Possible structural change upon

Several other systems have been shown to involve substantial structural changes upon electron transfer, possibly giving rise to potential inversion. They can be divided into two categories. In the first, which includes the reduction of fra/r.v-2,3-dinitro-2-butcnc that we just discussed, electron transfer and structural change are concerted. In the second, they occur in a stepwise manner. Examples of such reactions are given in Section 2.5.7, where the question of the concerted versus stepwise character of electron transfer and structural change in the case of electron transfer-triggered isomerization is also addressed. 

We see that electron transfer can be accompanied by loss of a proton and that E ° may become pH dependent. (See also Eq. 16-18.) Even with cytochrome c, although there is little structural change upon electron transfer, there is an increased structural mobility in the oxidized form.156 This may be important for coupling and could also facilitate associated proton-transfer reactions. For example, it is possible that in some cytochromes the imidazole ring in the fifth coordination position may become deprotonated upon oxidation. This possibility is of special interest because cytochromes are components of proton pumps in mitochondrial membranes (Chapter 18). 

Besides TRIR, it is also possible to employ excited-state resonance Raman spectroscopy (TR or TR2) to characterize excited-states of [Re(L)(CO)3(N,N)] 1 complexes [25, 27, 37, 48, 79, 81, 82], The spectra show bands due to vibrations of the N,N ligand and provide information on its structural changes upon excitation. Picosecond TR3 spectra of Re complexes give very weak signals [37], Measurements on the ns timescale are more informative. In the case of CT and TL states of [Re(py) (CO)3(dppz)]+ and the 3LLCT state of [Re(py-azacrown)(CO)3(bpy)]+ [16, 27], complementary vibrational information was provided by TR3 and TRIR measured in the fingerprint region. 

The most important information obtainable pertains to (i) the number and lattice potential of the hydrogen positions present in the structure (Sects. 4.2.1 and 4.2.2), (ii) the extent of distortion of the water molecules (Sect. 4.2.2), (iii) the strength and arrangement of H-bonds and other intermolecular interactions (Sects. 3.2, 4.2.3, 4.2.4, 4.2.6), (iv) the distinction between true hydrates and pseudohydrates (Sects. 3.1, 4.3), (v) the possible orientational disorder of water molecules and dynamic processes, such as rotational diffusion, involved therein (Sects. 2.2, 2.5, 3.3, 4.2.5, 4.4), and (vi) the phase transitions and other structural changes upon heating or cooling (Sect. 4.8). 

Q.28.14 An unknown dimeric protein, isolated from an oceanic crustacean, is known to have maximum activity at 15°C, but it deactivates at 0°C and 45°C. A researcher investigates possible structural changes with respect to temperature using circular dichroism. He discovers that at 45°C, there is a substantial loss of secondary structure that is irreversible upon cooling however, at 0°C, the secondary structure is almost unchanged and the protein s activity returns when the temperature is increased above 0°C. Propose an explanation for these observations. 

In genercil, cill structural changes vhich occur during a surface reaction (reconstruction, or removal of reconstruction) can have a marked effect on both the rate of adsorption and desorption. Possible candidates for these phenomena bo occur are all metal surfaces vhich can undergo reconstruction upon interaction with a chemicedly active ad-soj ate. Interesting systems here are (besides the cilready known Pt (100) orNi(IIO) faces) Ir(100)/0,00 or W(100)Al and Mo(100)/H. 

---