Flip-flop coordination

Phospholipids, which are one of the main structural components of the membrane, are present primarily as bilayers, as shown by molecular spectroscopy, electron microscopy and membrane transport studies (see Section 6.4.4). Phospholipid mobility in the membrane is limited. Rotational and vibrational motion is very rapid (the amplitude of the vibration of the alkyl chains increases with increasing distance from the polar head). Lateral diffusion is also fast (in the direction parallel to the membrane surface). In contrast, transport of the phospholipid from one side of the membrane to the other (flip-flop) is very slow. These properties are typical for the liquid-crystal type of membranes, characterized chiefly by ordering along a single coordinate. When decreasing the temperature (passing the transition or Kraft point, characteristic for various phospholipids), the liquid-crystalline bilayer is converted into the crystalline (gel) structure, where movement in the plane is impossible. 

Silylene complexes with two intramolecular coordinating arms can participate in dynamic behavior, involving a flip-flop alternating coordination, illustrated in... 

Fig. 16. The rope jumping motion of the N04 crown of the functionalized calix[4]arene 10 on the coordinated Ag(I) ion. The flip-flop movement results from the fact that the N04 ring has too small an aperture to encompass the metal ion. In a CH2CI2/CH3OH solution (4 1 v/v, deuterated solvents) the oscillating motion slows down with temperature and stops at -20 °C...

The polymer molecule then flip-flops back and forth between these thiol groups with the new monomer being coordinated at the free site. This mechanism is illustrated in Fig. 2. 

Figure 10 Flip-flop mechanism of anionic coordination polymerization of oxirane involving AIR3.

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