was measured with a physical property measurement system. The electrical resistivity shows the MIT at T c = 154 K on cooling ( Fig. 1) and exhibits large thermal hysteresis behavior indicating a first-order character of the MIT. Ca 1.9 Sr 0.1 RuO 4 crystals for scanning tunneling microscopy (STM), LEED, and HREELS measurements were mounted on the sample plates with conducting silver epoxy, and a small metal post was glued on top. The crystal was cleaved by knocking off the post in ultrahigh vacuum with a base pressure of 1.0 × 10 −10 torr, producing a flat shiny [001] surface that yielded a sharp LEED pattern. The STM images of the freshly cleaved surfaces show large micrometer-sized terraces. Both the LEED pattern and atomically resolved STM images indicate that the surface has a well-ordered lattice structure. All surface steps are integral multiples of~6.4 Å , which is the spacing between two nearest-neighbor RuO6 octahedron layers (Fig. 1) 16. S. Nakatsuji et al., Phys. Rev. Lett. 93, 146401 (2003). 17. Detailed spectral data analysis methods were as follows:The Drude weight is the integrated intensity obtained from the difference between the left and right sides of the quasi-elastic peak through a Lorentzian function. Yann Ferrand, Matthew P. Crump, Anthony P. Davis* Carbohydrate recognition is biologically important but intrinsically challenging, for both nature and host-guest chemists. Saccharides are complex, subtly variable, and camouflaged by hydroxyl groups that hinder discrimination between substrate and water. We have developed a rational strategy for the biomimetic recognition of carbohydrates with all-equatorial stereochemistry (b-glucose, analogs, and homologs) and have now applied it to disaccharides such as cellobiose. Our synthetic receptor showed good affinities, not unlike those of some lectins (carbohydrate-binding proteins). Binding was demonstrated by nuclear magnetic resonance, induced circular dichroism, fluorescence spectroscopy, and calorimetry, all methods giving self-consistent results. Selectivity for the target substrates was exceptional; minor changes to disaccharide structure (for instance, cellobiose to lactose) caused almost complete suppression of complex formation. Carbohydrates are challenging substrates for host-guest chemistry (1-4). They possess extended, complex structures that require large receptor frameworks for full encapsulation. The differences between them are often subtle (e.g., the stereochemistry of a single hydroxyl group), so that meaningful selectivity is hard to achieve. Most particularly they are found in water and, with their arrays of hydroxyl groups, they quite strongly resemble water. The first task of a receptor is to discriminate between solvent and substrate, and in the case of carbohydrates this is clearly nontrivial. There is evidence that even nature finds the problem difficult. Though critical for many biological processes (5-7), protein/carbohydrate binding is remarkably weak (8). For example, lectins, the most common class of natural recepto...

The ab initio design of synthetic molecular receptors for a specific biomolecular guest remains an elusive objective, particularly for targets such as monosaccharides, which have very close structural analogues. Here we report a powerful approach to produce receptors with very high selectivity for specific monosaccharides and, as a demonstration, we develop a foldamer that selectively encapsulates fructose. The approach uses an iterative design process that exploits the modular structure of folded synthetic oligomer sequences in conjunction with molecular modelling and structural characterization to inform subsequent refinements. Starting from a first-principles design taking size, shape and hydrogen-bonding ability into account and using the high predictability of aromatic oligoamide foldamer conformations and their propensity to crystallize, a sequence that binds to β-D-fructopyranose in organic solvents with atomic-scale complementarity was obtained in just a few iterative modifications. This scheme, which mimics the adaptable construction of biopolymers from a limited number of monomer units, provides a general protocol for the development of selective receptors.