Spin-state equilibrium

The result Eq. (13) or Eq. (14) describes both gradual and abrupt transitions, an example for a specific set of parameter values being displayed in Fig. 2. Here, the HS fraction Uh is shown as a function of temperature for the values of JJRT attached to the individual curves. The curve characterized by J2 = 0 corresponds to the case of a true spin-state equilibrium (zero interaction between the molecules), whereas increasingly higher values of produce a gradually... 

Perturbation of a spin-state equilibrium by ultrasound results in the absorption of sound, the absorption coefficient a (neper cm ) being determined as a function of frequency /(Hz) according to ... 

Fig. 10. Schematic potential energy diagram for the spin-state equilibrium .t, in the iron(II)...

Step 1. The substrate, RH, associates with the active site of the enzyme and perturbs the spin-state equilibrium. Water is ejected from the active site and the electronic configuration shifts to favor the high-spin form in which pentaco-ordinated heme Fe3+ becomes the dominant form-binding substrate. In this coordination state, Fe3+ is puckered out and above the plane in the direction of the sixth ligand site. The change in spin state alters the redox potential of the system so that the substrate-bound enzyme is now more easily reduced. 

Locuson, C.W., Gannett, P.M. and Tracy, T.S. (2006) Heteroactivator effects on the coupling and spin state equilibrium of CYP2C9. Archives of Biochemistry and Biophysics, 449, 115-129. 

The far-infrared spectrum of Fe(EtPhDtc)3 as a function of pressure shows that the intensity of the band assigned to the 2 V2 state increases relative to that assigned to the 6A t state upon increasing pressure (93b) (38). For the Fe(n-Pr2 -Dtc)3 complex, the Fe—S vibration at 367 cm"1 was assigned to the low-spin-state (6i4j) isomer. On the basis of these results, the spin-state equilibrium was adopted as the true model with a spin-state interconversion rate lower than the vibrational time scale ( 10" 3 sec). 

These complexes are paramagnetic and posses a singlet-triplet spin state equilibrium in solid and solution phases30, Pignolet, L. H., Patterson, G. S., Weiher, J. F., Holm, R. H. Inorg. Chem. 13, 1263 (19741... 

A considerable expansion of the use of the spin-state equilibrium as a probe of allosteric effects is desirable. Thus it should be possible to use it in assisting studies on oxidases such as trytophan pyrolase and peroxidase which are mainly high-spin iron(III) enzymes. In a later section we shall show how the discussion can be extended to proteins containing haem a. 

In the absence of substrate, the heme iron of P450cam is six-coordinate and low spin. Camphor binding expels all water molecules from the P450cam active site, and the iron becomes five-coordinate and high spin. 456 linear free-energy relationship has been observed between the P450cam spin state equilibrium and heme redox potential. The presence... 

T2 spin-state equilibrium. The two NMR spectra become only similar when the temperature is lowered to 233 K, where iron is essentially in its diamagnetic low-spin state. The localization of Fe" in the pseudo-octahedral site is further confirmed by cyclic voltammetry which shows the [RFe(L36)3] helicates being oxidized in a reversible one-electron process at Ei/2 0.82 V (vs SCE), independent of the nature of the R " ion (Piguet et al., 1997b). 

Rivera et have used variable temperature NMR hyperfine chemical shifts to probe the electron spin density in a low-spin Fe(III) porphyrinate that models the hydroperoxide intermediate (Felll-OOH) of heme oxygenase. Variable temperature NMR measurements, in conjunction with ENDOR, indicated that the ruffled (dxy) and planar (dxy) spin states are in thermal equilibrium. The authors suggest that this kind of spin-state equilibrium may be important for enzymatic function. The same group used NMR and pulsed ENDOR to characterize the electronic and spin states of C-labeled porphyrin models of the low-spin Fe(III) hydroperoxide intermediate of heme oxygenase. The ferric hydroperoxide intermediate in heme catabolism has also been studied by Caig-nan et using NMR. 

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