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| - Matrix Effects on Spin−Lattice Relaxation in the Triplet State of Tryptophan
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| - Spin−lattice relaxation (SLR) of the photoexcited triplet stateof tryptophan both as the free amino acid(Trp) and also incorporated at a buried site of ribonuclease T1 fromAspergillus oryzae (RNase T1) isinvestigated in the ethylene glycol (EG)−water mixed solvent system.Global analysis of microwave-induceddelayed phosphorescence (MIDP) transients measured at 1.2 K is employedto obtain the triplet state kineticparameters over a range of solvent composition between 10 and 90 vol %EG. These parameters are used tocalculate the phosphorescence decay profile of the triplet states thatcontribute to the MIDP. The experimentalphosphorescence profile is fitted at each solvent composition as thesum of the calculated nonexponentialdecay and a simple exponential decay with lifetime equal to the averagetryptophan lifetime. Thephosphorescence is modeled to originate from two distinct populationscharacterized by either slow SLR orrapid SLR relative to the triplet state lifetime. Only the formercontributes to the MIDP signals while thelatter is responsible for the exponential decay component. Thefraction of tryptophan undergoing slow SLR,Φs, is at a maximum (ca. 0.7) for EG in the range30−40% (v/v) for both Trp and RNase T1, diminishessharply with increasing water content, and diminishes slightly withincreasing EG content. The SLR of thispopulation is independent of solvent composition. Since thetryptophan triplet state is not kineticallyhomogeneous in EG−water, the method of microwave-saturatedphosphorescence decay analysis produceserroneous kinetic parameters for this system. The native RNase T1structure becomes unstable above 60 vol% EG as revealed by changes in phosphorescence and ODMR.
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