The Pr(i) sidechains of two adjacent valine residues, V8 and V44, define the surface of the rubredoxin from Clostridium pasteurianum and control access to its Fe(S-Cys)4 active site. To assess the effect of systematic change of the steric bulk of the alkyl sidechains, eight single and three double mutant proteins have been isolated which vary G (H), A (Me), V (Pr(i)), L (Bu(i)) and I (Bu(s)) at those positions. X-ray crystal structures of the Fe(III) forms of the V44A and V44I proteins are reported. Positive shifts in reversible potential of up to 116 mV are observed and attributed to increased polarity around the Fe(S-Cys)4 site induced by (1) changes in protein backbone conformation driven by variation of the steric demands of the sidechain substituents and (2) changes in solvent access to the side-chains of ligands C9 and C42. Data for the V44A mutant show that a minor change in the steric requirements of a surface residue can introduce a NH...Sgamma hydrogen bond at the active site and lead to a shift in potential of + 50 mV.
Previous studies of the reactivity properties of the complexes [Fe2S2(SR)4]2~a nd [Fe4S4(SR)4]2" have demonstrated the occurrence of thiolate ligand substitution reactions with retention of Fe2S2 and Fe4S4 core structures. Described in this report are reactions which provide chemical and electrochemical interrelationships of 1-Fe, 2-Fe, and 4-Fe synthetic analogues of the active sites of iron-sulfur proteins. These reactions include (i[Fe4S4(SPh)4]3~by electrochemical reactions; and (iii) tetranuclear dianion-tetranuclear trianion chemical reduction, [Fe4S4(SR)4]2" -* [Fe4S4(SR)4]3". These transformations are correlated with prior synthetic and ligand-exchange processes to provide an overall reaction scheme starting with a simple iron(II) salt, progressing through intermediate 1-Fe and 2-Fe species and terminating in the 4-Fe complexes with cubane-type stereochemistry. Reaction iii was accomplished using sodium acenaphthylenide in hexamethylphosphoramide as reductant. The product trianion was obtained as oxygen-sensitive, crystalline Me4N+ or Et4N+ salts and is an analogue of the 4-Fe active sites of reduced ferredoxin proteins. With its isolation synthetic analogues of five of the seven known physiological proteins have been obtained in substance.
The intensely blue salts B+
[MoO(SAr)4]- (B+ = quaternary cation; Ar =
phenyl, p-tolyl) have been isolated. Magnetic susceptibility, e.s.r.,
conductance and electronic spectral data indicate that the complexes are
essentially mononuclear both in the solid state and in solution. The solution
electronic spectra exhibit an intense maximum at 598 nm, which is tentatively
assigned to a sulphur-to-molybdenum charge- transfer transition.
An initial survey of the 95Mo
n.m.r. properties of a range of 12 polyoxomolybdates
is presented. The species resonate in the range - 18 to + 140 ppm. The observed line widths generally exceed 100 Hz for
six-coordinate molybdenum sites of low symmetry, but can be narrow (< 20 Hz
at 20�) for four-coordinate sites. Aqueous solutions of [Mo7O24]6-
at pH 6.1 exhibit exchange between Mo- and Mo7-based species.
Solutions of α-[Mo8O26]4- in MeCN feature a single narrow resonance (δ16, Wh/2 23 Hz at 20�) assigned to the
tetrahedral capping sites, while two broad resonances (δ19, Wh/2 240 Hz; δ109, Wh/2 C. 270 HZ) are seen in
equivalent solutions of β-[Mo8O26]4-. In
the cases cited above, 17O n.m.r. is used to confirm the integrity
of sample solutions and assists in correction of the 95Mo n.m.r.
properties of β-[Mo8O26]4 reported in a
previous communication.
The orange-red complexes (R4N)2
[Mo2O4(SPh)4] (R =
Me, Et), (Et4N)2 [Mo2O4(SCH2CH2-
CH2S)2] and (Et4N)2 [Mo2O4{SCH2(CHOH)2CH2S}2]
have been synthesized. Crystalline (Me4N)2 [Mo2O4(SPh)4], space group P1, contains two [(μ-O)2(OMo(SPh)2)2]2-
complexes in each of which two approximately square-pyramidal (Ob)2Mo(Ot)(SPh)2 coordination spheres (b, bridging; t,
terminal) share an Ob--Ob edge with cis-bent
stereochemistry. Important average dimensions are Mo-Mo 2.61, Mo-S 2.45, Mo-Ot
1.68, Mo-Ob 1.94 Ǻ, Mo 0.71 Ǻ above the (Ob)2S2
basal plane. There occur two different orientations of S-C vectors, relative to
the Mo-Mo vector, associated with two clearly distinct classes of S-Mo-S
angles, 81.9�0.5 and 73.8�0.4°. The latter abnormally small angle, concomitant
with an interligand S--S distance of 2.94 Ǻ, occurs only in the
conformation where S--S σ-orbital overlap is possible, and suggests the
existence of some interligand bonding. Crystal data: a 12.376, b 14.513, c
11.307 Ǻ, α 104.00, β 99.55, γ 71.32°, cell contents 2xMo2S4O4N2C32H44,
3207 reflections, R 0.033.
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