Spectroscopic and Theoretical Studies of Ga(III)protoporphyrin-IX and Its Reactions with Myoglobin

Abstract

Ga(III)protoporphyrin-IX (Ga-PP) has been proposed as a model for the key interporphyrin interactions in malaria pigment. Unlike the paramagnetic parent iron heme derivatives, Ga-PP is readily soluble in methanol (MeOH). We report optical, mass spectroscopic, and theoretical results for Ga-PP as well as its reactions with myoglobin. UV-visible absorption and MCD spectroscopy show that Ga-PP exhibits a typical spectrum for a main group metal: a Q-band at 539 nm and a B band at 406 nm when dissolved in MeOH. We also report optical data for Zn(II)protoporphyrin IX (Zn-PP) dissolved in MeOH, which exhibits a Q-band at 545 nm and a B band at 415 nm. ESI mass spectral data for Ga-PP dissolved in MeOH show the presence of predominantly monomers, with smaller fractions of dimers [(Ga-PP)(2)] and trimers. UV-visible and MCD absorption spectroscopy and ESI mass spectral data demonstrate the successful insertion of monomeric Ga-PP into apo-Mb. Ga-PP-Mb exhibits a B band at 417 nm and Q bands at 545 and 584 nm, which are all red-shifted from the free Ga-PP values. The calculated electronic structures and frontier molecular orbitals of Ga-PP, (Ga-PP)(2) and Zn-PP fit the previously reported trends in band energies and oscillator strengths as a function of molecular orbital energies. These new data can be applied to explain the experimentally observed optical spectroscopy. The observed Q-band energies are accounted for by calculated (HOMO-LUMO) gap of the frontier MOs, while the split in the two top occupied MOs accounts for the magnitude of the Q-band oscillator strength as well as the experimentally observed Q to B band energy separation. Although Ga-PP shares more spectroscopic properties with Zn-PP than it does with Fe(III)PPIX, the trivalent oxidation state allows this molecule to be used as a model for ferric hemes in heme proteins.

Evaluation of Gallium as a Tracer of Exogenous Hemoglobin–Haptoglobin Complexes for Targeted Drug Delivery Applications

“…Incorporation of Ga-PP into Hb in our work was also carried out at low temperature. The correct placement of the prosthetic group within the polypeptide chains was indirectly verified by UV-Vis absorption spectroscopy (Figure 1), which shows a significant red shift of the B-band of Ga-PP (from 406 to 415 nm), consistent with the formation of the Soret band; a very similar Ga-PP "insertion signature" was previously reported for myoglobin [32]. A more detailed examination of the composition of the products of Hb reconstitution with Ga-PP was carried out using native ESI MS, which revealed (α Ga β Ga ) 2 as a species giving rise to the most abundant ionic signal (Figure 2).…”

“…A recent study by Pinter et al demonstrated that Ga-PP can be readily incorporated in myoglobin, a protein that is highly homologous to the Hb α-and β-chains, although the Ga-PP reconstituted protein remained stable only at low temperatures [32]. Incorporation of Ga-PP into Hb in our work was also carried out at low temperature.…”

“…However, gallium appears to be the best substitute for the cognate metal due to the filled d-shell, very close size match (0.62 Å versus 0.65 Å for Fe 3+ ) and its trivalency (even though heme-bound iron exists in the ferrous state inside erythrocytes, hemolysis results in its rapid oxidation to Fe 3+ , leading to formation of met-hemoglobin, which is the form interacting with Hp in circulation). In fact, Ga-substituted protoporphyrin (Ga-PP) is commonly considered to be a good model to study the heme-globin interaction [32]. Furthermore, gallium is not an essential metal and, unlike most of its neighbors in the periodic table, it is not present in the human body at levels exceeding 0.2 ppb [33] (unless the subjects had chronic exposure to this metal [34]), which should minimize interferences in ICP MS and LA-ICP MS measurements.…”

Soluble Diamagnetic Model for Malaria Pigment: Coordination Chemistry of Gallium(III)protoporphyrin-IX

“…38 TD-DFT calculations were also carried out and the results compared with Ga(PPIX)-(OH) from our previous work. 10 The ESI mass spectral data, as with the Ga(PPIX)(OH), show predominantly the monomeric Ga(OEP) at 601.2834 (calculated: 601.2816 amu). The region at 1239.58 amu shows the presence of a number of μ-oxo axially coordinated species of the dimer, Figure 6.…”

“…In an effort to find a new probe for this ill-defined system, we have developed an analogous system using gallium(III) protoporphyrin IX species. We recently reported optical spectroscopy and myoglobin binding of Ga(PPIX) 10 as well as a dimer structure in which the interdimer hydrogen bond linkages were absent, leading to a compound with high solubility and axial lability. 11 In this paper, we lay the foundation for our modeling studies through a series of experiments which explore the complex exchange chemistry of these protoporphyrin derivatives in solution.…”