Bridge head ions

At —154 °C the NMR spectrum of the ion shows in the lowest field a signal of two carbons at +70 ppm. In the classical ion structure this j k should be an averaged signal of the secondary cation centre and the adjacent bridge-head carbon. If for the former one takes the chemical shift from the spectrum of an isopropyl cation (—125 ppm) and for the latter — from that of norbomane (+158 ppm). 

This explanation is supported by the behaviour of ck-435 and trans-bicyclo-[3,2,0]heptadienols 436 in fluorosulphonic acid. In this case the signal intensity ratio of the protons of bonded and unbonded olefine fragments, and of the bridgehead and bridge protons of the forming 7-norbomadienyl ion is 2 1.5 1.5 1.0, i.e. in 50% of the cases deuterium is in the bridge-head and in the other 50% — in die unbonded olefine fragment. 

With triethylamine the ion 99a forms a mixture of tricyclohexanes 504 and 505 their deuterium distribution shows the proton (deuterium) to detach in an unusual way — from the methyl groups added to the bridge-head carbons C and C ... 

It is troponin that responds to the increased Ca centration. The Ca ions bind to Tn-C, which then binds to Tn-I and causes a conformational change in Tn-T. This results in a conformational change in tropomyosin that exposes sites on the actin filament for binding with the myosin head of the cross-bridge. The process can be summarised as follows (see also Figure 13.16) ... 

The bridge provides sufficient hydrophilicity and spatial mobility for the ligands to overcome both difficulties of ion diffusion in polymeric media, and steric inhibition of the polymer-bound ligands. The selectivity of the polymeric ligands obeys the Irving-Williams rule in the transition metal series. Separation of the element at the head of this series. Cu(ll) is thu.s readily obtained, provided iron is retained as Fe(II). Since the complexation is pH-dependent, this separation is most effective if carried out at the lowest possible pH (pH 2). The complexation phenomenon is completely reversible, and the rates of metal binding and release are reasonably fast. 

---