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 a and a HOMO energies. Formation of the three-membered ring S isomeric structure decreases the energy of the HOMO a and a orbitals compared to the unmodified heme due to the electron-withdrawing, sulfur-containing, three-membered ring. Conversely, formation of the S isomeric structure decreases the energy of the HOMO a and a 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 d (d , d) transitions having components of a deoxy-like derivative.

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