3.3.1 Thin-layer chromatography
Thin-layer chromatography (TLC) was used to determine the fingerprint profile of the M. cardiospermoides crude extracts. Thin-layer chromatography is based on the relative affinity of compounds towards the stationary and mobile phases. Compounds with a higher affinity for the stationary phase travel slowly compared to those of a lower affinity (Valle Jr et al., 2015). Dried extracts were reconstituted in acetone or methanol to 10 mg/ml and 10 µl of each extract was spotted on aluminium-baked TLC plates.
The TLC plates were allowed to dry and then developed in three solvent systems of varying polarity, namely, ethyl acetate/methanol/water [EMW] (10:1.3:1) (polar/neutral); chloroform/ethyl acetate/formic acid [CEF] (10:8:2) (intermediate polarity/acidic); toluene/ethyl acetate/ammonium (18:2:0.2) (non-polar/basic). To detect the presence of non-fluorescing phytochemicals, vanillin-sulphuric acid reagent [0.1 g vanillin (Sigma®): 28 ml methanol: 1 ml concentrated sulphuric acid] or p- anisaldehyde-sulphuric acid [0.5 ml p-anisaldehyde: 15 ml glacial acetic acid: 85 ml methanol: 5 ml sulphuric acid] were sprayed on the TLC plates. The plates were subsequently heated at 110°C for 10 minutes for optimal colour development and photographed.
19 3.3.2. Qualitative phytochemical tests using standard chemical tests
The methanol and acetone extracts were screened for phytochemicals using various methods described by Yadav et al. (2014) as depicted in the table below.
Table 1: Tests for the presence of phytochemicals.
Phytoconstituent Test Observation
Tannins (Braymers’ Test) 2 ml extract + 2 ml H2O + 2-3 drops of FeCl3 (5%)
Green precipitate
Flavonoids 1 ml extract + 1 ml Pb(Ac)4
(10%)
Yellow colouration
Saponins 5 ml of extract + olive oil (few drops)
Emulsion forms
Phlabatannins (precipitate test)
2 ml extract + 2 ml HCl (1%) + heat
Red precipitate
Coumarins 2 ml extract + 3 ml NaOH
(10%)
Yellow colouration
Anthocyanins 2 ml extract + 2 ml HCl (2N) + NH3
Pinkish red to bluish violet Colouration
3.3.3. Quantitative phytochemical test
3.3.3.1 Determination of total phenolic content
Determination of the total phenolic content (TPC) was done using the Folin-Ciocalteau method as described by Singleton et al. (1999) with minor modifications. The assay is based on the reduction of the Folin-Ciocalteu reagent in the presence of phenolics resulting in the production of molybdenum–tungsten blue (Swain and Hillis 1959). A volume of 66.6µL of 10 mg/ml of either the acetone or methanol extract was added into a test tube. This was followed by the addition of 250 µL of 10% (v/v) Folin- Ciocalteau phenol reagent. The mixture was shaken and kept in the dark for 5 minutes.
A volume of 250 µL of 7% (w/v) sodium carbonate (Na2CO3) solution was then added to the mixture and incubated for a further 90 minutes at room temperature. A set of standard solutions (ranging from 0 – 0.5 mg/ml) of gallic acid in methanol were
20 prepared in the same manner. Absorbance was measured against the reagent blank at 750 nm using the GloMax®-Multi+ Detection System (Promega, Madison, USA).
Total phenolic content was calculated using the formula below and expressed as mg of GAE/g of extract.
𝑇𝑃𝐶 = 𝐶 × 𝑉 𝑀
Where ´C´ is the concentration of gallic acid mg/ml, ´V´ is the volume of plant extract in ml and ´M´ is the weight of the extract in grams.
3.3.3.2. Determination of total flavonoid content
Determination of the total flavonoid content (TFC) was done using the aluminium chloride colourimetric assay as described by Gorinstein et al. (2007) with minor modifications. In this assay, aluminium chloride forms acid-stable complexes with the ortho-dihydroxyl groups in A- or B-rings of flavonoids. Briefly, 50µL of 10 mg/ml of either the acetone or methanol extract was added into a test tube. This was followed by the addition of 1.4 ml of distilled water, 50 µL of 1 M potassium acetate and 50µL of 10% (w/v) aluminium chloride into the mixture which was then incubated at room temperature for 30 minutes. A set of reference standard solutions (ranging from 0 – 0.5 mg/ml) of quercetin dissolved in methanol were prepared in the same manner. The absorbance of the test and standard solutions was measured against the reagent blank at 420 nm using the GloMax®-Multi+ Detection System (Promega, Madison, USA). Total flavonoid content was calculated using the formula mentioned in section 3.3.3.1. and expressed as mg of QE/g of extract.
3.3.3.3. Determination of total tannin content
Determination of the total tannin content (TTC) was done using the Folin-Ciocalteau reagent method as described by Gorinstein et al. (2007) with minor modifications. The assay is based on the reduction of the Folin–Ciocalteu reagent in the presence of tannins resulting in the production of molybdenum–tungsten blue (Swain and Hillis 1959). A volume of 20 µL of 10 mg/ml of either the acetone or the methanol extract was added to a test tube containing 1.5 ml of distilled water. To this mixture, 0.1 ml of 10% Folin-Ciocalteau phenol reagent and 1 ml of 35% sodium carbonate (Na2CO3)
solution were added. This was made up to a final volume of 2 ml with distilled water.
The mixture was shaken and kept at room temperature for 30 minutes. A set of
21 standard solutions (ranging from 0 – 0.5 mg/ml) of tannic acid dissolved in acetone were prepared in the same manner. Absorbance for test samples and standard solutions was measured against the blank at 725 nm using a GloMax®- Multi+Detection System (Promega, Madison, USA). Total tannin content was calculated using the formula mentioned in section 3.3.3.1. and expressed as mg of TAE/g of extract.
3.3.3.4. DPPH free radical scavenging activity
Analysis of the free radical scavenging activity of the methanol or acetone extract was done using the DPPH free radical scavenging assay as described by Brand-Williams et al. (1995) with some modifications. This assay makes use of the organic chemical compound 2, 2-diphenyl-1-picryhydrazyl, which is a purple coloured stable free radical that provides an easy and rapid way to evaluate antioxidant activity. When an odd electron becomes paired in the presence of a free radical scavenger, the DPPH solution is decolourised as the colour change from deep violet to light yellow (Madhu 2013). The degree of reduction in absorbance measured is indicative of the radical scavenging (antioxidant) power of extract. Sample stock solutions (1 mg/ml) of the acetone or methanol extract were diluted to concentrations of 0, 20, 40, 60, 80 and 100 μg/ml, in 96-well plates. In each well, 100 µL of 200 µM DPPH dissolved in methanol was added. L-ascorbic acid (1 mg/ml) was prepared in the same manner.
Methanol was used as blank while the DPPH solution was used as a standard control.
The plates were shaken and incubated in the dark for 30 minutes. The absorbance reading was measured at 515 nm using the GloMax®-Multi+ Detection System (Promega, Madison, USA). The inhibitory effect of the extracts was calculated according to the following formula.
% 𝐼𝑛ℎ𝑖𝑏𝑖𝑡𝑖𝑜𝑛 =(𝐴𝑏𝑠𝑜𝑟𝑏𝑎𝑛𝑐𝑒 𝑜𝑓 𝑐𝑜𝑛𝑡𝑟𝑜𝑙 − 𝐴𝑏𝑠𝑜𝑟𝑏𝑎𝑛𝑐𝑒 𝑜𝑓 𝑠𝑎𝑚𝑝𝑙𝑒) × 100 𝐴𝑏𝑜𝑟𝑏𝑎𝑛𝑐𝑒 𝑜𝑓 𝑐𝑜𝑛𝑡𝑟𝑜𝑙
22