DNA photocleavage by novel intercalating 6‐(2‐pyridinium)phenanthridinium viologens

Abstract

A new type of DNA-intercalating viologen dications, derived from the N,N'-dialkyl-6-(2-pyridyl)phenanthridine structure (in which dialkyl is -CH2CH2-, -CH2CH2CH2-, or (-CH3)2, abbreviated dq2pyp, dq3pyp, and Me2pyp, respectively), are able to produce frank strand breaks in supercoiled plasmid DNA upon irradiation with visible light. The amount of photocleavage is similar for the three drugs. The observed DNA photosensitization appears to follow a single-strand cleavage model, as shown by a kinetic analysis of the reaction with dq2pyp. The photodynamic action of the drugs seems to be initiated by a light-induced electron transfer reaction from the nucleobases, given the singlet excited-state redox potentials (ca. + 2.1 V vs. SHE) and the low quantum yields of singlet molecular oxygen production of the drugs (0.1-0.2 in aerated D20). Key words. Intercalating viologen; DNA photocleavage;Photoinduced electron transfer reaction of Io 2 with guanine; (b) direct or OH'-mediated hydrogen abstraction from the deoxyribose moiety; (c) oxidation of guanine by the photoexcited drug. It is not uncommon to find the simultaneous participation of two of such processes in the sensitized photooxidation of nucleosides [4].In a recent paper [5], we have reported the intercalative interaction with DNA of a novel family of photoactive organic compounds, namely dialkylated 6-(2-pyridyl)phenanthridine (pyp) viologens (Scheme 1). The possibility of photochemistry with DNA was then suggested, based on their singlet excited state redox potentials and the efficient fluorescence quenching observed with GMP and AMP (kq = 1.2 x 10 l° and 6.8 x l09 M ~. s ], respectively). The goal of the present paper is to report the capability of dq2pyp, dq3pyp, and Me2pyp, to photosensitize DNA cleavage, as well as to obtain evidences about the possible mechanism responsible for their photodynamic activity.

Photocleavage of Nucleic Acids

“…The pentacycle 63 is an intriguing structure, combining a viologen-like moiety (rotationally locked by the dimethylene bridge) and an intercalating phenanthridinium group . The fluorescence of 63 is quenched in the presence of DNA, and irradiation leads to photonicking of supercoiled DNA.…”

DNA Photocleavage by a Supramolecular Ru(II)−Viologen Complex

“…Following electron transfer from photoproduced *Ru(bpy) 3 2+ , *Ru(phen) 3 2+ , and *Ru(phen) 2 (ddpz) 2+ (bpy = 2,2‘-bipyridine, phen = 1,10-phenanthroline, dppz = dipyrido[3,2-a:2‘,3‘-c]phenazine) to various electron acceptors, the resulting Ru(III) species ( E 1/2 ∼ 1.4 V vs NHE) is able to oxidize guanine bases in solution and in duplex DNA ( E 1/2 = 1.29 V vs NHE). , DNA cleavage was also observed for intercalated ethidium bromide with 4,4‘-dimethylviologen (V 2+ ) electron acceptor in solution, where the reduced acceptor was shown to participate in oxygen-mediated single-strand breaks of the duplex . In addition, molecules whose excited states are able to oxidize guanines have been reported to result in DNA cleavage and to covalently modify guanine nucleotides in duplex DNA. , …”

Electronic Spectra and Transition Moments of 6-(2‘-Pyridiniumyl)phenanthridinium Photoactive DNA Intercalators

“…As has been demonstrated, the quenching of the drugs fluorescence by DNA is due to an electron transfer from the nucleobases to the singlet excited state of the pyp viologens; this photooxidation has been proposed as the first step in the photosensitized DNA cleavage produced by these molecules and those of the singlet excited states of the drugs 1 (see above), these calculations explain the observed high efficiency of the photooxidation, since the singly occupied HOMO orbital is centered in the phenanthridinium moiety, and this ring becomes intercalated between two base pairs upon binding to DNA. , Therefore, a large overlap between the HOMO and the molecular orbitals of the DNA bases occurs in the complex, allowing a very effective electron transfer.…”

“…N , N‘ -Dialkyl-6-(2‘-pyridyl)phenanthridine compounds comprise an interesting novel group of chiral photoactive drugs targeted to nucleic acids (Figure ). − ,, As derivatives of N , N‘ -dialkyl-2,2‘-bipyridine, they belong to the family of organic dications called viologens , widely used as oxidants in chemistry and biology due to their ability to participate in reversible redox reactions . Their general structure (Figure ) also resembles that of the well-known DNA- and RNA-intercalator ethidium.…”

“…From laser flash photolysis experiments, in combination with spectroelectrochemical measurements, an electron transfer from the bases to the drug singlet excited state has been demonstrated to be responsible for the quenching mechanism . This photooxidation has been proposed as the initial step in the mechanism of the photosensitized DNA cleavage that is demonstrated by these drugs, a conclusion based on the negligible effects that sodium azide, superoxide dismutase, and catalase (all quenchers of singlet oxygen, superoxide anion, and oxygen peroxide, respectively) have in the cleavage reaction 1 General structure of the pyp viologens studied showing the two enantiomeric conformations ( S and R ) observed for each member.…”