Gauche-trans isomerization

The measured quadrupolar splittings lead directly to order parameters that are averaged by all motions faster than about a microsecond, the characteristic time scale for this measurement. Elaborate, usually model dependent, calculations are required to ascertain the contributions of any one of the possible acyl chain motions (trans-gauche isomerization, chain librations, etc.) to the measured spectral parameters (10,11). ... 

The vibrational spectroscopy time scale (10-300 x 10 sec) is appropriate for the direct sampling of the fastest motion expected to occur in phospholipid acyl chains, namely trans-gauche isomerization. To date, most Raman ai)d FT-IR studies of phospholipid phase behavior and lipid/protein interaction have focused mainly on qualitative measures of acyl chain organization. For example, the... 

The use of molecular dynamics simulations for chemical reactions with low barriers has been so far limited to isomerization reactions. The most common system studied has been the trans-gauche isomerization of -butane. The potential energy for rotation about the central C—C bond in -butane is shown in Figure 3. The trans conformation is at a dihedral angle defined to be 0° while there are two equivalent, higher energy gauche conformations at... 

The applications of this stochastic picture to the trans-gauche isomerization of -alkanes and the boat-chair isomerization of cyclohexane have demonstrated the usefulness of this approach. In both cases, the transmission coefficients calculated from stochastic dynamics agreed quite well with those from the (later) molecular dynamics calculations, given that there can be an uncertainty in the correct value of the collision frequency to use in comparing with the full molecular dynamics in solution. Stochastic dynamics therefore can allow the rapid calculation of reaction dynamics over a wide range of solvent densities and/or viscosities. 

Stochastic Molecular Dynamics Study of trans-gauche Isomerization Processes in Simple Chain Molecules. 

Let us now discuss the dynamics of annealed sanqiles in more detail. Figure 25 shows a drastic motional decrease for the crystalline component (open symbols) at the melting point. Apparently, all intermolecular motions abruptly cease below T . Similar is true for the local motions of the phenyl rings (see Fig. 22). Thus, only slow trans-gauche isomerization of the spacer (10 s < < 10 s) can be detected in... 

Fig. 3.3 Trans/gauche isomerism (a) a trans bond viewed normally to the plane of the three bonds required to define it (b) a view of (a) from the right-hand side, looking almost end-on to the trans bond (c) and (d) nearly end-on views of left- and right-handed gauche bonds, respectively. (Reproduced from The Vibrational Spectroscopy of Polymers by D. I. Bower and W. F. Maddams. Cambridge University Press 1989.)...

The rate of trans-gauche isomerization is evaluated using the following state function of the dihedral angles [49] ... 

---