As a means to study the relationship of diffusion through proteins with the protein dynamics the authors used the quenching of phosphorescence of an intrinsic tryptophan by molecules that are free to diffuse in solution. Tryptophans in rigid environments within proteins are well known to exhibit long-lived phosphorescence at room temperature. Phosphorescence can be used to study the triplet quenching reaction in this way: the reactive triplet species of tryptophan is formed by excitation to the singlet state followed by spontaneous conversion to the triplet state and the reaction of the triplet state with an external molecule is monitored by a decrease in phosphorescence lifetime. The phosphorescence lifetimes of tryptophan in proteins are often $GRT msec, as contrasted to fluorescence lifetimes on the order of nsec, therefore, phosphorescence is very sensitive to quenching reactions. In previous studies the quenching reaction of tryptophan by molecules with $GRT 4 atoms was examined with the conclusion they interacted with buried tryptophan by a reaction that occurs over distance, not requiring the physical diffusion of these molecules through the protein for the reaction to occur. In contrast, diatomic molecules appear to be able to diffuse through the protein matrix, although diffusion is hindered relative to aqueous medium. Little information is available about the diffusion of triatomic molecules H$-2$/O and CO$-2$/. The authors use their sulfur analogues, H$-2$/S and CS$- 2$/, respectively, to approach the question whether protein fluctuations allow their penetration.
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