24
in free energy for the reaction of the protein with
small molecules is made up of the statistical entropy
terms for v plus a term proportional to v. when the
total number of sites (or groups of sites) is large, the
calculations become tedious if the total number is
not known the calculations cannot be performed. Such
a situation does arise if one attempts to apply the
equations without modification to nucleic acid binding.
Since DNA polyanions commonly have molecular weights
/T
in excess of one million and may contain as many as 10
phosphates (67) Peacocke and Skerrett (20) redefined the
parameters of the Scatchard equations to allow their
application to nucleic acids. Their assignments and an
abbreviated derivation of the Scatchard equations are as
follows.
Let R represent the amount of small molecules, D,
bound per mole of nucleic acid phosphorus (generally a
fraction), and C represent the molar concentration of
free D. Consider a number, n^, of P classes of binding
sites for D on the DNA polyanion. If each class has
associated with it an intrinsic binding constant then
we may write for any general case
J=P
R = I
J=1
n .K .C
1 + K.C
1
(1-8)
This may be simplified when all sites are identical
R/C = Kn Kr
(1-9)