When a blood clot forms, the protein thrombin cleaves fibrinogen into fibrin that polymerizes into a branched network of fibers that forms the structure of the clot. The kinetics of fiber formation affect fiber diameter, fiber network density, rate of clot lysis and potentially clot stiffness. We previously showed that exposure to nitric oxide, an important regulator of blood flow, alters the characteristics of these fibrin clots. The addition of ProliNONOate to fibrinogen or thrombin together or separately lead to a decreased the rate of fiber formation, increased the time the clot took to gel, increased fiber diameters, and decreased clot density.We hypothesize the effects of nitric oxide on the fibrin clots are due to oxidation of the proteins due to the creation of peroxynitrite. Peroxynitrite is created when nitric oxide reacts with superoxide anion and is known to cause nitration of tyrosine resides and oxidation of methionine and cysteine residues. Therefore, we measured carbonyl formation and tyrosine nitration upon exposure to ProliNONOate.Here we show the addition of the peroxynitrite scavenging ferric porphyrin complex, Iron(III)-5,10,15,20-tetrakis94-sulonatophenyl) porphyrinato chloride (FeTPPS) to the samples immediately prior to the addition of proliNONOate reduces protein oxidation.The addition of FeTPPS reduced or eliminated protein carbonyl formation. Previously we reported the formation of 880 ± 480 mmol of carbonyls per mole of fibrinogen after the addition of ProliNONOate however, with the addition of FeTPPS no carbonyls were detected. Carbonyls detected on thrombin exposed to ProliNONOate were reduced from 390 ± 80 mmol/mol thrombin to 18 mmol/mole thrombin with the addition of FeTPPS.FeTPPS abrogated the changes to the rate of fibrin formation. When FeTPPS was added to fibrinogen before the addition of ProliNONOate there was no significate change in the rate of fiber formation. Without FeTPPS we previously reported a decreased rate of clot formation.Here we show the exposure of blood clot proteins fibrinogen and thrombin to NO donor ProliNONOate lead to peroxynitrite formation which caused the previously reported protein oxidation and altered fibrin fiber formation kinetics and thereby altered various fibrin clot properties.