The pH is a measure of proton availability in the solution (comes from combined effect of all acids & bases in a
solution)
* The more protons there are, the more acidic, and the lower the pH
* The fewer protons there are, the more basic/alkaline, and the higher the pH
The pKa is the pH at which half the copies of an acidic site is deprotonated (and thus in conjugate base state) and half is protonated (and this in conjugate acid form)
* At a pH below the pI, there’s more protons available to take, so more conjugate acid
* At a pH above the pI, there are fewer protons available, so more conjugate base
The pKR is just the pKa of an amino acid R-group (side chain)
The pI (isoelectric point) is the pH at which a molecule is net neutral (has no net charge)
* At a pH below the pI, a molecule is positively-charged
* At a pH above the pI, a molecule is negatively-charged
pI tells you about charge. pKa does NOT. Instead, it tells you about protonation. And at a single site. The molecule may be negative, neutral, or positive when that site is protonated.
The pI comes from the combination of acid/base groups in a molecule
The pH comes from the contributions of all acids/bases in a solution
Something can only act as an acid in its protonated state.
But the stronger the acid is, the less likely it will be to be in that state!
So the more likely it is to be in the deprotonated state, where it can only act as a base!
If the molecule is neutral in its protonated state (conjugate acid), it will be negatively-charged in its deprotonated state (conjugate base).
But if the molecule is positively-charged in its protonated state (conjugate acid) it will be neutral in its deprotonated state (conjugate base).
The happier a molecule is to be in a state, the more likely it will be in that state. Resonance and inductive effects can make molecules happy, so resonance etc. that stabilizes one state will make it favorable, even if it comes with charge.
We call amino acids basic or acidic based on what their neutral form acts as - even if their predominant form acts the opposite! So, for example, you pretty much always find arginine in its protonated, acid state because its neutral form is a stronger base, so we call it basic (and show it as blue). On the other hand, you pretty much always find aspartate in its deprotonated, base state because its neutral form is a stronger acid, so we call it acidic (and show it as red)
For a more in-depth discussion: • pH, pKa, pI, protein c...
more on pH, pKa, and the Henderson-Hasselbalch equation: bit.ly/phbuffers RU-vid: • Henderson-Hasselbalch ... & • Avoiding common proble...
more about all sorts of things: #365DaysOfScience All (with topics listed) 👉 bit.ly/2OllAB0 or search blog: thebumblingbioc...
29 сен 2024
