COMMUNICATIONS z and p anomers (10190) of 2,3,4,6-aceto-1(2-bromoethoxy)-~-galactose in 68% yield (8.3 8). The purified fi anomer could be obtained by flash chromatography. Stereochemical assignments were made by an X-ray crystal structure of the fi anomer. 2,3,4,6-tetraacetyl-l-(2-bromoethoxy)-~-galactose was treated with cyclene to produce the monosubstituted product. The acetate protecting groups were cleaved, and the three carboxyhc acid substituents were added by reaction with bromoacetic acid at pH 10.5. The product 4,7,10-tri(acetic acid)-l-(2-~-galactopyranosylethoxy)-I ,4,7,1O-tetraazacyclododecane was isolated by anion exchange fast performance liquid chromatography (FPLC, detection at 218 nm) in 37% overall yield. Gd3+ or Tb" ions were incorporated into complexes, and these (EGad-Gd and EGad-Tb) were purified by repeated collections on a reverse phase HPLC analytic C,, column with waterlacetonitrile (0-10% gradient) as the eluent (fluorescence detection = 274 nm and i,, = 315 nm) in 70% yield. The high resolution mass spectrum of the isolated solid provided a parent molecular ion (M+Na)+, which exhibited the correct exact mass and the predicted isotope ratios. X-ray data for C,,H,,O,,Br: CAD-4 diffractometer; monoclinic, colorless plates, space group F"2, (no. 4); a total of 5603 reflections measured, 2981 used for refinemen t . Fluorescence experiments [5,6] with EGad: The decay rate (inversely proportional to the lifetime) of the emission peak at 1 . -= 545 nm (& = 460 nm) was measured with a Hitachi f-4500 fluorescence spectrophotometer (2 s delay, 64 scans) in H,O, 50150 H,O/D,O, and D,O. An exponential curve fit (DeltaGraph 3, Delta Point Inc., Monterey, CA) was used to determine the decay rates. The slope of the decay rates versus D,O concentration was compared to the literature value of a slope of 0.2391q to obtain q.In three identical inversion-recovery (IR-NMR) high resolution experiments (Bruker AMX 500, 26°C) EGad(2 mM) was incubated with two different concentrations ofp-gal (1.7 p~ and 5.1 PM) heat-inactivated p-gal(l0 min at 80', 5.1 pM), and EGad alone (2 mM) in phosphate buffer (25 mM, pH 7.3) at 37 "C. Minimal enzyme concentrations were used to reduce potential interactions between the contrast agent and the enzyme. The solutions in a 40 pL round-bottomed NMR tube insert (Wilmad glass) were placed into a 5 mm NMR tube containing CD,CI. Tl measurements were made immediately following mixing and after complete cleavage of the galactopyranose (> 95 % after incubation for 7 days). The data was processed with the program Felix (BIOSYM/Molecular Simulations, San Diego, CA), and the peak heights were fitted to an exponentially rising curve in order to obtain T, (regression: R>0.999).
A combination of variable-temperature 1H NMR spectroscopy and molecular mechanics calculations have been used to probe the factors that determine the rate of macrocyclic ring rotation in benzylic amide [2]catenanes. The results show that the interlocked macrocycle dynamics are governed by a delicate combination of steric effects, intricate inter-macrocyclic arrays of hydrogen bonds, π−π stacking, and T herringbone-type interactions. A cascade of hydrogen-bond ruptures and formations is the principal event during circumvolution (complete rotation of one macrocyclic ring about the other) but is accompanied by a series of cooperative conformational and co-conformational rearrangements that help to stabilize the energy of the molecule. The experimental picture is consistent both when activation energies are measured from the coalescence of NMR signals and when rate constants are directly measured by spin polarization transfer by selective inversion recovery (SPT-SIR) methods. The nature of the circumrotational process means that the precise structure of the diacylaromatic units has a tremendous effect on the frequency of macrocyclic ring rotation: a 2,5-thiophene-based catenane rotates 3.2 million-fold faster than the analogous 2,6-pyridine-based system at room temperature! The polarity of the environment also plays a crucial role in determining the inter-ring dynamics: reducing the strength of the ground-state hydrogen-bonding network by employing hydrogen bond-disrupting solvents (methanol, DMSO) increases the rate of rotation by lowering the activation energy for circumvolution (normally in the region of 11−20 kcal mol-1) by up to 3.2 kcal mol-1. This allows exquisite control over the kinetics of the translational behavior of the individual components of an interlocked molecular system, a key requirement for their development as nanoscale shuttles, switches, and information storage systems.
The catenane layers, in turn, are held together by van der Waals interactions and two intermolecular hydrogen bonds between each catenane and its nearest neighbors in the layers above and below (Fig. 3). The interlayer hydrogen bonds are formed between the amide hydrogen atoms of the inverted amide groups and the carbonyl groups of the cisoid isophthaloyldiamide units. The remaining amide hydrogen atoms and carbonyl groups (NS' and 0 2 ; N5 and 02') appear, by the positioning of the heavy atoms, to be connected through an unusual hydrogen bonding architecture in which the amide proton either binds to the n-cloud of the O=C bond or binds in a nonlinear arrangement to the oxygen atom. Since the H atoms are not resolved in the structure, these two possibilities cannot currently be distinguished.The X-ray crystal structure supports the proposal that the driving force for catenane formation is hydrogen bonding between the newly formed 1,3-diamide units and carbonyl groups on the acid chloride or other intermediates. The stacking of the electron-rich xylylene and electron-poor isophthaloyl rings may also play a supporting role. In the mannerfirst introduced with the Stoddart catenanes, the formation of new functionalgroups in this reaction controls the self-assembly of a topologicully complex product.[31The following communication ["] shows that 1 is not an isolated example of a [2]catenane, but rather the simplest of a diverse family of catenanes derived in one step from aromatic 1,3-dicarbonyl compounds and benzylic diamines. Self-assembly processes currently provide the only viable route to these kinds of topologically complex molecules which exhibit, in both solution and the solid state, a range of structural and dynamic properties not available to topologically trivial molecules. Experimental Procedure[' ' 1 To a stirred solution of triethylamine (1.19 g, 18.9 mmol) in anhydrous chloroform (130mL, stabilized with amylenes not ethanol) [19] under argon were added 3 (0.87 g, 4.3 mmol) in anhydrous chloroform (130 mL) and 4 (0.58 g, 4.3 mmol) in anhydrous chloroform (130 mL) simultaneously, over 30 min by means of motordriven syringe pumps. The mixture was allowed to stir for about 12 h and then filtered. The filtrate was washed with 1 M aqueous hydrochloric acid (3 x 200 mL). then 5 % aqueous sodium hydroxide (3 x 200 mL), and finally water (3 x 200 mL). The organic layer WAS then dried over anhydrous magnesium sulfate and concentrated under reduced pressure to afford 0.23 g (20.1 %)of [2]catenane 1. M.p. 31 5 "C (decomp); 'HNMR (300 MHz. [DJDMSO): 6 = 4.01 (br. s, 16H, CH,), 6.75 (br. s, 16H, p-xylyl H), 7.50 (t, J,. = Js, = 8 Hz, 4H, isophthaloyl 5-H), 7.88 (dd, J4. = 8 Hz, J2.4 = 1 Hz, 8H, isophthaloyl 4-H and 6-H), 8.05 (d, J2.& = 1 Hz. 4H, isophthaloyl 2-H), 8.62 (s, 8H, CONH); "C NMR (75 MHz, [DJDMSO): 6 = 46. 98, 130.03, 130.81, 132.40, 133.78, 138.40, 141.35, 168.94; FAB-MS (mNBA matrix): m/z 1065 [ ( M + H)+], 533 [(M/2 + H)+]. The precipitate from the reaction contains polymers, larger macrocycles, and ca...
151 a) Ricin B was purchased from Sigma b) S. Olsnes. A Pihl. The M o k d u r A d o n s o/ Torinrs and Yirirses.
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