4s-states

EPR spectra of the native D. africanus Fdlll show an almost isotropic signal centered around g = 2.01 similar to the one observed in proteins containing [3Fe-4S] clusters (72). The temperature dependence of this signal and low-temperature MCD spectra and magnetization properties are identical to the ones reported for D. gigas Fdll (93). Upon one-electron reduction ag = 12 signal develops, characteristic of the [3Fe-4S] state. Two-electron reduction elicits an EPR active species with an axial properties and g-values of 1.93 and 2.05, consistent with the presence of a [4Fe-4S] center. 

Eurther decrease in redox potential, -200 mV, only affects the [3Fe-4S] cluster properties (one electron reduced in the [3Ee-4S] state). The EPR spectrum now shows a g = 12 feature due to a S = 2 spin system. Mossbauer data clearly illustrates this point, ruling out the possibility of a [3Ee-4S] to [4Ee-4S] cluster interconversion. The two [4Ee-4S] clusters remain in the diamagnetic state, indicating that they have a lower redox potential. 

Figure 3.5. Continued. The H2-NAD reaction is inhibited neither in air nor in the presence of CO. C,The possible reactions of hydrogen with the Fe-Fe site of active [Fe]-hydrogenases. In the oxidized state, the bimetallic center shows a S = 1/2 EPR signal, presumably due to an Fe -Fe pair (an Fe -Fe pair cannot be excluded). Whether the unpaired spin is localized on iron (Pierik et al. 1998a) or elsewhere (Popescu and Mtlnck 1999) is not known. Hydrogen is presumably reacting at the vacant coordination site on Fe2 (Fig. 3.1C). After the heterolytic splitting, the two reducing equivalents from the hydride are rapidly taken up by the Fe-Fe site (one electron) and the attached proximal cluster (one electron). Subsequently, the electron is transferred from the proximal cluster to the other Fe-S clusters in the enzyme. Under equilibrium conditions, the proximal cluster in the active enzyme appears to be always in the oxidized [4Fe-4S] state (Popescu and Mtlnck 1999). Protons are not shown.

An abbreviation for Ifigh-potential iron-sulfur protein, which is now regarded as a ferredoxin. In its role as a bacterial electron transfer component, this [4Fe-4S] cluster protein can undergo interconversion to the [4Fe-4S] and [4Fe-4S] states. 

Fig. 4.2 (a) Radial matrix element of the K 4s—>n p transition with n a continous variable. The quantum defect of the 4s state is 2.23. The quantum defects of the np states are 1.71, so the np states fall at the locations shown by the arrows near where the matrix element crosses zero, (b) H 2s-n p radial matrix element as a function of n. Note that the maximum amplitudes of the matrix element occur at integer values of n. ... 

Fig. 4.3 Oscillator strength distribution from the K 4s state to the np states and ep continuum. Note that the vertical scale is logarathmic, not Unear as in Fig. 4.1. The oscillator strengths to the high np states are orders of magnitude smaller than in H.

Figure 7-7. (a) Electron ionization efficiency curve for CH3OH+ ion (m/z = 32) from an Ar/MeOH expansion. Arrow indicates energy corresponding to the first excited 4s state of Ar (11.55 eV). (b) Electron ionization efficiency curve for CH3OH+ ion (m/z = 32) from an He/MeOH expansion. Arrow indicates onset of ionization (10.8 eV). Reprinted with permission from Vaidyanathan et al. 1991b. Copyright 1991 American Institute of Physics. 

The AEs measured by our group are consistent with an ionization mechanism which involves the creation of the excited metastable argon atom Ar(3P2j0) via electron impact. After generation of this Ar 4s state, its excitation energy may then be transferred to the methanol component of the heterocluster. The excitation energies for the formation of Ar 4s 3P2 0 states are 11.55 and 11.72 eV, respectively. 

Because it is B-specific and binds to native human PR, MAb PR-6 was used to study the nature of the association between the A- and B-proteins in the untransformed 8S state, and in the transformed 4S state [43]. This is of interest since there are conflicting models for the molecular interaction of the A- and B-proteins. One model holds that A and B dimerize [48,49] and that they are subunits of a larger holoreceptor the other model holds that A and B exist as separate 8S molecules [54,63]. 

Goddard has also computed GVB wavefunctions for CF, but details are not available.13 However, the GVB wavefunctions were compared with those of CH, and it was shown that additional repulsive interactions with the lone pairs on F should make the 2II state of CF much more strongly bound than the 4S state. (The calculated AE 2.8—2.9 eV.)... 

As is seen, the most noticeable differences between the all-electron and pseudopotential eigenvalues are observed for the molecular orbitals containing the s-type AOs of Pd by symmetry. It appears to be related to the non-core character of the 4s states in the second transition series atoms therefore, one could take into account for the subvalence shells when constructing pseudopotential [17] or to use some extended basis in such cases. 

The states can only be observed if the argon ground state has excited configurations with an occupied d orbital. These states do not obey the weak-coupling approximation. The most prominent transition is to the 3s 3p )4s state at 34.20 eV. It is shown in fig. 11.7(a). The narrow peak at about p=0.25 a.u. is best described by the Hartree—Fock 4d orbital, which is diffuse in coordinate space. This peak could be... 

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