Chemical Forums
Chemistry Forums for Students => High School Chemistry Forum => Topic started by: crays on May 22, 2008, 09:33:51 AM
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Hi guys, I got a question here. I tried asking my teacher but she can't answer it so i decided to ask for help here. Its about the protein molecules. As my book writes, Protein can be polar, non polar, acidic and basic. But why can non polar molecule, for example Glycine to have H+ and O- ? They're non polar right? Wouldn't that make them acidic ?
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But why can non polar molecule, for example Glycine to have H+ and O- ? They're non polar right? Wouldn't that make them acidic ?
Please elaborate. It doesn't make sense.
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You have mixed up a lot of concepts here.
I suggest you understand first what an amino acid is (of which glycine is one)
Then you understand how amino acids bond to form proteins. (there is a key chemical bond in this - search google or wiki or your textbook and find out).
Then you rephrase your question.
The above about amino acids and proteins is necessary understanding in this area.
Can you post a two-line definition of amino acids and proteins on this thread then look back at your question and see if it makes sense?
Clive
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Sorry for my bad English. I guess a picture would help me a little here.
My book's example of glycine is as so.(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fhome.doramail.com%2Fcraymel%2Fthis.jpg&hash=38f1425cd31cb2b244122f6964d5dd8c2ef333f7) Me and my classmate were both confused why there is a H+ and O- since Glycine is a non polar protein. My teacher could not explain as well.
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Aha!
I think I have a better idea of what you mean. Diagrams are good in Chemistry.
My thinking is this (it may be wrong)
Given the glycine molecule has formula
H2NCH2COOH and the carbon backbone you have given . Glycine is an amino acid NOT a protein you keep on mixing up the two concepts. Read up on the relationship between amino acids and proteins. Do this. The groups are key to this.
See the molecule image below (1) from wikipedia, which has an amine group (NH2) at one end and carboxylic acid group (COOH) at the other end
The NH2 can attach a H+ ion. Therefore glycine can act as a base.
The COOH can release a H+ ion. Therefore glycine can act as an acid. Read up about COOH and you will see you don't consider O- on its own. The COO- is considered more a single entity.
This gives your version of glycine.
Because of this behaviour, glycine is soluble in water.
I have copied the generic diagram(2) from http://www.chemguide.co.uk/organicprops/aminoacids/background.html
Note the structure where NH2 and COOH are close to each other, not at opposite ends of the molecule. I guess this has biological significance. Read the comment in chemguide on zwitterions. Glycine is the simplest amino acid and is the only one where NH2 and COOH are at opposite ends of the molecule ONLY because the central carbon chain has one carbon. This is not typical.
Given the above, where does that get you?
Clive
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Thank you for your reply, Its helpful. Sorry that i mixed up protein and amino acid. Just to make sure, so it means that, even in a solid state, the amino acid are all linked up together by an ionic bond ? and the H3N+ is actually an ionic bonding to another amino acid? But so far as i know, Nitrogen can only have 3 valance electron right? How can it have 3 hydrogen and a carbon bonded to it?
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...the amino acid are all linked up together by an ionic bond ? and the H3N+ is actually an ionic bonding to another amino acid?
The aa's are linked by covalent bonds - called amide (or peptide) bonds.
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so the H3N+ (the extra H) is actually a peptide bond?
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No!
When many amino acids come together they form protein.
The amino acids in protein are connected through petide/amide bond -CONH-
The polarity of amino acids are usually determine by its side group.
In glycine, the side group is -H, and the H atom is non polar.
An example of polar amino acid is serine, due to the presence of -CH2OH side group
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Thank you for your reply, Its helpful. Sorry that i mixed up protein and amino acid. Just to make sure, so it means that, even in a solid state, the amino acid are all linked up together by an ionic bond ? and the H3N+ is actually an ionic bonding to another amino acid? But so far as i know, Nitrogen can only have 3 valance electron right? How can it have 3 hydrogen and a carbon bonded to it?
Hi
I was trying to push you towards reading up on amino acids and proteins so you discovered the peptide link yourself. But the answer is in the other posts.
As a side comment, going back to the ionized form of glycine (and I don't think this is anywhere as important as the peptide link) there is ionic / electrostatic attraction in pure glycine. This has nothing to do with the peptide links in proteins
From the link I gave you to chemguide ...
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This is called a zwitterion.
This is the form that amino acids exist in even in the solid state. Instead of the weaker hydrogen bonds and other intermolecular forces that you might have expected, you actually have much stronger ionic attractions between one ion and its neighbours.
These ionic attractions take more energy to break and so the amino acids have high melting points for the size of the molecules.
..
Read Jim Clarke's chem guide on amino acids and proteins and understand ...
Clive
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<< snip >>
But so far as i know, Nitrogen can only have 3 valance electron right? How can it have 3 hydrogen and a carbon bonded to it?
Aha ! The tyranny of the full outer shell ( I have written on this concept before)
As is often the case, Jim Clarke's chemguide is clear on this
Read the entry
http://www.chemguide.co.uk/atoms/bonding/dative.html#top
(all of it)
The section
The reaction between ammonia and hydrogen chloride
is particularly relevant to the (good) question you have raised - which is entirely a logical thing to write - if incorrect.
The question is really about how does the N bond a H+ when the simple model says N in glycine cannot form any more bonds.
After reading the entry, can you answer your own question and post back on this forum?
Clive
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Oh sorry clive, i was just simply searching for the answer x(. Anyway, thanks for the link. I kinda get what it is now already. But again just to make sure, its the whole hydrogen nucleus that is transfered but the electron is left behind right?
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Oh sorry clive, i was just simply searching for the answer x(. Anyway, thanks for the link. I kinda get what it is now already. But again just to make sure, its the whole hydrogen nucleus that is transfered but the electron is left behind right?
My understanding of the dative bond is nothing gets "left behind". The two species "munge" (that's a technical term ;) ) together ( NH2 and COOH in the amino acid zwitterion ) and share the electrons.
Does that make sense?
I just read about polyamides.
Can you find out the similarities and differences between
1) Polypeptides or proteins
versus
2) Polyamides
I never thought of the polyamide configuration before. You learn something new .
Clive
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Thank you for your help. But refering back to my question why does it have an H+ on the amino group and a H taken away from the COOH group. is it because of the amino acid is linking each another ? so the NH2-C-COO(H)NH2-C-COOH the H is linked that way ?
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Thank you for your help. But refering back to my question why does it have an H+ on the amino group and a H taken away from the COOH group. is it because of the amino acid is linking each another ? so the NH2-C-COO(H)NH2-C-COOH the H is linked that way ?
Amino acids can undergo internal acid-base reaction forming dipolar ions known as the zwitterions in the solid state and at the isoelectric point.
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Thanks, i understand now.
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Hi Guys.. :)
I am a new member and was not aware that such a site exists. I was a science student and finished my schooling way back. My Job however took me away from science (esp. chemistry). So when I saw this site.. could not control myself from from relearning the old concepts...!
Hey Crays! .. though others have aptly explained the concept and you have acknowledged them too, please let me explain a little more.. and all please correct me where I am wrong... thus I can also relearn with Crays... :)
As per the structure of glycine: H2NCH2COOH
... Amine group :NH2- being basic wants to capture one Proton (H+) while Carboxylic group -COOH being acidic wants to donate one Proton, so that they can form the Zwitter Ion.
Now all this depends on the pH of the solution. If the pH < 7, then the solution is acidic (excess Protons) and thus glycine exists as a positive ionH3N(+)CH2COOH. For pH > 7 the solution is basic (excess Hydroxide Ions), thus glycine exists as a negetive ion H2NCH2COO(-). However, for every Amino Acid there is a specific range of pH where it behaves as if it a neutral species! This is when glycine exists as a zwitter ion.
You may think why I am refering a zwitter ion as neutral species ???
... because zwitter ion has equal positve charge and negetive charge, thus the net glycine stucture is Neutral. The pH at which this zwitter ion exists is called Isoelectric Point.
Hope... I still remember few concepts correctly :D