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# A chemical polypeptide question.

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posted on Feb, 2 2016 @ 03:11 PM
If the molecular formula of Leucine is H02CCH(NH2)CH2CH(CH3)2, the molecular mass is 131.18 grams/mole. If 10 Leucines are joined together by a dehydration reaction to make a polypeptide, what would the molecular mass of the polypeptide equal?
N= 14g/mole, C=12g/mole, O=16g/mole, and H=1g/mole

posted on Feb, 2 2016 @ 03:36 PM
I've got the feeling that's someone's homework question.
dehydration = removal of water = removal of -COOH and replacement with -CO-NH, freeing one H2O.

You start with 10 moles of Leucine (10 x 131.18). You are going to lose 9 moles of -COOH's, but you gain 9 moles of -CO-NH, and 9 moles of H2O. Whatever all that adds up to,

10 x H02CCH(NH2)CH2CH(CH3)2,
- 9 x -COOH
+ 9 x -CO-NH

posted on Feb, 2 2016 @ 03:46 PM

That makes sense, you explained it in a more digestible manner. Thank you for the help, I'm grateful.

posted on Feb, 2 2016 @ 05:52 PM

originally posted by: RobFox
If the molecular formula of Leucine is H02CCH(NH2)CH2CH(CH3)2, the molecular mass is 131.18 grams/mole. If 10 Leucines are joined together by a dehydration reaction to make a polypeptide, what would the molecular mass of the polypeptide equal?
N= 14g/mole, C=12g/mole, O=16g/mole, and H=1g/mole

You convert an acid to an amide by reaction with an amine and concomitant loss of water. A decapeptide loses 9 H2O. 10 Leucines mass ~1311.8 g/mol. 9 H2O = 162.14 g/mol: MW of decapeptide is 1311.8 - 162.1= 1149.7 g/mol

edit on 2/2/2016 by pteridine because: clarification

posted on Feb, 2 2016 @ 11:47 PM
I know what the chemical leucine is and some of what it does in the body.

I know that the guy who discovered it had a wife named Lucy.

I think maybe I should quit researching this kind of stuff. Maybe I should build a sauna.

posted on Feb, 3 2016 @ 08:19 AM

originally posted by: stormcell
I've got the feeling that's someone's homework question.
dehydration = removal of water = removal of -COOH and replacement with -CO-NH, freeing one H2O.

You start with 10 moles of Leucine (10 x 131.18). You are going to lose 9 moles of -COOH's, but you gain 9 moles of -CO-NH, and 9 moles of H2O. Whatever all that adds up to,

10 x H02CCH(NH2)CH2CH(CH3)2,
- 9 x -COOH
+ 9 x -CO-NH

Completely wrong.

posted on Feb, 23 2016 @ 07:41 PM

You started off strong but made your math way too complicated. As you say, it's a dehydration reaction; i.e. you lose a molecule of water for every peptide bond you make. All you then had to do was subtract the mass of 9 waters away from the mass of 10 leucines. That would have given you the right answer.

Alternatively, there are very easy to use calculators online that could give you the right answer. For example. Or for a more general molar mass calculator. I personally use ChemBioDraw, but this is a downloadable software that requires a licence.

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