Characteristic ion clusters as determinants for the identification of pyrrolizidine alkaloid N‐oxides in pyrrolizidine alkaloid–containing natural products using HPLC–MS analysis

Pyrrolizidine alkaloid (PA)–containing plants are widely distributed in the world. PAs are hepatotoxic, affecting livestock and humans. PA N‐oxides are often present together with PAs in plants and also exhibit hepatotoxicity but with less potency. HPLC–MS is generally used to analyze PA‐containing herbs, although PA references are unavailable in most cases. However, to date, without reference standards, HPLC–MS methodology cannot distinguish PA N‐oxides from PAs because they both produce the same characteristic ions in mass spectra. In the present study, the mass spectra of 10 PA N‐oxides and the corresponding PAs were systemically investigated using HPLC–MS to define the characteristic mass fragment ions specific to PAs and PA N‐oxides. Mass spectra of toxic retronecine‐type PA N‐oxides exhibited two characteristic ion clusters at m/z 118–120 and 136–138. These ion clusters were produced by three unique fragmentation pathways of PA N‐oxides and were not found in their corresponding PAs. Similarly, the nontoxic platynecine‐type PA N‐oxides also fragmented via three similar pathways to form two characteristic ion clusters at m/z 120–122 and 138–140. Further application of using these characteristic ion clusters allowed successful and rapid identification of PAs and PA N‐oxides in two PA‐containing herbal plants. Our results demonstrated, for the first time, that these characteristic ion clusters are unique determinants to discriminate PA N‐oxides from PAs even without the availability of reference samples. Our findings provide a novel and specific method to differentiate PA N‐oxides from PAs in PA‐containing natural products, which is crucial for the assessment of their intoxication. Copyright © 2012 John Wiley & Sons, Ltd.     

A tandem mass spectrometric investigation of N-(3-ferrocenyl-2-naphthoyl) dipeptide ethyl esters and N-(6-ferrocenyl-2-naphthoyl) dipeptide ethyl esters

N‐(3‐Ferrocenyl‐2‐naphthoyl) dipeptide ethyl esters 1–4 and N‐(6‐ferrocenyl‐2‐naphthoyl) dipeptide ethyl esters 5–8 were prepared by coupling either 3‐ferrocenylnaphthalene‐2‐carboxylic acid or 6‐ferrocenylnaphthalene‐2‐carboxylic acid to the dipeptide ethyl esters GlyGly(OEt) (1, 5), AlaGly(OEt) (2, 6), GlyPhe(OEt) (3, 7) and GlyLeu(OEt) (4, 8), using the standard N‐(3‐dimethylaminopropyl)‐N'‐ethylcarbodiimide hydrochloride, 1‐hydroxybenzotriazole protocol. Electrospray ionization mass spectrometry (ESI‐MS) and laser desorption ionization mass spectrometry (LDI‐MS) were employed in conjunction with tandem mass spectrometry in the analysis of N‐(3‐ferrocenyl‐2‐naphthoyl) dipeptide ethyl esters 1–4 and N‐(6‐ferrocenyl‐2‐naphthoyl) dipeptide ethyl esters 5–8. Radical cations, [M]^+? and [M + H]⁺ species were both observed in the mass spectra. Intense sodium [M + Na]⁺ and potassium [M + K]⁺ adducts were also present. An important diagnostic ion at m/z [M–65]⁺ was observed in both the MS and MS/MS spectra of the N‐(3‐ferrocenyl‐2‐naphthoyl) dipeptide derivatives. Sequence‐specific ions were generally not observed in the MS/MS spectra of the N‐(3‐ferrocenyl‐2‐naphthoyl) series due to formation of the diagnostic [M–65]⁺ ion. Sequence‐specific ions were observed in the MS/MS spectra of the N‐(6‐ferrocenyl‐2‐naphthoyl) dipeptide esters with charge retention on the derivatized N‐terminal of the dipeptide. Both series of compounds could be successfully analyzed by MALDI without the use of a matrix (LDI). Copyright © 2011 John Wiley & Sons, Ltd.     

甾体磷酰胺类缀合物在电喷雾质谱中的裂解特征

Novel steroidal phosphoramidate conjugates of 3'-azido-2',3'-dideoxythymidine(AZT)and amino acid esterswere synthesized and determined by positive and negative ion electrospray ionization mass spectrometry.The MSfragmentation behaviors of the steroidal phosphoramidate conjugates have been investigated in conjunction withtandem mass spectrometry of ESI-MS/MS.There were three characteristic fragment ions in the positive ion ESImass spectra,which were the Na adduct ions with loss of steroidal moiety,amino acid ester moiety from pseudomolecular ion(M Na)~ ,and the phosphoamino acid methyl ester Na adduct ion by α-cleavage of the phosphora-midate respectively.The main fragment ions in negative ion ESI mass spectra were the ion(M-HN_3)~-,the ion(M-AZT-H)~-,and the ion(M-steroidal moiety-H)~- besides the pseudo molecular ion(M-H)~-.Thefragmentation patterns did not depend on the attached amino acid ester moiety.     

Evaluation of the α‐phenylvinyl cation as a chemical ionization reagent for the differentiation of isomeric substituted phenols in an ITMS

Ion–molecule reactions between the α‐phenylvinyl cation and isomeric naturally occurring phenols were investigated using a quadruple ion trap mass spectrometer. The α‐phenylvinyl cation m/z 103, generated by chemical ionization from phenylacetylene, reacts with neutral aromatic compounds to form the characteristic species: [M + 103]⁺ adduct ions and the trans‐vinylating product ions [M + 25]⁺, which correspond to [M + 103]⁺ adduct after the loss of benzene. Isomeric differentiation of several ring‐substituted phenols was achieved by using collision‐induced dissociation of the [M + 103]⁺ adduct ions. This method also showed to be effective in the differentiation of 4‐ethylguaiacol from one of its structural isomers that displays identical EI and EI/MS/MS spectra. The effects of gas‐phase alkylation with phenylvinyl cation on the dissociation behavior were examined using mass spectrometryⁿ and labeled derivatives. Copyright © 2015 John Wiley & Sons, Ltd.     

Direct analysis in real time mass spectrometry (DART‐MS) of highly non‐polar low molecular weight polyisobutylenes

Low molecular weight polyisobutylenes (PIB) with chlorine, olefin and succinic acid end‐groups were studied using direct analysis in real time mass spectrometry (DART‐MS). To facilitate the adduct ion formation under DART conditions, NH₄Cl as an auxiliary reagent was deposited onto the PIB surface. It was found that chlorinated adduct ions of olefin and chlorine telechelic PIBs, i.e. [M + Cl]^? up to m/z 1100, and the deprotonated polyisobutylene succinic acid [M? H]^? were formed as observed in the negative ion mode. In the positive ion mode formation of [M + NH₄]⁺, adduct ions were detected. In the tandem mass (MS/MS) spectra of [M + Cl]^?, product ions were absent, suggesting a simple dissociation of the precursor [M + Cl]^? into a Cl^? ion and a neutral M without fragmentation of the PIB backbones. However, structurally important product ions were produced from the corresponding [M + NH₄]⁺ ions, allowing us to obtain valuable information on the arm‐length distributions of the PIBs containing aromatic initiator moiety. In addition, a model was developed to interpret the oligomer distributions and the number average molecular weights observed in DART‐MS for PIBs and other polymers of low molecular weight. Copyright © 2015 John Wiley & Sons, Ltd.     

Characterization of N‐Boc/Fmoc/Z‐N′‐formyl‐gem‐diaminoalkyl derivatives using electrospray ionization multi‐stage mass spectrometry

N‐Boc/Fmoc/Z‐N′‐formyl‐gem‐diaminoalkyl derivatives, intermediates particularly useful in the synthesis of partially modified retro‐inverso peptides, have been characterized by both positive and negative ion electrospray ionization (ESI) ion‐trap multi‐stage mass spectrometry (MSⁿ). The MS² collision induced dissociation (CID) spectra of the sodium adduct of the formamides derived from the corresponding N‐Fmoc/Z‐amino acids, dipeptide and tripeptide acids show the [M + Na‐NH₂CHO]⁺ ion, arising from the loss of formamide, as the base peak. Differently, the MS² CID spectra of [M + Na]⁺ ion of all the N‐Boc derivatives yield the abundant [M + Na‐C₄H₈]⁺ and [M + Na‐Boc + H]⁺ ions because of the loss of isobutylene and CO₂ from the Boc protecting function. Useful information on the type of amino acids and their sequence in the N‐protected dipeptidyl and tripeptidyl‐N′‐formamides is provided by MS² and subsequent MSⁿ experiments on the respective precursor ions. The negative ion ESI mass spectra of these oligomers show, in addition to [M‐H]^?, [M + HCOO]^? and [M + Cl]^? ions, the presence of in‐source CID fragment ions deriving from the involvement of the N‐protecting group. Furthermore, MSⁿ spectra of [M + Cl]^? ion of N‐protected dipeptide and tripeptide derivatives show characteristic fragmentations that are useful for determining the nature of the C‐terminal gem‐diamino residue. The present paper represents an initial attempt to study the ESI‐MS behavior of these important intermediates and lays the groundwork for structural‐based studies on more complex partially modified retro‐inverso peptides. Copyright © 2013 John Wiley & Sons, Ltd.     

Studies on the Mass Spectra of Monocyclic N‐Aryl‐δ‐Valerolactams

The fragmental behavior of some monocyclic N‐aryl‐δ‐valerolactams in EI‐MS was studied. Their molecular ion peak, together with some characteristic fragments such as [M‐29]⁺, [M‐56]^+?, [M‐69]⁺, and [M‐98]⁺, were always found in a series of N‐aryl‐δ‐valerolactams in EI‐MS spectra. Furthermore, the mechanism for the interpretation of each fragment is described.     

Antioxidant Pathways and One‐Electron Oxidation of Dopamine and Cysteine in Electrospray and On‐Line Electrochemistry Electrospray Ionization Mass Spectrometry

Dopamine [DA]⁺ (m/z 154), DA dimer [2DA‐H]⁺ (m/z 307) and DA quinone [DAQ]⁺ (m/z 152) are detected in positive ion mode electrospray ionization mass spectrometry (ESI MS) of dopamine in 50/1/49 (vol%) water/acetic acid/methanol. H/D exchange experiments support a covalent structure of DA dimer. Thus, ESI of DA may involve 1e^?, 1H⁺ oxidation processes followed by rapid radical dimerization. The DA quinone signal is low in ESI MS, which indicates a low efficiency of the 2e^?, 2H⁺ oxidation reaction. On‐line electrochemistry ESI MS (EC/ESI MS) with low electrochemical cell voltage floated on high ES voltage increases electrospray current and improves sensitivity for DA. The DA quinone signal increases and DA dimer signal decreases. A new configuration of the ESI MS instrument with a cone‐shaped capillary inlet significantly enhanced sensitivity of ESI and EC/ESI MS measurements. A DA quinone‐cysteine adduct [DAQ+Cys]⁺ was detected in solutions of DA with cysteine (Cys). ESI MS and EC/ESI MS indicate formation of the DA quinone‐cysteine adduct by 1e^? pathway. Oxidation pathways in ESI MS are relevant to biological reactivity of DA and Cys.     

Competition between Nucleophilic Substitution of Halogen (SNAr) versus Substitution of Hydrogen (SNArH)—A Mass Spectrometry and Computational Study

The mechanism of intramolecular gas‐phase reactions of N‐(2‐X‐5‐nitrophenyl)‐N‐methylacetamide carbanions (X=H, F, Cl) has been studied using negative ion electrospray mass spectrometry ((?)ESI‐MS) technique and modelled computationally. It was proven that all three anions form cyclic σ^H adducts, which undergo elimination of water. In the case of X=F, formation of the σ^F adduct, leading to S_NAr reaction, was a competing process. This is the first proof that also in the gas phase formation of σ^H adduct proceeds faster than σ^X adduct and only when X=F, rates of these two processes are comparable. The experimental results are in full agreement with quantum chemical calculations.     

Studies on the Mass Spectra of N‐Aryl α,β‐Unsaturated γ‐Lactams

The mass spectra of a series of N‐aryl α,β‐unsaturated γ‐lactams were studied. Besides the molecular ion, the three characteristic fragments such as [M⁺‐29], [M⁺‐55], and [M⁺‐82] were commonly found in a series of N‐Aryl α,β‐unsaturated γ‐lactams in EI/MS. Further more the mechanism for the interpretation of these fragments is also de scribed.     

Dehydrogenation and dehalogenation of amines in MALDI‐TOF MS investigated by isotopic labeling

Secondary and tertiary amines have been reported to form [M–H]⁺ that correspond to dehydrogenation in matrix‐assisted laser desorption ionization time of flight mass spectrometry (MALDI‐TOF MS). In this investigation, we studied the dehydrogenation of amines in MALDI‐TOF MS by isotopic labeling. Aliphatic amines were labeled with deuterium on the methylene of an N‐benzyl group, which resulted in the formation of [M–D]⁺ and [M–H]⁺ ions by dedeuteration and dehydrogenation, respectively. This method revealed the proton that was removed. The spectra of most tertiary amines with an N‐benzyl group showed high‐intensity [M–D]⁺ and [M–H]⁺ ion peaks, whereas those of secondary amines showed low‐intensity ion peaks. Ratios between the peak intensities of [M–D]⁺ and [M–H]⁺ greater than 1 suggested chemoselective dehydrogenation at the N‐benzyl groups. The presence of an electron donor group on the N‐benzyl groups enhanced the selectivity. The dehalogenation of amines with an N‐(4‐halobenzyl) group was also observed alongside dehydrogenation. The amino ions from dehalogenation can undergo second dehydrogenation. These results provide the first direct evidence about the position at which dehydrogenation of an amine occurs and the first example of dehalogenation of haloaromatic compounds in MALDI‐TOF MS. These results should be helpful in the structural identification and elucidation of synthetic and natural molecules. Copyright © 2013 John Wiley & Sons, Ltd.     

Structural identification of hindered amine light stabilisers in coil coatings using electrospray ionisation tandem mass spectrometry

Hindered amine light stabilisers (HALS) are the most effective antioxidants currently available for polymer systems in post‐production, in‐service applications, yet the mechanism of their action is still not fully understood. Structural characterisation of HALS in polymer matrices, particularly the identification of structural modifications brought about by oxidative conditions, is critical to aid mechanistic understanding of the prophylactic effects of these molecules. In this work, electrospray ionisation tandem mass spectrometry (ESI‐MS/MS) was applied to the analysis of a suite of commercially available 2,2,6,6‐tetramethylpiperidine‐based HALS. Fragmentation mechanisms for the [M + H]⁺ ions are proposed, which provide a rationale for the product ions observed in the MS/MS and MS³ mass spectra of N‐H, N‐CH₃, N‐C(O)CH₃ and N‐OR containing HALS (where R is an alkyl substituent). A common product ion at m/z 123 was identified for the group of antioxidants containing N‐H, N‐CH₃ or N‐C(O)CH₃ functionality, and this product ion was employed in precursor ion scans on a triple quadrupole mass spectrometer to identify the HALS species present in a crude extract from of a polyester‐based coil coating. Using MS/MS, two degradation products were unambiguously identified. This technique provides a simple and selective approach to monitoring HALS structures within complex matrices. Copyright © 2010 John Wiley & Sons, Ltd.     

Rapid characterization of triterpene saponins from Conyza blinii by liquid chromatography coupled with mass spectrometry

Conyza blinii Le'vl is a medicinal herb used for the treatment of inflammation in Chinese folk medicine. Its major bioactive constituents are triterpene saponins, most of which contain 6–8 sugar residues. In this report, electrospray ionization tandem mass spectrometry fragmentation behaviors of bisdesmosidic triterpene saponins (conyzasaponin A, B, and C) were studied in both positive and negative ion modes with an ion‐trap mass spectrometer. In full scan mass spectrometry, these saponins gave predominant [M–H]^? and [M+Na]⁺ ions, which determined the molecular weights. In tandem mass spectrometry (MSⁿ, n = 2–4), the [M–H]^? and [M+Na]⁺ ions yielded fragments [Y_0α–H]^? and [B_α+Na]⁺, which were diagnostic for the structures of the triterpene skeleton and sugar chains. The structural elucidation was approved by accurate mass data using IT‐TOF‐MS. An interpretation guideline based on MSⁿ (n = 2–4) diagnostic ions was proposed in order to elucidate the chemical structures of unknown triterpene saponins in C. blinii extract. The saponins in C. blinii were separated by liquid chromatography with a methanol/acetonitrile/water solvent system, and then analyzed by ion‐trap and IT‐TOF mass spectrometers. Based on the interpretation guideline, a total of 35 triterpenoid saponins were tentatively identified. Among them, 15 saponins had been previously reported, and the other 20 saponins were reported from Conyza species for the first time. This study indicates that LC/MS is a powerful technology for the rapid characterization of complicated saponins in herbal extracts. Copyright © 2010 John Wiley & Sons, Ltd.     

Structural characterization and identification of oleanane-type triterpene saponins in Glycyrrhiza uralensis Fischer by rapid-resolution liquid chromatography coupled with time-of-flight mass spectrometry

Oleanane‐type triterpene saponins (OTS) are major active ingredients in Glycyrrhiza uralensis. In this work, a rapid‐resolution liquid chromatography with time‐of‐flight mass spectrometry (RRLC/TOF‐MS) method has been developed to characterize and identify OTS from G. uralensis. The major diagnostic ions and fragmentation pathways from thirteen OTS have been characterized for the first time. At a low fragmentor voltage of 120 V in positive ion mode, the precursor ion [M + H]⁺ or/and [M + Na]⁺ was obtained for accurate determination of molecular formula. When the fragmentor voltage was increased to 425 V, abundant characteristic fragment ions were observed for structural characterization. Neutral losses of sugar moieties, such as glucuronic acid (GlcA, 176 Da), glucose (Glc, 162 Da) and rhamnose (Rha, 146 Da), were commonly observed in the MS spectra for prediction of the sugar number and sequences. Other typical losses included AcOH (60 Da), CH₂O (30 Da), 2 × H₂O (2 × 18 Da) and HCOOH (46 Da) from [Aglycone + H–H₂O]⁺ (named [B]⁺), corresponding to the presence of a C₂₂‐acetyl group, C₂₄‐hydroxyl group, C₂₂‐hydroxyl group or C₃₀‐carboxyl group on the aglycone moiety, respectively. In particular, characteristic ring cleavages of the aglycone moieties on A‐ and B‐rings were observed. Based on the fragmentation patterns of reference compounds, nineteen OTS have been identified in an extract of G. uralensis, thirteen of which were unambiguously identified and the other six were tentatively assigned. Copyright © 2010 John Wiley & Sons, Ltd.     

Identification of five hepatotoxic pyrrolizidine alkaloids in a commonly used traditional Chinese medicinal herb, Herba Senecionis scandentis (Qianliguang)

Senecio scandens Buch.-Ham is a plant source for a commonly used traditional Chinese medicinal (TCM) herb Qianliguang. A TCM herbal proprietary product containing Qianliguang as the major herb for the treatment of sinusitis has been used in China for several decades, and has also been exported to other regions and countries worldwide. In the present study, the aqueous extract of S. scandens collected in the Shanxi Province of China was determined, for the first time, to contain hepatotoxic and tumorigenic pyrrolizidine alkaloids (PAs) by using high-performance liquid chromatography/mass spectrometric (HPLC/MS) analysis in various scanning modes. A total of nine toxic and two non-toxic PAs were detected in the aqueous extract of S. scandens, of which six PAs, namely neoplatyphylline, senecionine, senecionine N-oxide, seneciphylline, seneciphylline N-oxide and senkirkine, were unequivocally characterized, while other PAs were tentatively assigned as jacobine, jacozine N-oxide (or erucifoline N-oxide), 7-tigloylplatynecine, usaramine and an isomer of yamataimine. The estimated total content of toxic PAs in S. scandens was 10.82 microg/g herb, which was significantly higher than that (> or =1 microg/g herb) recommended by Belgium and Germany not to be used clinically. Among the PAs definitively identified, senecionine, seneciphylline, and senkirkine are known tumorigens capable of inducing liver tumors in experimental animals, while seneciphylline N-oxide and senecionine N-oxide are probably tumorigenic due to their potential conversion into seneciphylline and senecionine via metabolic reduction in the body. Thus, the current finding of the presence of toxic/tumorigenic PAs in S. scandens challenges the safety of using this TCM herb and its proprietary products.     

Differentiation of lisinopril and its RSS diastereomer by liquid chromatography combined with collision‐induced dissociation mass spectrometry

A simple and sensitive liquid chromatography tandem multiple‐stage mass spectrometry (HPLC/MS/MS) method suitable for bulk lisinopril analysis was developed, by which lisinopril and its RSS isomer were separated and differentiated. In the collision‐induced dissociation (CID) mass spectra of the [M + H]⁺ ions, the abundance of the fragment ion of m/z 246 for lisinopril was about two times higher than the ion of m/z 245; however, the former fragment ion was noted to be a little lower than the latter for RSS isomer at all collision energies. In the CID mass spectra of the [M + Li]⁺ ion, the abundance of the rearrangement ion of m/z 315 for the RSS isomer was about three times higher than that for lisinopril. Furthermore, the difference was supported by the results of energy‐resolved mass spectrometry (ERMS) in the test range of collision energies. Similar differences were also observed between the CID mass spectra of lisinopril and RSS isomer methylester, which indicated that the RSS isomer could be rapidly characterized by the CID mass spectra of both the protonated and lithium adduct ion. Elemental compositions of all the ions were confirmed by Fourier Transform ion cyclotron resonance ESI mass spectrometry (FT‐ICR‐ESI/MS). In addition, theoretical computations were carried out to support the experimental results. Copyright © 2009 John Wiley & Sons, Ltd.     

Thermospray mass spectrometry of N-methylcarbamates

The thermospray mass spectrometry (TSP/MS) of five N-methylcarbamates is presented. This is the first time that ions other than [M + H]⁺ and [M + NH₄]⁺ have been reported using positive TSP/MS. Protonation of ROCONHCH₃ yields the [CH₃NH₂CO]^+· ion, with formation of the ion–molecule adduct [ROCONHCH₃ · CH₃NH₂CO]^+· through elimination of CO^+· from [CH₃NH₂CO]^+·, and the adduct [M + 75]^+·, [ROCONHCH₃ · OCONH₂CH₃]^+·, is also obtained.     

Tandem mass spectrometric approaches for the analysis of alkylguanines in human urine

Human exposure to carcinogenic alkylating agents can lead to the formation of covalently bound adducts in DNA, some of which are excreted in urine as alkylated purines following DNA degradation and repair. Tandem mass spectrometric methods have been developed for the qualitative and quantitative determination of such alkylpurines in human urine. Short-chain alkyl- and hydroxyalkylguanines have been synthesized with the substituents at the N-7-, O⁶- and N2-positions of guanine. Examination of the product ion scans of their molecular ions (electron impact (EI) ionization) revealed that the ion at m/z 151, [guanine]⁺, was common to all of the alkylguanines studied, with the exception of the methylated analogues. Precursor ion scans of this ion on partially purified human urine extracts showed the presence of several ions (e.g. m/z 179, 195) which were consistent with molecular ions for alkylguanines. The presence of these and other constituents was confirmed by product ion spectra of molecular ions (EI and fast atom bombardment), and by high-performance liquid chromatographic separation prior to tandem mass spectrometry (MS/MS). Evidence was obtained for the presence of N-7-methyl-, N²-dimethyl-, N²-dimethyl-, N²-ethyl- and N-7-(2-hydroxyethyl)guanine. Quantitative methods were established for these five alkyl guanines using gas chromatography mass spectrometry (GC/MS) and GC/MS/MS. Deuterated internal standards were synthesized and added to the urine prior to extraction of alkylpurines by Sep-Pak cartridge chromatography. The products were converted into their tert-butyldimethylsilyl derivatives and analysed by selected ion monitoring (SIM) of [M – 57]⁺ or by multiple reaction monitoring (MRM) of the fragmentation M^+˙ → [M – 57]⁺. The MRM method yielded values for N-7-methylguanine of 2.57 ± S.D. 1.32 mg day^?1 (n = 6), N2-methylguanine of 0.31 ± 0.10 mg day^?1 (n = 10) and N²-dimethylguanine of 0.21 ± 0.23 mg day^?1 (n = 10). N²-Ethyl- and N-7-(2-hydroxyethyl)guanine could only be detected by SIM at levels of ～0.5 and 2 μg day^?1, respectively. The MRM analyses, although inherently less sensitive than the SIM analyses, exhibit greater selectivity and consequently fewer contaminant ions.     

Synthesis and Structures of N‐Alkyl‐1,13‐dimethoxychromeno‐ [2,3,4‐kl]acridinium Salts: The Missing Azaoxa[4]helicenium

Helical structures are interesting due to their inherent chirality. Helicenium ions are triarylmethylium structures twisted into configurationally stable helicenes through the introduction of two heteroatom bridges between the three aryl substituents. Of the configurationally stable [4]helicenium ions, derivatives with sulfur, oxygen and nitrogen bridges have already been synthesised. However, one [4]helicenium ion has proven elusive, until now. We present herein the first synthesis of the 1,13‐dimethoxychromeno[2,3,4‐kl]acridinium (DMCA⁺) [4]helicenium ion. A series of six differently N‐substituted DMCA⁺ ions as their hexafluorophosphate salts are reported. Their cation stability was evaluated and it was found that DMCA⁺ is ideally suited as a phase‐transfer catalyst with a pK_R+ of 13.0. The selectivity of nucleophilic addition to the central carbon atom of DMCA⁺ has been demonstrated with diastereotopic ratios of up to 1:10. The single‐crystal structures of several of the DMCA⁺ salts were determined, and structural differences between N‐aryl‐ and N‐alkyl‐substituted cations were observed. The results of a comparative study of the photophysics of the [4]helicenium ions are presented. DMCA⁺ is found to be a potent red‐emitting dye with a fluorescence quantum yield of 20 % in apolar solvents and a fluorescence lifetime of 12 ns. [4]Helicenium ions, including DMCA⁺, all suffer from solvent‐induced quenching, which reduces the fluorescence quantum yields significantly (?_fl<5 %) in polar solvents. A difference in photophysical properties is observed between N‐aryl‐ and N‐alkyl‐substituted DMCA⁺, which has tentatively been attributed to a difference in molecular conformation.     

HPLC/QTOF‐MS/MS application to investigate phenolic constituents from Ficus pandurata H. aerial roots

Ficus pandurata H. aerial roots are used as a traditional Chinese medicine for the treatment of uarthritis, indigestion and hyperuricemia. However, the bioactive constituents responsible for the pharmacological effects of F. pandurata H. are unclear. A simple and efficient HPLC/QTOF‐MS/MS (high‐performance liquid chromatography/electrospray ionization with quadrupole time‐of‐flight tandem mass spectrometry) method was established to detect and identify active constituents in the n‐butanol extract of F. pandurata H. aerial roots. Chemical constituents were separated and investigated by HPLC/QTOF‐MS/MS in the negative‐ion mode. Thirty‐seven compounds, including hydroxycinnamic acid derivatives, hydroxybenzoic acid derivatives, hydroquinone glycosides, flavonoid glycosides, etc., were identified or tentatively characterized in the n‐butanol extract of F. pandurata H. aerial roots by comparing the UV spectra, accurate mass spectra and fragmentation pathways and retrieving the reference literatures. Moreover, the flavonoid trisaccharides and hydroxybenzoic acid derivatives were tentatively characterized in F. pandurata H. for the first time. The analytical tool used here is very valuable in the rapid separation and identification of the multiple and minor constituents in the n‐butanol extract of F. pandurata H. aerial roots. Copyright © 2014 John Wiley & Sons, Ltd.