9] Whereas N-C1Ac-~-piperidine-2-carboxylic acid (N-CIAc-L-pipecolic acid) is a good substrate, N-C1Ac-piperidine, N-C1Ac-piperidine-3-carboxylic acid, and N-ClAc-piperidine-4-carhoxylic acid are not hydrolyzed.[lo] A. Yaron, D. Mlynar, Biochem. Biophy.7. Res.
The kinetics of the inclusion of ionene-6,10 (3b) dibromide and of its monomeric model, 1,10-bis(trimethylammonium)decane (5) diiodide, by R-cyclodextrin (1a) were investigated by 1 H NMR spectroscopy in an aqueous solution. The inclusion of the monomer 5 is unusually slow and shows a high activation energy, Ea ) 63 kJ mol -1 , which was attributed to a high steric hindrance for the threading caused by the terminal trimethylammonium groups. Microcalorimetric titration of 5 with 1a revealed a stability constant, KS ) 1540 M -1 , of the inclusion compound. Because of its high kinetic stability, 5‚1a was termed a rotaxane. The inclusion of the ionene polymer by R-cyclodextrin takes days, months, or even years depending on the temperature. The kinetics could be described quantitatively by a Monte Carlo type of computer simulation of a consecutive hopping process of the rings along the polymer chain. The rate constants and activation energies for the polymer 3b were found to be similar to those for the monomer 5. A very high occupation of the polymer 3b (95% of the repeat units) was reached by the thermally induced threading of 1a. The resulting supramolecular structure of one polymer chain and about 65 rings was termed a polyrotaxane, as it can be isolated by GPC or dialysis.
Surface modification of acid-pretreated titanium with 3-aminopropyltriethoxylsilane (APTES) in dry toluene resulted in covalently bonded siloxane films with surface coverage that was relatively controllable by regulating the reaction conditions. A hetero-bifunctional cross-linker, N-succinimidyl-3-maleimidopropionate (SMP), reacted with the terminal amino groups, forming the exposed maleimide groups. Finally, a model cell-binding peptide, Arg-Gly-Asp-Cys (RGDC), was immobilized on the surface through covalent addition of the cysteine thiol groups to the maleimide groups. X-ray photoelectron spectroscopy, radiolabelling techniques, and ellipsometry were used to quantify and characterize the modified surfaces.
α-Cyclodextrin, α-CD, which does not adsorb on mica, can be bound to this surface as a polymeric inclusion compound that is formed by threading many α-CD rings on a cationic polyelectrolyte, poly(decamethylenebipyridinium dibromide). The major driving force of the adsorption of the polymer and the polymeric inclusion compound is ion exchange with surface ions of mica. Somewhat less polymer is adsorbed when it is engaged in the inclusion complex than as pure polymer. The rates of adsorption for both the polymer alone and for the polymer in the inclusion compound depend strongly on the particular cation initially present at the mica surface (Li+ or K+); it is much more rapid with Li+. The equilibrium between complexed α-CD and uncomplexed α-CD, found in solution, is modified upon adsorption of the complex to favor the release of α-CD from the complex. However, the α-CD is only slowly released from the complex on the surface and, as a result, mica-bound complex can be isolated.
The amphiphilic polymers, poly(iminoundecamethylene), poly((N-methylimino)undecamethylene), poly((N,N-dimethylammonio)undecamethylene), and poly((N-methylimino)undecamethylene-N-oxide) were synthesized from nylon-11 by BH 3 reduction of amide groups to amino groups and subsequent methylations. The poly(N-oxide) was obtained by H 2 O 2 oxidation of the polymeric tertiary amine. Since the polymers and their inclusion compounds were water-soluble, threading kinetics of R-cyclodextrin rings onto these polymers could be investigated by 1 H NMR. Kinetics were fitted by an empirical root exponential association function Y ) Y ∞ (1 exp(-sqrt(5.3t/t 90 ))). The time t 90 , necessary to reach 90% completion of the threading process, was taken as a measure of the steric hindrance exerted by the hydrophilic groups along the polymer chain. The vaIues of t 90 decreased by more than 3 orders of magnitude as the diameters of the hydrophilic groups decreased from 5.5 to 4.3 Å.
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