyl derivative. The coumarin structure can be considered a cyclized form of trihydroxyphenylalanine thus suggesting possible biosynthetic pathways for the chromophoric group of pyoverdins, the siderophores produced by the wild type. (2) 4-Hydroxy-2-nonenal (HNE), one of the lipid peroxidation products, was observed to be capable of modifying some proteins at the histidine residue. The reaction of HNE and Nalpha-acetylhistidine was studied in detail so as to gain some insight into the protein reaction. The reaction of HNE and acetylhistidine produced a complex mixture from which two different derivatives were obtained. 2D 1-H NMR analysis of these two derivatives led to complete assignment of their structures. These structural features provided conclusive evidence that the reaction occurred as a 1,4-addition of one of the imidazole nitrogens in the histidine molecule to the alpha beta-unsaturated carbonyl system in HNE.