Electron-water couplings, time

Short-time water caging and transient electron-OH couplings in liquid phase... 

Short-time water caging and transient electron-OH couplings in liquid phase B. Brozek-Pluska, A. Hallou, D. Glijer, B. Charles, Y. Gauduel 233... 

Fig. 2 shows calculated Tim and Xm values for the 1 and 2 flash experiments assuming the PRE results from one exchangeable H2O. xm is in the range 10-20 ps this is the expected range for protolysis reactions of water coordinated to (+3) or (+4) ions. Measured Tim values in S2 range from <10 ps when T<17°C to near 50 ps at 30°C, with an extremely rapid temperature dependence in this region, which is believed to result from very small electron exchange couplings within the OEC (3). Measured Tim values have been used to calculate electron spin relaxation times xsi of the Mn ion that acts as the H relaxation trap (Fig. 3). Calculations assume an inner sphere dipolar interaction to a single H2O. Four situations were considered (1) that the trap consists of a Mn(lll) monomer (S=2, pe=4.9 BM), (2) a manganese(IV) monomer (S=3/2, pe=3.9 BM), and...

This impressive reaction is catalyzed by stearoyl-CoA desaturase, a 53-kD enzyme containing a nonheme iron center. NADH and oxygen (Og) are required, as are two other proteins cytochrome 65 reductase (a 43-kD flavo-protein) and cytochrome 65 (16.7 kD). All three proteins are associated with the endoplasmic reticulum membrane. Cytochrome reductase transfers a pair of electrons from NADH through FAD to cytochrome (Figure 25.14). Oxidation of reduced cytochrome be, is coupled to reduction of nonheme Fe to Fe in the desaturase. The Fe accepts a pair of electrons (one at a time in a cycle) from cytochrome b and creates a cis double bond at the 9,10-posi-tion of the stearoyl-CoA substrate. Og is the terminal electron acceptor in this fatty acyl desaturation cycle. Note that two water molecules are made, which means that four electrons are transferred overall. Two of these come through the reaction sequence from NADH, and two come from the fatty acyl substrate that is being dehydrogenated. 

We have seen that copper(II) is a slowly relaxing metal ion. Magnetic coupling of copper to a fast relaxing metal ion increases the electron relaxation rate of copper, as clearly shown by the NMRD profiles of tetragonal copper(II) complexes reacting with ferricyanide (105) (Fig. 38). The electron relaxation time, estimated from the relaxation rate of the water protons coordinated to the copper ion, is 3 x 10 ° s, a factor of 10 shorter than in the absence of ferricyanide. 

The relaxivity enhancement of water protons in the aqueous solutions of paramagnetic complexes arises from time fluctuation of the dipolar coupling between the electron magnetic moment of the metal ion and the nuclear magnetic moment of the solvent nuclei (13,14). The dipolar interaction... 

---