A detailed analysis of the spin-crossover reaction of H2S binding to heme and the six-coordinated FeP(Im)-HS− porphyrin complex

• Published in: Journal of inorganic biochemistry Vol. 206; p. 111049
• Main Authors: Ostojić, B.D., Schwerdtfeger, P., Nakayama, A., Hasegawa, J., Đorđević, D.S.
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 01.05.2020
• Subjects: Electronic states; H2S binding; Heme; Hydrosulfide monoanion; Imidazole; Minimum energy intersystem crossing point; Electronic states; H2S binding; Minimum energy intersystem crossing point; Hydrosulfide monoanion; Imidazole; Heme
• ISSN: 0162-0134; 1873-3344
• DOI: 10.1016/j.jinorgbio.2020.111049

The potential energy surfaces of the H2S binding to iron-porphyrin (FeP) with the imidazole (Im) ligand via intersystem crossings are investigated by using density functional theory. The minimum energy intersystem crossing point (MEISCP) between the quintet and triplet states (MEISCPTQ) for the Fe(II)P(Im)-H2S complex is located at a Fe-S distance of 3.39 Å with only 1.1 kcal/mol above the quintet state minimum. The second spin-crossover point, where a change from the triplet to the singlet state occurs, comes at a much shorter Fe-S distance of 2.79 Å, and the MEISCPST is located at 3.7 kcal/mol above the triplet state minimum. The nature of the chemical bonding along the Fe-S reaction coordinate from the ground state singlet to the quintet state along the path to the separated species is analyzed. An inspection of the vibrational modes reveals that the largest contribution to the triplet-quintet transition around the quintet and triplet state minimum comes from the symmetric shrinking of the pyrrole units of the porphyrin ring, indicating that the related reaction coordinate plays a main role in the intersystem crossing. The fully optimized structures of the Fe(II)P(Im)-HS− complex corresponding to three different spin multiplicities (M = 1, 3, 5) are characterized by a bent Fe-H-S conformation. The binding of the hydrosulfide anion to Fe(II)P(Im) in the quintet state induces a 0.2 Å displacement of the Fe atom out of the nitrogen porphyrin (Npyr) plane. The fully optimized structure of the ground state of Fe(II)P(Im)-HS− agrees well with experimental data for the corresponding heme models. The binding of H2S to iron porphyrin complex with imidazole that proceeds via two intersystem crossings is investigated using quantum chemical calculations. Minimum energy intersystem crossing points are located and the results reveal that symmetric shrinking of the porphyrin ring system plays an important role during the binding process. [Display omitted] •The potential energy surfaces of H2S binding to model heme compound are investigated.•The minimum energy intersystem crossing points (MEISCP) are located.•The symmetric shrinking of the porphyrin (P) is important for intersystem crossing.•The nature of the chemical bonding along the Fe-S reaction coordinate is analyzed.•Optimized structures of FeP(Imidazole)-HS− for three different spins are determined.