This study shows that the role of DNA in the DNA-based enantioselective Diels-Alder reaction of azachalcone with cyclopentadiene is not limited to that of a chiral scaffold. DNA in combination with the copper complex of 4,4'-dimethyl-2,2'-bipyridine (Cu-L1) gives rise to a rate acceleration of up to 2 orders of magnitude compared to Cu-L1 catalysis alone. Furthermore, both the enantioselectivity and the rate enhancement prove to be dependent on the DNA-sequence. These features are the main reasons for the efficient and enantioselective catalysis observed with salmon testes DNA/Cu-L1 in the Diels-Alder reaction. The fact that absolute levels of stereocontrol can be achieved with a simple and weak DNA-binding complex like Cu-L1 is a clear demonstration of the power of the supramolecular approach to hybrid catalysis.

No abstract

The (13)C NMR spectra of two different iodoalkynes, 1-iodo-1-hexyne (1) and diiodoethyne (2), exhibit a strong solvent dependence. Comparisons of the data with several common empirical models, including Gutmann's Donor numbers, Reichardt's E(N)(T), and Taft and Kamlet's beta and pi, demonstrate that this solvent effect arises from a specific acid-base interaction. Solvent basicity measures such as Donor numbers and beta values correlate well with the alpha-carbon chemical shift of 1, but polarity measures such as E(N)(T) and pi do not correlate. The similarity of the solvent effect for 1 and 2 suggests that carbon-carbon bond polarization may not play a role in the change in chemical shift, as previously hypothesized.

A kinetic and structural investigation of DNA-Based asymmetric catalysis using firstgeneration ligands Rosati, Fiora; Boersma, Arnold J.; Klijn, Jaap E.; Meetsma, Auke; Feringa, B.L.; Roelfes, Gerard Link to publication in University of Groningen/UMCG research database Citation for published version (APA): Rosati, F., Boersma, A. J., Klijn, J. E., Meetsma, A., Feringa, B. L., & Roelfes, G. (2009). A kinetic and structural investigation of DNA-Based asymmetric catalysis using first-generation ligands. Chemistry, 15(37), 9596-9605.

The rate-determining deprotonation of 5-nitrobenzisoxazole (Kemp elimination) by hydroxide is efficiently catalyzed by vesicles formed from dimethyldioctadecylammonium chloride (C(18)()C(18)()(+)()). Gradual addition of sodium didecyl phosphate (C(10)()C(10)()(-)()) leads to the formation of catanionic vesicles, which were characterized by cryo-electron microscopy, and their main phase transition temperatures (DSC) and zeta-potentials. Increasing percentages of C(10)()C(10)()(-)() in the vesicular bilayers decrease the catalysis of the Kemp elimination. A detailed kinetic analysis, supported by consideration of substrate binding site polarities and counterion binding percentages, suggest that the catalytic effects of C(18)()C(18)()(+)()/C(10)()C(10)()(-)() catanionic vesicles are primarily determined by the binding of catalytically active hydroxide ions to the vesicular surface area. The formation of neutral microdomains between 10 and 30 mol % of C(10)()C(10)()(-)() in the bilayer, as revealed by DSC, is not apparent from the catalytic effects found for these vesicles. Interestingly, the catalytic effects observed for 50 mol % C(10)()C(10)()(-)() in the catanionic vesicles indicate an asymmetric distribution of C(18)()C(18)()(+)() and C(10)()C(10)()(-)() over the bilayer leaflets. The overall kinetic results illustrate the highly complex mix of factors which determines catalytic effects on reactions occurring in biological cell membranes.

The mechanism of hydroxide ion binding to nonionic surfaces is explored by variation of the properties of the water-aggregate interface and by variation of the type of the aggregate.

Sugar-based gemini surfactants (GSs) display rich pH-dependent phase diagrams and are considered to be promising candidates as gene- and drug-delivery vehicles for biomedical applications. Several sugar-based GSs form vesicles around neutral pH. The vesicular dispersions undergo transitions toward wormlike micelles and spherical micelles at acidic pH, whereas flocculation followed by redispersion upon charge reversal is observed at basic pH. The influence of various amounts of the double-tailed phospholipids DOPC (1,2-dioleoyl-sn-glycero-3-phosphocholine) and DOPE (1,2-dioleoyl-sn-glycero-3-phosphoethanolamine) and of the single-tailed surfactants lyso-PC (1-palmitoyl-2-hydroxy-sn-glycero-3-phosphocholine) and OTAC (octadecyltrimethylammonium chloride) on the phase behavior of GS1 (1,8-bis(N-octadec-9-yl-1-deoxy-D-glucitol-1-ylamino)3,6-dioxaoctane) was determined as a function of pH, in water and in water at physiological ionic strength. The pH corresponding to the phase transitions and the characteristics of the aggregates were determined by means of a combination of physical techniques: static and dynamic light scattering (SLS and DLS), fluorescence spectroscopy, cryo-TEM and diffusion- and (31)P NMR. The results show that the additives affect the phase behavior of the GS1 dispersions in a pH-dependent fashion. In the presence of double-tailed phospholipids, a higher degree of protonation of GS1 must be reached to observe micelle formation, whereas single-tailed surfactants affect these transitions only slightly. In the presence of increasing amounts of lyso-PC, the pH range of flocculation becomes more narrow, indicating the increased hydration of the vesicles. The pH of redispersion after charge reversal is particularly sensitive to the presence of positively charged additives. It is suggested that the cationic headgroups disturb the hydrogen-bond structure of water at the vesicular surface, hampering OH(-) binding. The effect of an increase in ionic strength to physiological values is found to be modest, except for the dispersions containing the positively charged additives.

The phase behavior of a series of carbohydrate-based gemini surfactants with varying spacer lengths was studied using static and dynamic light scattering between pH 2 and 12. Cryo-electron microscopy pictures provide evidence for the different morphologies present in solution. The spacer length of the gemini surfactants was varied from two to 12 methylene units. At near neutral pH, spherical vesicles were obtained for gemini surfactants with a spacer shorter than 10 methylene units, whereas nonspherical vesicles were obtained for spacer lengths of 10 and 12. Upon decreasing the pH, the vesicles underwent transitions toward worm-like micelles and spherical micelles for a spacer length of six and larger, whereas for shorter spacers, these transitions are not observed. For the shortest spacer at low pH, perforated vesicles are observed, and vesicles built from the gemini surfactant with a spacer of four methylene units only underwent a transition toward worm-like micelles. Upon increasing the pH to slightly basic values, flocculation followed by redispersion upon charge reversal was observed up to a spacer length of eight methylene units. The redispersal is explained by hydroxide-ion binding to the uncharged vesicular surface. By contrast, vesicles formed from the gemini surfactants with 10 and 12 methylene units only undergo a transition toward inverted phases. The observations can be understood in terms of the packing parameter.