Question

26. Melting Points of Lipids The melting points of a series of 18-carbon fatty acids are: stearic acid, 69.6 °C, oleic acid, 13.4 °C; linoleic acid, -5 °C; and linolenic acid, -11 °C. (a) What structural aspect of these 18-carbon fatty acids can be correlated with the melting point? (b) Draw all the possible triacylglycerols that can be constructed from glycerol, palmitic acid, and oleic acid. Rank them in order of increasing melting point. (c) Branched-chain fatty acids are found in some bacterial membrane lipids. Would their presence increase or decrease the fluidity of the membranes (that is, give them a lower or higher melting point)? Why?

Answer 1

a)

Melting points of unsaturated fatty acids are much lower than those of saturated fatty acids with the same number of carbon atoms. The m.p. decreases as the number of double bonds increases for fatty acids with the same number of carbon atoms. Therefore, having same carbon no but varying in unsaturation gives different melting point.

Stearic acid is a saturated fatty acid having formula C18H36O2.   (Fully saturated fatty acid)

Oleic acid has one double bond with molecular formula C18H34O2 (mono unsaturated fatty acid)

Linoleic acid has two double bond with molecular formula C18H32O2 (unsaturated fatty acid)

Linolenic acid has three double bond with molecular formula C18H30O2 (Polyunsaturated fatty acid)

b)  
palmitic acid oleic acid oleic acid oleic acid palmitic acid palmitic acid

c)

The introduction of cis-unsaturation, shorter-chained fatty acids and fatty acids with branches reduces the melting temperature of the cell membrane prominent to an increased fluidity at low temperature due to less packing efficiency created by branching.