Acidity and Hydrogen Exchange Dynamics of Iron(II)‐Bound Nitroxyl in Aqueous Solution

Abstract

Nitroxyl-iron(II) (HNO-Fe(II)) complexes are often unstable in aqueous solution, thus making them very difficult to study. Consequently, many fundamental chemical properties of Fe(II)-bound HNO have remained unknown. Using a comprehensive multinuclear ((1)H, (15)N, (17)O) NMR approach, the acidity of the Fe(II)-bound HNO in [Fe(CN)5(HNO)](3-) was investigated and its pK(a) value was determined to be greater than 11. Additionally, HNO undergoes rapid hydrogen exchange with water in aqueous solution and this exchange process is catalyzed by both acid and base. The hydrogen exchange dynamics for the Fe(II)-bound HNO have been characterized and the obtained benchmark values, when combined with the literature data on proteins, reveal that the rate of hydrogen exchange for the Fe(II)-bound HNO in the interior of globin proteins is reduced by a factor of 10(6).

Covalent attachment of the heme to Synechococcus hemoglobin alters its reactivity toward nitric oxide

“…The 15 N chemical shift of HNO is upfield from reported values (536 ppm in GlbN, compared to ~580 ppm [69]), likely affected by differences in Fe II –HNO bonding geometry. The 1 H- 15 N J coupling (| 1 J NH | ~ 71 Hz) observed for 1 H 15 NO bound to ferrous GlbNs indicates that protonation occurs at the nitrogen and is in agreement with the splittings determined previously in Hbs (| 1 J NH | ~ 66–72 Hz) and [Fe II –(CN) 5 HNO] 3− (| 1 J NH | ~ 71 Hz) [61, 69, 83, 84]. In the model complex (octaethylporphyrinato)5-Me-imidazole–Fe II –HNO (or (OEP)Fe(HNO)(5-MeIm)), the proton chemical shift and 1 H- 15 N splitting are slightly different: 1 H = 13.99 ppm and | 1 J NH | = 77 Hz, which may be due to a solvent effect (CDCl 3 versus H 2 O) and sample temperature (253 K versus 298 K) [85].…”

“…However, the typical Fe II –HNO yield was ~ 20% at neutral pH (Figure 6) and decreased to ~ 5% at pH 9.2 (data not shown). At basic pH, the decrease in HNO proton intensity could be due to deprotonation (partial formation of Fe II –NO − ), enhanced base-catalyzed hydrogen exchange dynamics [84], or decreased formation/trapping efficiency (or any combination of factors). Still, the data provide a tentative apparent pK a ≥ 8.5 for HNO when bound to Fe II GlbNs.…”

“…Still, the data provide a tentative apparent pK a ≥ 8.5 for HNO when bound to Fe II GlbNs. This lower limit is below that of [Fe II –(CN) 5 HNO] 3− , which yielded a pK a ≥ 11 [84] as determined by the absence of pH-dependent changes in the HNO 17 O signal intensity and chemical shift.…”

Probing nitrite ion dynamics in NaNO2 crystals by solid‐state 17O NMR

“…The δ 11 component is 138° off the N–O bond. It is interesting that this 17 O CS tensor orientation for the ion is similar to those reported for R–S–N=O and nitroxyl (H–N=O) . This is not totally unexpected because the N–O bond in the ion does contain a partial double‐bond character due to the resonance structures – O–N=O ↔ O=N–O − .…”

Solving the 170‐Year‐Old Mystery About Red‐Violet and Blue Transient Intermediates in the Gmelin Reaction

“…As ar esult of the Fe II coordination,t he 15 Na nd 17 Ochemical shifts for Aa nd Ba re more shielded than those reported for free S-and Cnitroso compounds, approximately d( 15 N) = 900-700ppm [25] and d( 17 O) = 1200-1300 ppm. [26] We also obtainedt he NMR signals for [Fe(CN) 5 [27] To further link the experimental NMR chemical shifts to molecular structure, we performed quantum chemical calculations, because no crystal structure has yet been reported in the literature for any metal coordination complexes containing thionitrite and perthionitrite ligands. 4À .H ere we followed the procedure suggested by Olabe and co-workers [14] to add three water molecules to each of the models in order to mimic the solvente nvironment.…”