XENOBIOTIC COMPOUNDS AND THEIR TYPES.pdf

Xenobiotic compounds are man-made chemicals that
are present in the environment at unnaturally high
concentrations. The xenobiotic compounds are either
not produced naturally, or are produced at much lower
concentrations than man.

2. Xenobioticcompoundsareman-madechemicalsthat
arepresentintheenvironmentatunnaturallyhigh
concentrations.Thexenobioticcompoundsareeither
notproducednaturally,orareproducedatmuchlower
concentrationsthanman.
Axenobiotic(Greek,xenos"foreign";bios"life")is
acompoundthatisforeigntoalivingorganism.
Microorganismsarealsoabletodegrademanyofthe
xenobioticcompounds,buttheyareunabletodegrade
manyothers.Thecompoundsthatresistbiodegradation
andtherebypersistsintheenvironmentforalong
periodtheyarecalledas“recalcitrant”.
XENOBIOTICCOMPOUNDS

3. (i)Theyarenotrecognizedassubstratebythemicroorganisms.
(ii)Theyarehighlystable,i.e.,chemicallyandbiologicallyinert
duetothepresenceofsubstitutiongroupslikehalogens,nitro-,
sulphonate,amino-,methoxy-andcarbonylgroups.
(iii)Theyareinsolubleinwater,orareadsorbedtoexternal
matriceslikesoil.
(iv)Theyarehighlytoxicorgiverisetotoxicproductsdueto
microbialactivity.
(v)Theyarelargemolecularsizeandthuspreventsentryinto
microbialcells.
(vi)Inabilityofthecompoundstoinducethesynthesisof
degradingenzymes,and
(vii)Lackofthepermeasesneededfortheirtransportintothe
microbialcells.
Whyxenobioticcompoundsarehighlyrecalcitrant?

4. Structuralfeaturesofxenobioticcompounds
(i)Presenceofhalogensintheplaceofhydrogeninthe
molecule;thecarbon-halogenbondishighlystableandits
cleavagerequiresconsiderableenergy,
(ii)SubstitutionofHbyothergroupslikenitro-,
sulphonate,methoxy-,amino-andcarbonylgroups,
(iii)Cyclicstructures,aromaticcompounds,cycloalkanes
andheterocycliccompoundsaremorerecalcitrantthan
linearchainoraliphaticcompounds,
(iv)Branchedlinearchainsresistbiodegradationetc.
Ingeneral,themorecomplexisthestructureofaxenobiotic
compound,themoreresistantitistobiodegradation.Many
otherxenobioticsresistbiodegradationduetotheirlarge
molecularsizeandinsolubilityinwater.

5. CLASSIFICATIONOFXENOBIOTICCOMPOUNDS

7. TYPESOFXENOBIOTICCOMPOUNDS
(i) Halocarbons
(ii) Polychlorinated biphenyls
(iii) Synthetic polymers
(iv) Alkyl benzyl sulphonates
(v) Oil mixture Polycyclic aromatic hydrocarbons(PAHs)
(vi) Others.

8. (i)Halocarbons:
Thesecompoundscontaindifferentnumbersofhalogen
(e.g.,CI,Br,F(fluorine),I)atomsintheplaceofHatoms.
Theyareusedassolvents(chloroform,CHCI
3),as
propellantsinspraycansofcosmetics,paintsetc.,in
condenserunitsofcoolingsystems(Freon,CCI
3F,CCl
2F
2,
CClF
3,CF
4),andasinsecticides(DDT,BHC,lindaneetc.)
andherbicides(dalapon,2,4-D,2,4,5-Tetc.).
TheC
1-C
2haloalkaneslikechloroform,freonsetc.are
volatileandescapeintotheatmospherewheretheydestroy
theprotectiveozone(O
3)layerleadingtoincreasedUV
radiation.Pesticides(herbicides,fungicidesand
insecticides)areappliedtocropsfromwheretheyleachinto
waterbodies;manyofthemaresubjecttobio-
magnification.

10. (ii)PolychlorinatedBiphenyls(PCB’s):
Thesecompoundshavetwocovalentlylinkedbenzene
ringshavinghalogenssubstitutingforhydrogen.
PCB’sareusedasplasticizers,insulatorcoolantsin
transformersandasheatexchangefluids.Theyareboth
biologicallyandchemicallyinerttovariousdegrees,which
increaseswiththenumberofchlorineatomspresentinthe
molecule.
Therecalcitrantnatureoftheabovetwogroupsof
compoundsisduetotheirhalogenationsandaswelltheir
cyclicstructure(PCB’s).

11. (iii)SyntheticPolymers:
Thesecompoundsareproducedasplastics,e.g.,
polyethylene,polystyrene,polyvinylchlorideetc.,and
nylonswhichareusedasgarments,wrappingmaterialsetc.
Theyarerecalcitrantmainlyduetotheirinsolubilityin
waterandmolecularsize.
(iv)AlkylbenzylSulfonates:
Thesearesurface-activedetergentssuperiortosoaps.The
sulphonate(—SO
3
–
)grouppresentatoneendresists
microbialdegradation,whiletheotherend(non-polaralkyl
end)becomesrecalcitrantifitsisbranched,(resistance
increaseswiththedegreeofbranching).Atpresent,alkyl
benzylsulphonateshavingnon-branchedalkylendsare
used;thesearebiodegradedbyβ-oxidationfromtheiralkyl
ends.

12. (v)OilMixtures:
Oilisanaturalproduct,hasmanycomponentsandis
biodegradable,thedifferentcomponentsbeingdegradedat
differentrates.Biodegradationisabletohandlesmalloil
seepages.Butwhenlargespillsoccurtheproblemof
pollutionbecomesacute.Oilisrecalcitrantmainlybecause
ofitsinsolubilityinwaterandduetothetoxicityofsomeof
itscomponents.
(vi)OtherXenobioticCompounds:
Anumberofpesticidesarebasedonaliphatic,cyclicring
structurescontainingsubstitutionofnitro-,sulphonate,
methoxy-,amino-andcarbonylgroups;inaddition,theyalso
containhalogens.Thesesubstitutionsmakethemrecalcitrant.