75
which both contain the same amount of DNA and have, there
fore, the same total ionic strengths. The extent of
binding in the former is 77% while in the latter it is
only 70%.
Rapid estimations of the values of m and q of equation
(1-21) may be obtained by selecting reasonable terms such
that mq = n (n is total phosphate per drug). It has been
shown that (85) in a medium containing 0.002M to 0.005M
Ki^PO^ (pH 5.9) the binding of 3-aminoacridinium cation to
DNA is best expressed by
, ->
BH + (P04)3
bh(po4) 3
(3-1)
for
the surface mode, and
BH+ + 2(P04)3
BH((P04)2 > 2~
(3-2)
for
the intercalative mode.
In equation (3-1),
three DNA
phosphates act as a single moiety, reacting with the small
molecular cation to form the surface-bound species. In
equation (3-2) 2 phosphates act as a single reacting
entity with two of these, in turn, interacting with one
cation to form the intercalated complex. The values of m
and q are 3 and 1, respectively, for the first case and 2
and 2 for the second. Substituting these into equation (1-21)
yields
Ks = [BHP]/[BH] ( [Pt/3]-[BHP])) (3-3)
and