Lycopodium alkaloids from Palhinhaea cernua

SHORT REPORT

Lycopodium alkaloids from Palhinhaea cernua

Fu-Wei Zhao^I,II; Qian-Yun Sun^III; Fu-Mei Yang^III; Ji-Feng Luo^I; Guang-Wan Hu^I; Fang Liu^IV; Yue-Hu Wang^I,*; Chun-Lin Long^I,V,*

^IKey Laboratory of Economic Plants and Biotechnology, Kunming Institute of Botany, Chinese Academy of Sciences, Kunming 650201, People's Republic of China

^IIGraduate University of Chinese Academy of Sciences, Beijing 100049, People's Republic of China

^IIIKey Laboratory of Chemistry for Natural Products, Guizhou Province and Chinese Academy of Sciences, Guiyang 550002, People's Republic of China

^IVCollege of Landscape and Horticulture, Yunnan Agricultural University, Kunming 650201, People's Republic of China

^VCollege of Life and Environmental Sciences, Minzu University of China, Beijing 100081, People's Republic of China

ABSTRACT

Two new Lycopodium alkaloids, acetyllycoposerramine M and palcernine A were isolated from whole plant extracts of Palhinhaea cernua L. together with ten previously identified compounds. The structures of the new compounds were elucidated by spectroscopic methods and single-crystal X-ray diffraction analyses using the Flack parameter.

Keywords: Lycopodiaceae, Palhinhaea cernua, Lycopodium alkaloids

RESUMO

Dois novos alcaloides tipo-licopódio, acetil-licoposerramina M e palcernina A foram isolados de extratos de plantas inteiras de Palhinhaea cernua L. juntamente com dez compostos previamente identificados. As estruturas dos dois novos compostos foram elucidadas por métodos espectroscópicos e difratometria de Raios X de monocristal usando o parâmetro Flack.

Introduction

Palhinhaea cernua L. (syn. Lycopodium cernuum (L.); Lepidotis cernua (L.) P. Beauv.; Lycopodiella cernua (L.) Pic. Ser.), found in both the Flora of North America and the Flora of China,¹ is a member of the Lycopodiaceae family. The species is traditionally used to heal contusions, scalding and rheumatism by the Chinese people and is broadly distributed in Eastern and Southern China.² The chemical constituents of the genus Lycopodium (sensu lato) plants are mainly Lycopodium alkaloids which are quinolizine or pyridine and α-pyridone type alkaloids.^3-10 Some of these alkaloids inhibit acetylcholinesterase (AChE) and provide a challenge for total synthesis.^9-10 In our previous research on this plant, palhinine A, a novel C₁₆N-type alkaloid was isolated.⁹ We now report the isolation and structural elucidation of two new Lycopodium alkaloids (1-2) along with ten previously identified compounds (Figure 1), namely, lycoflexine N-oxide (3)⁸, lycoflexine (4),¹¹ lycodine (5),¹²α-obscurine (6),¹² dehydroisofawcettiine (7),^13,14 lycopodine (8),¹⁴ 5-acetyllycofoline (9),¹⁵ acetylfawcettiine (10),^16,17 cernuine (11)¹⁸ and lycocernuine (12).¹⁸

Results and Discussion

Acetyllycoposerramine M(1) was obtained as a colorless columnar crystal (mp 197-198 ºC). A molecular formula (C₁₈H₂₇NO₃) was assigned by HREIMS (electrospray ionization mass spectrometric) analysis (observed [M]⁺ at m/z 305.1995, calcd. [M]⁺: 305.1991), implying six degrees of unsaturation. The IR (infrared) spectrum showed absorptions for ester (1695 cm^-1) and keto (1733 cm^-1) carbonyl groups. One singlet (δ_H 2.09, 3H) and one doublet (δ_H 0.88, 3H, J 6.3 Hz, H₃-16) for two methyl groups were observed in ¹H NMR (nuclear magnetic resonance) spectrum of 1 (Table 1). Meanwhile, signals of 18 carbons were recorded in the ¹³C NMR and DEPT (distortionless enhancement by polarization) spectra (Table 2), including the presence of one ester carbonyl (δ_C 170.3), one keto carbonyl (δ_C 214.4, C-5), one sp³ quaternary, five sp³ methine, eight sp³ methylene and two methyl groups. Specifically, the signals of an oxygenated methine group were recorded at δ_H 5.23 (ddd, J 2.9, 2.9, 2.9 Hz, H-11) and δ_C 72.3 (C-11). The above mentioned analyses suggested that 1 was a lyopodine alkaloid, and shared a similar planar structure with the known Lycopodium alkaloid, lycoposerramine M.¹⁹ Differentiations between the two compounds were observed in that the additive signals (δ_H 2.09, 3H, s; δ_C 22.0 and 170.3) for an acetyl group appeared in the ¹H and ¹³C NMR spectra of 1, and the presence of a chemical shift of H-11 in 1 was downshifted compared with that of the known compound (δ_H 4.22).¹⁹ Therefore, it could be speculated that 1 was the acetylated derivative of lycoposerramine M. This was confirmed by the presence of the cross peak between H-11 and ester carbon in the HMBC (heteronuclear multiple bond correlation) spectrum of 1 (Figure 2).

To elucidate the absolute configuration, the crystals of 1 were isolated from a mixed solution of petroleum ether/Me₂CO (9:1), and single-crystal X-ray crystallographic analysis was performed (Figure 3). The results of the analysis revealed the absolute configuration of 1 to be 4R, 7R, 11R, 12R, 15R in light of the Flack parameter of 0.0(2), using anomalous dispersion with copper radiation.^20-22 Finally, 1 was elucidated as acetyllycoposerramine M.

Palcernine A (2) was obtained as a colorless block crystal (mp 98-101 ºC). Its molecular formula (C₁₇H₂₄NO₃) was established on the basis of HRESIMS for the [M + H]⁺ ion at m/z 290.1754 (calcd. 290.1756), implying seven degrees of unsaturation. The IR spectrum showed the presence of hydroxy (3441 cm^-1), ketone (1709 cm^-1) and α,β-unsaturated ketone (1631 cm^-1) groups. A signal δ_H 0.90 (d, 3H, J 6.7 Hz) for a methyl group was observed in the ¹H NMR spectrum of 2 (Table 1), and 17 carbon signals for two ketone, one tetrasubstituted double bond, two sp³ quaternary, one methine, nine methylene and one methyl groups were recorded in the ¹³C NMR and DEPT of 2 (Table 2). Three fragments, a (C-1 to C-3), b (C-9 to C-12) and c (C-8–C-15–C-16) were elucidated by the ¹H-¹H COSY (correlation spectroscopy) experiments (Figure 4). Based on the HMBC correlations (Figure 4) of H-2a/C-4, H-17a/C-2 & C-3, H-17a & H-3a/C-5, H-10b/C-12, H-11β/C-7 & C-13, H-8α/C-6, C-7 & C-12, H-15/C-13, and H₃-16/C-14 & C-13, a fawcettimine skeleton of 2 was determined, and the double bond was located at C-6, and two ketone groups were located at C-5 and C-13, respectively. After analysis of the molecular formula of 2 the presence of a hydroxy functionality in 2 was confirmed, possibly being located at C-6.

The relative configuration of partial chiral center of 2 could be elucidated, integrating ROESY (rotating-frame Overhauser effect spectroscopy) experiment and analysis on coupling constant. In ROESY spectrum (Figure 4), the cross peaks of H-8α (δ_H 2.66, dd, J 15.1, 8.0 Hz)/H-11β (δ_H 1.65, dd, J 14.7, 11.8 Hz), H-8α/H-14α (δ_H 2.49, dd, J 14.8, 4.0 Hz) and H-11β/H-14α were observed. H-14α and H-15 should be in cis configuration due to the coupling constant of J_H-14α/H-15 4.0 Hz (Table 1). Therefore, the 16-CH₃ in 2 was β-oriented, which is rare because the 16-CH₃ is α-oriented in most of the Lycopodium alkaloids.^3-5 Based on the single-crystal X-ray crystallographic analysis (Figure 5), the absolute configuration of 2 was determined to be 4S, 12S, 15S in light of the Flack parameter of 0.1(2), using anomalous dispersion with copper radiation.^20-22 Accordingly, 2 was elucidated as shown in Figure 2 and named palcernine A.

Compounds 1-3 were tested for their inhibition activities against acetylcholinesterase, butyrylcholinesterase and human chronic myelogenous leukemia K562 cells, respectively by the improved Ellman's method,^23-27 and the MTT method.²⁸ However, using these methods no inhibitory activities for the compounds were detected.

Conclusions

Two new Lycopodium alkaloids, acetyllycoposerramine M and palcernine A were isolated from whole plant extracts of Palhinhaea cernua L. together with ten previously identified compounds. The structures of the new compounds were elucidated by spectroscopic methods and single-crystal X-ray diffraction analyses using the Flack parameter. The two new Lycopodium alkaloids along with lycoflexine N-oxide were tested for their inhibition activities against acetylcholinesterase, butyrylcholinesterase and human chronic myelogenous leukemia K562 cells, respectively by the improved Ellman's method, and the MTT method. However, using these methods, none of the inhibitive activities of the compounds were detected.

Experimental

General experimental procedures

Melting points (mp) were determined using an X-4 melting point apparatus (Yingyu Yuhua Apparatus Factory, Gongyi, P. R. China), and was not adjusted. Bruker SMART APEX-II and Bruker APEX DUO diffractometers using graphite-monochromated Cu K_α radiations were employed for the intensity data collection, and the structures of compounds were solved by direct methods (SHELXS97). Optical rotations were determined on a JASCO DIP-370 automatic digital polarimeter. UV spectra were recorded on a Shimadzu double-beam 210A spectrometer. IR spectra were recorded on a Bio-Rad FTS-135 infrared spectrophotometer. 1D and 2D NMR spectra were recorded on Bruker AM-400, DRX-500 and Bruker Avance 600 spectrometers with chemical shifts in ppm relative to TMS (tetramethylsilane) as internal standard. ESIMS and HRESIMS were measured using an API QSTAR Pulsar 1 spectrometer, as well as EIMS and HREIMS using a VG Autospec Premier spectrometer. Column chromatography was performed over Al₂O₃ (200-300 mesh, Wusi Lt. Co., Shanghai), silica gel G (80-100 and 300-400 mesh), silica gel H (10-40 µm) and Sephadex LH-20 (40-70 µm, Amersham Pharmacia Biotech AB, Uppsala, Sweden). HPLC (high-performance liquid chromatographic) separations were performed using an Agilent 1200 series pump equipped with a diode array detector and a semi-preparative Zorbax SB-C₁₈ (5 µm, Φ 9.4 × 250 mm) column. TLC (thin layer chromatography) was conducted on precoated GF₂₅₄ silica gel plates (Qingdao).

Plant material

Whole plant samples of P. cernua, a nodding club-moss was collected from Liping County of Guizhou Province, P. R. China and identified by Dr. Guang-Wan Hu at the Kunming Institute of Botany, Chinese Academy of Sciences. A voucher specimen (No. SZ09048) was deposited in the Key Laboratory of Economic Plants and Biotechnology, Kunming Institute of Botany.

Extraction and isolation

The air-dried and powdered sample (16.5 kg) was three times extracted with MeOH (4, 4, and 3 h) at 70 ºC. The extracts were partitioned between EtOAc and 1% HCl/H₂O. Water-soluble materials were adjusted to pH 10 with 17% NaOH/H₂O solution and extracted with CHCl₃, producing an alkaloid-rich extract (82.0 g). The latter was submitted to a silica gel column (CHCl₃/MeOH, 1:0 to 0:1) to produce four fractions: Fr. 1-Fr. 4.

Fr. 1 was partitioned over C₁₈ column eluting with MeOH/H₂O (20% to 100%, v/v). The fraction by 50% MeOH/H₂O was chromatographed over Sephadex LH-20 (MeOH), Al₂O₃ [petroleum ether/Me₂CO/NH₃^•H₂O, 80:10:1 (v/v/v)] and silica gel [petroleum ether/Me₂CO/NH₃•H₂O, 80:10:1 (v/v/v)] columns to obtain two sub-fractions: Fr. 1-50-1 and Fr. 1-50-2. Fr. 1-50-1 was deposited at room temperature, and compound 1 (246.2 mg) was crystallized out [petroleum ether/Me₂CO, 9:1 (v/v)]. The mother solution was evaporated and the residue was subjected to silica gel with petroleum ether/Me₂CO (8:1, v/v) to yield compound 7 (26.5 mg). Fr. 1-50-2 was submitted to Al₂O₃ and silica gel columns and preparative TLC [petroleum ether/Me₂CO/NH₃^•H₂O, 80:10:1 (v/v/v)] to yield compound 3 (11.0 mg). The fraction by 60% MeOH/H₂O was purified by Sephadex LH-20 (MeOH) and preparative TLC [petroleum ether/Me₂CO/NH₃^•H₂O, 80:10:1 (v/v/v)] to yield compound 5 (14.1 mg).

Fr. 2 was chromatographed over C₁₈ column eluting with MeOH/H₂O (30% to 100%, v/v), and the fraction by 50% MeOH/H₂O was subjected to Sephadex LH-20 (MeOH) and silica gel [petroleum ether/(C₂H₅)₂NH, 100:1 (v/v)] columns to give compounds 4 (98.6 mg) and 8 (16.5 mg).

Fr. 3 was chromatographed over C₁₈ column eluting with MeOH/H₂O (30% to 100%, v/v). The fraction by 50% MeOH/H₂O was separated into two sub-fractions (Fr. 3-50-1 and Fr. 3-50-2). Fr. 3-50-1 was successively subjected to Sephadex LH-20 (MeOH) and Al₂O₃ [CHCl₃/MeOH, 30:1 (v/v)] columns to afford compound 2 (12.5 mg). Fr. 3-50-2 was submitted to Sephadex LH-20 (MeOH) and silica gel [CHCl₃/MeOH/NH₃•H₂O, 300:20:1 (v/v/v)] columns to give compound 9 (26.4 mg). The fraction by 60% MeOH/H₂O was subjected Sephadex LH-20 (MeOH) and silica gel [CHCl₃/MeOH, 30:1 (v/v/v)] columns to obtain compounds 6 (138.6 mg) and 10 (407.3 mg).

Fr. 4 was chromatographed over C₁₈ column eluting with MeOH/H₂O (30% to 100%, v/v). The fraction by 40% MeOH/H₂O was purified by Sephadex LH-20 (MeOH), and compound 12 (7.4 mg) was obtained in light of recrystallization [petroleum ether/MeOH, 9:1 (v/v)]. The fraction by 50% MeOH/H₂O was separated on Sephadex LH-20 (MeOH) and silica gel [CHCl₃/MeOH, 20:1 (v/v)] columns to yield compound 11 (16.9 mg).

Acetyllycoposerramine M (1)

Colorless columnar crystal; mp. 197-198 ºC (from petroleum ether/Me₂CO, 9:1); [α] + 45.9 (c 0.35, CHCl₃); UV (CHCl₃) λ_max/nm (log ε) 241 (2.94); IR (KBr) v_max/cm^-1 1733, 1695, 1235 and 1196; ¹H and ¹³C NMR data, see in Table 1 and Table 2; EIMS m/z (%) 305 (M⁺, 83), 262 (38), 246 (95); HREIMS m/z (%) 305.1995 (M⁺, C₁₈H₂₇NO₃⁺, calcd. 305.1991).

Crystal data for 1

C₁₈H₂₇NO₃, MW = 305.41; colorless columnar, size 0.58 × 0.42 × 0.36 mm³, orthorhombic, space group P21 21 21; a = 8.43070 (10) Å, b = 10.9756 (2) Å, c = 17.8783 (3) Å, a = b = γ = 90.00º, V = 1655.31 (5) ų, T = 296(2) K, Z = 4, ρ_calcd = 1.226 Mg m^-3, µ(Cu K_α) = 0.7 mm^-1, F(000) = 664, 10967 reflections in h(-10/10), k(-13/12), l(-21/21), measured in the range 4.73º < θ < 66.50º, completeness θ_max = 99.1%, 2843 independent reflections, R_int = 0.0386, 2878 reflections with |F|²> 2σ |F|², 202 parameters, 0 restraints, GOF = 1.116. Final R index: R₁ = 0.0424, wR₂ = 0.1089. R index (all data): R₁ = 0.0425, wR₂ = 0.1092. Flack parameter 0.0(2), largest difference peak and hole = 0.222 and -0.277 e Å^-3. The intensity data for 1 were collected on a Bruker APEX DUO diffractometer with a graphite monochromater, Cu K_α radiation. The structure of 1 was solved by direct methods (SHELXS97), expanded using difference Fourier techniques, and refined by the program and full-matrix least-squares calculations. The non-hydrogen atoms were anisotropically refined, and hydrogen atoms were fixed at calculated positions. Crystallographic data for the structure of 1 were deposited in the Cambridge Crystallographic Data Center (deposition No. CCDC 817948). Copies of the data can be obtained free of charge from the CCDC via www.ccdc.cam.ac.uk.

Palcernine A (2)

Colorless block crystal; mp. 98-101 ºC (from Me₂CO/MeOH, 5:1); [α]– 21.0 (c 0.06, MeOH); UV (MeOH) λ_max/nm (log ε) 270 (3.12), 205 (3.44); IR (KBr) ν_max/cm^-1 3441, 1709, 1631, 1384 and 1102; ¹H and ¹³C NMR data, see in Table 1 and Table 2; ESIMS m/z 290 [M + H]⁺, 601 [2M + Na]⁺, 890 [3M + Na]⁺; HRESIMS m/z 290.1754 [M + H]⁺ (C₁₇H₂₄NO₃, calcd. 290.1756).

Crystal data for 2

C₁₇H₂₃NO₃, MW = 289.36; colorless blocks, size 0.22 × 0.12 × 0.08 mm³, orthorhombic, space group P21 21 21; a = 6.35940 (10) Å, b = 13.3290 (2) Å, c = 17.5422 (3) Å, a = b = γ = 90.00º, V = 1486.96 4) ų, T = 296(2) K, Z = 4, ρ_calcd = 1.293 Mg m^-3, µ(Cu K_α) = 0.7 mm^-1, F(000) = 624, 2774 reflections in h(-6/6), k(-14/13), l(-19/15), measured in the range 4.17º < θ < 64.73º, completeness θ_max = 71.1%, 1617 independent reflections, R_int = 0.0131, 1597 reflections with |F|²> 2σ |F|², 190 parameters, 0 restraints, GOF = 1.075. Final R index: R₁ = 0.0395, wR₂ = 0.1191. R index (all data): R₁ = 0.0400, wR₂ = 0.1203. Flack parameter 0.1(2), largest difference peak and hole = 0.318 and -0.201 e Å^-3. The intensity data for 2 were collected on a Bruker SMART APEX-II diffractometer with a graphite monochromater using Cu K_α radiation. The structure of 2 was solved by direct methods (SHELXS97), expanded using difference Fourier techniques, and refined by the program and full-matrix least-squares calculations. The non-hydrogen atoms were refined anisotropically, and hydrogen atoms were fixed at calculated positions. Crystallographic data for the structure of 2 were deposited in the Cambridge Crystallographic Data Center (deposition No. CCDC 831841). Copies of the data can be obtained free of charge from the CCDC via www.ccdc.cam.ac.uk.

Bioactivity test in vitro

Acetylcholinesterase and tacrine were purchased from Sigma-Aldrich Corporation, S-butyrylthiocholine iodide and 5,5-dithiobis-2-nitrobenzoic acid (DTNB) were purchased from Fluka Chemie GmbH and Acros Organics, respectively. Inhibitory activity of all new compounds against acetylcholinesterase was performed by the improved Ellman's method, and tacrine was used as positive (IC₅₀ = 0.20 µM).

Butyrylcholinesterase was obtained from the blood serum of SD rats which were purchased from Laboratory Animal Center, Guiyang Medical University, Guizhou Province, P. R. China. S-butyrylthiocholine iodide and DTNB were purchased from Acros Organics. Inhibitory activity of all new compounds against butyrylcholinesterase was performed by the improved Ellman's method, and tetraisopropylpyrophosphoramide was treated as positive (IC₅₀ = 1.35 µM).

K562 human chronic myelogenous leukemia cell line was purchased from the China Center for Type Culture Collection, Hubei Province, P. R. China. K562 cells were maintained at 37 ºC in RPMI-1640 medium (Hyclone) containing 10% fetal bovine serum (Hangzhou Sijiqing Biological Engineering Materials Co., Ltd., P. R. China) in an atmosphere of humidified 5% CO₂. The cytotoxic assay was carried out in quintuplicate in 96-well microplates (Corning), and the amount of viable cells at the end of the incubation period was determined by using MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] assay. K562 cells were exposed for 24 h to the solution of all new compounds. Non-treated culture cells were used as a negative control. IC₅₀ was calculated by the GWBASIC software as the concentration (µM) of the compounds causing a 50% inhibition of cell viability. Adriamycin was treated as positive control (IC₅₀ = 0.96 µM).

Supplementary Information

Supplementary information is available free of charge at http://jbcs.sbq.org.br, as PDF file.

Acknowledgements

This work was sponsored by the National Natural Science Foundation of China (No. 20972166), the National Basic Research Program of China (973 Program) (No. 2009CB 526512), the Ministry of Education of China through its 111 and 985 Projects (B08044 and MUC 985-9), and the Knowledge Innovation Project of the Chinese Academy of Sciences. We appreciate Dr. Jia-Liang Zhong (at the Shanghai Institute of Pharmaceutical Industry) and Xiao-Nian Li (at the Kunming Institute of Botany) who measured and elucidated the crystal structure. We are also grateful to Adam Nerin (Ph.D. candidate at the City University of New York, USA) and Jeffrey R. Boutain (Ph.D. candidate at the University of Hawaii at Manoa, USA) for editing the English.

Submitted: July 6, 2011

Published online: December 1, 2011

[Supplementary material]

• 1. Zhang, X. C.; Zhang, L. B.; Flora Reipublicae Popularis Sinicae; Science Press: Beijing, China, 2004, ch. 6(3).
• 2. Chinese Herbs Edition Board; Chinese Herbs, Shanghai Science and Technology Press: Shanghai, China, 2005, ch. 2.
• 3. Hirasawa, Y.; Kobayashi, J.; Morita, H.; Heterocycles 2009, 77, 679.
• 4. Ayer, W. A.; Nat. Prod. Rep. 1991, 8, 455.
• 5. Ma, X.; Gang, D. R.; Nat. Prod. Rep. 2004, 21, 752.
• 6. Ishiuchi, K.i.; Kubota, T.; Ishiyama, H.; Hayashi, S.; Shibata, T.; Kobayashi, J.; Tetrahedron Lett. 2011, 52, 289.
• 7. Ishiuchi, K.i.; Kubota, T.; Ishiyama, H.; Hayashi, S.; Shibata, T.; Mori, K.; Obara, Y.; Nakahata, N.; Kobayashi, J.; Bioorg. Med. Chem. 2011, 19, 749.
• 8. Katakawa, K.; Mito, H.; Kogure, N.; Kitajima, M.; Wongseripipatana, S.; Arisawa, M.; Takayama, H.; Tetrahedron 2011, 67, 6561.
• 9. Zhao, F. W.; Sun, Q. Y.; Yang, F. M.; Hu, G. W.; Luo, J. F.; Tang, G. H.; Wang, Y. H.; Long, C. L.; Org. Lett. 2010, 12, 3922.
• 10. Kitajima, M.; Takayama, H.; Topics in Current Chemistry; Springer-Verlag: Heidelberg, 2011, vol. 1.
• 11. Takayama, H.; Katakawa, K.; Kitajima, M.; Yamaguchi, K.; Aimi, N.; Tetrahedron Lett. 2002, 43, 8307.
• 12. Nakashima, T. T.; Singer, P. P.; Browne, L. M.; Ayer, W. A.; Can. J. Chem. 1975, 53, 1936.
• 13. Burnell, R. H.; Taylor, D. R.; Tetrahedron 1961, 15, 173.
• 14. MacLean, D. B.; Can. J. Chem. 1963, 41, 2654.
• 15. Anet, F. A. L.; Ahmad, M.; Khan, N. H.; Can. J. Chem. 1962, 40, 236.
• 16. Burnell, R. H.; Mootoo, B. S.; Taylor, D. R.; Can. J. Chem. 1960, 38, 1927.
• 17. Laemmerhold, K. M.; Breit, H.; Angew. Chem., Int. Ed. 2010, 49, 2367.
• 18. Braekman, J. C.; Nyembo, L.; Bourdoux, P.; Kahindo, N.; Hootele, C.; Phytochemistry 1974, 13, 2519.
• 19. Takayama, H.; Katakawa, K.; Kitajima, M.; Yamaguchi, K.; Aimi, N.; Chem. Pharm. Bull. 2003, 51, 1163.
• 20. Flack, H.; Acta Crystallogr., Sect. A: Found. Crystallogr. 1983, 39, 876.
• 21. Flack, H. D.; Bernardinelli, G.; J. Appl. Crystallogr. 2000, 33, 1143.
• 22. Sheldrick, G. M; SHELXS-97, Program for Crystal Structure Solution; University of Gottingen: Gottingen, Germany, 1997.
• 23. Ellman, G. L.; Courtney, K. D.; Andres, V. J.; Featherstone, R. M.; Biochem. Pharmacol. 1961, 7, 88.
• 24. Rhee, I. K.; Appels, N.; Hofte, B.; Karabatak, B.; Erkelens, C.; Stark, L. M.; Flippin, L. A.; Verpoorte, R.; Biol. Pharm. Bull. 2004, 27, 1804.
• 25. Sun, Q. Y.; Yang, F. M.; Chin. Pharm. Bull. 2008, 24, 1387.
• 26. Vogel, H. G.; Vogel, W. H.; Drug Discovery and Evaluation: Pharmacological Assays; Springer: New York, 2008.
• 27. Yang, F. M.; Sun, Q. Y.; Chin. Pharm. Bull. 2009, 25, 690.
• 28. Mosmann, T.; J. Immunol. Methods 1983, 65, 55.

Publication Dates

• Publication in this collection
  02 Mar 2012
• Date of issue
  Feb 2012

History

• Received
  06 July 2011
• Accepted
  01 Dec 2011