title: Protein-Assisted Formation of Molybdenum Heterometallic Clusters: Evidence for the Formation of S2MoS2-M-S2MoS2Clusters with M = Fe, Co, Ni, Cu, or Cd within the Orange Protein.
abstract: The Orange Protein (ORP) is a small bacterial protein, of unknown function, that harbors a unique molybdenum/copper (Mo/Cu) heterometallic cluster, [S2MoVIS2CuIS2MoVIS2]3-, noncovalently bound. The apo-ORP is able to promote the formation and stabilization of this cluster, using CuII- and MoVIS42-salts as starting metallic reagents, to yield a Mo/Cu-ORP that is virtually identical to the native ORP. In this work, we explored the ORP capability of promoting protein-assisted synthesis to prepare novel protein derivatives harboring molybdenum heterometallic clusters containing iron, cobalt, nickel, or cadmium in place of the "central" copper (Mo/Fe-ORP, Mo/Co-ORP, Mo/Ni-ORP, or Mo/Cd-ORP). For that, the previously described protein-assisted synthesis protocol was extended to other metals and the Mo/M-ORP derivatives (M = Cu, Fe, Co, Ni, or Cd) were spectroscopically (UV-visible and electron paramagnetic resonance (EPR)) characterized. The Mo/Cu-ORP and Mo/Cd-ORP derivatives are stable under oxic conditions, while the Mo/Fe-ORP, Mo/Co-ORP, and Mo/Ni-ORP derivatives are dioxygen-sensitive and stable only under anoxic conditions. The metal and protein quantification shows the formation of 2Mo:1M:1ORP derivatives, and the visible spectra suggest that the expected {S2MoS2MS2MoS2} complexes are formed. The Mo/Cu-ORP, Mo/Co-ORP, and Mo/Cd-ORP are EPR-silent. The Mo/Fe-ORP derivative shows an EPR S =3/2signal (E/D ≈ 0.27, g ≈ 5.3, 2.5, and 1.7 for the lower M= ±1/2doublet, and g ≈ 5.7 and 1.7 (1.3 predicted) for the upper M = ±3/2doublet), consistent with the presence of either one S =5/2FeIIIantiferromagnetically coupled to two S =1/2MoVor one S =3/2FeIand two S = 0 MoVIions, in both cases in a tetrahedral geometry. The Mo/Ni-ORP shows an EPR axial S =1/2signal consistent with either one S =1/2NiIand two S = 0 MoVIor one S =1/2NiIIIantiferromagnetically coupled to two S =1/2MoVions, in both cases in a square-planar geometry. The Mo/Cu-ORP and Mo/Cd-ORP are described as {MoVI-CuI-MoVI} and {MoVI-CdII-MoVI}, respectively, while the other derivatives are suggested to exist in at least two possible electronic structures, {MoVI-MI-MoVI} ↔ {MoV-MIII-MoV}.
authors.affiliation: UCIBIO, REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa , 2829-516 Caparica, Portugal.
authors.forename: Biplab K
authors.identifier: http://orcid.org/0000-0002-0985-0031
authors.initials: BK
authors.lastname: Maiti
authors.affiliation: UCIBIO, REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa , 2829-516 Caparica, Portugal.
authors.forename: Luisa B
authors.identifier: http://orcid.org/0000-0002-6901-6591
authors.initials: LB
authors.lastname: Maia
authors.affiliation: UCIBIO, REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa , 2829-516 Caparica, Portugal.
authors.forename: Sofia R
authors.identifier: http://orcid.org/0000-0002-2149-9416
authors.initials: SR
authors.lastname: Pauleta
authors.affiliation: UCIBIO, REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa , 2829-516 Caparica, Portugal.
authors.forename: Isabel
authors.identifier: http://orcid.org/0000-0003-0971-4977
authors.initials: I
authors.lastname: Moura
authors.affiliation: UCIBIO, REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa , 2829-516 Caparica, Portugal.
authors.forename: José J G
authors.identifier: http://orcid.org/0000-0002-4726-2388
authors.initials: JJ
authors.lastname: Moura