The iron(II) complex 1 of a macrocyclic tetracarbene binds NO to form a low-spin (S = (1)/2) {FeNO}(7) complex (2) with a linear FeNO unit and a short Fe-NO bond. IR, electron paramagnetic resonance, and Mössbauer spectroscopies as well as density functional theory calculations suggest some Fe(I)NO(+) character and reveal that the singly occupied molecular orbital of 2, resulting from the σ-antibonding interaction of Fe dz(2) and the NO lone pair, is largely iron-based. Reduction yields a quite stable {FeNO}(8) species (3); both 2 and 3 feature very low Mössbauer isomer shifts (∼0.0 mm·s(-1)).

Flavo-diiron nitric oxide reductases (FNORs) are a subclass of nonheme diiron proteins in pathogenic bacteria that reductively transform NO to NO, thereby abrogating the nitrosative stress exerted by macrophages as part of the immune response. Understanding the mechanism and intermediates in the NO detoxification process might be crucial for the development of a more efficient treatment against these bacteria. However, low molecular weight models are still rare, and only in a few cases have their reductive transformations been thoroughly investigated. Here, we report on the development of two complexes, based on a new dinucleating pyrazolate/triazacyclononane hybrid ligand L, which serve as model systems for nonheme diiron active sites. Their ferrous nitrile precursors [L{Fe(R'CN)}(μ-OOCR)](X) (1) can be readily converted into the corresponding nitrosyl adducts ([L{Fe(NO)}(μ-OOCR)](X), 2). Spectroscopic characterization shows close resemblance to nitrosylated nonheme diiron sites in proteins as well as previous low molecular weight analogues. Crystallographic characterization reveals an anti orientation of the two {Fe(NO)} (Enemark-Feltham notation) units. The nitrosyl adducts 2 can be (electro)chemically reduced by one electron, as shown by cyclic voltammetry and UV/vis spectroscopy, but without the formation of NO. Instead, various spectroscopic techniques including stopped-flow IR spectroscopy indicated the rapid formation, within few seconds, of two well-defined products upon reduction of 2a (R = Me, X = ClO). As shown by IR and Mössbauer spectroscopy as well as X-ray crystallographic characterization, the reduction products are a diiron tetranitrosyl complex ([L{Fe(NO)}](ClO), 3a') and a diacetato-bridged ferrous complex [LFe(μ-OAc)](ClO) (3a″). Especially 3a' parallels suggested products in the decay of nitrosylated methane monooxygenase hydroxylase (MMOH), for which NO release is much less efficient than for FNORs.

[1,2]Dithiino[4,3-b:5,6-b']dipyridine (1) and its protonated open form 3,3'-dithiol-2,2'-bipyridine (2) were synthesised and their interconversion investigated. The X-ray structure of 2 revealed an anti orientation of the two pyridine units and a zwitterionic form. In depth electrochemical studies in combination with DFT calculations lead to a comprehensive picture of the redox chemistry of 1 in the absence and presence of protons. Initial one-electron reduction at E =-1.20 V results in the formation of the radical anion 1 with much elongated S-S bond, which readily undergoes further reduction at E =-1.38 V. Water triggers a potential inversion (E≥-1.13 V for the second reduction) as the radical anion 1 is protonated at its basic N atom. DFT studies revealed that S-S bond breaking and twisting of the pyridine units generally occurs after the second reduction step, whereas the potential inversion induced by protonation is a result of charge compensation. The CV data were simulated to derive rate constants for the individual chemical and electrochemical reactions for both scenarios in the absence and presence of protons.

In the Gastein valley, Austria, radon-rich thermal water and air have been used for decades for the treatment of various diseases. To explore the exposure pathway of radon progeny adsorbed to the skin, progeny activities on the skin of patients exposed to thermal water (in a bathtub) and hot vapour (in a vapour chamber) were measured by alpha spectrometry. Average total alpha activities on the patients' skin varied from 1.2 to 4.1 Bq/cm(2) in the bathtub, and from 1.1 to 2.6 Bq/cm(2) in the vapour bath. Water pH-value and ion concentration did affect radon progeny adsorption on the skin, whereas skin greasiness and blood circulation did not. Measurements of the penetration of deposited radon progeny into the skin revealed a roughly exponential activity distribution in the upper layers of the skin. Based on the radon progeny surface activity concentrations and their depth distributions, equivalent doses to different layers of the skin, in particular to the Langerhans cells located in the epidermis, ranged from 0.12 mSv in the thermal bath to 0.33 mSv in the vapour bath, exceeding equivalent doses to the inner organs (kidneys) by inhaled radon and progeny by about a factor 3, except for the lung, which receives the highest doses via inhalation. These results suggest that radon progeny attachment on skin surfaces may play a major role in the dosimetry for both thermal water and hot vapour treatment schemes.

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