Search

Water-soluble organic carbon (WSOC), which is closely related to biogenic emissions, is of great importance in the atmosphere for its ubiquitous existence and rich abundance. Levoglucosan, a typical WSOC, is usually considered to be stable and thus used as a tracer of biomass burning. However, we found that levoglucosan can be photo-oxidized on mineral dust, with formic acid, oxalic acid, glyoxylic acid, 2,3-dioxopropanoic acid, dicarbonic acid, performic acid, mesoxalaldehyde, 2-hydroxymalonaldehyde, carbonic formic anhydride, and 1,3-dioxolane-2,4-dione detected as main products. Further, we observed the heterogeneous uptake of NH3 promoted by the carboxylic acids stemming from the photocatalytic oxidation (PCO) of levoglucosan. The mineral-dust-initiated PCO of levoglucosan and enhanced heterogeneous uptake of NH3, which are highly influenced by irradiation and moisture conditions, were for the first time revealed. The reaction mechanisms and pathways were studied in detail by diffuse reflection infrared Fourier transform spectroscopy (DRIFTS), high-pressure photon ionization time-of-flight mass spectrometry (HPPI-ToF-MS) and flow reactor systems. Diverse WSOC constituents were studied as well, and the reactivity toward NH3 is related to the number of hydroxyl groups of the WSOC molecules. This work reveals a new precursor of secondary organic aerosols and provides experimental evidence of the existence of organic ammonium salts in atmospheric particles.

It is still poorly understood how the first intermediates of CO2 reduction are formed and converted to multi-carbon products over Cu-based electrodes. Herein, Ag is used to decorate dendritic Cu and a high Faradaic efficiency (FE) for C2 H4 (25 %) is obtained on a CuAg electrode, which is about five times higher than dendritic Cu. The intermediates including *CO2 - , OH groups, Cu-CO, C-O rotation, and CHx species are investigated by in situ Raman spectroscopy. This work provides spectroscopic evidence that the first intermediate of CO2 reduction on Ag-decorated Cu is carboxylate anion *CO2 - bonded with the catalyst surface through the C and O atom. The formation and evolution process of the *CO2 - intermediate over the applied potential are investigated in depth as well. This research contributes to a better understanding of the mechanism of CO2 reduction and multi-carbon product formation pathways over Ag-decorated Cu.

Salicylic acid (SA) has been proven to be a multifunctional signaling molecule that participates in the response of plants to abiotic stresses. In this study, we used cold-sensitive cucumber and cold-tolerant pumpkin as experimental materials to examine the roles of SA in root–shoot communication responses to aerial or/and root-zone chilling stress in own-root and hetero-root grafted cucumber and pumpkin plants. The results showed that pumpkin (Cm) rootstock enhanced the chilling tolerance of grafted cucumber, as evidenced by the observed lower levels of electrolyte leakage (EL), malondialdehyde (MDA), and higher photosynthetic rate (Pn) and gene expression of Rubisco activase (RCA). However, cucumber (Cs) rootstock decreased the chilling tolerance of grafted pumpkins. Cs/Cm plants showed an increase in the mRNA expression of C-repeat-binding factor (CBF1), an inducer of CBF expression (ICE1), and cold-responsive (COR47) genes and CBF1 protein levels in leaves under 5/25 and 5/5°C stresses, or in roots under 25/5 and 5/5°C stresses, respectively, compared with the Cs/Cs. Chilling stress increased the endogenous SA content and the activity of phenylalanine ammonia-lyase (PAL), and the increase in SA content and activity of PAL in Cs/Cm plants was much higher than in Cs/Cs plants. Transcription profiling analysis revealed the key genes of SA biosynthesis, PAL, ICS, and SABP2 were upregulated, while SAMT, the key gene of SA degradation, was downregulated in Cs/Cm leaves, compared with Cs/Cs leaves under chilling stress. The accumulation of SA in the Cs/Cm leaves was mainly attributed to an increase in SA biosynthesis in leaves and that in transport from roots under aerial and root-zone chilling stress, respectively. In addition, exogenous SA significantly upregulated the expression level of cold-responsive (COR) genes, enhanced actual photochemical efficiency (ΦPSII), maximum photochemical efficiency (Fv/Fm), and Pn, while decreased EL, MDA, and CI in grafted cucumber. These results suggest that SA is involved in rootstock–scion communication and grafting-induced chilling tolerance by upregulating the expression of COR genes in cucumber plants under chilling stress.

Melatonin (MT) and nitric oxide (NO) are two multifunctional signaling molecules that are involved in the response of plants to abiotic stresses. However, how MT and NO synergize in response to cold stress affecting plants is still not clear. In this study, we found that endogenous MT accumulation under cold stress was positively correlated with cold tolerance in different varieties of cucumber seedlings. The data presented here also provide evidence that endogenous NO is involved in the response to cold stress. About 100 μM MT significantly increased the nitrate reductase (NR) activity,NR-relative messenger RNA (mRNA) expression, and endogenous NO accumulation in cucumber seedlings. However, 75 μM sodium nitroprusside (SNP, a NO donor) showed no significant effect on the relative mRNA expression of tryptophan decarboxylase (TDC), tryptamine-5-hydroxylase (T5H), serotonin-N-acetyltransferase (SNAT), or acetylserotonin O-methyltransferase (ASMT), the key genes for MT synthesis and endogenous MT levels. Compared with H2O treatment, both MT and SNP decreased electrolyte leakage (EL), malondialdehyde (MDA), and reactive oxygen species (ROS) accumulation by activating the antioxidant system and consequently mitigated cold damage in cucumber seedlings. MT and SNP also enhanced photosynthetic carbon assimilation, which was mainly attributed to an increase in the activity and mRNA expression of the key enzymes in the Calvin–Benson cycle. Simultaneously, MT- and SNP-induced photoprotection for both photosystem II (PSII) and photosystem I (PSI) in cucumber seedlings, by stimulating the PsbA (D1) protein repair pathway and ferredoxin-mediated NADP+photoreduction, respectively. Moreover, exogenous MT and SNP markedly upregulated the expression of chilling response genes, such as inducer ofCBFexpression (ICE1), C-repeat-binding factor (CBF1), and cold-responsive (COR47). MT-induced cold tolerance was suppressed by 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO, a specific scavenger of NO). However, p-chlorophenylalanine (p-CPA, a MT synthesis inhibitor) did not affect NO-induced cold tolerance. Thus, novel results suggest that NO acts as a downstream signal in the MT-induced plant tolerance to cold stress.

Micro- and nanoplastics are considered one of the top pollutants that threaten the environment, aquatic life and mammalian (including human) health. Unfortunately, the development of uncomplicated but reliable analytical methods that are sensitive to individual microplastic particles, with sizes smaller than 1 μm, remains incomplete. Here, we demonstrate the detection and identification of (single) micro- and nanoplastics, by using surface-enhanced Raman spectroscopy (SERS), with Klarite substrates. Klarite is an exceptional SERS substrate; it is shaped as a dense grid of inverted pyramidal cavities, made of gold. Numerical simulations demonstrate that these cavities (or pits) strongly focus incident light into intense hotspots. We show that Klarite has the potential to facilitate the detection and identification of synthesized and atmospheric/aquatic microplastic (single) particles, with sizes down to 360 nm. We find enhancement factors of up to two orders of magnitude for polystyrene analytes. In addition, we detect and identify microplastics with sizes down to 450 nm on Klarite, with samples extracted from ambient, airborne particles. Moreover, we demonstrate Raman mapping as a fast detection technique for sub-micron microplastic particles. The results show that SERS with Klarite is a facile technique that has the potential to detect and systematically measure nanoplastics in the environment. This research is an important step towards detecting nanoscale plastic particles that may cause toxic effects to mammalian and aquatic life when present in high concentrations.

Summary Recently, the environmental impacts of microplastics have received extensive attention owing to their accumulation in the environment. However, developing efficient technology for the control and purification of microplastics is still a big challenge. Herein, we investigated the photocatalytic degradation of typical microplastics such as polystyrene (PS) microspheres and polyethylene (PE) over TiO2 nanoparticle films under UV light irradiation. TiO2 nanoparticle film made with Triton X-100 showed complete mineralization (98.40%) of 400-nm PS in 12 h, while degradation for varying sizes of PS was also studied. PE degradation experiment presented a high photodegradation rate after 36 h. CO2 was found as the main end product. The degradation mechanism and intermediates were studied by in situ DRIFTS and HPPI-TOFMS, showing the generation of hydroxyl, carbonyl, and carbon-hydrogen groups during the photodegradation of PS. This study provides a green and cost-efficient strategy for the control of microplastics contamination in the environment., Graphical Abstract, Highlights • Efficient degradation of microplastics under UV light by TiO2 film • Triton-based TiO2 film showed higher photocatalytic performance • The role of radical species during microplastics degradation was elucidated • Degradation mechanism and reaction intermediates were explored, Catalysis; Environmental Chemistry; Nanomaterials

Converting solar energy into valuable hydrogen and hydrocarbon fuels through photoelectrocatalytic water splitting and CO2 reduction is highly promising in addressing the growing demand for renewable and clean energy resources. However, the solar-to-fuel conversion efficiency is still very low due to limited light absorption and rapid bulk recombination of charge carriers. In this work, we present chlorophyll (Chl) and its derivative sodium copper chlorophyllin (ChlCuNa), as dye sensitizers, modified BiVO4 to improve the photoelectrochemical (PEC) performance. The photocurrent of BiVO4 is surprisingly decreased after a direct sensitization of Chl while the sensitization of ChlCuNa obviously enhances photocurrent of BiVO4 electrodes by improved surface hydrophilicity and extended light absorption. ChlCuNa-sensitized BiVO4 achieves an improved H2 evolution rate of 5.43 μmol h−1 cm−2 in water splitting and an enhanced HCOOH production rate of 2.15 μmol h−1 cm−2 in CO2 PEC reduction, which are 1.9 times and 2.4 times higher than pristine BiVO4, respectively. It is suggested that the derivative ChlCuNa is a more effective sensitizer for solar-to-fuel energy conversion and CO2 utilization than Chl.

Sulfite is recently found to be promising in enhancing photocatalytic pollutants degradation, and it is a byproduct from flue gas desulfuration process. Herein, 4-chlorophenol (4-CP) photodegradation was systematically investigated in a sulfite mediated system with g-C3N4 as photocatalyst. The degradation efficacy was improved by about 3 times with addition of 25 mM Na2SO3. The dominant responsible reactive oxygen species for chlorophenols remediation in the presence of sulfite included O2·-, SO3·-, and SO4·- as confirmed by radical quenching experiments and electron spin resonances technology. In-situ DRIFTs results indicated the improved cleavage of C-Cl and C-H bonds with the simultaneous formation of C O and C C bonds when bisulfite was added. Degradation intermediates such as 4-chlorocatechol, hydroquinone, and muconic acid were detected by HPLC-MS. Furthermore, the photodegradation mechanisms were discussed in the presence of sulfite. Other chlorophenols (phenol, 2-CP, 2,4-DCP, and their mixture) were also efficiently removed in the system, suggesting that sulfite could be universally applied in photocatalytic wastewater purification.

Despite the increased awareness of heterogeneous reaction on mineral dust, the knowledge of how the intensity of solar irradiation influences the photochemistry activity remains a crucially important part in atmospheric research. Relevant studies have not seriously discussed the photochemistry under weak sunlight during haze, and thus ignored some underlying pollution and toxicity. Here, we investigated the heterogeneous formation of nitrate and nitrite under various illumination conditions by laboratory experiments and field observations. Observed by in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), water-solvated nitrate was the main surface product, followed by other species varying with illumination condition. The growth of nitrate formation rate tends to be slow after the initial fast with increasing light intensity. For example, the geometric uptake coefficient (γgeo) under 30.5 mW/cm2 (5.72 × 10−6) has exceeded the 50 % of that under 160 mW/cm2 (1.13 × 10−5). This case can be explained by the excess NO2 adsorption under weak illumination while the excess photoinduced active species under strong irradiation. Being negatively associated with nitrate (R2 = 0.748, P

Hydrogen sulfide (H2S) and Hydrogen peroxide (H2O2) are two crucial gaseous signaling molecules that participate in various physiological processes and abiotic stresses. However, how the synergy of H2S and H2O2 regulates photosynthesis have rarely been studied. This study aimed to reveal the mechanism underlying the interaction between H2S and H2O2 in plants response to photosynthesis. Cucumber (Cucumis sativus L., ‘Jinyou 35’) seedlings were used as the material and grown in a climate chamber at 26 °C/18 °C with a 600 μmol m−2·s-1 photon flux density (PFD). Sodium hydrosulfide hydrate (NaHS, an H2S donor), H2O2, and their scavengers or inhibitors were applied as foliar sprayed at the two-leaf seedling stage. The result showed that both NaHS and H2O2 increased the CO2 assimilation, which mainly attributed to an increase in the activity and gene expression of photosynthetic enzymes. NaHS and H2O2 also induced photoprotection for both photosystem Ⅱ (PSⅡ) and photosystem Ⅰ (PSⅠ) in cucumber seedlings, by activating the D1 protein repair pathway under chilling stress. Interestingly, 1.0 mM NaHS significantly enhanced the relative gene expression of respiratory burst oxidase homolog (RBOH), which in turn elevated endogenous H2O2 accumulation in cucumber seedlings. However, H2O2 had little effect on gene expression of L-/D-cysteine desulfhydrase (L-/D-CD) and endogenous H2S level. The H2S-induced adaptive response of photosynthesis to chilling stress was suppressed by diphenyleneiodonium (DPI, a H2O2 generation inhibitor) or dimethylthiourea (DMTU, a H2O2 scavenger). These data suggest that NaHS alleviates the negative effects of chilling stress on photosynthesis by improving photosynthetic carbon assimilation, carbon metabolism, and photoprotection for both PSⅡ and PSⅠ in cucumber seedlings. H2O2 may act as a downstream signal in H2S-induced protection of the photosynthetic apparatus in cucumber seedlings under chilling stress.

Full details of studies are disclosed on the total syntheses of binding pocket analogues of vancomycin bearing the peripheral l-vancosaminyl-1,2-d-glucosyl disaccharide that contain changes to a key single atom in the residue-4 amide (residue-4 carbonyl O → S, NH, H2) designed to directly address the underlying molecular basis of resistance to vancomycin. Also disclosed are studies piloting the late-stage transformations conducted on the synthetically more accessible C-terminus hydroxymethyl aglycon derivatives and full details of the peripheral chlorobiphenyl functionalization of all of the binding-pocket-modified vancomycin analogues designed for dual d-Ala-d-Ala/d-Ala-d-Lac binding. Their collective assessment indicates that combined binding pocket and chlorobiphenyl peripherally modified analogues exhibit a remarkable spectrum of antimicrobial activity (VSSA, MRSA, and VanA and VanB VRE) and impressive potencies against both vancomycin-sensitive and vancomycin-resistant bacteria (MICs = 0.06–0.005 and...

A highly efficient diversified methodology for preparation of benzimidazole, quinazolin-4(3H)-ones and 1,4-benzodiazepine-2,5-diones is established using a palladium-catalyzed transfer hydrogenation (CTH)/condensation cascade of o-nitroaniline and o-nitrobenzamides in a triethylamine–formic acid azeotropic mixture (2:5) under microwave irradiation.

Airborne mineral dust is a big contributor to atmospheric particulate matter. Complex chemistry of mineral dust surfaces might give rise to the conversion of some important atmospheric trace gases. Herein, for the first time we find that CO2 can be photochemically reduced to CO, which is an ozone precursor in the atmosphere, on mineral dust particles. In this study, we performed CO2 uptake experiments using a quartz reactor and investigated the uptake kinetics of CO2 on TiO2 particles and authentic mineral dust (Arizona Test Dust, illite, montmorillonite, and kaolin) under simulated atmospheric conditions using gas chromatography (GC). The impacts of different relative humidity (RH) values and irradiation intensities on CO2 photoreduction were studied. Moreover, the surface intermediate of the heterogeneous photoreduction of CO2 with mineral dust and its kinetic relevance were investigated using in situ DRIFTS and isotopic 13C labelling. Furthermore, field observations of increased CO concentrations in a mineral dust storm were interpreted as the results of the direct uptake of CO2 on the mineral dust surface and the following photochemical reaction of CO2 on atmospheric mineral dusts under solar irradiation. In summary, we provide evidence for a pathway in which CO2 interacts with mineral dust and is converted into CO under artificial solar light. Due to the abundance of CO2 and mineral dust in the lower atmosphere, this process could cause some impacts on the atmosphere and climate.

A strategy has been developed that culminated in a stereoselective total synthesis of the tetracyclic antimalarial Myrioneuron alkaloid myrioneurinol. The synthesis relies on three highly diastereoselective reactions, including an intramolecular chelation-controlled Michael spirocyclization of an N-Cbz-lactam titanium enolate to an α,β-unsaturated ester for construction of the A/D-ring system and the attendant C5 (quaternary), C6 relative stereochemistry; a malonate enolate conjugate addition to a nitrosoalkene in order to install the appropriate functionality and establish the configuration at C7; and an intramolecular aza-Sakurai reaction to form the B-ring and the accompanying C9 and C10 stereocenters.

Studies on the further development of the sequential glycosylations of the vancomycin aglycon catalyzed by the glycosyltransferases GtfE and GtfD and the observation of unusual, perhaps unexpected, aglycon substrate substituent effects on the rate and efficiency of the initial glycosylation reaction are reported.

The reaction has a vast substrate scope and tolerates multiple functional groups, thus allowing for the diversification of complex biomolecules and active pharmaceutical agents.

A practical synthetic method for the annulation of benzo-rings by the intramolecular coupling of an aryl iodide and a methylene C-H bond is described. The palladium-catalyzed C-H functionalization is directed by an aminoquinoline carboxamide group, which can be easily installed and removed. High yields and broad substrate scope were achieved. An additive of ortho-phenyl benzoic acid, identified from a systematic screening, functions as a critical ligand for the catalytic process under mild condition, even at near room temperature.

the most potent isolate (IC50= 0.8 mm ; IC= inhibitory concentration) from the celogentin/moroidin family, whose members possess inhibitory activity against tubulin polymerization. Its highly unusual architecture, which is characterized by the direct linkages of Trp C6 to Leu Cb, and Trp C2 to His N1 (Figure 1), and its biological activity have prompted a number of synthesis studies. Although N-linkedHis residues are known to occur in other macrocyclic peptides, the Leu-Trp linkage is extremely rare and poses a difficult synthetic challenge. To access the key Leu-Trp motif, Moody and Bentley, and Campagne et al., applied asymmetric hydrogenation conditions to dehydroamino acid precursors. Enantioand/or diastereoselectivities ranged from 1:1 to 16:1 for these 10–14 step sequences. More recently, Castle and coworkers developed a novel Knoevenagel condensation/radical conjugate addition approach to the Leu-Trp linkage. They completed the first celogentin synthesis through an elegant NCS-mediated Trp-His C N coupling by utilizing the major diastereomer product, albeit in modest enantioand diastereoselectivity. Herein, we report a highly stereoselective and efficient synthesis of celogentin C using a novel palladiumcatalyzed C H functionalization strategy. The highly constrained structure of 1 is probably assembled in vivo from the much simpler linear peptide precursor through a series of enzymatic oxidative cross-links (Figure 1). Inspired by these simple yet powerful transformations found in nature, we envisioned developing a synthetic equivalent of these processes in a direct approach to celogentin. Our synthetic strategy relied on the direct regioand stereoselective activation of the b C H bond of a Leu moiety and on the subsequent coupling of the derived C Pd species with a suitable Trp partner. The recent report by Corey et al. of the carboxamide-directed b C H functionalization of amino acids served as the starting point for our venture. Corey demonstrated that the b C H bond of the Nphthaloyl amino acid 8-aminoquinoline amide can be efficiently activated and then arylated with simple aryl iodides under Pd(OAc)2 catalysis, a procedure built on the seminal discovery of Daugulis and co-workers for the functionalization of inactivated sp C H bonds. The quinoline moiety serves as a chelating auxiliary for palladium coordination, and promotes the formation of trans-palladacycle intermediate 4. This palladium(II) intermediate presumably undergoes crosscoupling with an aryl iodide partner to provide the final arylated product which has an erythro stereochemical preference. To our delight, we were able to achieve the high-yielding and highly stereoselective 6-indolylation of N-phthaloyl leucine (Scheme 1a). Upon simple heating of precursors 2 (2.0 equiv) and 3 (1.0 equiv), with Pd(OAc)2 (0.2 equiv), and AgOAc (1.5 equiv), at 110 8C in tBuOH, the desired diastereomer 5 was formed exclusively, and the slight excess of 2 could be largely recovered. About 3% of deiodinated side product 6 was also generated. Although the quinoline carboxamide serves as an effective auxiliary in arylation chemistry, its efficient removal under mild conditions would be required for this process to become a useful tool for natural product synthesis. However, the cleavage of the amide linkage was particularly problematic, owing to both steric hindrance and the lability of the Nphthaloyl group. This phthaloyl group, which provides both bis-protection of the a-amino group and steric bias, is critical Figure 1. Celogentin C and our synthetic strategy.

The mannopeptimycins are a class of glycopeptide natural products with unusual structures and potent antibiotic activity against a range of Gram-positive multidrug-resistant bacteria. Their cyclic hexapeptide core features a pair of unprecedented β-hydroxyenduracididines (L- and D-βhEnd), an O-glycosylated D-Tyr carrying an α-linked dimannose, and a β-methylated Phe residue. The D-βhEnd unit also carries an α-linked mannopyranose at the most hindered N of its cyclic guanidine ring. Herein, we report the first total synthesis of mannopeptimycin α and β with fully elaborated N- and O-linked sugars. Critically, a gold-catalyzed N-glycosylation of a D-βhEnd substrate with a mannosyl ortho-alkynylbenzoate donor enabled the synthesis of the most challenging N-Man-D-βhEnd unit with excellent efficiency and stereoselectivity. The L-βMePhe unit was prepared using a Pd-catalyzed C-H arylation method. The L-βhEnd, D-Tyr(di-Man), and L-βMePhe units were prepared in gram quantities. A convergent assembly of the cyclic peptide scaffold and a single global hydrogenolysis deprotection operation provided mannopeptimycin α and β.

A nontoxic and inexpensive photocatalytic initiation of anti-Markovnikov hydrothiolation of olefins using visible light is reported. This method is characterized by low catalyst loading, thereby enabling a mild and selective method for radical initiation in thiol–ene reactions between a wide scope of olefins and thiols.

Title derivatives of quinazolin-4(3H)-one [(III), (V), and (VII)], benzimidazole [(IX) and (XI)], and 1,4-benzodiazepine-2,5-dione (XIII) are efficiently synthesized through a one-pot procedure in triethylamine—formic acid azeotropic mixture under microwave irradiation.

MMP-2 is a member of the matrix metalloproteinase family that has been implicated in tumor cell metastasis and angiogenesis. Here, we describe the solution structure of a catalytic domain of MMP-2 complexed with a hydroxamic acid inhibitor (SC-74020), determined by three-dimensional heteronuclear NMR spectroscopy. The catalytic domain, designated MMP-2C, has a short peptide linker replacing the internal fibronectin-domain insertion and is enzymatically active. Distance geometry-simulated annealing calculations yielded 14 converged structures with atomic root-mean-square deviations (r.m.s.d.) of 1.02 and 1.62 A from the mean coordinate positions for the backbone and for all heavy atoms, respectively, when 11 residues at the N-terminus are excluded. The structure has the same global fold as observed for other MMP catalytic domains and is similar to previously solved crystal structures of MMP-2. Differences observed between the solution and the crystal structures, near the bottom of the S1' specificity loop, appear to be induced by the large inhibitor present in the solution structure. The MMP-2C solution structure is compared with MMP-8 crystal structure bound to the same inhibitor to highlight the differences especially in the S1' specificity loop. The finding provides a structural explanation for the selectivity between MMP-2 and MMP-8 that is achieved by large inhibitors.

The monoterpene indole alkaloids, which are usually comprised of a tryptamine moiety appended to a single C9- or C10-terpenoid unit, constitute one of the largest known classes of natural products.[1] In 2004, Kam and Choo isolated a new type of monoterpenoid indole alkaloid, angustilodine (1), which contains a unique rearranged skeleton, from the leaves of the Malayan plant Alstonia angustiloba (Figure 1).[2] The structure of angustilodine was determined by detailed spectroscopic analysis to include an indole appended to a cis-fused 2-azadecalin ring system bearing a 7-membered ring ether bridge. An interesting conformational feature of this molecule established by 2D NMR studies is the observation that the piperidine ring exists as a boat. More recently, Morita and coworkers discovered the N-demethyl congener alstilobanine E (2), along with alstilobanine A (3), which lacks the bridging oxepane ring found in 1 and 2, in the same plant.[3] Unlike alkaloids 1 and 2, it was proposed that alstilobanine A has the piperidine ring in a chair conformation as shown in Figure 1. Alstilobanines A and E were found to possess modest relaxant activity against phenylephrine-induced contractions of thoracic rat aortic rings with endothelium. In this communication we describe the first approach to these alkaloids, culminating in a convergent total synthesis of racemic alstilobanine A (3).

The structure and stability of apocytochrome b562 were explored using absorption and circular dichroism spectroscopic methods. The polypeptide chain retains a well-defined structure when the prosthetic heme group is removed from cytochrome b562. Circular dichroism measurements estimate 60% helicity for apocytochrome b562, compared with 80% helicity found in holocytochrome b562. At low pH, apocytochrome b562 displays a midpoint pH of 2.9, while ferricytochrome b562 displays a midpoint pH of 2.3. The unfolding of the apoprotein by urea and heat can be well approximated by the two-state transition model. The stability of apocytochrome b562 is significantly reduced from that of the holoprotein. The free energy of stabilization (delta G degrees) and the midpoint transition temperature (Tm) for apocytochrome b562 are found to be 3.2 +/- 0.5 kcal/mol and 52.3 +/- 0.9 degrees C, respectively, compared with 6.6 +/- 0.5 kcal/mol and 67.2 +/- 0.5 degrees C for ferricytochrome b562. The smaller heat capacity change upon unfolding of apocytochrome b562 than that of ferricytochrome b562, estimated from the thermodynamic parameters, indicates that apocytochrome b562 possesses a smaller hydrophobic core than holocytochrome b562. Size-exclusion chromatography studies indicate that the apoprotein is slightly more extended in molecular dimension than ferricytochrome b562. The data suggest that apocytochrome b562 resembles a "molten globule" or a "collapsed form" of the holoprotein, in which secondary structure formation is largely complete while the global folding is either only partially complete or dynamically expanded.

To search for salt-dependent structure changes that might help to explain physicochemical differences observed in previous solution studies, two-dimensional proton NMR spectra of reduced and oxidized cytochrome c were recorded at relatively high and low salt concentrations. The results rule out substantial ionic strength dependent structure change in either redox form over the salt concentrations tested (5 mM phosphate to 5 mM phosphate plus 200 mM NaCl, at pH 7). Chemical shift changes were found for several residues within a limited segment of the oxidized protein, most prominently in the sequence Lys-86, Lys-87, Lys-88, Thr-89. A salt-dependent binding of phosphate anion(s) at this site, as observed earlier by others, is indicated. The binding of one or two phosphates at the cytochrome c surface can explain earlier small-angle X-ray scattering observations of an increase in the calculated radius of gyration of the oxidized protein at the same low-salt condition used here. Other observations, by ultraviolet resonance Raman and 1D NMR spectroscopies, of salt-dependent changes could not be corroborated, but may depend on the still lower salt used and the absence of phosphate. The results obtained support the view that the absence of sizeable redox-dependent structure change observed in X-ray and NMR studies at varying salt conditions is characteristic of the protein at all salt conditions above the low millimolar range. Physicochemical differences between oxidized and reduced cytochrome c apparently represent differences in stability without patent structure change.