title: Synthesis and Characterization of the Actinium Aquo Ion.
abstract: Metal aquo ions occupy central roles in all equilibria that define metal complexation in natural environments. These complexes are used to establish thermodynamic metrics (i.e., stability constants) for predicting metal binding, which are essential for defining critical parameters associated with aqueous speciation, metal chelation,in vivotransport, and so on. As such, establishing the fundamental chemistry of the actinium(III) aquo ion (Ac-aquo ion, Ac(H2O)x3+) is critical for current efforts to develop225Ac [t1/2= 10.0(1) d] as a targeted anticancer therapeutic agent. However, given the limited amount of actinium available for study and its high radioactivity, many aspects of actinium chemistry remain poorly defined. We overcame these challenges using the longer-lived227Ac [t1/2= 21.772(3) y] isotope and report the first characterization of this fundamentally important Ac-aquo coordination complex. Our X-ray absorption fine structure study revealed 10.9 ± 0.5 water molecules directly coordinated to the AcIIIcation with an Ac-OH2Odistance of 2.63(1) Å. This experimentally determined distance was consistent with molecular dynamics density functional theory results that showed (over the course of 8 ps) that AcIIIwas coordinated by 9 water molecules with Ac-OH2Odistances ranging from 2.61 to 2.76 Å. The data is presented in the context of other actinide(III) and lanthanide(III) aquo ions characterized by XAFS and highlights the uniqueness of the large AcIIIcoordination numbers and long Ac-OH2Obond distances.
authors.affiliation: Los Alamos National Laboratory , Los Alamos, New Mexico 87545, United States.
authors.forename: Maryline G
authors.initials: MG
authors.lastname: Ferrier
authors.affiliation: Los Alamos National Laboratory , Los Alamos, New Mexico 87545, United States.
authors.forename: Benjamin W
authors.initials: BW
authors.lastname: Stein
authors.affiliation: Los Alamos National Laboratory , Los Alamos, New Mexico 87545, United States.
authors.forename: Enrique R
authors.initials: ER
authors.lastname: Batista
authors.affiliation: Los Alamos National Laboratory , Los Alamos, New Mexico 87545, United States.
authors.forename: John M
authors.initials: JM
authors.lastname: Berg
authors.affiliation: Los Alamos National Laboratory , Los Alamos, New Mexico 87545, United States.
authors.forename: Eva R
authors.initials: ER
authors.lastname: Birnbaum
authors.affiliation: Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States; University of Wisconsin, Madison, Wisconsin 53711, United States.
authors.forename: Jonathan W
authors.initials: JW
authors.lastname: Engle
authors.affiliation: Los Alamos National Laboratory , Los Alamos, New Mexico 87545, United States.
authors.forename: Kevin D
authors.initials: KD
authors.lastname: John
authors.affiliation: Los Alamos National Laboratory , Los Alamos, New Mexico 87545, United States.
authors.forename: Stosh A
authors.initials: SA
authors.lastname: Kozimor
authors.affiliation: Stanford University , Stanford, California 94305, United States.
authors.forename: Juan S
authors.initials: JS
authors.lastname: Lezama Pacheco
authors.affiliation: Los Alamos National Laboratory , Los Alamos, New Mexico 87545, United States.
authors.forename: Lindsay N
authors.initials: LN
authors.lastname: Redman