Fundamental research on the mechanisms of plasma-protein interactions is performed. The possibility of controlling the modification processes of peptides is investigated by changing gas species and treatment time. Changes of the peptide composition are analyzed by X-ray photoelectron spectroscopy using [Arg(8)]-Vasotocin as peptide, under different discharge conditions. The XPS results show that the modifications depend on the discharging gas (such as argon, oxygen and nitrogen). Ultraviolet radiation emitted by the plasma contributes to the alteration of disulfide (S-S) bond in [Arg(8)]-Vasotocin, which might be responsible for the loss of biological function. (C) 2009 Elsevier B.V. All rights reserved.