Xenobiotic compounds and its types
Rohini Bansode1, Sandeep Kumar2, Lal Chand Malav2, Mahesh Kumar Malav2
1 Department of Plant Biotechnology, KAU, Kerala-680656
2 Indian Agricultural Research Institute, New Delhi 110012
Email id- contact.rohibiotech@gmail.com

Xenobiotic compounds are man- made chemicals that are present in the environment at higher concentration. These are either not produced naturally or are produced at much lower amount than that by man. The compounds that resist biodegradation and persist in the environment for long period of time are called recalcitrant.

They may be recalcitrant due to the reasons such as.,

1. They are not recognized as substrate by degradative microorganisms.
2. Highly stable in nature.
3. Insoluble in water.
4. They are highly toxic or release toxic products due to microbial activity.
5. They have large molecular weight which prevents entry to microbial cells.

Structural features of xenobiotic compounds:

1. Presence of halogens in place of hydrogen in the molecule and it requires more energy for cleavage.
2. Presence of other groups such as nitro, sulphonate, methoxy, amino etc.
3. Aromatic compunds, cycloalkanes and heterocyclic compounds are more recalcitrant.
4. Branched linear chains are resistant to biodegradation.

Types of recalcitrant Xenobiotic compounds

These are categorized into six types-

1. Halocarbons
These compounds consist of different numbers of halogen viz., CI, Br, F (fluorine), or I, atoms in the place of H atoms. They are used in various applications as solvents (chloroform, CHCI3), as propellants in spray cans of cosmetics, paints, etc. Also they are used in condenser units of cooling systems (Freons, CCI3F, CCI2F2, CCIF3, CF4), and as insecticides (DDT, BHC, lindane, etc.) and herbicides (dalapon, 2, 4-D, 2, 4, 5-T, etc). Chloroform, freons are volatile and escape into the atmosphere where they destroy the protective ozone (O3) layer leading to increased UV radiation.

2. Poly chlorinated Biphenyls (PCB's):
Poly chlorinated Biphenyl compounds have been covalently linked with two benzene rings having halogens in place of hydrogen. These are mainly used as plasticisers, insulator coolants in transformers and as heat exchange fluids.

3. Synthetic Polymers:
Synthetic polymers such as polyethylene, polystyrene, polyvinyl chloride, etc., and nylons, which are used as garments, wrapping materials, etc. They are recalcitrant mainly due to the following reasons such as insoluble in water and molecular size.

4. Alkylhenzyl Sulphonates:
These are surface-active detergents which are found to be superior compare to soaps. The sulphonate (-SO3-) group present at one end resists microbial degradation, while the other end becomes recalcitrant if it is branched. In this degree of resistance increases with the increasing branching length. At present, alkylbenzyl sulphonates having non-branched alkyl ends are used; these are biodegraded by beta-oxidation from their alkyl ends.

5. Oil Mixtures:
Oil is recalcitrant mainly because of its insolubility in water and due to the toxicity of some of its components. It is a natural product contains many components and is biodegradable. Biodegradation process is mainly used to handle small oil seepages. But when large spills occurs the problem of pollution becomes serious. These compounds are mainly recalcitrant in nature.

Other Xenobiotic Compounds:

More number of pesticides consists of aliphatic, cyclic ring structures containing replacement of nitro-, sulphonate, methoxy-, amino and carbomyl groups in addition to halogen group. These modifications make them recalcitrant.

About Author / Additional Info:
I am currently pursuing Ph.D in Plant Biotechnology from Kerala Agricultural University, Thrissur.