confusion about acidic/basic amino acids

[equityrange]

why is lysine considered to be basic? since it has NRH3+, shouldn't it be acidic because i can donate protons?

same thing with aspartic acid, isn't it a base because it can donate electrons?

http://www.bioss.ac.uk/~dirk/genomeOdyssey/AminoAcids.gif

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[mooshika]

This was very confusing for me too because no one really seemed to explain it clearly.

In the pH of the body, 7.4, some AA's will exist as either negative or positive ions, and neutral AA's will exist as "zwitterions" right around 7.0 to 7.4 pH at 98.6F with one - and one + so they are neutral and "isoelectric" The negative and positive charges are on the cbxlyc acid and the amino group respectively.

pH and polarity are measured by examining the "R" groups. You need to understand that AA's exist as "zwitterions" in their "natural" state, meaning whatever the pH is when the carboxylic acid has a - charge and the amino group had a + charge determines whether or not the AA is acidic or basic.

Also, you don't have to actually know the pH of the "isoelectric" point of the AA to determine if it is acidic or basic. If the AA has an overall - charge at neutral pH than it is basic. If it has an overall + charge at neutral pH then it is acidic.

The word in the "quotes" are very important for the MCAT they love to ask questions with these concepts. So know what they mean and just memorize the acidic and basic AA's. There's only 2 truly acidic and 3 truly basic AA's. HAL, basic and GA, acidic. (histidine, arganine, lysine) (glutamic acid, aspartic acid)

Hope that helps.

 

[RogueUnicorn]

Lysine is basic because at physiological pH, it is a proton acceptor to BECOME R-NH3+. like any acid base reaction, the resultant product is the conjugate acid of lysine, but we still call it lysine. You are correct in that this is now an acid, but it won't FUNCTION as an acid until the pH is about 10 or so.

 

[mooshika]

Lysine is basic because at physiological pH, it is a proton acceptor to BECOME R-NH3+. like any acid base reaction, the resultant product is the conjugate acid of lysine, but we still call it lysine. You are correct in that this is now an acid, but it won't FUNCTION as an acid until the pH is about 10 or so.

Lysine is considered a basic AA, as I wrote above. Also, be aware that the MCAT loves questions about electrophoresis and AA's at certain pH's as well as which AA's would travel furthest toward the cathode/anode, (which are labeled opposite from ox/redox anode and cathode) etc. As long as you are comfortable understanding the concepts regarding the relationship between "charge" "pH" and what "isoelectric" and "zwitterion" mean in an applied problem (MCAT primarily tests knowledge by presenting circumstances and asking you to apply the concepts and come up with the right answer.)