Solvent-induced construction of two zinc supramolecular isomers: synthesis, framework flexibility, sensing properties, and adsorption of dye molecules

Two zinc supramolecular isomers, formulated as a-[Zn(cis-cppca)(H2O)]n (1) and b-[Zn(trans-cppca)(H2O)]n (2), have been synthesized under solvothermal conditions and fully characterized (H2cppca 1⁄4 5-(3carboxy-phenyl)-pyridine-2-carboxylic acid). Solvent-induced conformational isomerism of cppca ligand and framework isomerism of 1 and 2 are both observed. Each distorted [Zn(cis-cppca)]2 unit joined four equivalent ones in an abnormal 2D puckered layer in compound 1; however, compound 2 displays a 3-D open framework with larger bi-edge channels constructed by 1D Zn–O–Zn chains and trans-cppca bridges, in which the internal edge is a nearly hexagonal channel with an approximate dimension of 14.51 Å and the external edge is a circular channel with an approximate dimension of 19.26 Å. Fluorescent properties indicated that compound 1 displays an apparent fluorescence emission in acetone solvent; however, compound 2 was very sensitive to acetone molecules which results in nearly 100% fluorescence quenching. Moreover, compounds 1 and 2 might be promising luminescent probes for the detection of Fe. The dye adsorption experiments demonstrated that compound 2 showed good adsorption properties for MB and RhB molecules.


Introduction
Supramolecular isomerism, which possesses the same molecular components but different network superstructures, represents an intriguing and important phenomenon in the construction of coordination compounds and has received considerable attention in recent decades. 1 Many coordination polymers with supramolecular isomerism have been reported; however, controllable synthesis of supramolecular isomers still remains a long-term challenge because the self-assembly processes are oen inuenced by compositional and process parameters such as reaction temperature, 2 solvent, 3 template, 4 light, 5 reaction time, 6 concentration effect, 7 and pH value. 2f,8 In a word, research on selfassembly, crystallization, and structure-property relationships for supramolecular isomerism is also meaningful for crystal engineering of coordination compounds. Supramolecular isomerism has been categorized by Moulton and Zaworotko into four different kinds: structural, conformational, catenane, and optical isomerism. 1a Hitherto, the commonly mentioned strategy for the construction of conformational isomerism is the introduction of exible ligands into the coordination frameworks due to the conformational exibility of such species. 8b,9 However, the conformational isomers of coordination compounds based on conformational exibility of rigid multidentate ligands are rarely observed. 10 As we know, there exists in C-N or C-C single bond between aromatic rings, thus such rigid ligands allow conformational changes due to a certain degree of rotations around the C-N or C-C bonds; for example, cis-and trans-conformations are easily observed in the conjugated Nheterocyclic dicarboxylic ligand, 5-(3-carboxy-phenyl)-pyridine-2carboxylic acid (H 2 cppca), because the C-C single bonds between phenyl and pyridyl rings can rotate freely. If the orientation of the nitrogen atom of the pyridine group and the carboxyl group associated with the benzene ring in the H 2 cppca ligand is in the same direction, the conformation is dened as 'cis' conguration and if they are in the opposite direction, the conguration is assigned as 'trans' conformation (Scheme 1). Based on the abovementioned analysis, the H 2 cppca ligand might be an effective component in the design of conformational isomers.
Following our interests regarding uorescent materials, 11 herein, we report two zinc supramolecular isomers formulated as a-[Zn(cis-cppca)(H 2 O)] n (1) and b-[Zn(trans-cppca)(H 2 O)] n (2). Compounds 1 and 2 were fully characterized by single-crystal Xray diffraction, elemental analysis, IR spectroscopy, thermal analysis, and powder X-ray diffraction. The uorescence properties of compounds 1 and 2 in the solid state and in various solvent emulsions also were investigated.

Materials and physical measurements
All reagents and solvents were commercially available and used as received without further purication. Elemental analysis (C, H and N) was performed on a PerkinElmer 240C elemental analyzer. Infrared (IR) spectra were obtained with KBr Pellets on a PerkinElmer Spectrum One FTIR spectrometer in the range of 4000-400 cm À1 with a resolution (4.0 cm À1 ) and the scan's numbers (10). Powder X-ray diffraction (PXRD) patterns of the samples were recorded by a RIGAKU-DMAX2500 X-ray diffractometer using Cu-Ka radiation (l ¼ 1.542Å) with a scanning rate of 10 min À1 and a step size of 0.02 . Thermogravimetric analysis (TGA) was performed on a PerkinElmer TGA-7000 thermogravimetric analyzer with a heating rate of 10 C min À1 . Emulsion and solid uorescence spectra of compounds 1 and 2 were obtained on a HITACHI F-2700 uorescence spectrophotometer at room temperature. Nitrogen sorption experiments were carried out on a surface area analyzer ASAP-2020. Scanning electron microscopy (SEM) was taken on a eld emission scanning electron microscope (FESEM, JSM 7001-F, JEOL, Japan).

Physical measurements
Elemental analysis (C, H, and N) was performed on a Perki-nElmer 240C elemental analyzer. Infrared (IR) spectra were obtained with KBr Pellets on a PerkinElmer Spectrum One FTIR spectrometer in the range of 4000-400 cm À1 with a resolution (4.0 cm À1 ) and the scan's numbers (10). Powder X-ray diffraction (PXRD) patterns of the samples were recorded by a RIGAKU-DMAX2500 X-ray diffractometer using Cu-Ka radiation (l ¼ 1.542Å) with a scanning rate of 10 min À1 and a step size of 0.02 . Thermogravimetric analysis (TGA) was performed on a PerkinElmer TGA-7000 thermogravimetric analyzer with a heating rate of 10 C min À1 . Solvent emulsion and solid uorescence spectra of compounds 1 and 2 were obtained on a HITACHI F-2700 uorescence spectrophotometer at room temperature.

Single crystal structure determination
Crystal structures were determined by single-crystal X-ray diffraction. Reection data were collected on a Bruker SMART CCD area-detector diffractometer (Mo-Ka radiation, graphite monochromator) at room temperature with u-scan mode. Empirical absorption correction was applied to all data using SADABS. The structure was solved by direct methods and rened by full-matrix least squares on F2 using SHELXTL 97 soware. 12 Non-hydrogen atoms were rened anisotropically. All C-bound H atoms were rened using a riding model with U iso (H) ¼ 1.2. All calculations were carried out using SHELXTL 97. A larger 1D channel in compound 2 was observed, in which no any guest molecules are lled (the REM highest difference peak is 0.406), and the nal structure of compound 2 wasn't treated through applying the SQUEEZE procedure. Crystallographic data and pertinent information are given in Table 1; selected bond lengths and angles in Table S1 †.

Synthesis of compounds 1 and 2
Starting from the same reactants (ZnCl 2 and H 2 cppca) in different solvent systems (H 2 O for compound 1, CH 3 CN/H 2 O mixed solvents for 2), compounds 1 and 2 were obtained; as indicated, the reaction solvents played an important role in formation of the isomers. When the same precursors reacted in a pure water system under 120 C, compound 1 with 2D abnormal puckered layer containing cis-cppca 2À was obtained. However, the 3D framework 2 with 1D bi-edge channels containing trans-cppca 2À was obtained from the CH 3 CN/H 2 O mixed solvent systems. If CH 3 CN was substituted by the other organic solvents such as methanol, ethanol, DMF, etc., no precipitates were obtained in mixed DMF/H 2 O; however, white powders were obtained in mixed CH 3 OH/H 2 O or CH 3 CH 2 OH/H 2 O, and their PXRD are apparently different from that of compounds 1 and 2 (Fig. S1 †). The acetonitrile molecules used in the reaction Scheme 1 cis-and trans-conformations of H 2 cppca. system were not incorporated into the nal crystal lattice of compound 2, but acted as important reaction modulators, which may have greatly affected the conformational isomerism of cppca 2À ligand and framework isomerism of compounds 1 and 2.
Crystal structure of compound 1. A single-crystal X-ray diffraction shows that compound 1 crystallizes in the monoclinic space group P2 1 /c. The asymmetric unit of compound 1 is composed of one Zn(II) center, one completely deprotonated cppca 2À anion and one coordinated water molecule. The Zn center with trigonal bipyramidal geometry coordinates with four oxygen atoms and one nitrogen atom from three independent cis-cppca 2À anions and one water molecule (Fig. 1a). The Zn-O/N bond distances range between 1.940(3) and 2.130(3)Å, and the O/N-Zn-O/N bond angles range from 77.90 (12) to 158.85(11) .
Two symmetry-related Zn centers and two cppca 2À ligands adopting m3-kN, O:kO 0 :kO 00 , O 000 coordination mode (Fig. S2a †) are connected into a [Zn(cppca)] 2 unit, in which a distorted 18membered ring with dimension of 5.15 Â 6.39Å 2 is observed (Fig. 1b). Each distorted [Zn(cppca)] 2 unit joined four equivalent ones into an abnormal 2D puckered layer through the Zn-O bonds, in which a 36-membered ring with the dimension of 7.83 Â 12.94Å 2 is observed ( Fig. 1c and d). Interestingly, the distorted 18-membered ring results in the formation of a 1D channel along the b axis ( Fig. S3 †). Notably, the 2D layers are interdigitated with each other into a 3D supramolecular network through p/p interactions among the phenyl rings of cppca 2À ligands in /ABAB$$$ arrangement; signicantly, the 1D channels and 36-membered rings are all blocked by the neighboring layer, resulting in the formation of a 3D nonporous supramolecular close-packed network (Fig. S4 †).
Crystal structure of compound 2. Differently from compound 1, compound 2 crystallizes in the trigonal space group R 3. The Zn center with slightly distorted octahedral geometry is coordinated with ve oxygen and one nitrogen from four independent cppca 2À anions and one water molecule, differently from the trigonal bipyramidal geometry in compound 1 (Fig. 2a). The Zn-O/N bond distances range between 2.036(2) and 2.173(2)Å, and the O/N-Zn-O/N bond angles range from 78.38(8) to 173.94(8) .
The Zn 2+ ions are connected into an innite 1D Zn-O-Zn chain by sharing a single oxygen atom (O1) and a syn-syn carboxylate groups from cppca 2À ligands and the corresponding Zn/Zn separation is 3.63Å (Fig. 2b). Each 1D Zn-O-Zn chain interconnects with three equivalent ones through double cppca 2À bridges adopting m 4 -kN, O:kO:kO 00 :O 000 (Fig. S1b †) and results in the formation of a 3-D open framework with larger 1-D channels (Fig. 2c). Interestingly, the 1D channel may be regarded as a bi-edge channel, the internal edge is a nearly hexagonal channel with an approximate dimension of 14.51Å and the external edge is a circular channel with an approximate dimension of 19.26Å (Fig. 2d). Notably, an elliptical channel  with the dimension of 4.51 Â 11.39Å 2 along the c direction is observed between the internal and external edges of the channel.
Notably, no guest solvents were observed in the channels; however, the framework of compound 2 shows a higher stability due to presence of the rigid bi-edge channel. Calculation performed using PLATON reveals a total solvent-accessible volume equaling to 3948.4Å 3 per unit cell, which counts for 44.6% of the cell volume, 13 offering potential possibilities of adsorption/ separation for gas and liquid guest materials, etc. Compound 2 is constructed from m 4 -cppca 2À and 4-connected Zn centers, so the resulting framework can be simplied into a binodal 4,4connected net with Schlai symbol {4 3 .6 2 .8} (Fig. S5 †), indicating a very rare ptr topology according to the classication by the Reticular Chemistry Structure Source. 14 Characterization The experimental and simulated PXRD patterns of compounds 1 and 2 are shown in Fig. S6. † The simulated PXRD patterns from the single-crystal X-ray diffraction data agree with the observed ones, indicating the phase purity of these crystalline products. Different intensities between the simulated and experimental patterns may be caused by a preferred orientation of the powder samples. In order to evaluate the stability of compound 1 and 2 for the solvents and water, the samples of compounds 1 and 2 (30 mg) were immersed in water, ethanol, acetone, acetonitrile (CH 3 CN), N,N-dimethylformamide (DMF), and trichloromethane (CHCl 3 ), respectively, then treated by ultrasonication for 30 minutes and kept for 12 hours. The power patterns of compounds 1 and 2 in the different organic solvents matched well with their simulated patterns, demonstrating the excellent stability of 1 under these conditions (Fig. S7 †). The SEM images of compounds 1 and 2 aer ultrasonication for 30 minutes indicated their crystalline properties still strongly remain (Fig. 3).
The FT-IR spectra of compounds 1 and 2 with the same peak positions and shapes are displayed in Fig. S8. † The wide peak at 3437 cm À1 belongs to O-H stretching of coordination water molecules. Absorption peaks located at 1636 and 1381 cm À1 can be ascribed to asymmetric and symmetric stretching vibrations of carboxylate groups, respectively.
Additionally, to investigate the thermal stability of 1 and 2, their thermal behaviors were studied from 22 C to 750 C in air (Fig. S9 †). The TGA curves of compounds 1 and 2 have similar thermal behaviors indicating a two-step weight loss. The rst weight loss corresponded to loss of water molecules in the range of 70-150 C (obsd 6.21% for 1, 5.58% for 2; calcd 5.56%); thus, the framework of compound 1 is thermally stable up to 320 C. However, compound 2 displays higher thermal stability than compound 1 because it doesn't begin to decompose until 400 C. The second weight loss of 1 and 2 were characteristic of the combustion of organic ligands (obsd. 68.12% for 1, 68.99% for 2; calcd. 69.41%). The total weight loss of compounds 1 and 2 amounted to 74.33% and 74.55%, respectively, and the remaining weight corresponds to the formation of ZnO (obsd. 25.67% for 1, 25.45% for 2; calcd. 25.03% for 1 and 2). Solid uorescence properties of compounds 1 and 2 Fluorescent sensors based on coordination compounds have been attracting more attentions owing to not only their fascinating and tunable structures but also high sensitivity, low cost, simplicity, portability, short response time, and so on. 15 Solidstate photoluminescence of compounds 1, 2, and H 2 cppca were performed at room temperature (Fig. 4). Luminescence spectra of H 2 cppca and compound 1, which are very similar in terms of position and band shape, both exhibit blue light emission with a maximum at about 400 nm when excited at 320 nm, attributed to intraligand charge transfer (LLCT), which may be due to p* / p or p* / n transition of the cis-cppca 2À ligands. Interestingly, a blue emission band at around 365 nm is observed in compound 2 when excited at 320 nm. Compared with 1, the emission maximum of compounds 2 is apparently blue-shied by 35 nm, as might be related to intraligand charge transfer of transcppca 2À ligands, different from that of compound 1. Notably, the uorescence intensities of compounds 1 and 2 are apparently stronger than that of H 2 cppca ligand under the same experimental conditions, which may be attributed to the unique coordination of ligand cppca 2À to the Zn(II) centers, thus increasing conformational rigidity, and thereby reducing nonradiative energy loss. 16 Sensing properties for small organic molecules The blue luminescence of compounds 1 and 2 encouraged us to use them as hosts for the sensing of small molecules. Different solvent emulsions were prepared as follows: the ground powders of compound 1 or 2 (0.5 mg) were immersed in water and common organic solvents (4.00 mL, acetone, acetonitrile (CH 3 CN), benzene, cyclohexane, ethanol (EtOH), 1,2-ethanediol, dimethylacetamide (DMA), formaldehyde (HCHO), H 2 O, methanol (MeOH), n-butylalcohol, N,N-dimethylformamide (DMF), tetrahydrofuran (THF), and trichloromethane (CHCl 3 )), followed by ultrasonication for 30 minutes, and then corresponding PL spectra were obtained. The emission spectra of compounds 1 and 2 (l ex ¼ 320 nm) in different solvent emulsions indicate that the maximum emission peaks and uorescent intensities largely depend on the solvent molecules. For 1, the maximum emission peaks in most solvent emulsions blue-shied 15 nm compared with its solid-state emission spectrum except that of 1-CHCl 3 and 1-benzene red-shi about 10 nm (Fig. 5a), and the maximum luminescence intensity of 1-solvent emulsions gradually decreased in the following order: EtOH > DMF > CH 3 CN > DMA > MeOH > CHCl 3 > n-butylalcohol > acetone > ethanediol > H 2 O > benzene > HCHO > THF > cyclohexane. Apparently, all the solvent emulsions of compound 1 exhibit blue light emission under UV light (Fig. 5b). For 2, the maximum emission peaks of 2-CHCl 3 and 2-cyclohexane emulsions blue-shied 10 nm compared with its solid-state emission spectrum; however, that of the other emulsions almost remain unchanged, and the decreasing order of luminescence intensity is: EtOH > CHCl 3 > CH 3 CN > n-butylalcohol > THF > benzene > MeOH > HCHO > ethanediol > cyclohexane > DMF > H 2 O > DMA > acetone (Fig. 6a). The above-mentioned phenomena may be ascribed to interactions between a homogeneously dispersible framework and solvent molecules with different polarities. 17 Obviously, ethanol emulsions for 1 and 2 both exhibit the strongest uorescence emission; notably, 1-acetone emulsion exerted the stronger luminescence emission, while acetone was the most effective quencher for compound 2, resulting in a nearly 100% uorescence quenching. Apparently, emissive visible blue light from the acetone emulsion of compound 2 is darker than that from the other emulsions under UV light (Fig. 6b). Such  solvent-dependent luminescence quenching properties of compound 2 might be a promising luminescent probe for detecting acetone molecules.
To gain a deeper insight regarding the sensing properties toward acetone for compound 2, 0.5 mg samples of compound 2 dispersed in ethanol were considered as the standard emulsion, and a batch of emulsions with gradually increased acetone concentration were prepared and monitored by uorescent spectra. As shown in Fig. 7a, the uorescence intensity of 2ethanol emulsion has a signicant decrease with the increase of acetone concentration. When the concentration of acetone increased to 0.868 M, 97.2% uorescence intensity was completely quenched. The quenching effect can be illustrated by the Stern-Volmer equation: I 0 /I ¼ 1 + K SV [M], where I 0 and I are the luminescence intensity of 2-ethanol emulsions in the absence and presence of acetone, respectively, [M] is the acetone concentration, and K SV is the quenching coefficient of the acetone. 18 The curve of (I 0 /I À 1) vs. acetone concentration is shown in Fig. 7b; when the acetone concentration is very low, a linear plot was observed and the corresponding slope (K SV value) is calculated as 7.532 M À1 . The linear correlation coefficient (R) is 0.99044; however, at higher concentrations, the plot deviates from linearity and bends upwardly. The above results indicate that the quenching effect of acetone for compound 2 ts the Stern-Volmer model well.
Under the same experimental conditions and excitation wavelength, compounds 1 and 2, with different topology structures, displayed different uorescence properties: compound 1 displayed an apparent uorescence emission in acetone solvent; however, compound 2 was very sensitive to the acetone molecules and resulted in a nearly 100% uorescence quenching at 0.868 M. This phenomenon rules out the possibility of competition for absorption of the light source energy between assynthesized complexes and acetone molecules. 19 Although the mechanism for such quenching effects by acetone molecules is still not clear, we speculate that it may be related to conformational isomerism of cppca 2À ligands in compounds 1 and 2.

Sensing properties for metal ions
Moreover, we also investigated the sensing (detection) ability of compounds 1 and 2 for different metal ions. Powders of compound 1 or 2 (0.5 mg) were immersed in different ethanol solutions (4 mL) including 10 À2 M of M(NO 3 ) x (M ¼ Na + , Ag + , Co 2+ , Cu 2+ , Zn 2+ , Cd 2+ , Mg 2+ , Pb 2+ , Ni 2+ , Al 3+ and Fe 3+ ), and the corresponding luminescent spectra of ethanol emulsions were measured aer ultrasonication for approximately 30 min. The luminescence intensities of 1-ethanol emulsions were largely dependent on the species of metal ion (Fig. 8a); the addition of Al 3+ and Na + signicantly enhanced the luminescence intensities of ethanol emulsions, while other metal ions show different levels of the quenching effect; particularly, Fe 3+ had a nearly 100% uorescence quenching. Moreover, the emissive visible blue light from 1-ethanol solution with Fe 3+ ions is darker than that from the other emulsions under UV light (Fig. 8b). To further study detection sensitivity toward the Fe 3+ ion, the luminescence intensity of 1-ethanol emulsion was measured with gradually increased Fe 3+ concentration. As shown in Fig. 9a, the luminescence intensity is heavily dependent on Fe 3+ concentration, and when Fe 3+ concentration reached 0.23 mM, the corresponding emission intensity attenuated by  is the molar concentration of Fe 3+ ion, and K SV is the quenching coefficient of Fe 3+ ion, a nearly linear plot of (I 0 /I À 1) vs. Fe 3+ concentration was observed at low concentrations and the corresponding quenching constant (K SV ) was 2.26 Â 10 5 M À1 (Fig. 9b). Similarly, Fe 3+ also displayed the most effective quenching effects for compound 2 and the corresponding K SV was 1.56 Â 10 5 M À1 (Fig. S10 and S11 †). These results reveal that compounds 1 and 2 might be promising luminescent probes for the detection of Fe 3+ .

Dye adsorption properties of compound 2
To evaluate gas accessible porosity of compound 2, the samples were immersed in MeOH for 12 hours and then outgassed overnight in vacuum at 80 C; unfortunately, PXRD analysis showed amorphization of the samples. Moreover, the nonactivated compounds 2 have no signicant adsorption for nitrogen (Fig. S12 †).
Considering the porous structure in the framework of 2, its dye adsorption properties were investigated at room temperature. Two organic dyes i.e., methylene blue (MB) and rhodamine B (RhB), with different molecular sizes and that are widely used for dye adsorption experiments, were selected and UV/vis spectrophotometry was used to monitor their concentrations to evaluate adsorption performance of compound 2. Typically, 10 mg of non-activated compound 2 was immersed into 20 mL of dye-containing aqueous solution (2 Â 10 À5 mol L À1 ) at room temperature for about 24 h. Apparently, aer compound 2 adsorbed MB and RhB, the corresponding white crystals gradually became blue and red, respectively, showing certain degree of adsorption for MB and RhB dyes. Moreover, the UV spectra of MB and RhB aqueous solutions revealed that the characteristic adsorption peaks were obviously decreased aer dye adsorption (Fig. 10). Calculations show that compound 2 effectively adsorbed 67.4% MB and 16.2% RhB from the corresponding dye aqueous solutions within 24 h. According to the following equation: Q ¼ V(C 0 À C)/m, [Q (mg g À1 ) is the adsorption capacity; V (L) is the volume of the dye solution; C 0 and C are the initial and the equilibrium concentrations of dye molecules (mg L À1 ); m (g) is the weight of the absorbent, and the adsorption capacities of MB and RhB are 5.02 and 1.56 mg g À1 , respectively]. Compound 2 demonstrates good adsorption properties for MB and RhB molecules, which mainly results from interactions between the dye molecules and compound 2, similarly to previous reports. 20 The difference in adsorption between MB and RhB may be ascribed to the shape and size of the dyes; the MB dye with a smaller molecular size may be more easily adsorbed, while larger RhB molecules are more slowly adsorbed.
The luminescent properties of compound 2 aer adsorbing MB and RhB were further investigated (abbreviated as MB@2 and RhB@2, respectively), and the corresponding spectra of MB@2 and RhB@2 are displayed in Fig. 11. The luminescence spectra of MB@2 and pure compound 2, which are very similar  in terms of position and band shape, both exhibit blue light emission with a maximum at about 365 nm when excited at 320 nm; however, RhB@2 exhibits two emission peaks at 365 and 582 nm in the solid state at room temperature when excited at 345 nm. The emission at 365 nm is attributed to the intraligand charge transfer of trans-cppca 2À ligands, whereas the emission at 582 nm presumably originates from the adsorbed rhodamine B molecules in RhB@2. Interestingly, RhB@2 exhibits red light emission under UV illumination at 365 nm; however, MB@2 revealed blue light emission. Moreover, the uorescence intensities of MB@2 and RhB@2 are higher than that of pure compound 2. This strategy will facilitate extensive research on photofunctional materials exhibiting dual emissions and lead to some practically useful materials in the near future.

Conclusions
Two zinc supramolecular isomers were synthesized under two different solvent systems and fully characterized. Compound 1 displayed an abnormal 2D puckered layer constructed by distorted [Zn(cppca)] 2 units; however, a 3-D open framework with larger bi-edge channels, in which the internal edge is a nearly hexagonal channel with an approximate dimension of 14.51Å and the external edge is a circular channel with an approximate dimension of 19.26Å, was observed in compound 2. The uorescent properties of compounds 1 and 2 were investigated in their solid states and in various solvent emulsions. Results indicated that compound 1 displayed an apparent uorescence emission in acetone solvent; however, compound 2 resulted in a nearly 100% uorescence quenching with acetone molecules. Moreover, compound 2 demonstrates good adsorption properties for MB and RhB molecules and the corresponding adsorption capacities are 5.02 and 1.56 mg g À1 , respectively.