Avidin binds tightly to biotin ligand producing virtually irreversible complex. This property of the protein makes it a convenient carrier for the attachment of various probes. Avidin conjugates thus obtained can be used to label biotinylated molecules of interest. It is seen (Table 1) that the attachment of Tb3+ luminescent chelates 2 and 4 to the protein at low concentration of the probes caused ca. 3-fold quenching comparing to emission of non-attached probes. For probe 2, increasing

the number of attached probes resulted in further progressive quenching (Fig. 5C), while for probe 4 the dependence of the cumulative fluorescent Sorafenib signal on the number of the crosslinked probes remained linear. Attachment of Eu3+-based probe 1 also resulted in 3-fold quenching, however when the number

of the conjugated probes increased, a significant super-linear luminescence enhancement was observed (Fig. 5C). This effect can be explained by enhancement of antenna-to-lanthanide energy transfer, which is supported by decrease of antenna fluorescence and simultaneous increase of lanthanide emission in the complex (Table 2). One factor that reduces the brightness of the probe could be quenching due to the contact between the antenna fluorophore and protein surface. This is supported by the superior properties of the probe 4 possessing a rigid spacer between the antenna MK2206 fluorophore and the crosslinking group. This spacer could prevent the quenching by restricting the fluorophore contacts with avidin. As expected, light emission of avidin conjugates increased in heavy water (Table 1). Thus 1.3 and 3-fold enhancement however was observed for Tb3+ and Eu3+ chelates correspondingly, which is close to enhancement factors for corresponding non-attached probes [13]. As seen from Fig. 5D, attachment of more than one BODIPY fluorophore to avidin dramatically decreased the cumulative fluorescent signal due to expected FRET quenching. Extensive modification of avidin could potentially interfere with biotin binding. To test the binding ability of the modified protein, we titrated the conjugate with biotinylated oligonucleotide carrying BHQ quencher. As seen from Fig. 6, incubation caused a dramatic

decrease in brightness suggesting quenching of the modified protein through binding of the biotinylated oligonucleotide. As expected, ca. 4-fold excess of the oligo was required to achieve maximal quenching, which corresponds to saturation of all biotin binding sites. To image the cells, we first treated them with acylating biotin derivative, which resulted in covalent attachment of the biotin residues to the cellular surface (Fig. 7A and B). As expected, subsequent incubation with luminescent labeled avidin conjugates resulted in the attachment to the cells as judged by visual inspection under UV light. For microscopic imaging of the cells in time-gated mode we used Total Internal Reflection Fluorescence Microscopy (TIRFM) [16] and [17].