The inhibition of protein−carbohydrate interaction provides a powerful therapeutic strategy for the treatment of myriad human diseases. To date, application of such approaches have been frustrated by the inherent low affinity of carbohydrate ligands for their protein receptors. Because lectins typically exist in multimeric assemblies, a variety of polyvalent saccharide ligands have been prepared in the search for high affinity. The cluster glycoside effect, or the observation of high affinity derived from multivalency in oligosaccharide ligands, apparently represents the best strategy for overcoming the “weak binding” problem. Here we report the synthesis of a series of multivalent dendritic saccharides and a biophysical evaluation of their interaction with the plant lectin concanavalin A. Although a 30-fold enhancement in affinity on a valence-corrected basis is observed by agglutination assay, calorimetric titration of soluble protein with a range of multivalent ligands reveals no enhancement in binding free energies. Rather, IC50 values from agglutination measurements correlate well with entropies of binding. This observation suggests that hemagglutination measures a phenomenon distinct from binding that is typified by a large favorable entropy and an unfavorable enthalpy:  this process is almost certainly aggregation. Supporting this assertion, we report crystal structures of multivalent ligands cross-linking concanavalin A dimers. To the best of our knowledge, these structures are the first reported of their kind. Our results indicate that hemagglutination assays evaluate the ability of ligands to inhibit the formation of cross-linked lattices, a process only tangentially related to reversible ligand binding. Cluster glycoside effects observed in agglutination assays must, therefore, be viewed with caution. Such effects may or may not be relevant to the design of therapeutically useful saccharides.

1. Streambed light regimes change dramatically when riparian trees gain leaves in spring and lose them in autumn. This study examined the effect of these changes on periphyton photosynthetic characteristics, primary production, and light utilisation efficiency in two eastern Tennessee streams. 2. Photosynthesis–irradiance responses were measured at intervals covering leaf emergence and abscission in spring and autumn. Photosynthetic efficiency (αchl) increased with declining streambed irradiances during spring leaf emergence, but returned to pre‐emergence values after autumn leaf fall. The onset of photosaturation (Ik) displayed the opposite pattern, decreasing during leaf emergence and increasing after leaf fall. Both αchl and Ik were closely associated (P < 0.01) with daily integrated streambed irradiance, as were periphyton carotenoids. Internal shading by photoprotective carotenoids is hypothesised to account for lower αchl when streambed irradiances are high. 3. An in situ shading experiment confirmed that the temporal changes observed in periphyton photosynthetic characteristics and carotenoids were primarily the result of changing light levels and not other environmental factors (e.g. nutrients, temperature). 4. Daily chlorophyll‐specific primary production (PPchl) was calculated with P–I models and recorded streambed irradiances. In both streams, PPchl was the highest in early spring when trees were leafless, and then declined markedly as leaves emerged, reaching a minimum in summer. PPchl increased after leaf abscission, but was still lower than it was in early spring, when the sun was higher and daylength was longer. A hyperbolic tangent equation fit to PPchl and daily integrated irradiance (r2=0. 85) suggested that primary production was light saturated at 4–8 mol m–2 d–1. 5. Light utilisation efficiency (Ψ) increased 10‐fold during leaf emergence. Photosaturation at high irradiances and photoacclimation at lower irradiances were responsible for a negative hyperbolic relationship between Ψ and daily integrated irradiance.

The vulnerability of periphyton and grazers to current and future levels of ultraviolet radiation (UV) has important ramifications for energy flow in shallow waters. We reduced ambient UV levels in a Tennessee stream with UV‐opaque Plexiglas suspended over the stream and compared periphyton under these Plexiglas filters to periphyton under UV‐transparent polyvinylidene film. The experiments were performed three times, from autumn 1993 to summer 1994. The last experiment included a treatment with reduced snail densities, so potential interactions between grazing and UV could be identified. Periphyton biomass and photosynthesis were not significantly affected by reduced UV in any of the three experiments, indicating ambient UV effects were minor, at most. Grazing by snails (Elimia clavaeformis) tightly controlled periphyton biomass and primary production in all three experiments. Snail densities were not significantly affected by ambient UV, eliminating the possibility of a solar cascade of indirect effects. Snails may be less vulnerable to UV than are soft‐bodied grazers such as chironomids, and periphyton in southern U.S. streams may be adapted to naturally high levels of UV. The results of this study clearly imply that previous reports of UV impacts on periphyton and grazers cannot be generalized.

Crystals have been obtained of succinylated concanavalin A complexed to a novel bidentate synthetic ligand. The crystals are the first example of a lectin with a synthetic multivalent ligand and the first report of crystallization of succinylated concanavalin A. The crystals were obtained by sitting-drop vapour diffusion equilibrating with a solution of 20% polyethylene glycol, pH 5,293.5 K. Crystals are orthorhombic, belonging to space group C222~ with unit-cell dimensions of a = 99.1, b = 127.4, c = 118.9 A. The asymmetric unit contains a dimer, with over 65% of the volume occupied by water. The ligand cross links concanavalin A monomers. Succinylated concanavalin A is known to be a dimcr in solution, yet it is found as the typical concanavalin A tetramer in the crystal. The contacts holding together the tetramer appear extensive and suggest that a fine balance between dimer and tetramers exists. Data to 2.65 A, have been collected and the structure determined by the molecular replacement method.