Protonation of Planar and Nonplanar Porphyrins: A Calorimetric and Computational Study

Herein, we report the first calorimetric study of the protonation of planar and nonplanar free-base porphyrins: H₂OETPP (strongly saddled by its substituents), H₂T(tBu)P (strongly ruffled by its substituents), and the nominally planar porphyrins (npPs) H₂OEP, H₂TPP, H₂T(nPe)P, and H₂T(iPr)P. The observed enthalpies of protonation in solution (ΔH_protsoln) for formation of the dications in 1,1,2,2-tetrachloroethane with 2% trifluoroacetic acid are -45 ± 1 kcal mol^-1 for the npPs, -52.0 kcal mol^-1 for H₂T(tBu)P, and -70.9 kcal mol^-1 for H₂OETPP. The corresponding enthalpies of protonation (ΔH_DFT) obtained from DFT calculations (-27 ± 5, -42, and -63 kcal mol^-1, respectively) reproduce this trend. The much more negative enthalpy of protonation seen for H₂OETPP is consistent with this molecule being pre-deformed into the saddle structure favored by porphyrin dications. Except for OETPP, the calculated enthalpies of the first protonations (ΔH₁) are significantly more positive than the enthalpies of the second protonations (ΔH₂). In addition, the structural strain energies for the first protonations (ΔE_st(1)) are also significantly more positive than ΔE_st(2). According to the calculations, the monocations thus have higher proton affinities than the corresponding free-base porphyrins due to a structural strain effect, which is consistent with the generally elusive nature of the porphyrin monocation. The recent observations of monocations for free-base porphyrins with a high degree of saddling can be rationalized in terms of ΔH₁ and ΔH₂ being similar; so, the monocation is no longer an unstable intermediate.

Steric bulkiness of pyrrole substituents and the out-of-plane deformations of porphyrins: nickel(II) octaisopropylporphyrin and its meso-nitro derivative

Sterically bulky substituents at the β-carbons of the pyrrole rings of porphyrins are sufficient to cause large out-of-plane porphyrin distortions even in the absence of substituent groups at the meso carbons. It is well established that substituents at the meso-positions only or at both the β-pyrrole and the meso-positions are sufficiently bulky to result in large non-planar distortions of the macrocycle. However, no systematic studies of the effects of bulky β-pyrrole substituents alone have been reported. Herein, molecular simulations and X-ray crystallography of nickel(II) 2,3,7,8,12,13,17,18-octa(isopropyl)porphyrin reveal that large out-of-plane distortions (>1.5 Å) are induced by the steric repulsion of the β-isopropyl groups but fail to lead to a single strongly energetically favored conformer. The molecular simulations indicate that multiple conformers differing in the orientation of the isopropyl groups and the macrocycle conformation coexist in solution and this is confirmed by resonance Raman spectroscopy. Large downshifts in the structure-sensitive lines result from the non-planar distortion, and line broadenings result from structural heterogeneity. The heterogeneity originates from tradeoffs between energy contributions from steric repulsion and macrocycle distortion. Simulations for 5-nitro-2,3,7,8,12,13,17,18-octa(isopropyl)porphyrin suggest two possible orientations of the nitro group with respect to the macrocycle mean plane — one nearly vertical (as in the crystal structure) and another that is nearly parallel. INDO/S semiempirical calculations indicate an orbital of the NO2 group resides between the porphyrin frontier orbitals with significant mixing of the nitro and porphyrin orbitals.KEYWORDS: porphyrin, non-planar, resonance Raman, X-ray crystallography, crystal structure, isopropyl, nitro, conformer, molecular mechanics, molecular simulations, density functional theory, steric crowding, conformational heterogeneity.

Identification of a reduction product of aristolochic acid: implications for the metabolic activation of carcinogenic aristolochic acid

Aristolochic acids are nephrotoxic and carcinogenic natural products that have been implicated both in endemic nephropathy in the Balkan region and in ailments caused by ingestion of herbal remedies. Aristolochic acids are metabolized to active intermediates that bind to DNA. In this study, reduction of aristolochic acid I with zinc in acetic acid afforded a new product that was characterized as 9-methoxy-7-methyl-2H-1,3-oxazolo[5',4'-10,9]phenanthro[3,4-d]-1,3-dioxolane-5-carboxylic acid, designated as aristoxazole, along with the expected aristolactam I. This new compound is a condensation product of aristolochic acid and acetic acid that may be related to the aristolochic acid-DNA adducts. The proposed mechanism of formation of aristoxazole involves nucleophilic attack of acetic acid on the nitrenium ion of aristolochic acid I. On the basis of these studies, a route to the metabolic activation of aristolochic acids and formation of adducts with DNA in in vitro systems is proposed and discussed.

Bilirubin present in diverse angiosperms

BACKGROUND AND AIMS

Bilirubin is an orange-yellow tetrapyrrole produced from the breakdown of heme by mammals and some other vertebrates. Plants, algae and cyanobacteria synthesize molecules similar to bilirubin, including the protein-bound bilins and phytochromobilin which harvest or sense light. Recently, we discovered bilirubin in the arils of Strelitzia nicolai, the White Bird of Paradise Tree, which was the first example of this molecule in a higher plant. Subsequently, we identified bilirubin in both the arils and the flowers of Strelitzia reginae, the Bird of Paradise Flower. In the arils of both species, bilirubin is present as the primary pigment, and thus functions to produce colour. Previously, no tetrapyrroles were known to generate display colour in plants. We were therefore interested in determining whether bilirubin is broadly distributed in the plant kingdom and whether it contributes to colour in other species.

METHODOLOGY

In this paper, we use HPLC/UV and HPLC/UV/electrospray ionization-tandem mass spectrometry (HPLC/UV/ESI-MS/MS) to search for bilirubin in 10 species across diverse angiosperm lineages.

PRINCIPAL RESULTS

Bilirubin was present in eight species from the orders Zingiberales, Arecales and Myrtales, but only contributed to colour in species within the Strelitziaceae.

CONCLUSIONS

The wide distribution of bilirubin in angiosperms indicates the need to re-assess some metabolic details of an important and universal biosynthetic pathway in plants, and further explore its evolutionary history and function. Although colour production was limited to the Strelitziaceae in this study, further sampling may indicate otherwise.

Animal pigment bilirubin discovered in plants

The bile pigment bilirubin-IXalpha is the degradative product of heme, distributed among mammals and some other vertebrates. It can be recognized as the pigment responsible for the yellow color of jaundice and healing bruises. In this paper we present the first example of the isolation of bilirubin in plants. The compound was isolated from the brilliant orange-colored arils of Strelitzia nicolai, the white bird of paradise tree, and characterized by HPLC-ESMS, UV-visible, (1)H NMR, and (13)C NMR spectroscopy, as well as comparison with an authentic standard. This discovery indicates that plant cyclic tetrapyrroles may undergo degradation by a previously unknown pathway. Preliminary analyses of related plants, including S. reginae, the bird of paradise, also revealed bilirubin in the arils and flowers, indicating that the occurrence of bilirubin is not limited to a single species or tissue type.

Thin-layer chromatography and electrospray mass spectrometry coupled using a surface sampling probe

A combined surface sampling probe/electrospray emitter was used for the direct readout of thin-layer chromatography plates by electrospray mass spectrometry. The technique was demonstrated with reversed-phase C18 plates using a three-dye mixture composed of methylene blue, crystal violet, and rhodamine 6G for positive ion mode detection and a separate dye mixture containing fluorescein, naphthol blue black, and fast green FCF for negative ion mode detection. Acquisition of mass spectra of components of individual bands on the plate was shown by manual stepping to and sampling from specific locations within the bands. Computer-controlled scanning of development lanes on the plate was illustrated by using multiple ion monitoring in both positive and negative ion modes. Commercial TLC plates were used and no post-separation processing other than drying of the plates was required prior to mass spectrometric analysis. Readout resolution, the limits of scan speed, detection levels, TLC phase, and eluting solvents were investigated and discussed.