The discovery that Yb(3+) bound to the Ca(2+) ion-binding sites of the single tryptophan- containing parvalbumin from codfish emits near infrared radiation (977 nm) opens up an entire new class of lanthanide ion (Ln3+) probe experiments for studying calcium-binding proteins. The interpretation of this finding in terms of through-protein electron transfer from excited singlet tryptophan (Trp) to bound Yb(3+) ions introduces new redox probes for measuring electron transfer rates of proteins. The forward electron transfer produces a tryptophan cation radical, Trp+ with some of the Yb(3+) so produced remaining in its 2F5/2 excited state from which it emits 977 nm light. The Eu(3+) ion also acts as an electron acceptor and measuring electron transfer rates to both of these ions allows the straightforward determination of the nuclear reorganization energy and tunneling matrix element (HAB) of semi-classical electron transfer theory. Experiments show that electron transfer rates can be obtained by measuring Trp fluorescence decay rates using picosecond laser spectroscopy. Such experiments are proposed for a variety of tryptophan- and tyrosine- containing proteins including parvalbumin isotopes, S100B, calcylcin, calmodulin, staphylococcal nuclease, and mutants thereof. The electron transfer rates of slower reactions will be measured on molecules initially in a triplet excited state by monitoring Trp+ absorption as a function of time. The results will be used to test theories of through-protein electron transfer. The new phenomenon of Yb(3+) emission in proteins will be investigated by surveying a variety of systems. Emission in the near infrared region has many advantages in biological research, since it can be carried out on systems that are not transparent in the visible, luminescent labels with potential imaging or biosensor applications are envisaged. Eu(3+) luminescence of the (7)F(0)->(5)D(0) excitation transition and parallel microtitration calorimetry experiments will be exploited in on-going studies of the calcium-binding proteins calmodulin, calcineurin, S100B, calcylin, and annexin V including the study of peptide binding to some of these, yielding information on metal ion binding constants, ligand environments, numbers of metal-coordinating water molecules and distances between binding sites.
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