pH-Related and Site-Specific Excited-State Proton Transfer from Pterin to Acetate

Novel role of folate (vitamin B9) released by fermenting bacteria under Human Intestine like environment

The anaerobic region of the gastrointestinal (GI) tract has been replicated in the anaerobic chamber of a microbial fuel cell (MFC). Electroactive biomolecules released by the facultative anaerobes (Providencia rettgeri) under anoxic conditions have been studied for their potential role for redox balance. MALDI study reveals the presence of vitamin B9 (folate), 6-methylpterin, para-aminobenzoic acid (PABA) and pteroic acid called pterin pool. ATR-FTIR studies further confirm the presence of the aromatic ring and side chains of folate, 6-methylpterin and PABA groups. The photoluminescence spectra of the pool exhibit the maximum emission at 420, 425, 440, and 445 nm when excited by 310, 325, 350, and 365 nm wavelengths (day 20 sample) highlighting the presence of tunable bands. The cyclic voltammetric studies indicate the active participation of pterin pool molecules in the transfer of electrons with redox potentials at - 0.2 V and - 0.4 V for p-aminobenzoate and pterin groups, respectively. In addition, it is observed that under prolonged conditions of continuous oxidative stress (> 20 days), quinonoid tetrahydrofolate is formed, leading to temporary storage of charge. The results of the present study may potentially be useful in designing effective therapeutic strategies for the management of various GI diseases by promoting or blocking folate receptors.

Insights into Molecular Structure of Pterins Suitable for Biomedical Applications

Pterins are an inseparable part of living organisms. Pterins participate in metabolic reactions mostly as tetrahydropterins. Dihydropterins are usually intermediates of these reactions, whereas oxidized pterins can be biomarkers of diseases. In this review, we analyze the available data on the quantum chemistry of unconjugated pterins as well as their photonics. This gives a comprehensive overview about the electronic structure of pterins and offers some benefits for biomedicine applications: (1) one can affect the enzymatic reactions of aromatic amino acid hydroxylases, NO synthases, and alkylglycerol monooxygenase through UV irradiation of H₄pterins since UV provokes electron donor reactions of H₄pterins; (2) the emission properties of H₂pterins and oxidized pterins can be used in fluorescence diagnostics; (3) two-photon absorption (TPA) should be used in such pterin-related infrared therapy because single-photon absorption in the UV range is inefficient and scatters in vivo; (4) one can affect pathogen organisms through TPA excitation of H₄pterin cofactors, such as the molybdenum cofactor, leading to its detachment from proteins and subsequent oxidation; (5) metal nanostructures can be used for the UV-vis, fluorescence, and Raman spectroscopy detection of pterin biomarkers. Therefore, we investigated both the biochemistry and physical chemistry of pterins and suggested some potential prospects for pterin-related biomedicine.

Synthesis, Redox and Spectroscopic Properties of Pterin of Molybdenum Cofactors

Pterins are bicyclic heterocycles that are found widely across Nature and are involved in a variety of biological functions. Notably, pterins are found at the core of molybdenum cofactor (Moco) containing enzymes in the molybdopterin (MPT) ligand that coordinates molybdenum and facilitates cofactor activity. Pterins are diverse and can be widely functionalized to tune their properties. Herein, the general methods of synthesis, redox and spectroscopic properties of pterin are discussed to provide more insight into pterin chemistry and their importance to biological systems.

The integrative biology of pigment organelles, a quantum chemical approach

Coloration is a complex phenotypic trait involving both physical and chemical processes at a multiscale level, from molecules to tissues. Pigments, whose main property is to absorb specific wavelengths of visible light, are usually deposited in specialized organelles or complex matrices comprising proteins, metals, ions and redox compounds, among others. By modulating electronic properties and stability, interactions between pigments and these molecular actors can lead to color tuning. Furthermore, pigments are not only important for visual effects but also provide other critical functions, such as detoxification and antiradical activity. Hence, integrative studies of pigment organelles are required to understand how pigments interact with their cellular environment. In this review, we show how quantum chemistry, a computational method that models the molecular and optical properties of pigments, has provided key insights into the mechanisms by which pigment properties, from color to reactivity, are modulated by their organellar environment. These results allow to rationalize and to predict the way pigments behave in supramolecular complexes, up to the complete modelling of pigment organelles. We also discuss the main limitations of quantum chemistry, emphasizing the need for carrying experimental work with identical vigor. We finally suggest that taking into account the ecology of pigments (i.e. how they interact with these various other cellular components and at higher organizational levels) will lead to a greater understanding of how and why animals are vividly and variably colored, two fundamental questions in organismal biology.

Structure-guided evolution of Green2 toward photostability and quantum yield enhancement by F145Y substitution

Quantum yield is a determinant for fluorescent protein (FP) applications and enhancing FP brightness through gene engineering is still a challenge. Green2, our de novo FP synthesized by microfluidic picoarray and cloning, has a significantly lower quantum yield than enhanced green FP, though they have high homology and share the same chromophore. To increase its quantum yield, we introduced an F145Y substitution into Green2 based on rational structural analysis. Y145 significantly increased the quantum yield (0.22 vs. 0.18) and improved the photostability (t_1/2 , 73.0 s vs. 46.0 s), but did not affect the excitation and emission spectra. Further structural analysis showed that the F145Y substitution resulted in a significant electrical field change in the immediate environment of the chromophore. The perturbation of electrostatic charge around the chromophore lead to energy barrier changes between the ground and excited states, which resulted in the enhancement of quantum yield and photostability. Our results illustrate a typical example of engineering an FP based solely on fluorescence efficiency optimization and provide novel insights into the rational evolution of FPs.

Excited state intramolecular proton transfer (ESIPT) luminescence mechanism for 4-N,N-diethylamino-3-hydroxyflavone in propylene carbonate, acetonitrile and the mixed solvents

In this work, density functional theory (DFT) and time density functional theory (TDDFT) methods were employed to investigate the nature of the double fluorescence emission of DEAHF in these three solvents. We analyzed the geometric structures, vibrational frequencies, frontier molecular orbitals (MOs), molecular electrostatic potential surface (MEPS), calculated absorption and fluorescence spectra and the potential-energy curves for DEAHF. All the results show that the intramolecular hydrogen bond of DEAHF is strengthened from S₀ to S₁ and the electron density redistribution occurs between the proton acceptor and donor, which can facilitate ESIPT. Moreover, the geometric structures, absorption and emission spectra, MEPS and potential-energy curve of DEAHF are identical. It reveals theoretically that ACN and PC can maintain the polarity of the solvent with 1:1 mixing, which is consistent with the experimental results.

Excited state hydrogen bond and proton transfer mechanism for (2‑hydroxy‑4‑methoxyphenyl)(phenyl)‑methanone azine: A theoretical investigation

A novel fluorescence molecule (2‑hydroxy‑4‑methoxyphenyl)(phenyl)‑methanone azine (HMPM) has been explored theoretically in this present work. Based on density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods, we investigate the excited state hydrogen bonding behaviors and excite state intramolecular proton transfer (ESIPT) process for HMPM molecule. Via simulating the reduced density gradient (RDG) versus sign(λ₂)ρ, we firstly verify the double intramolecular hydrogen bonds (O1H2⋯N3 and O4H5⋯N6) for HMPM system. Comparing with the changes about these two hydrogen bonds (i.e., bond distances, bond angles and infrared (IR) vibrational spectra), we find that they should be enhanced in the first excited state upon the photo-excitation. The shortened hydrogen bonding distance of H2⋯N3 and H5⋯N6 provide the possibility for ESIPT reaction. Given the photo-excitation process, we confirm the charge redistribution around the hydrogen bonding moieties plays an important role as a driving force for the ESIPT process. Further, via constructing S₀-state and S₁-state potential energy surfaces (PESs), we confirm the excited state double proton transfer (ESDPT) is excludable since the high optimized energy and high potential energy barrier. While the low potential barrier for excited state single proton transfer path results in the ultrafast ESIPT reaction, which explains why the initial HMPM fluorescence peak cannot be detected in previous experimental phenomenon. This work not only clarifies the excited state dynamical behavior for HMPM system, but also explains previous experimental phenomenon and attributions about steady state spectra. We hope this work can facilitate novel applications based on the novel HMPM system in future.

pH-related fluorescence quenching mechanism of pterin derivatives and the effects of 6-site substituents

2-Amino-4-hydroxypteridine (pterin) and its derivatives serve as photooxidants and exhibit strong fluorescence. When they interact with hydrogen acceptors such as acetate and phosphate, their fluorescences are significantly quenched in acidic conditions (pH 4.9–5.5) but are retained in basic conditions (pH 10.0–10.5). This pH-related fluorescence quenching mechanism of pterin and its derivatives are fully investigated by using density functional theory (DFT) and time-dependent density functional theory (TD-DFT). Pterin and its derivatives are demonstrated to show favorable excited-state proton transfer (ESPT) abilities in acidic conditions that induce the experimentally observed fluorescence quenching. In contrast, the ESPT processes are found to be retarded due to the lack of strong hydrogen-bonding interactions in basic environments, which sustain their fluorescence. Interestingly, these ESPT processes are found to show different site specificities depending on the 6-site substituents. The introduction of electron-donating substituent activates the N1 site, making it the preferred ESPT site. By contrast, the introduction of an electron-withdrawing substituent activates the N5 site, making it the favorable ESPT site. The substitutions of different functional groups are found to affect the locations of acidic centers during the excitation and relaxation processes. This further affects the hydrogen-bonding patterns and ultimately brings site specificity to the ESPT process.

Theoretical investigation on ESIPT mechanism for a novel Sal-3,4-benzophen system

In this work, we theoretically investigate the properties of excited state process for a novel salicylidene sal-3,4-benzophen (Sal-3,4-B) system, which contains two intramolecular hydrogen bonds (O1-H2[Formula: see text]N3 and O4-H5[Formula: see text]N6). Based on the density functional theory (DFT) and time-dependent DFT (TDDFT) methods, we find these two hydrogen bonds should be strengthened in the S1 state, while the O4-H5[Formula: see text]N6 one could be largely affected upon the excitation process. Analyses about infrared (IR) vibrational spectra about hydrogen bond moieties also confirm this viewpoint. Frontier molecular orbitals (MOs) depict the nature of electronic excited state and support the excited state intramolecular proton transfer (ESIPT) reaction.Two kinds of stepwise potential energy curves of Sal-3,4-B in the S1 state demonstrate that only one proton could be transferred. Also based on constructing potential energy curves, the synergetic situation could be eliminated. Due to the specific ESIPT mechanism for Sal-3,4-B, we successfully explain the previous experiment and provide a reasonable attribution to the second emission peak of experiment.

Exploring the excited state behavior for 2-(phenyl)imidazo[4,5-c]pyridine in methanol solvent

In this present work, we theoretically investigate the excited state mechanism for the 2-(phenyl)imidazo[4,5-c]pyridine (PIP-C) molecule combined with methanol (MeOH) solvent molecules. Three MeOH molecules should be connected with PIP-C forming stable PIP-C-MeOH complex in the S₀ state. Upon the photo-excitation, the hydrogen bonded wires are strengthened in the S₁ state. Particularly the deprotonation process of PIP-C facilitates the excited state intermolecular proton transfer (ESIPT) process. In our work, we do verify that the ESIPT reaction should occur due to the low potential energy barrier 8.785 kcal/mol in the S₁ state. While the intersection of potential energy curves of S₀ and S₁ states result in the nonradiation transition from S₁ to S₀ state, which successfully explain why the emission peak of the proton-transfer PIP-C-MeOH-PT form could not be reported in previous experiment. As a whole, this work not only put forward a new excited state mechanism for PIP-C system, but also compensates for the defects about mechanism in previous experiment.

A theoretical study about the excited state intermolecular proton transfer mechanisms for 2-phenylimidazo[4,5-b]pyridine in methanol solvent

Within the framework of DFT and TDDFT methods, we have investigated the novel system 2-phenylimidazo[4,5-b]pyridine (PIP) with respect to the dynamical behavior of its excited state in methanol (MeOH) solvents.

Elaborating the excited-state proton transfer behaviors for novel 3H-MC and P2H-CH

We have explained the ESPT mechanism and shown the excited state dynamical overall perspective for 3H-MC and P2H-CH.

Computational insights into the photocyclization of diclofenac in solution: effects of halogen and hydrogen bonding

The effects of noncovalent interactions, namely halogen and hydrogen bonding, on the photochemical conversion of the photosensitizing drug diclofenac (DCF) in solution were investigated computationally.