Hemoglobin (Hb) is the heme-containing O 2 transport protein essential for life in all vertebrates. The resting high-spin (S = 2) ferrous form, deoxy-Hb, combines with triplet O 2 , forming diamagnetic (S = 0) oxy-Hb. Understanding this electronic structure is the key first step in understanding transition metal-O 2 interaction. However, despite intense spectroscopic and theoretical studies, the electronic structure description of oxy-Hb remains elusive, with at least three different descriptions proposed by Pauling, Weiss, and McClureGoddard, based on theory, spectroscopy, and crystallography. Here, a combination of X-ray absorption spectroscopy and extended X-ray absorption fine structure, supported by density functional theory calculations, help resolve this debate. X-ray absorption spectroscopy data on solution and crystalline oxy-Hb indicate both geometric and electronic structure differences suggesting that two of the previous descriptions are correct for the Fe-O 2 center in oxy-Hb. These results support the multiconfigurational nature of the ground state developed by theoretical results. Additionally, it is shown here that small differences in hydrogen bonding and solvation effects can tune the ground state, tipping it into one of the two probable configurations. These data underscore the importance of solution spectroscopy and show that the electronic structure in the crystalline form may not always reflect the true ground-state description in solution.H emoglobin (Hb) is the iron-containing heme protein essential for oxygen transport in all vertebrates. The protein is an assembly of four globular subunits, each containing a heme B (protoporphyrin IX group) (1). The first characterization of Hb was done in the mid-1800s (2, 3), with the magnetic properties first elucidated in early 1900s. Later, Pauling and Coryell (4) showed that whereas resting venous Hb is paramagnetic, the oxygenated arterial form (oxy-Hb) is diamagnetic in nature. The first structure determination (X-ray diffraction) was done by Perutz et al. (5) in 1960, who was awarded the Nobel prize for this work. In 1974, the geometry of O 2 binding was shown to be end-on η 1 -Fe-O 2 , based on X-ray diffraction data on a biomimetic model complex (6), which was later confirmed by diffraction data on oxyHb. However, the debate over the electronic structure of O 2 binding in Hb and the nature of the Fe-O 2 bond continues today.Although the diamagnetic nature of oxy-Hb has been described by several different models, three have persisted. Starting from deoxy-Hb (Fe 2+ , S = 5/2) and O 2 (neutral, S = 1) and using a valence bond theory, Pauling proposed that the oxy-Hb is a Fe 2+ , S = 0 system that accepts one lone pair from the sp 2 hybridized O 2 , forming a dative bond (Scheme 1) (7). The Weiss model involves one-electron reduction of O 2 by the ferrous heme in deoxy-Hb (8), which leads to antiferromagnetic coupling between the Fe 3+ (S = 1/2) and O In this study, quantitative Fe K-edge XAS and density functional theory (DFT) calculations have ...

The shifts in 25(OH)D distribution brought about by standardization indicate its importance in drawing correct conclusions about potential population deficiencies and insufficiencies and in permitting the comparison of distributions between national surveys.