Triazole biotin: a tight-binding biotinidase-resistant conjugate.

The natural amide bond found in all biotinylated proteins has been replaced with a triazole through CuAAC reaction of an alkynyl biotin derivative. The resultant triazole-linked adducts are shown to be highly resistant to the ubiquitous hydrolytic enzyme biotinidase and to bind avidin with dissociation constants in the low pM range. Application of this strategy to the production of a series of biotinidase-resistant biotin-Gd-DOTA contrast agents is demonstrated.

Carbon monoxide and carbon dioxide insertion chemistry of f-block N-heterocyclic carbene complexes.

The reactions of f-block silylamido N-heterocyclic carbene (NHC) complexes ([M(L)(N{SiMe(3)}(2))(2)], M = Y, Ce, and U, L = bidentate alkoxy-tethered NHC ligand) with CO and CO(2) have been studied and compared to each other, to those of selected [M(L)(2)(N{SiMe(3)}(2))] complexes, and to those of [M(N{SiMe(3)}(2))(3)] to identify the effect of the labile NHC group on the small molecule activation chemistry. The small molecules COS and N(2)CPh(2) have also been studied.

Lanthanide/actinide differentiation with sterically encumbered N-heterocyclic carbene ligands.

A study is reported on the relative stability of trivalent bis(ligand) complexes of the form [M(L(R))(2)N''] for trivalent group 3, lanthanide and actinide cations, using the sterically demanding N-heterocyclic carbene ligand L(R) = [OCMe(2)CH(2){CNCH(2)CH(2)NR}] (R = (i)Pr L(P), Mes L(M), Dipp L(D); N'' = N(SiMe(3))(2)). For the small Y(III) cation (r(6-coord) = 1.040 A) and the smallest L(R), R = (i)Pr, mono, bis, and tris(L(P)) complexes can be made; [Y(L(P))(2)N''] and [Y(L(P))(3)] have been characterised. For the larger ligands, L(M) and L(D), only the mono(L(R)) complexes [Y(L(M))N''(2)] and [Y(L(D))N''(2)] can be made. For the larger Ce(III) (r(6-coord) = 1.15 A), mono(L(R)) and bis(L(R)) complexes [Ce(L(M))N''(2)], [Ce(L(D))N''(2)], [Ce(L(M))(2)N''], and [Ce(L(D))(2)N''] can be made; structural characterisation of the latter two confirm the high degree of steric congestion. The new complex [U(L(M))N''(2)] has also been isolated. Despite the very similar radii of Ce(III) and U(III) (r(6-coord) = 1.165 A), the complexes [U(L(R))(2)N''] cannot be isolated; a surprising display of the difference between the 4f and 5f metal series. However, the six-coordinate, bis(ligand) U(IV) complexes can readily be isolated if smaller ancillary ligands are used; [U(L(M))(2)I(2)] and [U(L(D))(2)I(2)] have been fully, including structurally, characterised.