Strategy for drawing amino acid titration curves:

https://www.youtube.com/watch?v=dcNpoyiuHfc&ab_channel=thebumblingbiochemist

https://www.youtube.com/watch?v=APX4cjovul4&ab_channel=thebumblingbiochemist

1. Identify # of ionizable groups
   • at least 2 (N- & C-termini), possibly 3 (R group, aka sidechain)
   • this #= # of OH⁻ equivalents (eq.) required to fully deprotonate it
2. Draw your x-axis, extending that many equivalents-worth
   • label each 0.5 increment
3. Sketch the state of each ionizable group at each increment & label its charge
   • remember groups will deprotonate in order of increasing pKa (most acidic first)
4. Plot points intersecting each at each 1/2 OH⁻ (0.5, 1.5, 2.5 – if applicable)
   • the corresponding pH = pKa, so [HA] = [A⁻] 
5. Draw a flattish “buffering zone” around this point
6. Draw points at each 1 OH⁻ (0, 1, 2, 3 (if applicable)) halfway between the the pKas on either side
   • the steepest part of the inflection will be here
   • note: one of these pH’s will be the pI!
7. Connect the dots and lines, drawing the steepest part of the inflection curves through these equivalence points
8. Label the pI (pH at 0 net charge point)
   • will be the average of the pKa’s on either side of it
   • average of the 2 most similar pI’s

In the video, I start with an overview, then…

At 19:40, I show an example of how to draw an amino acid titration curve for an amino acid with a non-ionizable R group (in this case glycine). https://youtu.be/APX4cjovul4?t=1180

At 30:14, I show an example of how to draw an amino acid titration curve for an amino acid with a basic R group (in this case lysine) https://youtu.be/APX4cjovul4?t=1814

At 40:22, I show an example of how to draw an amino acid titration curve for an amino acid with an acidic R group (in this case glutamate) https://youtu.be/APX4cjovul4

Titration curves are nice for some things, especially helping you get a sense as to what’s going on at different pH’s, but if you want to actually calculate what the pH would be or what proportion would be protonated at a given pH, for an in-between equivalents point, the Henderson-Hasselbach equation is the place to go!  http://bit.ly/phbuffers  YouTube: https://youtu.be/VRJV2FTOUeM  

P.S. Huge thanks to Dr. Jeff Sigman for helping teach me his method!

Much more on amino acids:  http://bit.ly/allaminoacids    ; YouTube: https://youtu.be/Os6VVovCR8U 

More on acids, bases, & pKa calculations:   http://bit.ly/phbuffers   YouTube: https://youtu.be/VRJV2FTOUeM &  https://youtu.be/I-n9YUFLjZ4

more about all sorts of things:  #365DaysOfScience All (with topics listed) 👉 http://bit.ly/2OllAB0 or search blog: https://thebumblingbiochemist.com