Charge of glycine at pH 5.4 help please

[dannybht]

Why would the charge of glycine be positively charged at pH 5.4. I know that the general rule is that if pH is less than the isoelectric point pH (which is 6 for glycine) the species become a cation. The contradicting part is that pka of the amino group is when protonated is 9.78 and pka of carboxylic acid terminal is 2.33. Wouldn't a pH at 5.4 cause it to become neutral overall? I'm stuck on this for awhile now.

---

Members don't see this ad.

 

[doy]

Nope. Glycine would now be a neutral molecule at pka 5.4. The carboxyl end is now deprotonated and amino end still carries a positive charge.
I think the +/- pH pI thing would apply to proteins and not just individual amino acids. This is because acidic and basic sites on the protein due obey the pH,pI rule and the overall charge of the amino acid could then be calculated.

 

[dannybht]

I'm still confused though..
The answer says its positively charged at 5.4...

 

[dannybht]

The book answer says its positively charged though...

 

[ashtonjam]

What if it was talking about a solution of glycine? Some molecules could be positively charged while most remain neutral. This would actually affect glycine's migration on a pH gradient gel because some species would be + charged and they would move away from neutral pH until they hit the pI of 6.

Since you haven't hit the equivalence point (pI) yet, glycine is not 100% zwitterionic. A bunch of the molecules might be zwitterionic, but a few aren't and that is enough to affect the overall charge of a glycine sample. At the equivalence point, exactly 100% of the molecules are the same, but before and after it, that is not the case.

Although my first reaction would be to say that Glycine is neutral as well...

EDIT: After looking at that page you posted, the solution seems to be in line with using the pIs to determine charge. Asparagine's pI is at exactly 5.4 so it is truly neutral. Aspartate's pI is 2.77 so at pH = 5.4 it is negatively charged. Glycine's pI is at 6, so at a pH of 5.4 it is positively charged. Source: http://www.mhhe.com/physsci/chemistry/carey5e/Ch27/ch27-1-4-2.html

 

[dannybht]

Yeah this question was about a buffered solution so basically gel electrophoresis via ph gradient. So only when a question relates to gel electrophoresis you use the pH pI rule then? Thanks for the help. This question confused me because there were other questions that contradicted what I learned.