Substituent Effects in the Photophysical and Electrochemical Properties of Meso-Tetraphenylporphyrin Derivatives

Porphyrins were identified some years ago as a promising, easily accessible, and tunable class of organic photoredox catalysts, but a systematic study on the effect of the electronic nature and of the position of the substituents on both the ground-state and the excited-state redox potentials of these compounds is still lacking. We prepared a set of known functionalized porphyrin derivatives containing different substituents either in one of the meso positions or at a β-pyrrole carbon, and we determined their ground- and (singlet) excited-state redox potentials. We found that while the estimated singlet excited-state energies are essentially unaffected by the introduction of substituents, the redox potentials (both in the ground- and in the singlet excited-state) depend on the electron-withdrawing or electron-donating nature of the substituents. Thus, the presence of groups with electron-withdrawing resonance effects results in an enhancement of the reduction facility of the photocatalyst, both in the ground and in the excited state. We next prepared a second set of four previously unknown meso-substituted porphyrins, having a benzoyl group at different positions. The reduction facility of the porphyrin increases with the proximity of the substituent to the porphine core, reaching a maximum when the benzoyl substituent is introduced at a meso position.

Synthesis, Characterization, X-ray Molecular Structure, Antioxidant, Antifungal, and Allelopathic Activity of a New Isonicotinate-Derived meso-Tetraarylporphyrin

The present article describes the synthesis of an isonicotinate-derived meso-arylporphyrin, that has been fully characterized by spectroscopic methods (including fluorescence spectroscopy), as well as elemental analysis and HR-MS. The structure of an n-hexane monosolvate has been determined by single-crystal X-ray diffraction analysis. The radical scavenging activity of this new porphyrin against the 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical has been measured. Its antifungal activity against three yeast strains (C. albicans ATCC 90028, C. glabrata ATCC 64677, and C. tropicalis ATCC 64677) has been tested using the disk diffusion and microdilution methods. Whereas the measured antioxidant activity was low, the porphyrin showed moderate but encouraging antifungal activity. Finally, a study of its effect on the germination of lentil seeds revealed interesting allelopathic properties.

Self-Assembled Chiral Film Based on Melanin Polymers

Chirality plays a fundamental role in natural phenomena, yet its manifestation on solid surfaces remains relatively unexplored. In this study, we investigate the formation of chiroptical melanin-based self-assembled films on quartz substrates, leveraging mussel-inspired surface chemistry. Water-soluble porphyrins serve as molecular synthons, facilitating the spontaneous formation of hetero-aggregates in phosphate-buffered saline containing L- or D-DOPA. Spectroscopic analysis reveals chiral transfer from DOPA enantiomers to porphyrin hetero-aggregates, followed by the disruption of these latter and subsequent generation of chiral melanin structures in solution. Quartz substrates inserted into these solutions spontaneously accumulate homogeneous melanin-like films over days, demonstrating the feasibility of self-assembly. The resulting films exhibit characteristic UV/Vis and CD spectra, with distinct signals indicating successful chiral induction. Interestingly, the AFM characterizations reveal a distinct surface morphology, and in addition, some thermal and mechanical properties have been taken into account. Overall, this study sheds light on the formation, stability, and chiroptical properties of melanin-based films, paving the way for their application in various fields.

Interaction of Aromatic Amino Acids with Metal Complexes of Tetrakis-(4-Sulfonatophenyl)Porphyrin

The interaction of a series of metal derivatives of 5, 10, 15, 20-tetrakis(4-sulfonato-phenyl)porphyrin (MTPPS4, M = Cu(II), Pt(II), Ni(II), Zn(II) and Co(II)), including the metal free porphyrin (TPPS4), with the aromatic amino acids L-tryptophan (L-Trp), L-and D-phenylalanine (L-and D-Phe) and L-histidine (L-His) have been investigated through UV/Vis spectroscopy. The amino acid L-serine (L-Ser) has been included as reference compound. The spectroscopic changes induced by adding the amino acids have been exploited to evaluate the extent of interaction between the molecular components in the supramolecular adducts. The binding constants have been estimated for most of the investigated systems, assuming a simple 1:1 equilibrium. The bathochromic shifts of the B-bands, the extent of hypochromicity and the binding constants have been analyzed through two chemical descriptors. All the data point to the important role played by the steric hindrance introduced by axial ligands coordinated to the metal ions and to the degree of hydrophobicity and size of the aromatic moiety in the amino acids.

Twisted Structures in Natural and Bioinspired Molecules: Self-Assembly and Propagation of Chirality Across Multiple Length Scales

Several biomolecules can form dynamic aggregates in water, whose nanometric structures often reflect the chirality of the monomers in unexpected ways. Their twisted organization can be further propagated to the mesoscale, in chiral liquid crystalline phases, and even to the macroscale, where chiral, layered architectures contribute to the chromatic and mechanical properties of various plant, insect, and animal tissues. At all scales, the resulting organization is determined by a subtle balance among chiral and nonchiral interactions, whose understanding and fine-tuning is fundamental also for applications. We present recent advances in the chiral self-assembly and mesoscale ordering of biological and bioinspired molecules in water, focusing on systems based on nucleic acids or related aromatic molecules, oligopeptides, and their hybrid stuctures. We highlight the common features and key mechanisms governing this wide range of phenomena, together with novel characterization approaches.

Synthesis of New Amino-Functionalized Porphyrins:Preliminary Study of Their Organophotocatalytic Activity

The design, synthesis, and initial study of amino-functionalized porphyrins as a new class of bifunctional catalysts for asymmetric organophotocatalysis is described. Two new types of amine-porphyrin hybrids derived from 5,10,15,20-tetraphenylporphyrin (TPPH₂), in which a cyclic secondary amine moiety is covalently linked either to a β-pyrrolic position (Type A) or to the p-position of one of the meso phenyl groups (Type B), were prepared by condensation, reductive amination, or amidation reactions from the suitable porphyrins (either formyl or methanamine derivatives) with readily available chiral amines. A preliminary study of the possible use of Type A amine-porphyrin hybrids as asymmetric, bifunctional organophotocatalysts was performed using the chiral, imidazolidinone-catalyzed Diels-Alder cycloaddition between cyclopentadiene 28 and trans-cinnamaldehyde 29 as a benchmark reaction. The yield and the stereochemical outcome of this process, obtained under purely organocatalytic conditions, under dual organophocatalysis, and under bifunctional organophotocatalysis, were compared.

Enantiomeric Resolution and Enantiomer Isolation of H2 TPPS4 J-Aggregate from Aqueous Solution Is Enabled by Vortexes

Protonated achiral H₂ TPPS4 spontaneously self-arranges at acids pH and high ionic strength to build mesoscopic J-aggregates that are intrinsically chiral. According to the symmetry rule aggregation leads to a racemate that, however, can be unbalanced by chemical (chiral pollutants) or physical stimuli (as vortexing the solution). Vortexing the title racemate, in principle, might either induce chiral separation or chiral enrichment. Indeed, herein it is shown that vortices enable the resolution of this racemic solution exploiting the tendency to deposit, onto the quartz cuvette walls, of the enantiomer favored by the stirring sense. Simultaneously, over time, it was found that the opposite chiral conformation becomes prevalent in solution realizing a significant enantiomeric resolution. Therefore, after removing all stirring-favored chiral J-aggregate from the solution, the recovering and isolating of the desired enantiomers from the cuvette walls was successfully obtained without complex procedures. In this sense, it has been demonstrated that the stirring forces are executively able to fulfil the chiral separation in H₂ TPPS4 J-aggregates, employed as model of a self-assembled system in aqueous solution.

Switchable aqueous catalytic systems for organic transformations

In living organisms, enzyme catalysis takes place in aqueous media with extraordinary spatiotemporal control and precision. The mechanistic knowledge of enzyme catalysis and related approaches of creating a suitable microenvironment for efficient chemical transformations have been an important source of inspiration for the design of biomimetic artificial catalysts. However, in "nature-like" environments, it has proven difficult for artificial catalysts to promote effective chemical transformations. Besides, control over reaction rate and selectivity are important for smart application purposes. These can be achieved via incorporation of stimuli-responsive features into the structure of smart catalytic systems. Here, we summarize such catalytic systems whose activity can be switched 'on' or 'off' by the application of stimuli in aqueous environments. We describe the switchable catalytic systems capable of performing organic transformations with classification in accordance to the stimulating agent. Switchable catalytic activity in aqueous environments provides new possibilities for the development of smart materials for biomedicine and chemical biology. Moreover, engineering of aqueous catalytic systems can be expected to grow in the coming years with a further broadening of its application to diverse fields.

Possible chemical and physical scenarios towards biological homochirality

The single chirality of biological molecules in terrestrial biology raises more questions than certitudes about its origin. The emergence of biological homochirality (BH) and its connection with the appearance of life have elicited a large number of theories related to the generation, amplification and preservation of a chiral bias in molecules of life under prebiotically relevant conditions. However, a global scenario is still lacking. Here, the possibility of inducing a significant chiral bias "from scratch", i.e. in the absence of pre-existing enantiomerically-enriched chemical species, will be considered first. It includes phenomena that are inherent to the nature of matter itself, such as the infinitesimal energy difference between enantiomers as a result of violation of parity in certain fundamental interactions, and physicochemical processes related to interactions between chiral organic molecules and physical fields, polarized particles, polarized spins and chiral surfaces. The spontaneous emergence of chirality in the absence of detectable chiral physical and chemical sources has recently undergone significant advances thanks to the deracemization of conglomerates through Viedma ripening and asymmetric auto-catalysis with the Soai reaction. All these phenomena are commonly discussed as plausible sources of asymmetry under prebiotic conditions and are potentially accountable for the primeval chiral bias in molecules of life. Then, several scenarios will be discussed that are aimed to reflect the different debates about the emergence of BH: extra-terrestrial or terrestrial origin (where?), nature of the mechanisms leading to the propagation and enhancement of the primeval chiral bias (how?) and temporal sequence between chemical homochirality, BH and life emergence (when?). Intense and ongoing theories regarding the emergence of optically pure molecules at different moments of the evolution process towards life, i.e. at the levels of building blocks of Life, of the instructed or functional polymers, or even later at the stage of more elaborated chemical systems, will be critically discussed. The underlying principles and the experimental evidence will be commented for each scenario with particular attention on those leading to the induction and enhancement of enantiomeric excesses in proteinogenic amino acids, natural sugars, and their intermediates or derivatives. The aim of this review is to propose an updated and timely synopsis in order to stimulate new efforts in this interdisciplinary field.

Supramolecular Chirality in Porphyrin Self-assembly Systems in Aqueous Solution

Abstract:

The self-assembly process appears as a powerful and attractive strategy for constructing complex supramolecules by the spontaneous organization of appropriate building blocks. In this scenario, water-soluble porphyrinoids lend themselves as ideal paradigms to disclose the self-assembly phenomenon by exploiting their well-known tendency to build aggregates in aqueous media via weak non-covalent forces. Nevertheless, the spontaneous organization of achiral porphyrins can result in a final chiral superstructure moving away from single-molecule behaviour to supramolecular chirality. Therefore, over the years numerous attempts have been implemented to investigate how a porphyrin aggregate, made up of achiral monomers, becomes not-symmetric and which processes govern the bias for a certain enantiomeric assembly rather than another. Thus, in this mini-review, we exclusively discuss the main strategies for designing and building chiral aggregates in water from achiral porphyrin monomers, with particular regard to their chiroptical features.

Asymmetric hydroamination with far fewer chiral species than copper centers achieved by tuning the structure of supramolecular helical catalysts

Mixing a BTA ligand (in black), a “sergeant” (in blue) and an achiral BTA additive (in orange) affords the amination product in 75% e.e. even though only one “sergeant” for ca. 10 copper centers are present in the supramolecular helical catalyst.

Absolute Asymmetric Catalysis, from Concept to Experiment: A Narrative

The generally accepted hypothesis to explain the origin of biological homochirality (that is to say, the fact that proteinogenic amino acids are left-handed, and carbohydrates right-handed, in all living beings) is to assume, in the course of prebiotic chemical evolution, the appearance of an initial enantiomeric excess in a set of chiral molecular entities by spontaneous mirror-symmetry breaking (SMSB), together with suitable amplification and replication mechanisms that overcome the thermodynamic drive to racemization. However, the achievement of SMSB in chemical reactions taking place in solution requires highly specific reaction networks showing nonlinear dynamics based on enantioselective autocatalysis, and examples of its experimental realization are very rare. On the other hand, emergence of net supramolecular chirality by SMSB in the self-assembly of achiral molecules has been seen to occur in several instances, and the chirality sign of the resulting supramolecular system can be controlled by the action of macroscopic chiral forces. These considerations led us to propose a new mechanism for the generation of net chirality in molecular systems, in which the SMSB takes place in the formation of chiral supramolecular dissipative structures from achiral monomers, leading to asymmetric imbalances in their composition that are subsequently transferred to a standard enantioselective catalytic reaction, dodging in this way the highly limiting requirement of finding suitable reactions in solution that show enantio­selective autocatalysis. We propose the name ‘absolute asymmetric catalysis’ for this approach, in which an achiral monomer is converted into a nonracemic chiral aggregate that is generated with SMSB and that is catalytically active.

Our aim in this Account is to present a step-by-step narrative of the conceptual and experimental development of this hitherto unregarded, but prebiotically plausible, mechanism for the emergence of net chirality in molecular reactions.

1 Introduction: The Origin of Biological Homochirality and Spontaneous Mirror-Symmetry Breaking

2 Experimental Chemical Models for Spontaneous Mirror-Symmetry Breaking: The Soai Reaction and Beyond

3 Spontaneous Mirror-Symmetry Breaking in Supramolecular Chemistry: Plenty of Room at the Top

4 Absolute Asymmetric Catalysis: An Alternative Mechanism for the Emergence of Net Chirality in Molecular Systems

5 Experimental Realization of Top-Down Chirality Transfer to the

Molecular Level

6 Conclusions and Outlook

Chiral Amphiphilic Secondary Amine-Porphyrin Hybrids for Aqueous Organocatalysis

Two chiral proline-derived amphiphilic 5-substituted-10,15,20-tris(4-sulfonatophenyl)porphyrins were prepared, and their pH-dependent supramolecular behavior was studied. In neutral aqueous solutions, the free-base form of the hybrids is highly soluble, allowing enamine-based organocatalysis to take place, whereas under acidic conditions, the porphyrinic protonated core of the hybrid leads to the formation of self-assembled structures, so that the hybrids flocculate and their catalytic activity is fully suppressed. The low degree of chirality transfer observed for aqueous Michael and aldol reactions strongly suggests that these reactions take place under true "in water" organocatalytic conditions. The highly insoluble catalyst aggregates can easily be separated from the reaction products by centrifugation of the acidic reaction mixtures, and after neutralization and desalting, the sodium salts of the sulfonated amine-porphyrin hybrids, retaining their full catalytic activity, can be recovered in high yield.

Emergence of Homochiral Benzene-1,3,5-tricarboxamide Helical Assemblies and Catalysts upon Addition of an Achiral Monomer

Chirality amplification refers to the ability of a small chiral bias to fully control the main chain helicity of polymers and assemblies. Further implementation of functional chirally amplified helices as switchable asymmetric catalysts, chiral sensors, and circularly polarized light emitters will require a greater control of the energetics governing these chirality amplification effects. In this work, we report on the counterintuitive ability of an achiral molecule to suppress conformational defects in supramolecular helices, thus leading to the emergence of homochirality in a system containing a very small chiral bias. We focus our investigation on supramolecular helices composed of an achiral benzene-1,3,5-tricarboxamide (BTA) ligand, coordinated to copper, and an enantiopure BTA comonomer. Amplification of chirality as probed by varying the amount (sergeants and soldiers effect) or the optical purity (diluted majority-rules effect) of the enantiopure comonomer are modest in this initial system. However, both effects are hugely enhanced upon addition of a second achiral BTA monomer, leading to a perfect control of the helicity either by means of a remarkably low amount of sergeants (0.5%) or a small bias from a racemic mixture of enantiopure comonomers (10% ee). Such an enhancement in the amplification of chirality is only achieved by mixing the three components, i.e. the two achiral and the enantiopure comonomers, highlighting a synergistic effect upon coassembly of the three monomers. Investigation of the role of the achiral additive by multifarious analytical techniques supports its ability to stabilize the helical coassemblies and suppress helix reversals: i.e., conformational defects. Implementation of these helical copper precatalysts in the hydrosilylation of 1-(4-nitrophenyl)ethanone confirms that the effect of the achiral BTA additive is also operative under the conditions of the catalytic experiment. A highly enantioenriched product (90% ee) is produced by a supramolecular catalyst operating with ppm levels of chiral species.