Page 13 - Volume 4, Issue 2, July 2011

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-menthane type compounds against mosquito

41

phytochemical analysis are in agreement

with literature data (13, 17).

Essential oils are a mixture of different

ingredients, mostly terpenes, which in the

analysed

Mentha

spp. essential oils includ-

ed pulegone, piperitone, isomenthone, car-

vone, menthone and menthol and they are

comparable to findings in the literature (8,

13).

Besides menthone, the rest isolated in-

gredients were evaluated for the first time

against

Cx. pipiens

larvae. Among them,

pulegone demonstrated the strongest larv-

icidal activity. The activity of all essential

oils is in agreement with the proportion/

toxicity rate of their individual major com-

ponents, apart from the case of EOpip. The

EOpip consists of menthone and menthol

(in a total of 65%) and their LC

50

values are

higher than 100 mgl

-1

, so it was expected to

have the same toxicity pattern with the ma-

ternal essential oil. According to our experi-

mental and literature data (10) EOpip did not

follow this pattern and found to be one of

the two most drastic essential oils (LC

50

val-

ue near 40mgl

-1

). Two suggestions could be

made: either the rest of the ingredients pos-

sess independently strong larvicidal abili-

ty or some kind of synergistic phenomenon

took place. These results are in accordance

with those reported by Amer and Mehlhorn

(2) where peppermint had moderate larvi-

cidal activity against mosquito larvae (near

53% mortality after 24 h treatment).

Some interesting

conclusions can be

drawn concerning the relationship be-

tween the structure of the isolated

p

-men-

thane compounds and their larvicidal ef-

fect against

Cx. pipiens

. Although, pulegone

and piperitone are isomers, the first is 5-fold

more active than the later. This differentia-

tion probably stands on account of the loca-

tion of the C-C double bond that these two

molecules contain. We assume that in pule-

gone the C-C double bond position on the

chain group (isopropylidene

versus i

sopropyl

group) enhances the toxicity. This hypothesis

is strengthened by the case of piperitenone,

where toxicity ranges between pulegone

and piperitone. Piperitenone is structurally

similar to both aforementioned molecules.

Particularly, it combines all of the former

molecules characteristics to one structure

as it bares two double bonds, where the first

one is internal at the cyclohexane ring (re-

sembling to piperitone) and the second one

external (resembling to pulegone). There-

fore, two arguments can be made: either

piperitenone’s internal double bond reduc-

es the total toxicity or its external double

bond increases the toxicity. In any case, tox-

icity seems to be in direct connection to car-

bon-carbon double bond’s position.

The effectiveness order of the above

mentioned molecules, compared to men-

thone (saturated cetone), is the following:

pulegone> piperitenone > menthone > pip-

eritone. This ranking indicates that unsatu-

ration might be a key factor but not nec-

essarily the most determinant; it should be

taken into account in combination with the

position factor. Our results concerning car-

vone (a molecule isomer to piperitenone)

also support this hypothesis: carvone is less

effective than piperitenone while both of

them are showing reduced toxicity com-

pared to pulegone. This activity differen-

tiation probably results from the different

location of C-C double bond (isopropenyl

group) as well as to its endo-exocyclic di-

enone character. By inference, it seems that

in the case of the tested monoterpene ce-

tones, the presence of the isopropylidene

group increases the strength of larvicity,

comparing to isopropenyl and/or isopropyl

group effectiveness.

Finally, the use of the two enantiomers,

isomenthone and menthone, revealed that

the latter was more toxic, indicating that

enantioselectivity may play also an impor-

tant role for the toxicity of essential oils. This

role of enantioselectivity has already been

reported in previous projects and is a well-

known fact: naphthoquinones (14), linalool

(15) and limonene (16).

Essential oils have often proved to be

more effective than their ingredients, in-

dicating synergistic phenomena (9). Plants

usually produce essential oils as a mixture

of many ingredients with strong interac-

tions among them. Results from this study