Nitric Oxide: The Key Molecule for Polyphenols Antimicrobial Action

The role of hydroxycinnamic acids as antioxidants, in vivo, has been widely discussed, but, recently, a great debate has focused on their antimicrobial action. In general, for the hydroxycinnamic acids’ action, the presence of NO, which is known to be an antimicrobial agent, seems compulsory; its production goes through the intermediacy of the nitrosonium ion, and a very low pH, for instance, as in the stomach, is requested. However, the action of the hydroxycinnamic acids seems to take place even in different biological compartments, i.e., characterized by different pHs and conditions, and then, for NO production, an alternative mechanism could be involved. In this light, evidence for the NO formation, via an E.T. mechanism, even in mildly acidic conditions (pH = 6.4), was obtained by reacting an aqueous buffer solution of acidic nitrite (HNO2) with the hydroxycinnamic acids ferulic, caffeic, p-coumaric and sinapic. Experiments conducted by EPR spectroscopy, let to detect the NO formation, and the efficiency of the process depending on the available amount of free polyphenol, and the intrinsic nature of the hydroxycinnamic acids. Thus, the production of NO through a non-enzymatic mechanism, in light acidic conditions, would account for the antimicrobial action of hydroxycinnamic acids, even in unconventional biological compartments, and for NO as the key-molecule.