Both the arrest and thecreationofmovement are fundamental aspects of dynamics on macroscopic as well as microscopic levels. Brakes and motors dominate the operation of machines, be they those of daily life, such as vehicles and appliances, or those of living systems. like muscles and Ilagellae. On the molecular level motion is thenorm;spontanwus freerotationaroundsinglebonds is thus the rule, not the exception. In machines of ordinary experience, such as automobiles, the brake is often as important as the accelerator. We now report the first molecular analog: a reversible molecular brake. Figure I presents theconcept inbothgeneral andspecific terms. 1 (biako ofl) 3 (brako on) -"W 2 (brake ott) 4 (brake on) Figure 1. Conceptualandaetualdepictionoftheoperationofa molecular brake. With the brakedisengaged. the wheel-(a) representedasa threetoothed gear ( I ) and (b) constructed as a triptycene (2)' -spins rapidly. Engagement ofthe brake (3and4) slowsorstopsrotation. With the actual system, the brake is activated remotely (2 -4) by addition of Hg2+ ion. In the absence of Hgz+ (or other metal ions), the triptycene wheel spins rapidly a t 30 OC, as evidenced by the simplicity of the 'H NMR spectrum of 2 (Figure 2b). wherein by virtue of C, symmetry arising from relatively rapid rotation, the 12 triptycene aromatic protons give rise to only four sets (asterisked) of resonances. Addition of Hg(OzCCF3)z to 2 results in profound changes in the 30 OC (and other) 'H NMR spectrum (Figure 3a). Most noteworthy are the change in the extraordinarychemicalshiftoftheB-ringmethoxyin 2 (6 2.1 to a normal 6 4. I 3 value (not shown) and the obvious broadening of the four resonances attributable to the hydrogens in the three benzo rings of the triptycene. Variable-temperature 'H NMR experiments3 (see Figure 3) document the engagement of the brake. In particular, a t -30 OC (contrast Figure 2),thethreearomaticringsofthetriptyceneare nolongerequivalent becauseofthearrest ofrotation on theNMR . . ( I ) For the use of triptycenaas gean see, inter alia (a) Guenzi. A,: Johnson. C.(2) The 9-ring mcthary group is evidently (see models) in the shielding zone of the lriplycene knzo rings; the B-ring melhory group in 9 is similarly shielded (6 2.W).'(3) See supplementary material for specifis.
Three-dimensional parametric computer aided design (CAD) geometry definitions of the human carotid artery bifurcation are presented for both Y-shaped and tuning-fork models. Drawing on methods largely developed in aerodynamic design, these parametric CAD geometries are deployed within a response surface methodology to systematically map the variation of spatially integrated mean shear stress with the angles of the internal carotid artery (ICA) and the external carotid artery (ECA).Although the absolute values of this shear stress metric agree in some regions of the design space, significant differences exist in the shapes of the response surfaces for the alternative CAD models. The tuning-fork data reveals unexpected results in the location of the lowest value of the metric (at large ICA angles and small ECA angles) and also in the presence of two regions of high metric values -one, unsurprisingly, at large ICA and ECA angles but another exists close to the baseline geometry at the centre of the design space. In contrast, the Y-shaped data is such that a very spiky response surface is produced dominated by changes in the ICA angle. Also, the minimum is located at small ICA and large ECA angles. Finally, evidence is presented for strong recirculation at the outflow of the sinus bulb for small ICA angles that is nonexistent for large ICA angles.
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