Excited-State Absorption Spectroscopy of Ortho-Metalated Ir( I I I) Complexes

Excited-State Absorption Spectroscopy of Ortho-Metalated Ir( I I I) Complexes

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6104 J. Phys. Chem. 1987, 91, 6104-6106 of 10.2 and 9.9. The Eo values found for tryptophan were pH- independent below the pK of the radical (pK = 4.7) and decreased by 60 mV/pH at higher pH. For p-methoxyphenol, pulse radiolysis st~dies'~J~ have derived redox potentials of 0.32, 0.40, and 0.46 V at pH 13.5 and 0.6 V at pH 7. The cyclic voltammetry studies gave values within the error limits of the pulse radiolysis results. For tyrosine, cyclic voltammetry gave values of 0.93 and 0.72 V, respectively, at pH 7 and 13 compared to values of 0.85 and 0.64 V, respectively, obtained by pulse radio1ysis.I Considering some of the assumptions used to derive the latter values, the agreement is acceptable. For tryptophan, cyclic voltammetry gave higher values than ob- tained from any of the pulse radiolysis studies and in neutral solution the discrepancy is pronounced. In particular, the measured by cyclic voltammetry at pH 7 is 1.015 V compared to the pulse radiolysis values of 0.64 V obtained by Jovanovic et al.' and 0.87 V reported by Butler et al.* In neutral solution, redox potentials measured for substituted indoles and phenols (Table I) are clearly related to Hammett coefficients for the substituent. However, the amino acid side chain present in both tryptophan and tyrosine raises the of the molecule above that expected for a simple alkyl side chain.15 This is an important finding that merits further study; it appears to confirm the hypothesis' that the amino acid side chain can exert a strong influence on the redox properties of indole. Also, it indicates the dangers of estimating redox potentials for amino acids from simple model compounds and Hammett coefficients.* Comparison of the data obtained for tryptophan and simple indoles (Table 11) infers that the redox potential for tryptophan cannot (13) Steenken, S.; Neta, P. J. Phys. Chem. 1979, 83, 1134. (14) Steenken, S.; Neta, P. 1982, 86, 3661. (1 5) Comparison of the measured values for tryptophan and tyrosine with the alkyl-substituted model compounds given in Table I infers that the amino acid side chain raises by about 160 mV. be 0.64 V, as obtained recently by pulse radiolysis,' and the cyclic voltammetric value of 1.015 V seems to be the correct figure. It is clear from Figure 2 that the efficiency and direction of electron transfer between tryptophan and tyrosine are related to pH. That tyrosine radicals oxidize tryptophan at pH C 3 and pH > 12 has been demonstrated by pulse radiolysis studies.',* It has also been demonstrated that tryptophan radicals will oxidize tyrosine in neutral solution, although the bimolecular rate constants are low.' According to Figure 2, there is a small thermodynamic driving force for electron transfer in neutral solution. tryp + tyrH = trypH + tyr. AGO = -8 kJ mol-' Such a modest driving force would explain the relative slowness of electron transfer at pH 7.
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Excited-State Absorption Spectroscopy of Ortho-Metalated Ir( I I I) Complexes

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