**1.1 Porphyrins and hemeproteins**

Porphyrins are essential compounds for the metabolism of living organisms. Porphyrins result from the substitution of porphine, which is a macrocycle formed by four pyrrole rings linked via methine bridges (**Figure 1a**). The tetrapyrrole ring has space for the coordination of a central transition metal ion with the four nitrogen atoms of the pyrrole rings to form a metalloporphyrin (**Figure 1b**) [1]. The properties of porphyrins can be modulated by substitutions at the β- and *meso*-positions, the central transition metal ions, and the metal ion axial ligands (**Figure 1b**). Another modification of porphyrin ring is the insertion of a carbene in a free-base ring to form the N,N'vinyl-bridged porphyrin and the insertion of a carbene into

#### **Figure 1.**

*Porphyrin structure. (a) Porphine; (b) generic structure of a metalloporphyrin with meso- and β-substituents; (c) iron protoporphyrin IX in oxyhemoglobin and oxymyoglobin exhibiting the heme iron axial ligands, lateral chain of histidine at the fifth coordination position, and molecular oxygen at the sixth coordination position; and (d) structure of chlorophyll, a magnesium porphyrin responsible for light harvesting in the photosynthesis process.*

the metal-nitrogen bond of a metalloporphyrin [2, 3]. Carbenes can also be added to the porphyrin ring to form homoporphyrin that is also known as expanded porphyrins [4, 5]. The replacement of a nitrogen by C, O, S, Se, and Te results in core-modified porphyrins that are a platform for organometallic chemistry [6]. Two porphyrins are key groups for the energetic metabolism, oxygen transport, and photosynthesis: the iron protoporphyrin IX, the heme group, and chlorophyll (**Figure 1c** and **d**, respectively).

Metalloporphyrins are found in biological systems as the prosthetic group of proteins. Hemeproteins encompass a diversity of proteins associated with the heme group (iron protoporphyrin IX) such as respiratory cytochromes (cyt), cytoglobins (Cgb), neuroglobins (Ngb), myoglobin (Mb), hemoglobin (Hb), cytochrome P450 (CYP), cytochrome b5 (cytb5), and others [7, 8]. The biological activity of hemeproteins is modulated by the microenvironment and iron axial ligands provided by the apoprotein. The modulation of heme iron properties by the microenvironment of proteins results in the same prosthetic group responding for oxygen transport and storage [9], electron transport, NO• trapping, and a variety of catalytic activities such as redox reactions, hydrogen peroxide cleavage, hydroxylation of aromatic

**145**

**Figure 2.**

*Scheme inspired in the study of Zhang et al. [27].*

*Technological Applications of Porphyrins and Related Compounds: Spintronics and Micro…*

impact on spectroscopic properties and functions (**Figure 2**).

For both solar cells and PDT applications, it is essential that the electron promotion to the lowest excited state can be achieved by the absorption of red light. For energy, the chirality is also interesting because of the chiral-induced spin selectivity (CISS) effect. One example is the generation of hydrogen (H2) from water splitting by semiconductors. In a standard water splitting system by a semiconductor, the

*Schematic representation of the 18 π electron aromatic ring of a metallated porphyrin with the four nodes of the HOMOs and five nodes of the LUMOs (black-dotted lines). The ML values of HOMO and LUMO pairs are ±4 and ± 5, respectively. The electron density of occupied π MOs is represented by the blue and green shading. The red and yellow shading represents the electron density map of the unoccupied π\* MOs (molecular orbitals).* 

compounds, and others [8]. **Figure 1c** shows the heme group of hemoglobin with histidine imidazole ring as the heme iron axial ligand at the fifth coordination position and molecular oxygen coordinated at the sixth coordination position. Other important biological metalloporphyrins are chlorophylls (magnesium complexes, **Figure 1d**), the plant pigment responsible for plant light harvesting, and cyanocobalamin, a vitamin B12 (cobalt complex, not shown) that participates in the lipid metabolism [10]. The remarkable chemical and photophysical properties of porphyrins have attracted the interest of researchers worldwide [1]. Biological and technological applications of porphyrins can involve the use of native hemeproteins, metallo-substituted hemeproteins, and the product of the tryptic digestion of horse heart cytochrome *c*, microperoxidases [11–19]. Inspired by nature, researchers have synthesized a diversity of nonnatural porphyrins. Theoretical studies of porphyrins have also gained relevance [4, 20–23]. Synthesis of porphyrins is principally motivated by improved use in photodynamic therapy, energy, and catalysis [24–26]. The catalytic and photochemical properties of porphyrins are dependent on the presence and type of the central metal ion with axial ligands, the peripheral decoration, and microenvironment of the ring [11, 27]. In this regard, Zhang et al. [27] demonstrated that the peripheral decoration of porphyrins with simple electron withdrawing and donating groups affects the four Gouterman orbitals with a significant

*DOI: http://dx.doi.org/10.5772/intechopen.86206*

### *Technological Applications of Porphyrins and Related Compounds: Spintronics and Micro… DOI: http://dx.doi.org/10.5772/intechopen.86206*

compounds, and others [8]. **Figure 1c** shows the heme group of hemoglobin with histidine imidazole ring as the heme iron axial ligand at the fifth coordination position and molecular oxygen coordinated at the sixth coordination position. Other important biological metalloporphyrins are chlorophylls (magnesium complexes, **Figure 1d**), the plant pigment responsible for plant light harvesting, and cyanocobalamin, a vitamin B12 (cobalt complex, not shown) that participates in the lipid metabolism [10]. The remarkable chemical and photophysical properties of porphyrins have attracted the interest of researchers worldwide [1]. Biological and technological applications of porphyrins can involve the use of native hemeproteins, metallo-substituted hemeproteins, and the product of the tryptic digestion of horse heart cytochrome *c*, microperoxidases [11–19]. Inspired by nature, researchers have synthesized a diversity of nonnatural porphyrins. Theoretical studies of porphyrins have also gained relevance [4, 20–23]. Synthesis of porphyrins is principally motivated by improved use in photodynamic therapy, energy, and catalysis [24–26]. The catalytic and photochemical properties of porphyrins are dependent on the presence and type of the central metal ion with axial ligands, the peripheral decoration, and microenvironment of the ring [11, 27]. In this regard, Zhang et al. [27] demonstrated that the peripheral decoration of porphyrins with simple electron withdrawing and donating groups affects the four Gouterman orbitals with a significant impact on spectroscopic properties and functions (**Figure 2**).

For both solar cells and PDT applications, it is essential that the electron promotion to the lowest excited state can be achieved by the absorption of red light. For energy, the chirality is also interesting because of the chiral-induced spin selectivity (CISS) effect. One example is the generation of hydrogen (H2) from water splitting by semiconductors. In a standard water splitting system by a semiconductor, the

#### **Figure 2.**

*Solid State Physics - Metastable, Spintronics Materials and Mechanics of Deformable...*

the metal-nitrogen bond of a metalloporphyrin [2, 3]. Carbenes can also be added to the porphyrin ring to form homoporphyrin that is also known as expanded porphyrins [4, 5]. The replacement of a nitrogen by C, O, S, Se, and Te results in core-modified porphyrins that are a platform for organometallic chemistry [6]. Two porphyrins are key groups for the energetic metabolism, oxygen transport, and photosynthesis: the iron protoporphyrin IX, the heme group, and chlorophyll

*Porphyrin structure. (a) Porphine; (b) generic structure of a metalloporphyrin with meso- and β-substituents; (c) iron protoporphyrin IX in oxyhemoglobin and oxymyoglobin exhibiting the heme iron axial ligands, lateral chain of histidine at the fifth coordination position, and molecular oxygen at the sixth coordination position; and (d) structure of chlorophyll, a magnesium porphyrin responsible for light harvesting in the photosynthesis process.*

Metalloporphyrins are found in biological systems as the prosthetic group of proteins. Hemeproteins encompass a diversity of proteins associated with the heme group (iron protoporphyrin IX) such as respiratory cytochromes (cyt), cytoglobins (Cgb), neuroglobins (Ngb), myoglobin (Mb), hemoglobin (Hb), cytochrome P450 (CYP), cytochrome b5 (cytb5), and others [7, 8]. The biological activity of hemeproteins is modulated by the microenvironment and iron axial ligands provided by the apoprotein. The modulation of heme iron properties by the microenvironment of proteins results in the same prosthetic group responding for oxygen transport

ties such as redox reactions, hydrogen peroxide cleavage, hydroxylation of aromatic

trapping, and a variety of catalytic activi-

**144**

**Figure 1.**

(**Figure 1c** and **d**, respectively).

and storage [9], electron transport, NO•

*Schematic representation of the 18 π electron aromatic ring of a metallated porphyrin with the four nodes of the HOMOs and five nodes of the LUMOs (black-dotted lines). The ML values of HOMO and LUMO pairs are ±4 and ± 5, respectively. The electron density of occupied π MOs is represented by the blue and green shading. The red and yellow shading represents the electron density map of the unoccupied π\* MOs (molecular orbitals). Scheme inspired in the study of Zhang et al. [27].*

sunlight absorption produces the electron hole pair. The water oxidation by holes (h+ ) produces hydroxyl free radicals as intermediates of molecular oxygen evolution. The formation of hydrogen molecules requires that protons (H+ ), resulting from the combination of hydroxyl radicals as molecular oxygen, accept the electrons promoted to the conduction band. However, hydrogen gas production competes with the combination of hydroxyl radicals as hydrogen peroxide that is favored by spin-antiparallel photogenerated holes. In the absence of a spin filter, the combination of spin-antiparallel hydroxyl radicals produces singlet molecular oxygen and requires an overpotential of 1 eV, since molecular oxygen is a triplet species in the fundamental state. Chiral molecules act as a spin filter in the electron transfer favoring the production of spin-parallel hydroxyl free radicals and consequently oxygen evolution simultaneously with H2 production [28]. In the literature, the association of porphyrins and/or hemeproteins with nanostructures, especially for photodynamic therapy purposes, is reported [14, 29]. The reason for this association refers to an enormous quantity of studies and recent findings involving nanostructure properties and manipulation, particularly the potential for drug delivery systems [30]. Nanostructured materials have at least one dimension between 1 and 100 nm. They usually have different (electronic, mechanic, magnetic, optical, etc.) properties from the bulk material, which results in multiple potential applications [31].
