Extremophiles, especially those in Archaea, have a myriad of adaptations that keep their cellular proteins stable and active under the extreme conditions in which they live. Rather than having one basic set of adaptations that works for all environments, Archaea have evolved separate protein features that are customized for each environment. We categorized the Archaea into three general groups to describe what is known about their protein adaptations: thermophilic, psychrophilic, and halophilic. Thermophilic proteins tend to have a prominent hydrophobic core and increased electrostatic interactions to maintain activity at high temperatures. Psychrophilic proteins have a reduced hydrophobic core and a less charged protein surface to maintain flexibility and activity under cold temperatures. Halophilic proteins are characterized by increased negative surface charge due to increased acidic amino acid content and peptide insertions, which compensates for the extreme ionic conditions. While acidophiles, alkaliphiles, and piezophiles are their own class of Archaea, their protein adaptations toward pH and pressure are less discernible. By understanding the protein adaptations used by archaeal extremophiles, we hope to be able to engineer and utilize proteins for industrial, environmental, and biotechnological applications where function in extreme conditions is required for activity.

Tetranuclear Fe clusters have been synthesized bearing a terminal Fe III-oxo center stabilized by hydrogen bonding interactions from pendant tert-butyl amino pyrazolate ligands. This motif was supported in multiple Fe oxidation states, ranging from [Fe II 2Fe III 2] to [Fe III 4]; two oxidation states were structurally characterized by single crystal X-ray diffraction. The reactivity of the Fe III-oxo center in proton coupled electron transfer (PCET) with X-H (X = C, O) bonds of various strengths was studied in conjunction with analysis of thermodynamic square schemes of the cluster oxidation states. These results demonstrate the important role adjacent metal centers have on modulating the reactivity of a terminal metaloxo.

A new series of tetranuclear iron clusters displaying an interstitial μ4-F ligand was prepared for comparison to μ4-O analogs. With a single NO coordinated as a reporter of small molecule activation, the μ4-F clusters were characterized in five redox states, from FeII3{FeNO}8 to FeIII3{FeNO}7, with N–O stretching frequencies ranging from 1680 cm−1 to 1855 cm−1, respectively. Despite accessing more reduced states with an F− bridge, a two electron reduction of the distal Fe centers is necessary for the μ4-F clusters to activate NO to the same degree as the μ4-O system; consequently, NO reactivity is observed at more positive potentials with μ4-O than μ4-F. Moreover, the μ4-O ligand better translates redox changes of remote metal centers to diatomic ligand activation. The implication for biological active sites is that the higher charge bridging ligand is more effective in tuning cluster properties, including the involvement of remote metal centers, for small molecule activation

We report the synthesis of site-differentiated heterometallic clusters with three Fe centers and a single Mn site that binds water and hydroxide in multiple cluster oxidation states. Deprotonation of FeIII/II3MnII–OH2 clusters leads to internal reorganization resulting in formal oxidation at Mn to generate FeIII/II3MnIII–OH. 57Fe Mӧssbauer spectroscopy reveals that oxidation state changes (three for FeIII/II3Mn–OH2 and four for FeIII/II3Mn–OH clusters) occur exclusively at the Fe centers; the Mn center is formally MnII when water is bound and MnIII when hydroxide is bound. Experimentally determined pKa (17.4) of the [FeIII2FeIIMnII–OH2] cluster and the reduction potentials of the [Fe3Mn–OH2] and [Fe3Mn–OH] clusters were used to analyze the O–H bond dissociation enthalpies (BDEO–H) for multiple cluster oxidation states. BDEO–H increases from 69, to 78, and 85 kcal/mol for the [FeIIIFeII2MnII-OH2], [FeIII2FeIIMnII-OH2], and [FeIII3MnII-OH2] clusters, respectively. Further insight of the proton and electron transfer thermodynamics of the [Fe3Mn–OHx] system was obtained by constructing a potential–pKa diagram; the shift in reduction potentials of the [Fe3Mn–OHx] clusters in the presence of different bases supports the BDEO–H values reported for the [Fe3Mn–OH2] clusters. A lower limit of the pKa for the hydroxide ligand of the [Fe3Mn–OH] clusters was estimated for two oxidation states. These data suggest BDEO–H values for the [FeIII2FeIIMnIII–OH] and [FeIII3MnIII–OH] clusters are greater than 93 and 103 kcal/mol, which hints to the high reactivity expected of the resulting [Fe3Mn=O] in this and related multinuclear systems.

The synthesis of (thiolfan*)Zr(NEt2)2 (thiolfan* = 1,1′-bis(2,4-di-tert-butyl-6-thiophenoxy)ferrocene) and its catalytic activity for intramolecular hydroamination are reported.

Review surveying biomimetic modeling and molecular understanding of heteronuclear metalloenzyme active sites involved in dioxygen, nitric oxide, and sulfite reduction.

Proteins from extremophiles have the ability to fold and remain stable in their extreme environment. Here, we investigate the presence of this effect in the cysteinyl-tRNA synthetase from Halobacterium salinarum ssp. NRC-1 (NRC-1), which was used as a model halophilic protein. The effects of salt on the structure and stability of NRC-1 and of E. coli CysRS were investigated through far-UV circular dichroism (CD) spectroscopy, fluorescence spectroscopy, and thermal denaturation melts. The CD of NRC-1 CysRS was examined in different group I and group II chloride salts to examine the effects of the metal ions. Potassium was observed to have the strongest effect on NRC-1 CysRS structure, with the other group I salts having reduced strength. The group II salts had little effect on the protein. This suggests that the halophilic adaptations in this protein are mediated by potassium. CD and fluorescence spectra showed structural changes taking place in NRC-1 CysRS over the concentration range of 0–3 M KCl, while the structure of E. coli CysRS was relatively unaffected. Salt was also shown to increase the thermal stability of NRC-1 CysRS since the melt temperature of the CysRS from NRC-1 was increased in the presence of high salt, whereas the E. coli enzyme showed a decrease. By characterizing these interactions, this study not only explains the stability of halophilic proteins in extremes of salt, but also helps us to understand why and how group I salts stabilize proteins in general.