Bivalent Noninteractive Ligand Binding (lig)


An example of bivalent ligand bindingis modeled by examining the acid-base titration properties ofg glycine. Fully protonated -- "saturated" -- glycine undergoes a two-step dissociation reaction with decreasing hydronium ion concentration. Fully deprotonated -- "desaturated" -- glycine undergoes a two-step association reaction with increasing hydronium ion concentration.

Two-step dissociation
step1d 2d 
- H+

+ H+
- H+

+ H+
step2a 1a 
Two-step association
Kd-cooh = [H+] * [-OOC-CH2-NH3+] / [HOOC-CH2-NH3+]
(step 1d)
Kd-nh2 = [H+] * [-OOC-CH2-NH2] / [-OOC-CH2-NH3+]
(step 2d)

Ka-nh2 = [-OOC-CH2-NH3+] / [H+] * [-OOC-CH2-NH2]
(step 1a)
Ka-cooh = [HOOC-CH2-NH3+] / [H+] * [-OOC-CH2-NH3+]
(step 2a)

Continue the acid-base titration analysis of glycine with an Excel spreadsheet where the first worksheet ("Sat Plots") shows two saturation plots -- one with Ya, the association fraction, plotted against pH and the other with Ya plotted directly against [H+] on a log scale.

Glycine Acid-Base Titration

Bivalent (Noninteractive) Ligand Binding

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Duane W. Sears
Revised: August 10, 1998