Artificial Metalloglycoclusters:  Compact Saccharide Shell to Induce High Lectin Affinity as Well as Strong Luminescence

Tris-bipyridine ferrous and ruthenium complexes carrying various saccharide appendages have been investigated to develop sensory systems for monitoring saccharide-binding phenomena. Ferrous O-glycoclusters having spacer moieties inserted between saccharide appendages and the complex core showed enhanced affinities to lectins, but ferrous N-glycoclusters, in which the saccharide-appendages are directly linked to the complex core via amide linkage, had low lectin-affinities. Molecular dynamics calculation indicated that the O-glycoclusters have flexible and densely packed saccharide clusters, in contrast to the octahedrally fixed saccharide arrays of N-glycoclusters. Flexibility of saccharide clusters is essential for their enhanced affinity, probably to induce conformational change to fit the recognition sites of lectins. According to these insights, ruthenium O-glycoclusters have been designed as luminescence biosensors. The ruthenium complexes carrying α-manno clusters exhibited excellent affinities (IC_min = 9.0 × 10^-8 M) to concanavalin A (ConA). It is suggested from conformational analysis that densely packed mannoclusters can be fit properly to the recognition site of ConA. The binding was enthalpicaly driven (ΔH° = −21.8 kcal/mol). This binding behavior is quite similar to that of 1−3/1−6 trimannoside to ConA. They have strongly amplified luminescence (Φ_em = 0.15), and their luminescence intensities were changed (∼40%) upon binding to the specific lectins. The ruthenium glycoclusters can be a suitable sensory system for saccharide-binding phenomena.