3.1. Mass spectra analysis of detected quinoline alkaloids:
Two types of oxygenated quinoline alkaloids were detected in Suaeda fruticosa extract. One is oxygenated in one position while the other is oxygenated in two positions.
3.1.1. Mass spectra analysis of quinoline alkaloids oxygenated at one possiton:
From this type two quinoline alkaloids were detected. 3-hydroxyquinoline (1) and 6-hydroxyquinoline (2). Their mass spectra (Fig. 2) illustrated a similarity and the only difference was the relative intensity of M-28 peak and this exclude the probapility of 2,4 and 8 oxygenated quinoline[15].
Also, M-55 peak appeared at high intensity compared with 5 and 7 oxygenated quinoline, so the possible oxygenation positions are 3 and 6(Kaczmarek and Steinegger, 1958). The difference between 3-hydroxy and 6-hydroxy is the presence of mass peak at 104.0484 which indicates the loss of H2O followed by loss of C2. This is only possible for 3-hydroxyquinoline. Also, the retention time indicates a very low difference in polarity between the two compounds, therefeore 3-hydroxy is more polar than 6-hydroxy. The mass fragmentation pattern of the two compounds was given in Fig. 3.
3.1.2. Mass spectra analysis of quinoline alkaloids oxygenated at two possitons:
Four quinoline alkaloids were detected to be oxygenated at two positions but the only two possible positions were 2 and 3. Fig. 4 represents the mass spectra of the four compounds. The main peaks in all compounds are 144.0450, 116.0494 and 89.0387 Figure 5 shows the structure of these peaks and how they are formed .On other hand, the chvracterestic fragmentation in compounds 3 and 5 was due to the loss of H2O [M-18] and this indicates the presence of hydroxyl group in both compounds. In addition, some charcterstic peaks were found to prove the position of substituents.
For compound 3, three charcterstic peaks proved the porposed structure of the compound. The first peak appeared at m/z= 147.0462 which results from NH loss and this indicates an amid group in the compound. The other peaks at m/z=118.0660 and m/z=91.0555 indicate the presence of α-hydroxy to carbonyl group.
In compound 5, the appearance of two peaks at m/z=119.0783 and 106.0727 gives an evidance for the porposed strcture as explained in fig. 6.
Two main peaks proving the proposed structure for compound 6. The first one [M-C2H4] at m/z=162.0552 indicates that the two methoxy groups are adjacent and this peak was followed by the loss of CO2 to give a peak at m/z=118.0652 like compound 3, after that the loss of HCN. The other peak at m/z=146.0937 which due to [M-CO2] reveals a methoxy group at position 2 to allow rearrangement and the loss of CO2 as described in Fig. 7.
3.2. Biosynthetic pathway for the detected quinoline alkaloids
LC-MS/MS analysis of Suaeda fruticosa showed the presence of quinoline alkaloids, anthranilic acid and indole and the absence of tryptophane. Moreover, all detected quinoline alkaloids do not have hydroxyl group at position 4 which indicats that the biosynthetic pathway of quinoline alkaloids in Suaeda fruticosa is different from the reported one in other plants. The presence of 3-hydroxyquinoline and indole supports the hypothesis of sugested biosynthetic pathway shown in schem 1. Since this pathway was reported for indol, the only difference is in the cyclization mechanism in the last step in indole to give 5 memberd ring and 3-phosphglyceraldehyde but in the suggested pathway gives 3-hydroxyquinoline and phosphoacetaldehyde.
Its noteworthy that 3-hydroxyquinoline is a precursore for all other quinoline alkaloids present in Suaeda fruticosa as shown in schem 2 and all steps are common steps in plants which include oxidation, hydration and methylation.