SummaryMultivalent biomolecular interactions allow for a balanced interplay of mechanical stability and malleability, and nature makes widely use of it. For instance, systems of similar thermal stability may have very different rupture forces. Thus it is of paramount interest to study and understand the mechanical properties of multivalent systems through well-characterized model systems. We analyzed the rupture behavior of three different bivalent pyridine coordination complexes with Cu2+ in aqueous environment by single-molecule force spectroscopy. Those complexes share the same supramolecular interaction leading to similar thermal off-rates in the range of 0.09 and 0.36 s−1, compared to 1.7 s−1 for the monovalent complex. On the other hand, the backbones exhibit different flexibility, and we determined a broad range of rupture lengths between 0.3 and 1.1 nm, with higher most-probable rupture forces for the stiffer backbones. Interestingly, the medium-flexible connection has the highest rupture forces, whereas the ligands with highest and lowest rigidity seem to be prone to consecutive bond rupture. The presented approach allows separating bond and backbone effects in multivalent model systems.

The adsorption thermodynamics of 4-(dimethylamino)pyridine (DMAP) and its five divalent derivatives di-DMAP-n (2 ≤ n ≤ 6) with gradually increasing methylene-spacer lengths n binding to planar gold surfaces has been studied by surface-enhanced Raman spectroscopy (SERS) and density functional theory (DFT). SERS intensities of the totally symmetrical breathing mode of the pyridine ring at approximately 1007 cm −1 are used to monitor the surface coverage of the DMAP and di-DMAP-n ligands on gold surfaces at different concentrations. The equilibrium constant as a measure of the binding affinity is obtained from these measurements by using a modified Langmuir isotherm. Due to multivalent binding to the gold substrate, a characteristic enhancement of the binding affinity of di-DMAP-n compared to the monovalent DMAP is observed for all divalent species. First principles calculations of the di-DMAP-n ligands on an ideal Au(111) surface model as well as step terrace models have been performed to understand the adsorption structures and the multivalent binding enhancements. Furthermore, Raman spectra of the adsorbed molecules have been studied by first principles calculations to correlate the binding affinities to experimentally determined adsorption constants. The joint experimental and theoretical investigation of an oscillatory behavior of the binding affinity as a function of the methylene-spacer length in mono-and divalent 4-(dimethylamino)pyridines reveals that the molecular architecture plays an important role for the structure-function interplay of multivalently bound adsorbates.

In a brief exploratory study we investigated Heck reactions of several alkenyl nonaflates with a protected dehydroalanine derivative. Whereas simple alkenyl nonaflates provided the expected 1,3-dienes in good to excellent yields, more complex oxa-bridged bicyclic substrates gave poorer results. Nevertheless, this approach is of interest for the synthesis of tetradehydroamino acids. Palladium-catalyzed carbonylations of alkenyl nonaflates employing molybdenum hexacarbonyl as convenient carbon monoxide source leading to esters or to carboxylic acids were also studied. Microwave conditions seem to be superior to purely thermal conditions. With an oxygenbridged bicyclic alkenyl nonaflate the expected carboxylic acid was isolated, however, the formation of a tropone derivative as a result of an acid-promoted fragmentation was also observed.

Syntheses of several new functionalized cage compounds are described. The key steps of the reaction sequence are addition of lithiated methoxyallene 2 to cage diketone 1, preparation of dehydrated intermediate 5, and its ozonolysis leading to diester 7. Alternatively, 5 could be hydrolyzed to provide cage compound 6 with a bisenone subunit. Via diol 9 chiral crown ether 11 could be prepared in low yield. A first stereoselective epoxidation of chalcone 12 with tert-butyl hydroperoxide in the presence of 11 gave the epoxide 13 in reasonable yield, but with a low level of enantioselectivity.

No abstract