Extended Data Fig. 2. Mechanistic Investigations.
a, Visualization of transport through a hydrophobic phase in U-tube experiments by taking photographs of the tri-phasic U-tube system at the start of an experiment (t = 0, left) and after 24 h (right). In the U-tube experiments, the trans compartment contained initially only the fluorescent HPTS dye and the cis compartment was administered with combinations of carrier (boron cluster), peptide cargo (WR7), and DPX, which serves as a quencher of HPTS. Although the trans phase is initially strongly fluorescent (left photograph), transport of DPX from the cis phase (10 mM Tris, 107 mM NaCl, pH 7.4) across the chloroform phase leads to a fluorescence quenching. b, HPTS fluorescence measured in the trans buffer of the U-tube (2 mM HPTS, 10 mM Tris, 107 mM NaCl, pH 7.4) on addition of the following agents to the cis buffer: 40 μM B12Br122− and 20 μM WR7 (▲); 20 mM DPX (×); 20 μM WR7 and 20 mM DPX (♦); 40 μM B12H122−, 20 μM WR7, and 20 mM DPX (★); 40 μM B12Br122− and 20 mM DPX (•); 40 μM B12Br122−, 20 μM WR7, and 20 mM DPX (■). The experiments demonstrate that there is (i) no fluorescence quenching in the absence of added DPX quencher (▲), (ii) no transport of the DPX quencher in the absence of carrier and peptide cargo (×), (iii) no transport of the DPX quencher in the presence of peptide cargo but in the absence of a carrier (♦), (iv) slow and inefficient transport of DPX in the presence of peptide cargo and the least efficient carrier, B12H122− (★), (v) faster and more efficient transport of DPX in the absence of peptide cargo and the very efficient carrier, B12Br122− (•), and (vi) the fastest and most efficient transport of DPX in the presence of peptide cargo and the very efficient carrier, B12Br122− (■). The experiments show that B12Br122− is itself an effective carrier of DPX, but that the carrier/peptide cargo system leads to the most effective transport. c,d, Calorimetric evidence for direct interaction of B12Br122− with peptide cargos as obtained from raw ITC data (top) and apparent reaction heats obtained from the integration of the calorimetric traces (bottom) for the sequential injections of 2 mM of B12Br122− into c, 250 µM WR7 and d, 450 µM WK7. 15% Error for Ka and ± 0.5 kcal mol−1 for ΔH and TΔS (duplicate measurements). The calorimetric titrations show enthalpically driven non-stoichiometric intermolecular interactions between the cluster and the peptides.