Comparison of hydroxycarboxylato imidazole molybdenum(iv) complexes and nitrogenase protein structures: indirect evidence for the protonation of homocitrato FeMo-cofactors

Abstract

Glycolato and R,S-lactato imidazole molybdenum(iv) complexes [Mo3SO3(glyc)2(im)5]·im·H2O (1), Na2[Mo3SO3(R,S-lact)3(im)3]·10H2O (2), and [Mo6O10(R,S-lact)2(im)10]·16H2O (3) have been isolated and characterized (H2glyc = glycolic acid, H2lact = lactic acid, im = imidazole). α-Alkoxy coordination with molybdenum [Mo-Oα-alkoxy 1.993(7)av Å] in 1 and 2 showed obvious differences to their counterpart with α-hydroxy coordination [MoIV3S4(PPh3)3(Hlact)2(lact)] [2.204(4)av Å] as shown in M. N. Sokolov, S. A. Adonin, A. V. Virovets, P. A. Abramov, C. Vicent, R. Llusar and V. P. Fedin, Inorg. Chim. Acta, 2013, 395, 11-18. This was also true for the 36 reported structures of FeMo-cofactors in the RCSB protein data bank (Mo-Oav 2.272 Å), which can serve as indirect evidence for the protonation of homocitrate in FeMo-co. The C-OHα-hydroxy bonds were longer than the short C-Oα-alkoxy bonds. Trinuclear Mo3SO3 cores were stabilized by imidazoles and/or α-hydroxycarboxylates, whereas only two glycolates were present in 1. α-Hydroxycarboxylates in 1 and 2 acted as bidentate ligands of Mo(iv) atoms through α-alkoxy and α-carboxy groups, while the imidazoles coordinated monodentately with nitrogen atoms. The lactates in 3 coordinated with Mo(iv) atoms through two oxygen atoms of α-carboxy groups, leaving the α-hydroxy group free. Furthermore, novel hexanuclear oxomolybdenum(v) malate Na6[(Mo2O4)3(mal)4]·5H2O (4) was also isolated (H3mal = malic acid). Solid-state and solution 13C NMR resonances of carbon atoms in α-alkoxy groups appeared in a high-field region (71.6, 77.4 ppm), indicating that α-alkoxy groups were easy to protonate.

Preliminary Assignment of Protonated and Deprotonated Homocitrates in Extracted FeMo-Cofactors by Comparisons with Molybdenum(IV) Lactates and Oxidovanadium Glycolates

“…Structural investigations revealed that the longer protonated V–O α-hydroxy bonds [2.234(2) Å and 2.244(2) Å] in 1 and 3 are close to those of FeV-co 2.17 Å 1 (FeMo-co 2.17 Å 2 ), while deprotonated V–O α-alkoxy bonds [ 2 , 1.930(2); 3 , 1.927(2) Å] were obviously shorter. This shows a similar elongated trend as the Mo–O distances in the previously reported deprotonated vs. protonated molybdenum lactates, 3 and these vanadium and molybdenum complexes have the same local V/Mo-homocitrate structures as those of FeV/Mo-cos of nitrogenases. The IR spectra of these oxidovanadium and the previously synthesized molybdenum complexes including different substituted ≡C–O(H) model compounds show red-shifts for ≡C–OH vs .…”

Assignment of protonated R-homocitrate in extracted FeMo-cofactor of nitrogenase via vibrational circular dichroism spectroscopy

“…Nevertheless, besides the metal oxidation states of FeMo-co are ambiguous [13][14][15][16][17] , the precise local structure of chelated homocitrate is controversial. With a computational study for a proposed protonated state of FeMo-co 18 , structural comparisons of oxidomolybdenum(IV) complexes with the local structures of FeMo-cofactors in Protein Data Bank (PDB) gave an indirect evidence for the protonation of α-alkoxy group in R-homocitrate 19 . Comparisons of infrared spectroscopy (IR) between FeMo-co and a series of model compounds provided a direct evidence for the two possibilities of αalkoxy and/or α-hydroxy groups in R-homocitrate 20 .…”

Molybdenum imidazole citrate and bipyridine homocitrate in different oxidation states – balance between coordinated α-hydroxy and α-alkoxy groups

“…71 Previously, the mimic complexes [Mo IV 3 SO 3 (glyc) 2 (im) 5 ]$ im$H 2 O (13) and Na 2 [Mo IV 3 SO 3 (R,S-lact) 3 (im) 3 ]$10H 2 O (14) in Table 1 have been used for comparison with FeMo-cofactors in nitrogenases, which provide indirect evidence for the protonation of homocitrate in the FeMo-cofactor Mo III/IV Fe 7 S 9 C(Scys)(N-His)(homocit). 72 Here the longer Mo IV -O distances in 1 and 2 [2.192(4) avÅ in 1 and 2.201(4) avÅ in 2] further support this proposal that the oxygen atoms in the Mo III/IV -O (a-alkoxy/ hydroxy) of FeMo-co (16 and 17) should be protonated, [20][21][22][23][24][25]73 which are obviously longer than the Mo VI Fig. 5 and S22.…”

“…The characteristic Mo IV -O-Mo VI , 13-15 and 18-34 have also been listed for comparison inTable S4. †42,43,47,59,72,[74][75][76][77][78][79][80][81] Solution 13 C and 1 H NMR spectraSolution 13 C and 1 H NMR spectra are shown in Fig. 4, S12-S21 and Table S5, † and provide valuable information on the coordination environments of 1, 2 and 4.…”