Short-lived Oxidation Products Derived from Enkephalines

ed sulfur radical cation (SETA)(S.-.S)+ was observed after pulse irradiation of solutions contain1 50K ing high concentration of SETA at pH 6. This reveals another interesting feature that indicates that hydroxysulfuranyl radicals (SETA)>S--OHare not converted efficiently into (SETA)(S.. S)+ but they would rather convert into the 380(-nm species. The intensity of absorption bands and the decay kinetics measured at Amax= 90 and 380 nm arc affected in the same way by the presence of oxygen, suggesting that both absorption bands can be assigned to the same radical, i.e. the a-(alkylthio)alkyl radical. Theoretical caluclations show that the C-centered radical resulting from the H-abstraction from the carbon of methylene group located between the sulfur atom and the acetyl group H3 C-CH2 -SCH-C(=O)-CH 3 is the most 2.034 2.010 1 .987 [g] stable C-centered radical derived from S-ethylthioacetone (SETA). Therefore, it is reasonable to Fig.3. EPR spectrum of S-ethylthioacetone recorded a 150 K assign the 380-nm absorption band, exclusively to This spectrum was assigned to the carbon-centered the H3 C-CH2 -SCH-C(=O)-CH 3 radical. radical (H3 C-CH2 -SCH-C(=O)-CH3 ) which might ESR Studies result from deprotonation of the monomeric radical The main component of the ESR signal recorded cation (SETA)>S-+ (Fig.3). in y-irradiated degassed frozen sample of S-ethylthioacetone (SETA) studied over the temperature range References of 77-95 K was an anisotropic singlet with g-valucs: [1. Bobrowski K., Pogocki D. Schoncich Ch.: J. Phys. Chem., 21, gl=2.023, g2=2.011, and g3=2.000. This spectrum 13677-13684 (1993). was attributed to the monomeric sulfur radical cation 121. Schdneich Ch., Bobrowski K.: J. Am. Chem. Soc., 115, (SETA)>S t formed by direct ionization of a thio6538-6547 (1993). to 13] Bobrowski K., Hug G.L., Marciniak B_, Schoneich Ch., ether sulfur. Warming the samples of SETA to Winiowski P.: Res. Chem. Intermcd., 25, 285-297 (1999). 100-110 K resulted in the appearance of a broad [4]. Stadtman E: In: Free Radicals, Oxidative Stress, and Antioxisinglet ESR signal with gav= 2 .01. The singlet can be dants. Ed. Ozben. Plenum Press, New York 1998, pp.131-143. attributed to the dimeric sulfur-centered radical cat[5]. Hovorka S. Schbneich Ch.: J. Pharm. Sci.. 90, 253269 (2001). [61. Varmenot N., Remita S., Abedinzadeh Z., Wigniowski P., ion (SETA)(S.. S)+. As the temperature was increasStrzelczak G.. Bobrowski K.: J. Phys. Chem. A,1Q5, 6867-6875 ed to 150 K the ESR spectrum represents a doublet (2001). with gav=2.0045 and a hyperfine splitting aH=1.4 mT.