Correction: Spironaphthoxazine switchable dyes for biological imaging.

[This corrects the article DOI: 10.1039/C8SC00130H.].

Anion transport with halogen bonds.

This review covers the application of halogen bonds to transport anions across lipid bilayer membranes. The introduction provides a brief description of biological and synthetic transport systems. Emphasis is on examples that explore interactions beyond the coordination with lone pairs or hydrogen bonds for the recognition of cations and anions, particularly cation-π and anion-π interactions, and on structural motifs that are relevant for transport studies with halogen bonds. Section 2 summarizes the use of macrocyclic scaffolds to achieve transport with halogen bonds, focusing on cyclic arrays of halogen-bond donors of different strengths on top of calixarene scaffolds. This section also introduces methods to study anion binding in solution and anion transport in fluorogenic vesicles. In Sect. 3, transport studies with monomeric halogen bond-donors are summarized. This includes the smallest possible organic anion transporter, trifluoroiodomethane, a gas that can be bubbled through a suspension of vesicles to turn on transport. Anion transport with a gas nicely illustrates the power of halogen bonds for anion transport. Like hydrogen bonds, they are directional and strong, but compared to hydrogen-bond donors, halogen-bond donors are more lipophilic. Section 3 also offers a concise introduction to the measurement of ion selectivity in fluorogenic vesicles and conductance experiments in planar bilayer membranes. Section 4 introduces the formal unrolling of cyclic scaffolds into linear scaffolds that can span lipid bilayers. As privileged transmembrane scaffolds, the importance of hydrophobically matching fluorescent p-oligophenyl rods is fully confirmed. The first formal synthetic ion channel that operates by cooperative multiion hopping along transmembrane halogen-bonding cascades is described. Compared to homologs for anion-π interactions, transport with halogen bonds is clearly more powerful.

Transmembrane anion transport mediated by halogen-bond donors.

In biology and chemistry, the transport of anions across lipid bilayer membranes is usually achieved by sophisticated supramolecular architectures. Significant size reduction of transporters is hampered by the intrinsically hydrophilic nature of typical anion-binding functionalities, hydrogen-bond donors or cations. To maximize the atom efficiency of anion transport, the hydrophobic nature, directionality, and strength of halogen bonds seem promising. Unlike the ubiquitous, structurally similar hydrogen bonds, halogen bonds have not been explored for anion transport. Here we report that transport across lipid bilayers can be achieved with small perfluorinated molecules that are equipped with strong halogen-bond donors. Transport is observed with trifluoroiodomethane (boiling point=-22 °C); that is, it acts as a 'single-carbon' transporter. Contrary to the destructive action of small-molecule detergents, transport with halogen bonds is leakage-free, cooperative, non-ohmic and highly selective, with anion/cation permeability ratios <37.

Recent progress with functional biosupramolecular systems.

The objective of this account is to summarize our recent progress with functional biosupramolecular systems concisely. The functions covered are artificial photosynthesis, anion transport, and sensing in lipid bilayer membranes. With artificial photosynthesis, the current emphasis is on the construction of ordered and oriented architectures on solid surfaces. Recent examples include the zipper assembly of photosystems with supramolecular n/p-heterojunctions and oriented antiparallel redox gradients. Current transport systems in lipid bilayers reveal new interactions at work. Examples include anion-macrodipole or anion-π interactions. Current attention with membrane-based sensing systems shifts from biosensor approaches with enzymatic signal generation to aptamers (i.e., the DNA version of immunosensing) and differential sensing with dynamic polyion-counterion transporters. The functional diversity accessible with biosupramolecular systems is highlighted, as is the critical importance of cross-fertilization at intertopical convergence zones.