[jena]

Hi

My Question:

The titration curve of the amino acid glycine reveals a pK1 of 2.34, a pI of 5.97, and a pK2 of 9.60. When dissolved in water, which ionic species is most likely to predominate at pH = 5.97?

I really don't understand this question, I was confused during the lecture.

Please help and Thank You

[Yggdrasil]

Glycine has two acidic groups, an alpha-amino group and an alpha-carboxyl group.  At low pH, the alpha-amino group will be protonated and have a positive charge, while the carboxy group will be protonated and have no charge.  Conversely, at high pH, the alpha-amino group will be deprotonated and be neutral, while the carboxyl group will de deprotonated an have a neutral charge.  The pKas correspond to the pH values at which each acidic group will be protonated in half of the molecules in the population and deprotonated in the other half.

The pI of a protein/amino acid indicates the pH at which the population of molecules has a net charge of zero.  This would correspond to the zwitterionic form of glycine which carries a positive charge on the amino group and a negative charge on the carboxyl group to give a net charge of zero.