Introduction. Monoterpenes, which are the main components of essential oils, act as allelopathic agents, attractants in plant-plant or plant-pathogen/herbivore interactions or repellants. Their antioxidative capacity is believed to be responsible for the health promoting properties of fruits and vegetables. Several investigations have studied the antioxidant activity of number of monoterpenes in vitro but it not any data about monoterpene (+)-3-carene and its derivatives. On the other side monoterpenes have been recently used to prepare the cationic or anionic moiety of room-temperature ionic liquids. In many cases, these new solvents, based on modifications of monoterpenes, contain chiral centres and/or specific functional groups. Well known, one of the tasks of the synthesis of a bioactive compound is preparation of required enantiomer in optically pure form. Bicyclic monoterpene (+)-3-carene with 3,7,7-trimethylbicyclo[4.1.0]heptane framework is widely used for resolving this type of problems. A structural future of (+)-3-carene is the presence of the reactive C=C double bond and bicyclic bridging system and opens perspectives for synthesis with retention of the natural framework. Aim. The research aims to indicate which of the structural elements of monoterpene (+)-3-carene are responsible for their antioxidant activity. In this study we have synthesized several (1S,3S,4S,6R)-4-(1H-imidazol-1-yl)-3,7,7-trimethylbicyclo[4.1.0]heptan-3-ol derivatives.
Materials and methods. In order to determine the reaction stoichiometry, different molar ratios, expressed as moles of antioxidant per mole of DPPH▪, were tested, ranging from 0.1 to 10. For each molar ratio, the remaining concentration of DPPH▪ at the plateau was determined and graphed, and EC50 was read on the graph as the molar ratio which reduces half of the initial DPPH▪ concentration, was determined from the graph. A lower EC50 value is associated with a stronger DPPH radical scavenging capacity under the same testing conditions. These results for clarity were also expressed in terms of antiradical power (ARP) calculated as ARP=1/EC50 in which larger ARP values represented a larger scavenging capacity. The number of reduced DPPH▪ molecules per one molecule of antioxidant was
defined as ζ = 1/(2 x EC50). The results obtained for EC50, ζ and ARP of the studied compounds were compared to those of ascorbic acid as well as ionol (control).
Results and discussion. It was observed that 1-{(1R,3S,4S,6S)-4-hydroxy-4,7,7-trimethylbicyclo[4.1.0]heptan-3-yl}-3-methyl-1H-imidazol-3-ium (S)-2-[(S)-1,2-dihydroxyethyl]-4-hydroxy-5-oxo-2,5-dihydrofuran-3-olate 2 shows a similar radical scavenging capacity to that of ascorbic acid, which is probably due to the presence of AscH in its structure. It was observed that 3-ethyl-1-{(1R,3S,4S,6S)-4-hydroxy-4,7,7-trimethylbicyclo[4.1.0]heptan-3-yl}-1H-imidazol-3-ium (S)-2-[(S)-1,2-dihydroxyethyl]-4-hydroxy-5-oxo-2,5-dihydrofuran-3-olate 4 shows a greater radical scavenging capacity than ascorbic acid, which allows us to conclude that the ethyl- group contributes to a greater dislocation of the hydrogen atoms and contributes to their greater readiness to react with the DPPH radical. The radical scavenging capacity of remaining compounds decreases in the range: ionol > 1-{(1R,3S,4S,6S)-4-Hydroxy-4,7,7-trimethylbicyclo[4.1.0]heptan-3-yl}-3-methyl-1H-imidazol-3-ium iodide 1 > 1-{(1R,3S,4S,6S)-4-hydroxy-4,7,7-trimethylbicyclo[4.1.0]heptan-3-yl}-3-propyl-1H-imidazol-3-ium iodide 5 > 3-ethyl-1-{(1R,3S,4S,6S)-4-hydroxy-4,7,7-trimethylbicyclo[4.1.0]heptan-3-yl}-1H-imidazol-3-ium iodide 3. In terms of stoichiometry it was observed that one molecule of compound 4 deactivates two radical molecules, suggesting that the 2 hydrogen atoms of -OH groups in the structure of AscH are responsible for compound 4 radical scavenging activities. Ionol shows a 1:1 stoichiometry, which is in accordance with its chemical formula (one -OH group attached to the aromatic ring capable of donating the H atom). All compounds are characterized by more complex reaction mechanisms, according to fractional ζ numbers. It may be seen that compound 2 has ζ=1.46 which is the highest ζ value among remaining compounds, probably due to the presence of AscH in its structure. 1, 3 and 5 have ζ values less than 1, suggesting that it takes more than one molecule of studied compound to deactivate one molecule of DPPH, which is probably due to the fact that an active form capable of donating an H atom has to be formed in the reaction medium prior to the DPPH scavenging step. Conclusions. It was identified which of the structural elements of (1S,3S,4S,6R)-4-(1H-imidazol-1-yl)-3,7,7-trimethylbicyclo[4.1.0]heptan-3-ol is responsible for their antioxidant activity.