Abstract
The photoexcited triplet state of the thiouredopyrenetrisulfonate (TUPS) covalently linked to (1) α-acetyl lysine (3*TUPS-L) and (2) lysine residue 122 of azurin (3*TUPS-A) were studied by time-resolved EPR spectroscopy. The triplet spectra of 3*TUPS-L and 3*TUPS-A dissolved in water−glycerol (1:1) mixture, at freezing temperatures, show different temperature dependence. While in the case of 3*TUPS-L, no apparent dynamic effects were noticed, 3*TUPS-A exhibits a strong effect at 230 K. The dynamic effects were treated by line shape analysis with the discrete jumps model, suggesting that the environment in which 3*TUPS-A is embedded is not frozen, allowing for fast molecular motion with a correlation time of ∼1.8 × 10-10 s. These results are explained by the unique properties of the protein-bound water layers, which are kept in the liquid state even at relatively low temperatures, where the bulk water is frozen.
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