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Optical isomerism in Limonene

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iansonuk
955031.  Sun Dec 09, 2012 6:23 am Reply with quote

This may have been mentioned before, but if so I haven't found it:

QI repeated an anecdote I've often encountered to the effect that the difference in odour between oranges and lemons is due to the fact that the two fruits have the 'left handed' and 'right handed' versions of the molecule 'limonene'.

This strikes me as being extremely dubious because biological molecules do not typically occur in nature in more than one form (usually the right-handed' 'D' form).

Indeed Chris McManus in "Right Hand, Left Hand: The Origins of Asymmetry in Brains, Bodies, Atoms and Cultures" says that this is "wrong, a result of impurities introduced into the L- and D-forms while being synthesised".

I for one believe him.

 
Efros
955041.  Sun Dec 09, 2012 7:10 am Reply with quote

Indeed this piece suggests that (-)-Limonene (the L form) smells more like turpentine. Biological molecules can occur in nature in the D an L configuration it depends on the energetics of conversion between the two forms, typically though metabolic and other enzymes only deal with the D form, the L form does not fit the lock as it is the wrong key (enzymatic lock and key principle). Consequently if the D and L form can exist due to low energy transformations, and an enzyme is acting on the D form then the L form will be constantly changing into the D form to maintain the racemic mixture, so eventually all of the racemate will be used by the enzyme.

 
Celebaelin
955044.  Sun Dec 09, 2012 7:19 am Reply with quote

Efros wrote:
Indeed this piece suggests that (-)-Limonene (the L form) smells more like turpentine. Biological molecules can occur in nature in the D an L configuration it depends on the energetics of conversion between the two forms, typically though metabolic and other enzymes only deal with the D form, the L form does not fit the lock as it is the wrong key (enzymatic lock and key principle). Consequently if the D and L form can exist due to low energy transformations, and an enzyme is acting on the D form then the L form will be constantly changing into the D form to maintain X the racemic mixture X, so eventually all of the racemate will be used by the enzyme.

equilibrium

Concise - 8/10

 
Efros
955045.  Sun Dec 09, 2012 7:23 am Reply with quote

Celebaelin wrote:

equilibrium

Concise - 8/10


Wanton use of colour 0/10

:-b

 
Celebaelin
955048.  Sun Dec 09, 2012 7:25 am Reply with quote

Gah! In house styles : (

 
Jenny
955100.  Sun Dec 09, 2012 10:53 am Reply with quote

Welcome Iansonuk :-)

 
Jerrythesheep
956404.  Fri Dec 14, 2012 5:00 pm Reply with quote

Hi,

The following link may or may not help.

http://coedpages.uncc.edu/cstem/Document%20Hold-OLD/Jenna%20Beckham%20paper.doc

I secretly suspect that the difference in smell is more to do with relative concentrations of the R and S forms of limonene but both do occur in nature.

I would add that R and S are determined by a set of priority rules (Cahn, Ingold, Prelog) and that there is no relationship between R and S and left and right as determined by direction of rotation of polarized light by the pure enantiomers.

Baa,k

Jerrythesheep

 

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