The Table below lists binding and dissociation rate constants for ligand binding

Question

The Table below lists binding and dissociation rate constants for ligand binding to the isolated subunits of human hemoglobin at pH 7.0 and 20 degrees C.

---------------------- Beta Subunit-------------------Alpha Subunit

Ligand

Assocation ( x 10^-6 M-1 s-1)

60

How does changing the alkyl group on the isocyanide change the association and dissociation rates of these molecules? Can you give a reason for this change?

Ligand

Assocation ( x 10^-6 M-1 s-1)

Dissociation ( s-1) Assocation ( x 10^-6 M-1 s-1) Dissociation ( s-1) O2

60

16 50 28 CO 4.5 0.008 4.5 0.013 NO 30 2.2 x 10^-5 30 4.6 x 10^-5 Me-CH2NC 0.79 7.0 0.39 3.9 n-Bu-CH2NC 0.34 1.2 0.059 0.31 t-Bu-CH2NC 0.0088 0.23 0.0012 0.06

Explanation / Answer

As number of alkyl groups and branching on isocyanide increases the association as well as dissociation rate constant decreases because the addition of alkyl substituents to the CNR C1 atom decreases the bond strength of the zwitterionic isocyanide group through internal steric and inductive effects.

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