Sugar-controlled association-dissociation equilibria between DNA and boronic acid-appended porphyrin

5,10,15,20-Tetrakis{1-[2-(dihydroxyboryl)benzyl]pyridin-1-ium-4-yl} porphyrin tetrabromide 1 has been synthesized to control the DNA-binding properties by physiologically-non-toxic saccharides. At pH 8.0 1 is dicationic and strongly bound to DNA whereas it becomes neutral through saccharide complexation with two boronic acid pendants and the complex is dissociated. Examination with absorption and CD spectroscopy established that poly(dGdC)·poly(dGdC) intercalates 1 whereas poly(dAdT)·poly(dAdT) binds 1 to the outside of the double strand. Since 1 in the presence of calf thymus DNA gave the CD spectrum similar to that in the presence of poly(dAdT)·poly(dAdT), 1 should be bound to the AT region in an outside binding manner. When D-fructose was added, both the absorption and CD spectra changed simply from the 1·DNA complex to 1·D-fructose complex. This indicates that D-fructose can dissociate the 1·DNA complex in a one-step manner. A similar change is observed for D-glucose when the 1 concentration is low, but the dissociation occurred in a two-step manner when the concentration of 1 is high, suggesting the D-glucose-induced reorganization of 1 in DNA. This is a novel method for controlling the DNA-binding properties of porphyrins by saccharides.