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211.
It was recently shown experimentally that 5-(guanidiniocarbonyl)-1H-pyrrole-2-carboxylate 1, a self-complementary zwitterion, dimerizes even in water with an unprecedented high association constant of K = 170 M(-1) (J. Am. Chem. Soc. 2003, 125, 452-459). To get an insight into the importance of the various noncovalent binding interactions and of their interplay (electrostatic interactions, hydrogen binding, cooperative effects), we employ density functional theory to study the stability of several "knock-out" analogues in which single hydrogen bonds within these multiple point binding motif are switched off by replacing N-H hydrogen-donor groups with either methylene groups or an oxygen ether bridge. The influence of a highly polar solvent on the dimer stabilities is also examined. These calculations reproduce the experimental data for zwitterion 1. A comparison of 1 with the arginine dimer shows that the energy contents of the monomers also significantly influence the dimer stabilities. The analysis of the various "knock-out" analogues reveals as a main conclusion that simple models either based just on hydrogen-bond counting or on the assumption that the charge interaction by itself is the main and dominant factor fail to explain the stability of such self-assembled dimers. Our computations show that the hydrogen-bond network, the electrostatic attraction, and also their mutual interactions are responsible for the high stability of zwitterion 1. 相似文献
212.
We report the use of a variety of polyelectrolyte multilayers (PEMs) as selective skins in composite membranes for nanofiltration (NF) and diffusion dialysis. Deposition of PEMs occurs through simple alternating adsorption of polycations and polyanions, and separations can be optimized by varying the constituent polyelectrolytes as well as deposition conditions. In general, the use of polycations and polyanions with lower charge densities allows separation of larger analytes. Depending on the polyelectrolytes employed, PEM membranes can remove salt from sugar solutions, separate proteins, or allow size-selective passage of specific sugars. Additionally, because of the minimal thickness of PEMs, NF pure water fluxes through these membranes typically range from 1.5 to 3 m3/(m2 day) at 4.8 bar. Specifically, to separate sugars, we employed poly(styrene sulfonate) (PSS)/poly(diallyldimethylammonium chloride) (PDADMAC) films, which allow 42% passage of glucose along with a 98% rejection of raffinose and a pure water flux of 2.4 m3/(m2 day). PSS/PDADMAC membranes are also capable of separating NaCl and sucrose (selectivity of approximately 10), while high-flux chitosan/hyaluronic acid membranes [pure water flux of 5 m3/(m2 day) at 4.8 bar] may prove useful in protein separations. 相似文献
213.
Merlin Heß Tom E. Stennett Felipe Fantuzzi Rüdiger Bertermann Marvin Schock Marius Schfer Torsten Thiess Holger Braunschweig 《Chemical science》2020,11(34):9134
Sequential treatment of [Rh(COE)2Cl]2 (COE = cyclooctene) with PiPr3, alkyne derivatives and tBuN BMes (Mes = 2,4,6-trimethylphenyl) provided functionalized rhodium η4-1,2-azaborete complexes of the form (η4-azaborete)RhCl(PiPr3). The scope of this reaction was expanded to encompass alkynes with hydrogen, alkyl, aryl, ferrocenyl, alkynyl, azaborinyl and boronate ester substituents. Treatment of these complexes with PMe3 led to insertion of the rhodium atom into the B–C bond of the BNC2 ring, forming 1-rhoda-3,2-azaboroles. Addition of N-heterocyclic carbenes to azaborete complexes led to highly unusual rearrangements to rhodium η2,κ1-allenylborylamino complexes via deprotonation and C–N bond cleavage. Heating and photolysis of an azaborete complex also led to rupture of the C–N bond followed by subsequent rearrangements, yielding an η4-aminoborylallene complex and two isomeric η4-butadiene complexes.Rhodium η4-azaborete complexes can be transformed into a variety of species with ring-opened, BN-containing ligands by treatment with Lewis bases. 相似文献
214.
215.
Rehm T Stepanenko V Zhang X Würthner F Gröhn F Klein K Schmuck C 《Organic letters》2008,10(7):1469-1472
The self-assembly of the l-alanine derived zwitterion 3 leads to the formation of soft vesicles in solution even though this surprisingly small molecule does not possess the classical amphiphilic features of other vesicle-forming monomers. 相似文献
216.
Schmuck C Rupprecht D Junkers M Schrader T 《Chemistry (Weinheim an der Bergstrasse, Germany)》2007,13(24):6864-6873
Covalent fusion of two artificial recognition motifs for arginine and aspartate resulted in a new class of ditopic RGD receptor molecules, 1-4. The two binding sites for the oppositely charged amino acid residues are linked by either flexible linkers of different length (in 1-3) or a rigid aromatic spacer (in 4). These spacers are shown to be critical for the complexation efficiency of the artificial hosts. If the linkers are too flexible, as in 1-3, an undesired intramolecular self-association occurs within the host and competes with, and thereby weakens, substrate binding. The rigid aromatic linker in 4 prevents any intramolecular self-association and hence efficient RGD binding is observed, even in buffered water (association constant of K(a) approximately 3000 m(-1)). A further increase in hydrophobic contacts, as in host 16, can complement the specific Coulomb attractions, thereby leading to an even more stable complex (Ka=5000 m(-1)). The recognition events have been studied with NMR spectroscopy, UV/Vis spectroscopy, and fluorescence titrations. 相似文献
217.
A combinatorial library of 625 octapeptides was screened for their efficiency to catalyze phosphorester cleavage in water. Octapeptides such as 1c, which contains a catalytically active histidine and two cationic residues (Gua) as potential anion-binding sites, show substrate hydrolysis in water. Metal-free catalysis with a rate enhancement of up to a factor of 175 over the uncatalyzed background reaction is observed. 相似文献
218.
A Tailor‐Made Specific Anion‐Binding Motif in the Side Chain Transforms a Tetrapeptide into an Efficient Vector for Gene Delivery
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Mao Li Stefanie Schlesiger Prof. Shirley K. Knauer Prof. Carsten Schmuck 《Angewandte Chemie (International ed. in English)》2015,54(10):2941-2944
Arginine‐rich cell‐penetrating peptides are widely utilized as vectors for gene delivery. However, their transfection efficacy still needs to be optimized. To accomplish this, guanidinocarbonylpyrrole groups, which are tailor‐made anion binding sites, were introduced into the side chains of tetralysine to obtain the peptide analogue 1 . In contrast to the common strategy of adding a lipophilic tail to peptide vectors, this novel method most likely enhances transfection efficacy through more specific interactions between the binding motifs and DNA or the cell membrane. Tetrapeptide analogue 1 is thus the smallest peptidic transfection vector that has been reported to date. The transfection efficacy of 1 , which on average has less than two positive charges under physiological conditions, is even better than that of polyethylenimine (PEI). Furthermore, 1 exhibits only negligible cytotoxicity, which makes it an interesting candidate for further development. 相似文献
219.
Jong‐Bum Kim Wenxi Huang Matthew D. Miller Gregory L. Baker Merlin L. Bruening 《Journal of polymer science. Part A, Polymer chemistry》2003,41(3):386-394
Although atom transfer radical polymerization (ATRP) is often a controlled/living process, the growth rate of polymer films during surface‐initiated ATRP frequently decreases with time. This article investigates the mechanism behind the termination of film growth. Studies of methyl methacrylate and methyl acrylate polymerization with a Cu/tris[2‐(dimethylamino)ethyl]amine catalyst system show a constant but slow growth rate at low catalyst concentrations and rapid growth followed by early termination at higher catalyst concentrations. For a given polymerization time, there is, therefore, an optimum intermediate catalyst concentration for achieving maximum film thickness. Simulations of polymerization that consider activation, deactivation, and termination show trends similar to those of the experimental data, and the addition of Cu(II) to polymerization solutions results in a more constant rate of film growth by decreasing the concentration of radicals on the surface. Taken together, these studies suggest that at high concentrations of radicals, termination of polymerization by radical recombination limits film growth. Interestingly, stirring of polymerization solutions decreases film thickness in some cases, presumably because chain motion facilitates radical recombination. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 386–394, 2003 相似文献
220.