A reaction pathway to compound 0 intermediates in oxy-myoglobin through interactions with hydrogen sulfide and His64.

Myoglobin (Mb) binds oxygen with high affinity as a low spin singlet complex and thus functions as an oxygen storage protein. Yet, hybrid Density Functional Theory/Molecular Mechanical (DFT/MM) calculations of oxy-Mb models predict that the O2 bond is much less resistant to breaking in the presence of hydrogen sulfide (H2S) compared with water. Specifically, a hydrogen atom from H2S can be transferred to the distal oxygen atom through homolytic cleavage of the S-H bond to form the intermediate Compound (Cpd) 0 structure and a thiyl radical. In the presence of a neutral His64 (Nε protonation, His64-ε) and H2S, only a metastable Cpd 0 would be formed as the active site is devoid of any additional proton donor to fully break the O2 bond. In contrast, the calculations predict that the triplet state is significantly favored over the open shell singlet diradical state throughout the entire reaction coordinate in the presence of H2S and a positively charged His64. Furthermore, a positively charged His64 can readily donate a proton to Cpd 0 to fully break the O2 bond resulting in a configuration analogous to reported reaction models of a hemoglobin mutant bound to H2O2 with H2S present. Typically, exotic techniques are required to generate Cpd 0 but under the conditions just described the intermediate is readily detected in UV-Vis spectra at room temperature. The effect is observed as a 2 nm red shift of the Soret band from 414 nm to 416 nm (pH 5.0, His64-εδ) and from 416 nm to 418 nm (pH 6.6, His64-ε).

Charge Transfer and π to π* Transitions in the Visible Spectra of Sulfheme Met Isomeric Structures.

Since the 1863 discovery of a new green hemoglobin derivative called "sulfhemoglobin", the nature of the characteristic 618 nm absorption band has been the subject of several hypotheses. The experimental spectra are a function of the observation time and interplay between two major sulfheme isomer concentrations (a three- and five-membered ring adduct), with the latter being the dominant isomer at longer times. Thus, time-dependent density functional theory (TDDFT) was used to calculate the sulfheme excited states and visualize the highest occupied molecular orbitals (HOMOs) and lowest unoccupied MOs (LUMOs) of both isomers in order to interpret the transitions between them. These two isomers have distinguishable a1u and a2u HOMO energies. Formation of the three-membered ring SA isomeric structure decreases the energy of the HOMO a1u and a2u orbitals compared to the unmodified heme due to the electron-withdrawing, sulfur-containing, three-membered ring. Conversely, formation of the SC isomeric structure decreases the energy of the HOMO a1u and a2u orbitals due to the electron-withdrawing, sulfur-containing, five-membered ring. The calculations reveal that the absorption spectrum within the 700 nm region arises from a mixture of MOs but can be characterized as π to π* transitions, while the 600 nm region is characterized by π to dπ (d yz, d xz) transitions having components of a deoxy-like derivative.