How do you calculate the pKa to Ka and vice versa in your head?? Any shortcuts/simple steps?
When given the pKa of the COOH group, NH3+ group, and any side chains (whether basic OR acidic), how do you calculate an amino acid's pI?
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− Subtraction = Calculate +/- Plus/minus toggles the pos/neg sign of the displayed number mc Memory clear mr Memory recall m- Memory minus m+ Memory plus CE Clear entry AC All clear √x Square root % Percentage π pi = 3.1415926536 x y Exponent R2 Round to 2 decimals (cents)
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When the pH = pKa the solution is a buffer, and therefore half of the amino groups are protonated and half are deprotonated. The acid group will be deprotonated - not protonated, making it's charge -1. The pH is much greater than the pKa. The amino group will be half protonated (+1) and half deprotonated (0) for a net of "+0.5". This works out to a total of -0.5.
The only time the pka and pH are one in the same is at the half equivalence point. The pka is NOT the pH, but a constant value you can use to get the concentration of H: k_a = [H+] [A-]/ [HA] When you take the log of this equation, and move values around: log (k_a) = log [H+] + log [A-]/ [HA] -log [H+] = -log (k_a) + log [A-]/ [HA] p H = p ka + log [A-]/ [HA] (how I wish SDN had LaTeX) You see ...