X-ray absorption spectroscopic investigation of the electronic structure differences in solution and crystalline oxyhemoglobin.
Hemoglobin (Hb) is the heme-containing O2 transport protein essential for life in all vertebrates. The resting high-spin (S = 2) ferrous form, deoxy-Hb, combines with triplet O2, forming diamagnetic (S = 0) oxy-Hb. Understanding this electronic structure is the key first step in understanding transition metal-O2 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 McClure-Goddard, 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-O2 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.
Department of Chemistry, Stanford University, CA 94305.
Samuel A
SA
Wilson
Evan
E
Green
Irimpan I
II
Mathews
Maurizio
M
Benfatto
Keith O
KO
Hodgson
Britt
B
Hedman
Ritimukta
R
Sarangi
D018360:Crystallography, X-Ray; D015394:Molecular Structure; D010108:Oxyhemoglobins / Q000737:chemistry*; D011487:Protein Conformation; D056928:X-Ray Absorption Spectroscopy / Q000379:methods
D010108:Oxyhemoglobins