Polarization collapse

Simultaneously with the refinement of the EC structure, additional data on the conformation of taxol bound to MT came from ss-NMR REDOR measurements [84]. As introduced above, the REDOR experiment exploits the dipolar interaction between nuclear spins to determine distances between atoms closer than 12 A. A PTX analogue with suitable spin labeling for REDOR was prepared and complexed to MT. Labeling of the amide carbonyl and C3 methine with 13C, and position p- of the 2-O-benzoyl with 19F (Fig. 9, compound 3), allowed the measurement of the two 19F-13C distances I and II (Fig. 9), that could serve, in principle, to discriminate among candidates for PTX conformation. The respective values of 9.8 A 0.5 A and 10.3 A 0.5 A were interpreted by the authors as supportive of the hydrophobi-cally collapsed polar conformation for PTX [84]. 

It should be noted that in 2004, Ganesh et al. and in 2000, Snyder et claimed in their articles that only the T-conformation was the conformation to be adopted by paclitaxel when binding to tubulin, whereas collapsed polar and nonpolar conformations do not work. Their conclusion was supported by computer simulation and NMR experiments of semisynthetic taxoids, in which the C-4 alkyl terminal and the C3 -Ph ortfio-position were linked. ... 

The observation (Porter ef a ., 1972) that added BrCCla almost completely suppresses the polarization of the olefin, while leaving the polarization of trans-4 unalfected, points to the secondary radical pair as the principal immediate precursor of a-methylstyrene. A rate constant for the decomposition of thediazenyl radical of 10 -10 sec has been estimated. Cage collapse and free-radical formation are also thought to occur and appropriately polarized products have been identified (see above). 

Dendrimers can also be prepared with an inverse relationship between their hydrophobic and hydrophilic constituents, i.e. with a hydrophobic periphery and a hydrophilic interior. They can then behave as reverse micelles and are able to concentrate polar molecules from solutions of nonpolar solvents. The shape of these molecules, when dissolved in a solvent that matches the hydrophobic nature of the periphery, is spherical with chain-ends extended towards the solvent. The interior may then collapse to a minimum volume, so that unfavourable interactions that might result from penetration by solvent molecules are minimized. 

Molecules with polar atoms or charged groups can form intramolecular interactions such as internal H-bonds or cation-rr interactions. As a consequence energy minimization in vacuo often results in electrostatically collapsed conformations due to the dominance of these intramolecular interactions. Several compounds in... 

The competition between ion-pair and radical-pair collapse can be predictably modulated by the polarity of the solvent and by the addition of inert salt. 

On the other hand, in the polar acetonitrile diffusive separation leads to the free cation radical and NO which then undergo (homolytic) collapse. The sub-... 

The explanation for the dichotomy between aromatic nitration versus de-alkylative oxidation in equation (84) lies in the dynamics of ion-radical triad (which is predictably modulated by solvent polarity and added inert salt). For example, the nonpolar dichloromethane favors aromatic nitration via a radical-pair collapse of ArH+, NO, 239 i.e.,... 

Under even more intense photoexcitation ( 10mJ/cm2), the coherent A g and Eg phonons of Bi and Sb exhibit a collapse-revival in their amplitudes (Fig. 2.10) [42,43], This phenomenon has a clear threshold in the pump density, which is common for the two phonon modes but depends on temperature and the crystal (Bi or Sb). At first glance, the amplitude collapse-revival appears to be analogous to the fractional revival in nuclear wavepackets in molecules [44,45]. However, the pump power dependence may be an indication of a polarization, not quantum, beating between different spatial components of the coherent response within the laser spot [46],... 

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