Blue-light photodegradation of ferricyanide under protein relevant conditions

Ferricyanide is commonly used as a reoxidant in photochemical studies of redox proteins including cytochromes, photosystem II and flavoproteins. A low-spin d5 complex, [Fe(III)(CN)6]3- is a powerful electron acceptor which efficiently reoxidises photo-generated radical species. Unfortunately, ferricyanide itself absorbs strongly in the blue and a better understanding of its own photochemistry is required. Here, we present a combined UV/Vis and infrared spectroscopic study of the blue light photo-induced degradation of ferricyanide under conditions commonly employed in photochemical studies of proteins. Clear differences are observed in the photochemistry in pure water, Tris buffer and 20% glycerol solution, which are interpreted in terms of solvent-ligand exchange and ligand to metal charge transfer. The implications for photochemical studies of proteins employing ferricyanide as a reoxidant are discussed.

Ferricyanide photo-aquation pathway revealed by combined femtosecond Kβ main line and valence-to-core x-ray emission spectroscopy

Reliably identifying short-lived chemical reaction intermediates is crucial to elucidate reaction mechanisms but becomes particularly challenging when multiple transient species occur simultaneously. Here, we report a femtosecond x-ray emission spectroscopy and scattering study of the aqueous ferricyanide photochemistry, utilizing the combined Fe Kβ main and valence-to-core emission lines. Following UV-excitation, we observe a ligand-to-metal charge transfer excited state that decays within 0.5 ps. On this timescale, we also detect a hitherto unobserved short-lived species that we assign to a ferric penta-coordinate intermediate of the photo-aquation reaction. We provide evidence that bond photolysis occurs from reactive metal-centered excited states that are populated through relaxation of the charge transfer excited state. Beyond illuminating the elusive ferricyanide photochemistry, these results show how current limitations of Kβ main line analysis in assigning ultrafast reaction intermediates can be circumvented by simultaneously using the valence-to-core spectral range.

Charge-transfer and impulsive electronic-to-vibrational energy conversion in ferricyanide: ultrafast photoelectron and transient infrared studies

The photophysics of ferricyanide in H₂O, D₂O and ethylene glycol was studied upon excitation of ligand-to-metal charge transfer (LMCT) transitions by combining ultrafast photoelectron spectroscopy (PES) of liquids and transient vibrational spectroscopy.

Light-induced relaxation dynamics of the ferricyanide ion revisited by ultrafast XUV photoelectron spectroscopy

The photoexcited ferricyanide undergoes an ultrafast spin crossover followed by Jahn–Teller distortion.

Photooxidation and photoaquation of iron hexacyanide in aqueous solution: A picosecond X-ray absorption study

We present a picosecond Fe K-edge absorption study of photoexcited ferrous and ferric hexacyanide in water under 355 and 266 nm excitation. Following 355 nm excitation, the transient spectra for the ferrous and ferric complexes exhibit a red shift of the edge reflecting an increased electron density at the Fe atom. For the former, an enhanced pre-edge transition is also observed. These observations are attributed to the aquated [Fe(CN)5OH2](3-) species, based on quantum chemical calculations which also provide structural parameters. Upon 266 nm excitation of the ferric complex, a transient reminiscent of the aquated species is observed (appearance of a pre-edge feature and red shift of the edge) but it is different from that obtained under 355 nm excitation. This points to a new reaction channel occurring through an intermediate state lying between these two excitation energies. Finally, 266 nm excitation of the ferrous species is dominated by the photooxidation channel with formation of the ferric complex as main photoproduct. However, we observe an additional minor photoproduct, which is identical to the 266 nm generated photoproduct of the ferric species, suggesting that under our experimental conditions, the pump pulse photooxidises the ferrous complex and re-excites the primary ferric photoproduct.

Photochemical changes in cyanide speciation in drainage from a precious metal ore heap

In drainage from an inactive ore heap at a former gold mine, the speciation of cyanide and the concentrations of several metals were found to follow diurnal cycles. Concentrations of the hexacyanoferrate complex, iron, manganese, and ammonium were higher at night than during the day, whereas weak-acid-dissociable cyanide, silver, gold, copper, nitrite, and pH displayed the reverse behavior. The changes in cyanide speciation, iron, and trace metals can be explained by photodissociation of iron and cobalt cyanocomplexes as the solutions emerged from the heap into sunlight-exposed channels. At midday, environmentally significant concentrations of free cyanide were produced in a matter of minutes, causing trace copper, silver, and gold to be mobilized as cyanocomplexes from solids. Whether rapid photodissociation is a general phenomenon common to other sites will be important to determine in reaching a general understanding of the environmental risks posed by routine or accidental water discharges from precious metal mining facilities.

Inorganic iron complexes derived from the nitric oxide donor nitroprusside: competitive N-methyl-D-aspartate receptor antagonists with nanomolar affinity

Aquopentacyanoferrate(II), [Fe(II)H2O(CN)5]3-, is one of the photodegradation products of the vasodilator and nitric oxide donor nitroprusside. Earlier observations concerning the light dependence of N-methyl-D-aspartate (NMDA) receptor blockade by nitroprusside prompted us to examine the effects of this iron complex on the NMDA receptor. [Fe(II)H2O(CN)5]3- and two other related species, aminopentacyanoferrate(II) and aminopentacyanoferrate(III), were found to be highly potent, competitive, and selective NMDA receptor antagonists. In a binding assay for the transmitter recognition site on the NMDA receptor, these iron complexes displaced the radioligand [3H]CGP 39653 with nanomolar affinities. They did not displace radioligands labeling the channel ([3H]MK-801) or the glycine co-agonist ([3H]glycine) sites of the NMDA receptor, nor did they have any relevant affinities for a number of other neurotransmitter (alpha-adrenergic, 5-hydroxytryptamine, dopamine, opiate) receptors. The iron complexes blocked NMDA-induced depolarizations in rat cortical slices at submicromolar concentrations, whereas responses to alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and kainate were not affected. In another functional receptor assay (potentiation of [3H]MK-801 binding by glutamate under non-equilibrium conditions), Schild analysis demonstrated the competitive nature of the NMDA receptor antagonism. The pA2 values obtained from these experiments were similar to the pK(i) values derived from radioligand ([3H]CGP 39653) binding assays. To explain the high affinity and selectivity of these compounds for the NMDA receptor, a novel mechanism of antagonist-receptor interaction is proposed, involving a ligand exchange process in which a loosely bound species (here H2O or NH3) in the coordination sphere of the iron complex is replaced by a functional group of an amino acid side chain placed at the glutamate recognition site of the NMDA receptor, thereby hindering agonist binding.