Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. E-mail: david.parker@dur.ac.uk; sergey.shuvaev@dur.ac.uk Abstract: The design principles, mechanism of action and performance of europium(III) complexes that serve as strongly emissive and responsive molecular probes in water are critically discussed. Examples of systems designed to assess pH, selected metal ions and anions, including chiral species, as well as selected small molecules and biopolymers are considered, and prospects evaluated for improved performance in more complex biological media such as in bio--fluids and within living cells. Modulation of the emission spectral form, lifetime and degree of circular polarisation can be used to quantify the spectral response and permit calibration. IntroductionThe genesis of lanthanide bio--imaging probes can be traced to two discoveries: the observation of the red emission of europium by Crookes in 1885 and the report of its sensitised emission made by Weissman in 1942 [1] . Each discovery epitomises the advantageous optical features of lanthanide complexes - narrow bands in emission with energies only slightly affected by the nature of the ligand, and a large separation (a "pseudo--Stokes' shift") between the absorbing (ligand--antenna) and emitting (lanthanide ion) photon energies. These favourable optical properties create a myriad of opportunities to devise responsive systems by judicious ligand design. Europium(III) complexes have particular advantages over other lanthanide complexes in biologically relevant applications: the higher transparency of biological tissue towards the red region of the visible spectrum; their high inherent brightness and low sensitivity to quenching by oxygen; the presence of three excited states ( 5 D2, 5 D1 and 5 D0) spanning the range 17500--21000 cm --1 that are lower in energy than the triplet levels of many different types of ligands; the presence of an environmentally insensitive magnetic--dipole transition 5 D0 to 7 F1 in the luminescence spectrum, (Figure 1), that can be used as an internal reference to the hypersensitive transition bands around 620 ( 5 D0 --7 F2) and 700 nm ( 5 D0 to 7 F4), and the high emission anisotropy factors, gem, in circularly polarised luminescence (CPL) spectra.

The design, synthesis and application of a nine-coordinate gadolinium(III)-containing spin label, [Gd.sTPATCN]-SL, for use in nanometer-distance measurement experiments by EPR spectroscopy is presented. The spin label links to cysteines via a short thioether tether and has a narrow central transition indicative of small zero-field splitting (ZFS). A protein homodimer, TRIM25cc, was selectively labeled with [Gd.sTPATCN]-SL (70%) and a nitroxide (30%) under mild conditions and measured using the double electron electron resonance (DEER) technique with both commercial Qband and home-built W-band spectrometers. The label shows great promise for increasing the sensitivity of DEER measurements through both its favorable relaxation parameters, and the large DEER modulation depth at both Q-and W-band for the inter-Gd(III) DEER measurement which, at 9%, is the largest recorded under these conditions.

Peptides are interesting tools to rationalize uranyl-protein interactions, which are relevant to uranium toxicity in vivo. Structured cyclic peptide scaffolds were chosen as promising candidates to coordinate uranyl thanks to four amino acid side chains pre-oriented towards the dioxo cation equatorial plane. The binding of uranyl by a series of decapeptides has been investigated with complementary analytical and spectroscopic methods to determine the key parameters for the formation of stable uranyl-peptide complexes. The molar ellipticity of the uranyl complex at 195 nm is directly correlated to its stability, which demonstrates that the β-sheet structure is optimal for high stability in the peptide series. Cyclodecapeptides with four glutamate residues exhibit the highest affinities for uranyl with log KC =8.0-8.4 and, therefore, appear as good starting points for the design of high-affinity uranyl-chelating peptides.