A novel DOTA-like building block with a picolinate arm for the synthesis of lanthanide complex-peptide conjugates with improved luminescence properties.

Combination of complexes of lanthanide cations (Ln) for their luminescent properties and peptides for their recognition properties is interesting in view of designing responsive luminescent probes. The octadentate DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) chelate is the most popular chelate to design Ln complex-peptide conjugates. We describe a novel building block, DO3Apic-tris(allyl)ester, which provides access to peptides with a conjugated nonadentate chelate, namely DO3Apic, featuring a picolinate arm in place of one of the acetate arms compared to DOTA, for improved luminescence properties. This building block, with allyl protecting groups, is readily obtained by solid phase synthesis. We show that it is superior to its analogue with tBu protecting groups for the preparation of peptide conjugates because of the difficult removal of the tBu protecting groups for the latter. Then, we compare two luminescent zinc fingers (LZF) comprising (i) a zinc finger peptide for selective Zn binding, (ii) a Eu complex and (iii) an acridone antenna (ACD) for long-wavelength sensitization of Eu luminescence. The first one, LZF3, incorporates a DOTA chelate for Eu whereas the other, LZF4, incorporates a DO3Apic chelate. Both act as Zn-responsive luminescent probes but we show that changing DOTA for DO3Apic results in a higher Eu luminescence lifetime and in a doubling of the quantum yield, confirming the interest of the DO3Apic chelate and the DO3Apic(tris(allyl)ester building block for the preparation of Ln complex-peptide conjugates. Additionally, the DO3Apic chelate provides self-calibration for LZF4 luminescence upon excitation of its picolinamide chromophore, making LZF4 a ratiometric sensor for Zn.