Journal of Porphyrins and Phthalocyanines

The spectra of porphyrins meso-substituted by 4-dimethylaminophenyl groups and protonated at the central pyrrole nitrogens show strong new bands in the visible (450–600 nm) and far-red (700–780 nm). On complete protonation the observed spectra are essentially normal except for enhanced intensity in the farthest red band at ∼ 650 nm. The new bands observed with partial protonation have been qualitatively interpreted as hyperporphyrin spectra due to transitions from 'the filled π orbitals on the amino nitrogen atoms… to the porphyrin LUMOs' [Ojadi ECA, Linschitz M, Gouterman M, Walter RI, Lindsey JS, Wagner RW,]. We now report spectra calculated using the CAChe computer program package. The best structures are determined by energy minimization using a semi-empiral AM1 algorithm; a semi-empirical INDO/CI calculation is then used to obtained the excited state transition energies and intensities. The calculated spectra, although generally blue-shifted compared to experiment, reproduce the hyperporphyrin spectra in those cases where they are observed. In these cases, in contrast to species with normal porphyrin spectra, the HOMO or HOMO-1 is not an orbital on the porphyrin ring but is rather a π-orbital on the unprotonated amino nitrogen that extends over the associated phenyl ring. The calculations suggest that the hyperporphyrin transitions are of Ph(π) → Por(π*) origin, where Ph(π) is an orbital on the phenylamine substitutent and Por(π*) is a LUMO on the porphyrin. It is further suggested that in the fully protonated species the intensity enhancement of the farthest red band in an otherwise normal porphyrin spectrum may relate to phenyl character in the HOMO or HOMO-1.

Resonance Raman spectroscopy is an ideal tool to investigate the structural properties of chromophores embedded in complex (biological) environments. This holds particularly for metalporphyrins which serve as prosthetic group in various proteins. Resonance Raman dispersion spectroscopy involves the measurement of resonance excitation and depolarization ratios of a large number of Raman lines at various excitation energies covering the spectral region of the chromophore's optical absorption bands. Thus, one obtains resonance excitation profiles and the depolarization ratio dispersion of these bands. While the former contains information about the structure of excited electronic states involved in the Raman scattering process, the latter reflects asymmetric perturbations which lower the porphyrin macrocycle symmetry from ideal D_4h. The article introduces and compares different quantum mechanical approaches designed to quantitatively analyze both resonance excitation and the relationship between symmetry lowering and depolarization ratio dispersion.

A combination of density functional theory and multi-reference configuration interaction methods (DFT/MRCI) has been applied to the calculation of electronic absorption spectra in a series of porphyrin-type molecules. The calculated excitation energies and oscillator strengths for free-base porphyrin (PH₂) are in excellent agreement with experiment for both lower and higher excited states which are characterized by a significant contribution of double excitations (>20%). The 4¹B_2u, 4¹B_3u, and 5¹B_2u states are assigned to the L-band and the 7¹B_3u state to the M-band. The results for the hydroporphyrins chlorin (CH₂) and bacteriochlorin (BH₂) are in agreement with the experimentally observed increase in intensity for the Q-bands relative to PH₂. For BH₂ we predict a red shift of the Q_x-band (0.2 eV) and a blue shift of the B-band (0.5–0.7 eV) in comparison to both PH₂ and CH₂. For porphyrazine (PzH₂) and the commercial pigment phthalocyanine (PcH₂) the calculated oscillator strengths of the Q- and B-bands are of comparable size explaining the intense color of PcH₂. For the metalloporphyrins with magnesium (PMg) and zinc (PZn), the x- and y-polarized components of the Q- and B-bands collapse, due to the higher D_4h symmetry of the molecules. The calculations reproduce the slight, experimentally observed increase in the oscillator strength of the Q-band and the decrease for the B-band. These effects are ascribed to the electropositive nature of the metals relative to hydrogen. Except for the Q-bands, which are adequately described by the 'four-orbital model,' it is essential to account for excitations outside the four frontier orbitals as well as double and triple excitations for accurate reproduction of experimental data. We compare our results both with experiment and, where available, recent first-principle SAC-CI, MRMP, and TDDFT calculations.

Molecular orbital (MO) calculations of more than 60 porphyrinic π conjugated structures have been performed within the framework of the Pariser–Parr–Pople approximation. The results of a series of compounds are introduced and summarized in order to show how they vary depending on the systematic change in molecular structure. These are shown schematically or itemized. Our results are compared with the spectra of the corresponding known compounds and with the MO results reported by previous workers, if available. In addition, the results have continually been compared, where possible, with those of tetraazaporphyrin or phthalocyanine systems. In naphthalene- or anthracene-fused compounds, these molecule-centered orbitals often appear, and these are indicated using either triangles or circles in figures if they appear between HOMO – 3 and LUMO + 3 orbitals.

The excited states of four hemes, c-552, c-554, c-556, and c-559, in a c-type cytochrome subunit of the photosynthetic reaction center (PSRC) of Rhodopseudomonas (Rps.) viridis were studied using the symmetry-adapted cluster (SAC)-configuration interaction (CI) method. c-552 has two imidazole ligands, while the other three hemes have imidazole and methionine ligands. The electronic states of these four hemes are affected significantly by both the ligands and protein and water environments. The ligand effect classifies the four hemes into two groups, while the environmental effect is unique in each heme. The HOMO–LUMO energy separation of c-552 is much smaller than those of the other hemes, and therefore, the low-lying excited states of c-552 are more stable than those of the other hemes. The low-lying excited states and their oscillator strengths of c-559 are significantly modified by the environment when compared with c-556 and c-554. Some observed UV peaks of the PSRC of Rps. viridis were found to have been assigned reasonably to the excited states of these four hemes in a previous report. The calculated linear dichroism angles also supported our assignment.

The effects exerted by fused aromatic rings on the UV-vis spectra of porphyrins are surveyed. Modified porphyrin chromophores with fused benzene, 1,2-naphthalene, 9,10-phenanthrene or phenanthroline rings are surprisingly little affected even when a maximum number of ring fusions are incorporated. Linearly annealed naphtho- or anthraporphyrins show large red shifts to the Q bands but the Soret absorptions are weakened and undergo only minor bathochromic shifts. Fluoranthoporphyrins give multiple bands in the Soret region, but the Q band region is virtually unaffected by this tetracyclic ring system. On the other hand, metal chelates of fluoranthoporphyrins show surprisingly strong bands near 600 nm. Benzothiadiazole rings split and weaken the Soret band, but the Q bands region is unexceptional. However, metal coordination again produces relatively intense bands near 600 nm. The most significant results were obtained for porphyrins with fused acenaphthylene rings. Monoacenaphthoporphyrins (41) have three Soret bands at 387, 431 and 454 nm, and the longest wavelength Q band is shifted to 658 nm. opp-Diacenaphthoporphyrin (43) further shifts these bands with two Soret absorbances at 443 and 470 nm, and an additional strong peak is observed at 692 nm. The metal complexes of these systems also show strong bands between 602 and 656 nm. Still larger effects are produced by tetraacenaphthoporphyrin (47), the dication for which in trifluoroacetic acid (TFA)–chloroform has a Soret absorption at 528 nm. Tetraaryltetraacenaphthoporphyrins (48) are even more red shifted, showing Soret bands between 556 and 570 nm for the free bases and 565 to 588 nm for the related dications. The lead(II) chelate for tetraphenylporphyrin (48a) shows an additional 'hyper' spectral shift that brings the Soret band to 604 nm, and this effect can also be achieved by introducing four meso-phenylethynyl substituents onto the tetraacenaphthoporphyrin nucleus (49). In addition, by combining these two factors for the lead(II) chelate of 49, a record-breaking value for the Soret band of 642 nm can be achieved. Spectral shifts due to ring annelation in porphyrin analogues are also discussed, including those for oxybenziporphyrins, oxypyriporphyrins, carbaporphyrins and sapphyrins.

We provide a review on the status of theoretical modeling of tautomerization in free-base porphyrin. We focus our discussion on several aspects, namely: (1) potential surfaces for both the stepwise and concerted mechanisms calculated at different levels of theory, (2) solvent effects and (3) kinetics. The importance of quantum mechanical tunneling in this double hydrogen atoms transfer process is analyzed.

The connection is made between macrocycle deformations obtained from normal-coordinate structural decomposition (NSD) and the vibronic molecular states and spectra of porphyrins. NSD is a procedure that provides a description of the time-averaged distortion of a porphyrin in terms of normal-coordinate displacements from a D_4h symmetric reference structure. Expressions for the optical absorption spectra with vibrational structure are developed with these NSD-determined deformations as parameters. Using these expressions, the effects of macrocycle distortion on the UV-vis absorption spectra of porphyrins are determined.

Non-local Density Functional Theory (DFT) is applied to the calculation of geometry and vibrational frequencies of Fe^II(porphine)(imidazole)(CO), a model for CO adducts of heme proteins. Bond distances and angles are in agreement with crystallographic data, and frequencies are correctly calculated for C–O and Fe–C stretching and for Fe–C-O bending. This last mode is actually the out-of-phase combination of Fe–C–O bending and Fe–C tilting coordinates, which are heavily mixed because of a large bend–tilt interaction force constant. The in-phase combination is predicted at a very low frequency, 73 cm^-1, and to have a low infrared intensity; attempts to detect it in far-IR spectra via ¹³C¹⁸O isotope sensitivity have been unsuccessful. The stretch–bend interaction lowers the energy required for FeCO distortion. A soft potential may account for the wide range of crystallographically determined Fe–C–O displacements and orientations in myoglobin (Mb). The minimum energy path for displacement of the O atom from the heme normal was calculated by relaxing the structure while constraining only the O atom displacement from the heme normal. Energies of 0.2 to 3.5 kcal mol^-1 are required for the range of reported displacement, 0.3–1.3 Å. However, vibrational spectroscopy limits the allowable displacement to the low end of this range. The O atom displacement is computed via DFT to be 0.6 Å for a 7 ° angle of the C–O stretching IR dipole relative to the heme normal, the maximum value compatible with IR polarization measurements on MbCO. FeCO distortion is predicted to diminish both ν_CO and ν_FeC, thereby producing deviations from the well-established backbonding correlation; the scatter of the data permits a maximum displacement of 0.5 Å. This displacement would cost about 1.6 kcal mol^-1 of steric energy. A small distortion energy is consistent with the CO affinity changes produced by mutations of the distal histidine residue in Mb. Taking the leucine mutant as reference, we estimate the 1.6 kcal mol^-1 affinity loss in the wild-type protein to be the resultant of a 0.0–1.6 kcal steric inhibition, a 0.5 kcal mol^-1 attraction of the distal histidine sidechain for the bound CO [weak H-bond], and a 0.5–2.1 kcal mol^-1 attraction of the same side-chain for a water molecule in the deoxy protein. The observed 2.3 kcal mol^-1O₂ affinity increase in the wild-type protein relative to the leucine mutant then implies a 2.8–4.4 kcal mol^-1 attraction of the histidine sidechain for bound O₂, consistent with a substantial H-bond interaction with the distal histidine. Thus steric inhibition can account for only a minor fraction of the discrimination factor against CO and in favor of O₂ which is produced by the heme–myoglobin interaction.

We review contributions made towards the elucidation of CO and O₂ binding geometries in respiratory proteins. Nuclear magnetic resonance, infrared spectroscopy, Mössbauer spectroscopy, X-ray crystallography and quantum chemistry have all been used to investigate the Fe–ligand interactions. Early experimental results showed linear correlations between ¹⁷O chemical shifts and the infrared stretching frequency (ν_CO) of the CO ligand in carbonmonoxyheme proteins and between the ¹⁷O chemical shift and the ¹³CO shift. These correlations led to early theoretical investigations of the vibrational frequency of carbon monoxide and of the ¹³C and ¹⁷O NMR chemical shifts in the presence of uniform and non-uniform electric fields. Early success in modeling these spectroscopic observables then led to the use of computational methods, in conjunction with experiment, to evaluate ligand-binding geometries in heme proteins. Density functional theory results are described which predict ⁵⁷Fe chemical shifts and Mössbauer electric field gradient tensors, ¹⁷O NMR isotropic chemical shifts, chemical shift tensors and nuclear quadrupole coupling constants (e²qQ/h) as well as ¹³C isotropic chemical shifts and chemical shift tensors in organometallic clusters, heme model metalloporphyrins and in metalloproteins. A principal result is that CO in most heme proteins has an essentially linear and untilted geometry (τ = 4 °, β = 7 °) which is in extremely good agreement with a recently published X-ray synchrotron structure. CO/O₂ discrimination is thus attributable to polar interactions with the distal histidine residue, rather than major Fe–C–O geometric distortions.

Computational studies exploring the extent to which differences in proximal axial ligands modulate structure, spectra, and function of peroxidases have been performed. To this end, three heme models of compound I were characterized differing only in the axial ligand. The axial ligands considered were L=ImH, Im^-, that are alternative protonation models for a typical peroxidase with an imidazole ligand such as horseradish peroxidase (HRP-I), and L=SCH- that is a model for an unsual peroxidase, chloroperoxidase (CPO-I). Density functional calculations (DFTs) were performed to determine the optimized geometries and electronic structure of each of these three species. Their electronic spectra were also calculated at the DFT optimized geometries, using the INDO/S/CI method. The results of these studies led to the following conclusions: (1) the presence of the nearby Asp in a typical peroxidase does indeed decrease the energy required to deprotonate the imidazole making the two forms essentially degenerate, (2) neither the state of protonation of the imidazole ligand nor the change in axial ligand from an imidazole in typical peroxidases such as HRP to a mercaptide in CPO significantly alters the characteristics of the lowest energy spin state or the electronic structure of compound I in a way that can obviously affect function, (3) both the Im^- and ImH forms of the peroxidase compound I (HRP-I) lead to the same dramatic reduction in intensity relative to the ferric resting form observed experimentally. However, only in the ImH form of HRP-I does the calculated relative shift of one component of the Soret bands relative to CPO-I agree with that observed in the transient spectra of HRP-I compared to CPO-I. These results taken together strongly indicate that factors other than the nature of the proximal axial ligand are the main determinants of function.

Density functional theory (DFT) calculations have been carried out for a variety of iron and manganese porphyrin complexes with metal oxidation states of +3 and above. In general, DFT gives very good descriptions of molecular structure and electron distributions, but appears less reliable in predictions of the relative energetics of different spin states of transition metal compounds. For instance, the fact that our calculations favor a quartet state for the five-coordinate complex, (P)FeCl, over a sextet state may be regarded as a shortcoming of currently available exchange-correlation functionals. This problem is corrected at the considerably more demanding spin-restricted open-shell coupled-cluster level of theory. For both +3 and +4 metal oxidation states, the manganese 3d subshells appear to be significantly more spatially contracted than the 3d subshells of analogous iron compounds. This appears to be a key factor underlying the anomalously low MnO stretching frequencies of Mn(IV)-oxo porphyrins, compared to the FeO stretching frequencies of Fe(IV)-oxo porphryins. Similarly, a dramatic ruffling-induced redistribution of unpaired spin density in the case of an A_2u-type iron(IV)-oxo porphyrin cation radical, recently reported by Deeth, has been ascribed to a metal(d_xy)–porphyrin(a_2u) orbital interaction that becomes symmetry-allowed on ruffling of the porphyrin ring. If the axial ligand in a compound I species is strongly basic such as imidazolate (e.g. horseradish peroxidase) or mercaptide (chloroperoxidase and cytochrome P450), it too can be strongly noninnocent and carry a significant unpaired electron spin population. Similarly, theoretical modeling of a synthetic 'perferryl' complex, reported by Morishima and co-workers, has revealed a noninnocent axial methoxide ligand.

Oxoferryl porphyrin π-cation radical active sites of compound I intermediates which are found in enzymes such as peroxidases and catalases have been extensively modeled by oxidized synthetic metalloporphyrins. The electronic symmetry states of these compounds were initially assigned on the basis of electronic absorption data. In recent years new experimental and theoretical results have become available which have led to a re-evaluation and modification of the original assignments. A historical perspective of these developments is provided in the context of recent NMR, resonance Raman, and other spectroscopic data and theoretical calculations for the synthetic models and enzymatic systems.