Does hydrogen peroxide exist "free" in biological systems?

Hydrogen peroxide (H2O2) can diffuse far from the site of production to intracellular locations where biological effects may be greater. The diffusion range is extended by H2O2 carriers formed spontaneously by hydrogen bonding with monomeric and polymeric compounds, including amino and dicarboxylic acids, peptides, proteins, nucleic acid bases, and nucleosides. Hydrogen peroxide adducts (HPAs) are readily synthesized, e.g., crystalline histidine (His)-H2O2 adducts. An equilibrium exists between an adduct-forming compound and H2O2. The detection and relative stabilities of HPAs are measured by the degree of decomposition of H2O2 as influenced by test compounds in buffered solution competing with glucose or fructose for H2O2. The HPAs delay decomposition of H2O2 up to several hundredfold. The overall charge on an HPA, i.e., its ability to penetrate cell membranes, influences the cytotoxic and clastogenic effects of H2O2. Growth inhibition of Salmonella typhimurium LT2 by H2O2 is enhanced by neutral HPAs but decreased by anionic HPAs. Addition of catalase 1, 10, or 30 min after inoculation of S. typhimurium LT2 reduces or nearly eliminates partial growth inhibition by H2O2, but a neutral HPA, especially His-H2O2, transported H2O2 into the cells within 1 min, and in about 10 min completely inhibited growth. The stability of HPAs decreases with increasing pH or increasing temperature, while added Fe(II) in the presence and absence of EDTA accelerates H2O2 and HPA decomposition. Calculations indicate H2O2 hydrogen bonds with nucleic acid-base pairs with no apparent bond strain and energy stabilization comparable to normal hydrogen bonding.