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The results revealed that, under these conditions, the rate of Tyr consumption was slower than in the absence of I? (Fig.?5b). Accordingly, the rate of H2O2 production was also slower in the presence of I? than in its absence. Thus, taking into account that no quenching of the singlet excited state of Ptr was previously observed at an I? concentration of 500?��m [30], the inhibition of the photosensitized http://www.selleckchem.com/products/Adriamycin.html degradation of Tyr by I? strongly suggests the participation of the triplet excited state of Ptr (3Ptr*). Taking into account that (1) the process needs O2 to take place, (2) the triplet excited of the photosensitizer (3Ptr*) is involved and (3) Ptr produces efficiently 1O2 upon UV-A irradiation [32], it may be assumed that the photooxidation of Tyr is a 1O2�Cmediated reaction (type II mechanism). However, the fact that the efficiency of the process was greater in air-equilibrated solutions than in O2-saturated solutions (Fig.?5a) is not consistent with a process wherein Tyr would be consumed by 1O2. Furthermore, we have previously demonstrated that pterins act mainly through ET-initiated processes (type I mechanisms), even in cases where the substrate reacts rapidly with 1O2 [15, 33]. The oxidation of Tyr and its derivatives by 1O2 has been extensively studied [34] and values for the rate constant of the chemical reaction between 1O2 and Tyr (krTyr) have been reported in several studies (krTyr? http://www.selleckchem.com/products/obeticholic-acid.html the reported values that krTyr depends on the pH/pD conditions, its value increasing at higher pH. Under our experimental conditions krTyr should be lower than 106?m?1?s?1. The contribution of 1O2 in the photosensitized oxidation of Tyr was estimated considering a krTyr of 106?m?1?s?1 to calculate the initial rate of the chemical reaction between 1O2 and Tyr (d[Tyr]/dt)0, Eq.?(1): (1) where [1O2]SS is the steady-state concentration of 1O2 during irradiation of a solution containing Ptr and Tyr and can be calculated using Eq.?(2), (2) where qp,a and ��? are the photon http://www.selleck.cn/products/sch772984.html flux absorbed by Ptr (9.1?��?10?6 einstein L?1?s?1) and the quantum yield of 1O2 production (��??=?0.18 at pH?=?5.5 [32]), respectively; kd is the rate constant for the unimolecular deactivation of 1O2 which is solvent dependent [37] (kd?=?�Ӧ�?1, �Ӧ�: 1O2 lifetime in the absence of quencher; kd has a value of approx. 2.6?��?105?s?1 in H2O); ktPtr and ktTyr are the overall rate constants of 1O2 quenching by Ptr and Tyr, respectively (kt?=?kr?+?kq, where kq is the rate constant of 1O2 physical quenching by the substrate). Considering ktPtr?��?106?m?1?s?1 as previously determined [32] the quenching of 1O2 by Ptr is negligible under our experimental conditions (i.e. ktPtr [Ptr]???kd). On the other hand, taking into account values of ktTyr reported in the literature (ktTyr?=?2.7?��?107?m?1?s?1, pH?=?7.