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Home » Chemistry Homework Help » Biochemistry » Derived Lipids
Derived Lipids
This group consists of (1) fatty acids and (2) sterols. The chemistry of fatty acids has been already considered.

Sterols: these are derivatives of a complex ring system called cyclopentanoperhy drophenanthrene’ ring system. The meaning of the name and the method of naming the individual compounds of the system as A, B, C, D rings and numbering of the seventeen positions in the ring system are depicted in figure.

Cholesterol ergo sterol bile acids sex hormones adrenal cortical hormones and the D- vitamins are some of the important sterol derivatives. The term sterol means solid alcohol.

Cholesterol: since it was first isolated form gall stones, it was called cholesterol which means solid alcohol form bile brain nervous tissues adrenal glands and egg yolk are rich sources. White matter contains as much as 14% gray matter 5% spinal cord 12% and liver about 1% cholesterol. It has a molecular formula C27H45OH and has the structure shown in figure.

Saturation of the double bond between C5 and C6 will give dihydrocholesterol. Coprosterol is the cis isomer of dihydrocholesterol and is found in feces.

While the ring structure shown in figure in common use, the three dimensional structure of the steroid molecule is more correctly represented if the rings are replaced by chair or boat forms between the two the chair form is more stable.

Further the CH3 in position 10 is considered to be in the beta poison projects above the plane of paper. If H in position 5 is in the same plane as the CH3 the rings are in the cis- configuration and the steroid is said to belong to the beta series, and the bond linking h is shown as an unbroken line.

If the H in position 5 is in the opposite plane to that of CH3 below the plane of the paper), the bond is represented by a broken line and the steroid belongs to the Alfa series. Rings A and B are in the Trans position to each other. These concepts and structure are illustrated in figure for cholesterol (in which the double bond of cholesterol between C5 and C6 is saturated.

Cholesterol belongs to the Alfa series. The H at position 5 is in Tran’s position to the CH3 at position 10. The OH at position 3 is however in cis relationship with the CH3 at position 10. It has a double bond between C5 and C6 in the ring B.

Properties of cholesterol: it is a white crystalline substance showing the usual solubility properties of the lipids. The crystals are rhombic plates with one of the angles broken. It has a melting point of 149.

It is usually prepared in the laboratory by extraction from brain or spinal cord with acetone. It is a poor conductor of electricity and functions probably as an insulating mechanism for the nerve impulses. As a precursor of bile salts steroid hormones and vitamin D3 it is of great significance. It is present in blood to the extent of 150 – 250 mg per 100 ml. and its variations are of considerable clinical significance.

Chemical properties: the double bond can be saturated by addition of hydrogens to form the dihydroderivative. It can also be halogenated. The OH group in position can be esterifies with fatty acids to form cholesterol esters. Three- fourths of the cholesterol of plasm exists as ester. Lanoline the cholesterol ester has been already considered under waxes. The OH combines with digitizing to form an insolution digitionide. This property is used to separate cholesterol from an insoluble digitonide. This property is using to separate cholesterol form its esters which do not react with digitionin.

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