High spins temperature-dependent measurements

Fe(isoxazole)6]2+ and [Fe(isoxazole)4(H20)2]2+, together with four non-co-ordinated tetrafluoroborate anions (Fig. 15). The structural details for the low-field trans [Fe(isoxazole)4(H20)2]2+ are consistent with a high-spin Fe(II) chromophore (average Fe-O=2.09 A and Fe-N=2.19 A), whereas those for [Fe(isoxazole)6]2+ show a marked temperature dependence (average Fe-N=1.98 A at 115 K and 2.17 A at 231 K) related to the reversible low-spin to high-spin transition. From magnetic susceptibility measurements, the transition temperature has been found to be T1/2=137 K. 

Removal of calcium from HRP C has a significant effect not only on enzyme activity and thermal stability, but also on the environment of the heme group. The calcium-depleted enzyme has optical, EPR, and H NMR spectra that are different from those of the native enzyme (211). Temperature dependence studies indicate that the heme iron exists as a thermal admixture of high- and low-spin states. Kinetic measurements at pH 7 show that ki, the rate constant for compound I formation, is only reduced marginally from 1.6 0.1 x 10 to 1.4 x lO M s , whereas k, the rate constant for compound II reduction, is reduced from 8.1 1.6 x 10 to 3.6 x lO M s (reducing substrate p-aminobenzoic acid), 44% of its initial value (211). There can be little doubt that this is the main reason for the loss of enzyme activity on calcium removal. 

Figure 17 shows the temperature dependence of the lateral forces measured at the scanning rate of 10 nms for a high-density PMMA brush (a = 0.8 chains nm ) and an equivalent spin-coated film. The a-relaxation process was clearly observed accompanying a small peak, which was assigned to a surface /1-process. They commented that the surface molecular motion of the brush layer possibly differs from that of the spin-coated film but that it was rather difficult to conclude this because of shghtly scattered data. 

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