Bivalent Noninteractive Ligand Binding (lig)

Glycine:

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

step 1d   2d   HOOC-CH2-NH3+ - H+ + H+ -OOC-CH2-NH3+ - H+ + H+ -OOC-CH2-NH2 step 2a   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

• "Quit" Excel before moving to the next exercise.

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©	Duane W. Sears Revised: August 10, 1998