Crank-like motions

One explanation of the cooperativity of conformational transitions postulates that conformational transitions occur in pairs. Hie two transitions involved occur nearby in space and time. In this manner, the ends of chains are not required to move. Figure 7 shows several proposals of this type. Figure 7a shows the crankshaft motion proposed by Schatzld [44]. Figure 7b, c shows two crank-like transitions proposed by Helfand [78, 85]. The crankshaft motion occurs about two parallel, collinear bonds. Thus the chain aids can remain precisdy stationary. As discussed in Sect 42, the crankshaft motion would result in an activation energy of two barrier hdg ts. This is inconsistent with... 

The acromioclavicular joint is a planar joint. Occasionally there is an intra-articular plate between the surfaces that acts much as a disc. The acromioclavicular joint permits motion of the lateral end of the clavicle in an anteroposterior or cephalad-caudal direction, as well as rotation. More motion is possible at the lateral end of the clavicle because of the crank-like shape of the bone. 

This is enlightening but still a number of questions are left to be answered. A translation of the tails reduces, but does not eliminate, the enormous frictional resistance of the tails. The motion as illustrated is still not local. How does full localization occur Furthermore, while many cooperative crank-like pairs of transitions were observed, individual conformational transitions were also observed. The implication is that swinging of the tails between initial and final states can occur. How Finally, why is the activation energy one barrier height even when two transitions occur ... 

The high friction associated with the large aryl groups keeps them fairly stationary during a conformational change in the butylene segment. This enhances the need for transitions to occur in a very localized fashion. The NMR looks at orientational relaxation of the C-H bonds in the —CH2—CH2—CH2—CH2— sequence. By deuteration of the central two carbons the authors show that the central C-H bonds reorient more rapidly than the outer two. A crank-like counterrotational transition of the first and third C-C bonds does, indeed, reorient the two central C-H bonds without reorienting the two outer ones. The outer ones translate in such a motion, just like the tails. 

The multi-stacked actuator that is introduced in Chapter 7 can generate linear motion like natural muscles. Consequently, it is necessary to transfer the linear motion into a rotational one. Therefore, a simple slider crank mechanism is used to convert the linear motion of the multi-stacked actuator to rotation. The Maxwell stress and the active elastic force of the actuator cause the piston to translate along a vertical axis. This action causes the link to rotate by an angle 0 as shown in Fig. 9.27. 

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