Structure refinements of protein-ligand complex by the maximum entropy method
Makoto Sakata, T. Kajikawa, Eiji Nishibori, Hideo Ago, Kengi Hamada, J. Ishijima, Masashi Miyano · Acta Crystallographica Section A Foundations of Crystallography · 2005
A six-coordinated iron(II) complex, t-Fe(tzpy) 2 (NCS) 2 (tzpy = 3-(2-pyridyl) [1,2,3]triazolo [1,5-a]pyridine), undergoes a graduate spin transition from a paramagnetic high spin state ( 5 T 2 , S = 2, HS-1) above 200K to a diamagnetic low spin state ( 1 A 1 , S = 0, LS-1) below 75 K according to the magnetic measurement.The crystal structures of both HS-1 and LS-1 are studied at 298 and 40K respectively.Significant differences in Fe-N distances and in coordination geometries of Fe do occur during the spin transition.The electronic configuration of Fe in both HS and LS states is monitored by Fe K-& L-edge absorption.In addition, the ligand C-N stretching frequency can also be followed through the spin transition.A light-induced-excited-spin-state-trapping phenomenon (LIESST) is observed by pumping the crystal with 532nm laser at 40K, where a relatively long-lived high spin state (HS-2) occurred.The molecular and crystal structure of this HS-2 state is investigated using pump and probe mode.The excitation can be easily detected by XRD, XAS and IR spectroscopy.The molecular structure and electronic configuration of Fe of HS-2 state are very similar to those of HS-1.The relaxation from such HS-2 state to the low spin state (LS-2) is followed via C-N stretching frequency at various temperature.