[IamDeem]

Estimate the pI of the oligopeptide: S – E – P – T – E – M – D – E – R



CAn you show your work on this one and talk to me like a 2nd grader.  I really dont get this topic.

[IamDeem]

these are the pKa values I have for

H3N - S  E  P  T  E  M  D  E  R - COOH
10      -  4  -  -   4 -   4  4  12   2


So now what do i dowith these numbers?

[aHerraez]

At pH<2, all groups will be protonated. Total net charge = ??
At 2<pH<4, the terminal carboxyl will have lost its proton, so total net charge = ??
At 4<pH<10, the next group (terminal amino) will also be unprotonated...
etc.

Just find in which interval the net charge will be zero and calculate the average of the two flanking pKas. That's the pI