Positive charge on "basic R group of amino acid"

[lukasbecker]

My bio professor said that the NH3 groups are basic because the have a positive charge. Um...that isn't the Lewis base definition. It isn't the Bronstead Lowry either. What does she mean!

[hypervalent_iodine]

Well, they are positively charged at physiological pH because the amine is basic and picks up a H⁺ from solution, not the resulting -NH₃⁺.

 

You are right that the amine side chain (-NH₂) is a Lewis base. Given that the definition of a Lewis base is an electron lone pair donator can you think of why the amine fits the Lewis base description? If you know how to draw Lewis dot structures, try it for the amine. If not, let me know and I'll walk you through it

Edited September 4, 2011 by hypervalent_iodine

[lukasbecker]

Well, they are positively charged at physiological pH because the amine is basic, not the resulting -NH₃⁺. You are right that the amine side chain (-NH₂) is a Lewis base - can you think of why that might be? If you know how to draw Lewis dot structures, try it for the amine. If not, let me know and I'll walk you through it

 

 

This is what her slide says, "Lysine and Arginine are "basic" amino acids because of the + charge on their R groups." I see how the NH2 and NH3 of the amino acid is positive, but I still fail to see how the amino acid is considered basic. They are listed as basic, but why do they behave that way, and by which base definition? Thanks for helping.

[hypervalent_iodine]

This is what her slide says, "Lysine and Arginine are "basic" amino acids because of the + charge on their R groups." I see how the NH2 and NH3 of the amino acid is positive, but I still fail to see how the amino acid is considered basic. They are listed as basic, but why do they behave that way, and by which base definition? Thanks for helping.

 

The positive charge on the amine of the side chains is a result of the uncharged amine being being a Lewis base. The reason it is basic is because an uncharged amine has a lone pair of electrons (hence why it is a Lewis base). It can use that lone pair of electrons to react with a Bronsted acid and pick a H⁺. The slide seems a little deceiving in my opinion as it is not because it is positively charged that it is a base, rather it is because it is a base that it is positively charged at low pH.

 

To show what's happening, I drew some diagrams for you. For lysine, we can see the amine picking up the extra proton by way of the lone pair of electrons (drawn as two dots above the amine nitrogen):

 

 

 

 

And for arginine we have fairly much the same thing, except now there are resonance structures (didn't draw the arrow pushing on this one, but you get the idea):

 

 

[lukasbecker]

Thank you! This makes perfect sense, and the slide is misleading.

 

 

[hypervalent_iodine]

No problems, glad I could help