Binding of Stimuli-Responsive Ruthenium Aqua Complexes with 9-Ethylguanine.

Stimuli-responsive ruthenium complexes proximal - and distal -[Ru(C 10 tpy)(C 10 pyqu) OH 2 ] 2+ ( proximal - 1 and distal - 1 ; C 10 tpy = 4'-decyloxy-2,2':6',2″-terpyridine and C 10 pyqu = 2-[2'-(6'-decyloxy)-pyridyl]quinoline) were experimentally studied for adduct formation with a model DNA base. At 303 K, proximal - 1 exhibited 1:1 adduct formation with 9-ethylguanine (9-EtG) to yield proximal -[Ru(C 10 tpy)(C 10 pyqu)(9-EtG)] 2+ ( proximal - RuEtG ). Rotation of the guanine ligand on the ruthenium center was sterically hindered by the presence of an adjacent quinoline moiety at 303 K. Results from 1 H NMR measurements indicated that photoirradiation of a proximal - RuEtG solution caused photoisomerization to distal - RuEtG , whereas heating of proximal - RuEtG caused ligand substitution to proximal - 1 . The distal isomer of the aqua complex, distal - 1 , was observed to slowly revert to proximal - 1 at 303 K. In the presence of 9-EtG, distal - 1 underwent thermal back-isomerization to proximal - 1 and adduct formation to distal - RuEtG . Kinetic analysis of 1 H NMR measurements showed that adduct formation between proximal - 1 and 9-EtG was 8-fold faster than that between distal - 1 and 9-EtG. This difference may be attributed to intramolecular hydrogen bonding and steric repulsion between the aqua ligand and the pendant moiety of the bidentate ligand..