A theoretical study on the structure-activity relationships of metabolites of folates as antioxidants and its implications for rational design of antioxidants

To elucidate the structure-activity relationships of metabolites of folates as antioxidants, the O-H bond dissociation enthalpies (BDEs) and ionization potentials (IPs) for these compounds were calculated by density functional theory (DFT) on B3LYP/6-31+G(,3pd) level. Accordingly, the antioxidant activity difference for metabolites of folates can be elucidated by O-H BDE and IP values and can be further explained in terms of electronic effect and intramolecular hydrogen bond effect of substituents. Furthermore, the potential of the active center of metabolites of folates, 4-hydroxypyrimidine (4-HP), as lead antioxidant, was evaluated by comparing the BDEs and IPs of 4-HP with those of 5-hydroxypyrimidine (5-HP). It was revealed that 4-HP and 5-HP held identical IPs, but the O-H BDE of the former was 22.84 kcal/mol higher than that of the latter, which meant 4-HP was inert in H-atom donation. Nevertheless, the O-H BDE of 4-HP was very sensitive to the substituents, which made NH2-derivatives of 4-HP very active as antioxidants. Therefore, 4-HP is also a potential lead antioxidant and deserves attention in rational design of antioxidants.     

Quantitative elucidation of the molecular mechanisms of hydroxyl radical quenching reactivity of phenolic compounds

Reported discrepancies have confused the understanding of the molecular mechanisms of antioxidant reactivity somewhat. The consequent problems necessitate systematic investigations on the molecular orbital features of antioxidants and their correlation with antioxidant potentials. In the present work, phenolic compounds as typical antioxidants were selected to investigate their hydroxyl radical-scavenging properties, and the related mechanisms of action were studied theoretically by computational chemistry. A good correlation was observed between antioxidant activity and theoretical parameters, such as O-H bond dissociation energy (BDE), ionization potential (IP), enthalpy of electron transfer (E(a)), chemical hardness (HOMO-LUMO gap), and spin delocalization of the phenoxyl radicals (D(s)(r)). The results demonstrate that the molecular mechanisms regulating the antioxidant action were more complex than hydrogen or electron-transfer processes and explain previous contradictions. Meanwhile, a satisfactory quantitative structure-activity relationship (QSAR) model was established which should be of predictive value in evaluating or screening hydroxyl radical-scavenging antioxidants.     

A DFT Study on the Structural and Antioxidant Properties of Three Flavonols

The structural and antioxidant activity properties of three flavonols kaempferol, galangin and morin have been investigated at density functional level of theory with the aim of verifying experimental findings. The potentialities of antioxidant activity are highly related to their capabilities to scavenge free radicals. Two potential working mechanisms of the hydrogen-atom transfer and single-electron transfer are reported by which antioxidants can play their role. Two parameters of the O-H bond dissociation enthalpy (BDE) and ionization potential (IP) in the presence of water medium are computed to estimate the antioxidant capacities. Results indicate that the order of antioxidant efficacies predicted theoretically in this work is in agreement with that reported by experimental results of oxygen radical-scavenging capacity (ORAC) assay. This demonstrates the importance of the hydrogen-atom and single-electron transfer mechanisms to explain their capacities to scavenge peroxyl radical.     

A theoretical investigation on DPPH radical-scavenging mechanism of edaravone

The mechanism of edaravone (3-methyl-1-phenyl-2-pyrazolin-5-one) to scavenge DPPH radical is clarified by density functional theory (DFT) calculations. It is revealed that H-atom-abstraction rather than electron-transfer reaction is involved in the radical-scavenging process of edaravone, and H-atom at position 4 is readily to be abstracted. The C-H bond dissociation enthalpy (BDE) of edaravone is higher than the O-H BDE of alpha-tocopherol, accounting for the activity difference between the two antioxidants. As substituents have little influence on the C-H BDE, 2-pyrazolin-5-one is recognized as the active center for edaravone.     

Antioxidant activity of phenolic and related compounds: a density functional theory study on the O-H bond dissociation enthalpy

We report here on calculations at the hybrid DFT/HF (B3-LYP/6-31G(d, p)) level of the O-H bond dissociation enthalpy (O-H BDE) of phenylpropenoic acids (caffeic, ferulic, p-coumaric and cinnamic) and phenolic acids and related compounds (gallic, methylgallate, vanillic and gentisic) in order to gain insight into the understanding of structure-antioxidant activity relationships. The results were correlated and discussed mainly on the basis of experimental data in a companion work (Galato D, Giacomelli C, Ckless K, Susin MF, Vale RMR, Spinelli A. Antioxidant capacity of phenolic and related compounds: correlation among electrochemical, visible spectroscopy methods and structure-antioxidant activity. Redox Report 2001; 6: 243-250). The O-H BDE values showed remarkable dependence on the hydroxyl position in the benzene ring and the existence of additional interaction due to hydrogen bonding. For parent molecules, the experimental antioxidant activity (AA) order was properly obeyed only when intramolecular hydrogen bonding was present in the radicalized structures of o-dihydroxyl moieties. In structurally related compounds, excellent correlation with experimental data was in general observed (0.64 < rho < 0.99). However, it is shown that excellent correlation can also be obtained for this series of compounds considering p-radicalized structures which were not stabilized by intramolecular hydrogen bonding, but this had no physical meaning. These findings suggested that the antioxidant activity evaluation of phenolic and related compounds must take into consideration the characteristics of each particular compound.     

DFT-Based Quantum Chemical Studies on Conformational, Electronic and Antioxidant Properties of Isobavachalcone and 4-Hydroxyderricin

Isobavachalcone and 4-hydroxyderricin which exhibit numerous biological activities are two major chalcone constituents isolated from the roots of Angelica keiskei KOIDZUMI. The conformational and antioxidant activity properties have been investigated by quantum chemical calculations based on the density functional theory, with the aim of verifying dominant antioxidant mechanisms. Three parameters of the O-H bond dissociation enthalpy (BDE), ionization potential (IP) and acidity in the presence of an implicit solvent for methanol are computed to estimate the antioxidant capacities. Results reveal that the order of antioxidant efficacies predicted by BDE and IP, different from that predicted by acidity, is in agreement with that obtained by experimental data. This demonstrates the importance of the hydrogen atom transfer and single electron transfer mechanisms to explain their capacities to scavenge 2, 2-diphenyl-1-picrylhydrazyl (DPPH) free radical.     

The ortho hydroxy-amino group: another choice for synthesizing novel antioxidants

Four ortho hydroxy-amino derivatives have been designed based on the structures of flavonoids to explore the effect of the ortho hydroxy-amino group on the antioxidant properties of molecules, and their bond dissociation enthalpies (BDE), ionization potentials (IP), the highest occupied molecular orbitals (HOMO), and spin densities have been calculated. The results reveal that the ortho hydroxy-amino group plays an important role in promoting the antioxidant properties of molecules because of its lowering effect on BDE, IP, and spin density. The derivatives with ortho hydroxy-amino group show stronger antioxidant activity than the derivatives with mono hydroxy or ortho dihydroxy group. Thus, the ortho hydroxy-amino group can be used as another potential functional group to synthesize novel antioxidants as guessed.     

The influence of pH on antioxidant properties and the mechanism of antioxidant action of hydroxyflavones

The effect of the pH on antioxidant properties of a series of hydroxyflavones was investigated. The pKa of the individual hydroxyl moieties in the hydroxyflavones was compared to computer-calculated deprotonation energies. This resulted in a quantitative structure activity relationship (QSAR), which enables the estimation of pKa values of individual hydroxyl moieties, also in hydroxyflavones for which these pKa values are not available. Comparison of the pKa values to the pH-dependent antioxidant profiles, determined by the TEAC assay, reveals that for various hydroxyflavones the pH-dependent behavior is related to hydroxyl moiety deprotonation, resulting in an increase of the antioxidant potential upon formation of the deprotonated forms. Comparison of these experimental results to computer calculated O-H bond dissociation energies (BDE) and ionization potentials (IP) of the nondeprotonated and the deprotonated forms of the various hydroxyflavones indicates that especially the parameter reflecting the ease of electron donation, i.e., the IP, and not the BDE, is greatly influenced by the deprotonation. Based on these results it is concluded that upon deprotonation the TEAC value increases (radical scavenging capacity increases) because electron-, not H*-, donation becomes easier. Taking into account that the mechanism of radical scavenging antioxidant activity of the neutral form of the hydroxyflavones is generally considered to be hydrogen atom donation, this implies than not only the ease of radical scavenging, but also the mechanism of antioxidant action changes upon hydroxyflavone deprotonation.     

Theoretical study on the antioxidant properties of 2′-hydroxychalcones: H-atom vs. electron transfer mechanism

The free radical scavenging activity of six 2′-hydroxychalcones has been studied in gas phase and solvents using the density functional theory (DFT) method. The three main working mechanisms, hydrogen atom transfer (HAT), stepwise electron-transfer-proton-transfer (ET-PT) and sequential-proton-loss-electron-transfer (SPLET) have been considered. The O-H bond dissociation enthalpy (BDE), ionization potential (IP), proton affinity (PA) and electron transfer energy (ETE) parameters have been computed in gas phase and solvents. The theoretical results confirmed the important role of the B ring in the antioxidant properties of hydroxychalcones. In addition, the calculated results matched well with experimental values. The results suggested that HAT would be the most favorable mechanism for explaining the radical-scavenging activity of hydroxychalcone in gas phase, whereas SPLET mechanism is thermodynamically preferred pathway in aqueous solution.     

三种黑米花色苷自由基清除能力的理论评价

用一种组合的密度泛函理论方法计算了三种黑米花色苷的O-H键解离焓和电离势,以此作为理论指标评价其自由基清除能力。计算结果发现花色苷的电离势和总抗氧化能力存在良好的线性关系,提示电离势表征的质子转移伴随电子转移的反应机制在清除自由基的过程中发挥着重要作用。     

Autophagy-Associated Atrophy and Metabolic Remodeling of the Mouse Diaphragm after Short-Term Intermittent Hypoxia

Background

Short-term intermittent hypoxia (IH) is common in patients with acute respiratory disorders. Although prolonged exposure to hypoxia induces atrophy and increased fatigability of skeletal muscle, the response to short-term IH is less well known. We hypothesized that the diaphragm and limb muscles would adapt differently to short-term IH given that hypoxia stimulates ventilation and triggers a superimposed exercise stimulus in the diaphragm.

Methods

We determined the structural, metabolic, and contractile properties of the mouse diaphragm after 4 days of IH (8 hours per day, 30 episodes per hour to a FiO2 nadir=6%), and compared responses in the diaphragm to a commonly studied reference limb muscle, the tibialis anterior. Outcome measures included muscle fiber size, assays of muscle proteolysis (calpain, ubiquitin-proteasome, and autophagy pathways), markers of oxidative stress and mitochondrial function, quantification of intramyocellular lipid and lipid metabolism genes, type I myosin heavy chain (MyHC) expression, and in vitro contractile properties.

Results

After 4 days of IH, the diaphragm alone demonstrated significant atrophy (30% decrease of myofiber size) together with increased LC3B-II protein (2.4-fold) and mRNA markers of the autophagy pathway (LC3B, Gabarapl1, Bnip3), whereas active calpain and E3 ubiquitin ligases (MuRF1, atrogin-1) were unaffected in both muscles. Succinate dehydrogenase activity was significantly reduced by IH in both muscles. However, only the diaphragm exhibited increased intramyocellular lipid droplets (2.5-fold) after IH, along with upregulation of genes linked to activated lipid metabolism. In addition, although the diaphragm showed evidence for acute fatigue immediately following IH, it underwent an adaptive fiber type switch toward slow type I MyHC-expressing fibers, associated with greater intrinsic endurance of the muscle during repetitive stimulation in vitro.

Conclusions

Short-term IH induces preferential atrophy in the mouse diaphragm together with increased autophagy and a rapid compensatory metabolic adaptation associated with enhanced fatigue resistance.     

酚类抗氧化剂清除自由基活性的理论表征与应用

筛选高效、低毒、价廉的抗氧化剂一直是自由基生物学的核心课题之一。由于筛选工作的复杂性与艰巨性,近来开始用理论方法加快这一过程。已发现许多理论参数与抗氧化剂清除自由基速率常数的对数有良好的相关性,其中尤以有征Ｏ－Ｈ解离能的△ＨＯＦ值研究得最深入。借助这一理论指标可解释酚类抗氧化剂的构效关系。对复杂抗氧化体系中抗氧化剂的活性差异也能给予较好的预测,而且还可指导新型抗氧化剂的理论构建,至此这一理论的有效     

Effect of different C3-aryl substituents on the antioxidant activity of 4-hydroxycoumarin derivatives

The antioxidant activity of 4-hydroxycoumarin synthetic derivatives and 4-methylumbelliferone were determined taking 4-hydroxycoumarin as the reference compound. Six 3-aryl-4-hydroxycoumarin derivatives were synthesized from 4-hydroxycoumarin as precursor in order to evaluate changes in their antioxidant properties due to C3-aryl substituent nature. Free radical scavenging capacities of these compounds against two different species DPPH(·) and ABTS(·+) and the protecting ability towards the β-carotene-linoleic acid co-oxidation enzymatically induced by lipoxygenase were measured. In addition, the relationship between the activities of these molecules against DPPH radical and the bond dissociation energy of O-H (BDE) calculated using methods of computational chemistry was evaluated.     

Computation of the bond dissociation enthalpies and free energies of hydroxylic antioxidants using the ab initio Hartree–Fock method

Abstract

Introduction

A new method for calculating theoretical bond dissociation enthalpy (BDE) and bond dissociation free energy (BDFE) of hydroxylic antioxidants is forwarded. BDE and BDFE may be understood as activation energies accompanying the formation of transition states, which may undergo downhill homolytic dissociation. The new method does not involve the complete fission of O–H bonds.

Method

Theoretical gas phase BDE values were calculated with the ab initio unrestricted Hartree–Fock (UHF) method, as changes in enthalpy between ground singlet states (GS) and triplet dissociative states (DS). Similarly, gas phase BDFEs were estimated from the corresponding changes in Gibbs free energy. The results were then compared with reliable experimental reports.

Results

The proposed theoretical approach of BDE and BDFE determination was tested using 10 simple phenols, 5 flavonoids, and l-ascorbic acid derivatives. The agreement between our calculated gas phase results and the adopted experimental values were generally within 0.5 kcal mol^?1, with a very few exceptions.

Discussion

Generally, steric interactions as well as intramolecular hydrogen bonding involving the dissociating OH group should be minimized in the GS. The DS are both electronically and vibrationally exited transition states. They have one unpaired electron on the carbon atom, which bears the homolytically dissociating OH group and are second order saddle points with a fixed <C–O–H bond angel of 180°.

Conclusion

It was concluded that ab initio UHF was well suited for the estimation of gas phase BDE and BDFE. The method presented has a good potential for application across a range of hydroxylic antioxidants. Currently, work is underway to extend its application in other class of antioxidants.     

Intermittent hypoxia augments carotid body and ventilatory response to hypoxia in neonatal rat pups.

Carotid bodies are functionally immature at birth and exhibit poor sensitivity to hypoxia. Previous studies have shown that continuous hypoxia at birth impairs hypoxic sensing at the carotid body. Intermittent hypoxia (IH) is more frequently experienced in neonatal life. Previous studies on adult animals have shown that IH facilitates hypoxic sensing at the carotid bodies. On the basis of these studies, in the present study we tested the hypothesis that neonatal IH facilitates hypoxic sensing of the carotid body and augments ventilatory response to hypoxia. Experiments were performed on 2-day-old rat pups that were exposed to 16 h of IH soon after the birth. The IH paradigm consisted of 15 s of 5% O2 (nadir) followed by 5 min of 21% O2 (9 episodes/h). In one group of experiments (IH and control, n = 6 pups each), sensory activity was recorded from ex vivo carotid bodies, and in the other (IH and control, n = 7 pups each) ventilation was monitored in unanesthetized pups by plethysmography. In control pups, sensory response of the carotid body was weak and was slow in onset (approximately 100 s). In contrast, carotid body sensory response to hypoxia was greater and the time course of the response was faster (approximately 30 s) in IH compared with control pups. The magnitude of the hypoxic ventilatory response was greater in IH compared with control pups, whereas changes in O2 consumption and CO2 production during hypoxia were comparable between both groups. The magnitude of ventilatory stimulation by hyperoxic hypercapnia (7% CO2-balance O2), however, was the same between both groups of pups. These results demonstrate that neonatal IH facilitates carotid body sensory response to hypoxia and augments hypoxic ventilatory chemoreflex.     

Exploring a possible way to synthesize novel better antioxidants based on vitamin E: a DFT study

The promoting effect of heterocyclic ring and heteroatom on the antioxidant properties of vitamin E has been investigated systemically by using density functional theory. The calculated results show that the heteroatom plays a more important role in promoting the antioxidant properties than the heterocyclic ring, indicating that replacing O atom by S or Se is impossible to synthesize the novel better antioxidants. Furthermore, the bond dissociation enthalpy (BDE) and ionization potential (IP) of the corresponding molecules are decreased dramatically when the O atom is replaced by NH. In addition, the calculated results also reveal that reducing the atom number of heterocyclic ring is an ideal way to synthesize novel antioxidants with better antioxidant activity than vitamin E.     

Hypoxia-inducible factor 1 mediates increased expression of NADPH oxidase-2 in response to intermittent hypoxia

Sleep‐disordered breathing with recurrent apnea is associated with intermittent hypoxia (IH). Cardiovascular morbidities caused by IH are triggered by increased generation of reactive oxygen species (ROS) by pro‐oxidant enzymes, especially NADPH oxidase‐2 (Nox2). Previous studies showed that (i) IH activates hypoxia‐inducible factor 1 (HIF‐1) in a ROS‐dependent manner and (ii) HIF‐1 is required for IH‐induced ROS generation, indicating the existence of a feed‐forward mechanism. In the present study, using multiple pharmacological and genetic approaches, we investigated whether IH‐induced expression of Nox2 is mediated by HIF‐1 in the central and peripheral nervous system of mice as well as in cultured cells. IH increased Nox2 mRNA, protein, and enzyme activity in PC12 pheochromocytoma cells as well as in wild‐type mouse embryonic fibroblasts (MEFs). This effect was abolished or attenuated by blocking HIF‐1 activity through RNA interference or pharmacologic inhibition (digoxin or YC‐1) or by genetic knockout of HIF‐1α in MEFs. Increasing HIF‐1α expression by treating PC 12 cells with the iron chelator deferoxamine for 20 h or by transfecting them with HIF‐1alpha expression vector increased Nox2 expression and enzyme activity. Exposure of wild‐type mice to IH (8 h/day for 10 days) up‐regulated Nox2 mRNA expression in brain cortex, brain stem, and carotid body but not in cerebellum. IH did not induce Nox2 expression in cortex, brainstem, carotid body, or cerebellum of Hif1a^+/? mice, which do not manifest increased ROS or cardiovascular morbidities in response to IH. These results establish a pathogenic mechanism linking HIF‐1, ROS generation, and cardiovascular pathology in response to IH. J. Cell. Physiol. 226: 2925–2933, 2011. © 2011 Wiley‐Liss, Inc.     

Mis-sizing of stent promotes intimal hyperplasia: impact of endothelial shear and intramural stress

Stent can cause flow disturbances on the endothelium and compliance mismatch and increased stress on the vessel wall. These effects can cause low wall shear stress (WSS), high wall shear stress gradient (WSSG), oscillatory shear index (OSI), and circumferential wall stress (CWS), which may promote neointimal hyperplasia (IH). The hypothesis is that stent-induced abnormal fluid and solid mechanics contribute to IH. To vary the range of WSS, WSSG, OSI, and CWS, we intentionally mismatched the size of stents to that of the vessel lumen. Stents were implanted in coronary arteries of 10 swine. Intravascular ultrasound (IVUS) was used to size the coronary arteries and stents. After 4 wk of stent implantation, IVUS was performed again to determine the extent of IH. In conjunction, computational models of actual stents, the artery, and non-Newtonian blood were created in a computer simulation to yield the distribution of WSS, WSSG, OSI, and CWS in the stented vessel wall. An inverse relation (R(2) = 0.59, P < 0.005) between WSS and IH was found based on a linear regression analysis. Linear relations between WSSG, OSI, and IH were observed (R(2) = 0.48 and 0.50, respectively, P < 0.005). A linear relation (R(2) = 0.58, P < 0.005) between CWS and IH was also found. More statistically significant linear relations between the ratio of CWS to WSS (CWS/WSS), the products CWS × WSSG and CWS × OSI, and IH were observed (R(2) = 0.67, 0.54, and 0.56, respectively, P < 0.005), suggesting that both fluid and solid mechanics influence the extent of IH. Stents create endothelial flow disturbances and intramural wall stress concentrations, which correlate with the extent of IH formation, and these effects were exaggerated with mismatch of stent/vessel size. These findings reveal the importance of reliable vessel and stent sizing to improve the mechanics on the vessel wall and minimize IH.