CHEMICAL CONSTITUENTS AND ANTIOXIDANT ACTIVITY OF n-HEXANE EXTRACT OF Impatiens bicolor

Plants belonging to the genus Impatiens (Balsaminaceae) are rich in organic acids, anthraquinones, and flavonoids [1–3]. It is well known that these classes of aromatic compounds possess widespread pharmacological activities. Previous investigations showed that various plant extracts of Impatiens bicolor Royle display antimicrobial and antifungal activity [4, 5], as well as acetylcholinesterase inhibition and antioxidant activities [6]. Thus, there is a considerable research interest in the assay of composition and/or antioxidant properties of various extracts of I. bicolor. This study presents the very first investigation on the chemical composition of plant extracts of I. bicolor, as well as their antioxidant activities, and it can provide a broad base for the possibility of further detailed pharmacological studies on this species. The sub-extracted samples of the n-hexane fraction of I. bicolor were subjected to detailed GC and GC/MS analysis in order to determine the impact of chemical composition on their antioxidant activity. About 42 compounds were identified in eight samples, representing 56.64–93.85% in total. The percentage compositions of the samples were computed in each case from GC peak areas and are shown in Table 1. In general, all investigated samples can be characterized as mixtures of aliphatic compounds (20.11–93.85%), with oxygenated aliphatic compounds as the most abundant (5.97–89.38%). The fatty acid esters (FAEs) were found to be major components of the investigated subfractions of the n-hexane extract, comprising trans-methyl 13-octadecenoate (2.44–47.29%), methyl heptadecanoate (8.76–18.82%), methyl octadecanoate (4.96–16.49%), methyl docosanoate (3.14–12.64%), methyl tetracosanoate (1.04–11.23%), and methyl eicosanoate (3.83–10.93%). These compounds were mainly found in samples eluted with solvents with relatively low polarity (MQ-1 to MQ-5), while samples eluted with higher polarity (MQ-6 to MQ-8) contained significant amounts of aromatic compounds and shorter-chain oxygenated aliphatic compounds. The antioxidant activity of sub-extracted samples of the n-hexane fraction of I. bicolor were evaluated by DPPH radical-scavenging test. Assessed samples were able to reduce the stable violet DPPH radical to the yellow DPPH-H, reaching 50% of reduction with IC 50 values ranging from 23.22 0.75 g/mL for sample MQ1 to 59.00 2.01 g/mL for sample MQ8 (Table 2). Those values are comparable to ascorbic acid (IC 50 = 7.80 0.14 g/mL), whose antioxidant properties are already known [7, 8]. Despite the fact that major natural antioxidants belong to the class of phenolic compounds, such as thymol and carvacrol [9, 10], the investigated samples, rich in FAEs, showed radical scavenging activity. The IC 50 values presented in the Table 2 are comparable with the content of FAEs in the investigated sample. However, the percentage of these esters decreases from sample MQ1 to sample MQ8, and due to the polarity of eluent, the antioxidant activity decreases. Sample MQ1 was the exception, with a content of FAEs of 58.33%, which is much lower than in the next sample, MQ2, but sample MQ1 showed uniqueness in the content of 2-(1-naphthyl)acetophenone, a member of a class of already known natural antioxidants [11, 12].