5068 Journal of the American Chemical Society zyxw / 100.16 / August zy 2, 1978 zyxwvutsrqpo Electrochemistry, Synthesis, and Spectra of Pentaammineruthenium( 111) Complexes of Cytidine, Adenosine, and Related Ligands zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONM Michael J. Clarke Contribution zyxwvut from the Department of Chemistry, Boston College, Chestnut Hill, Massachusetts 02167. Received August 16, 1977 Abstract: The synthesis of pentaammineruthenium(II1) complexes of cytidine, adenosine, and tubercidin by redox catalysis is reported. The spectroscopic and chemical properties of these complexes favor coordination site assignments as the exocyclic nitrogens of cytidine, adenosine, and tubercidin, but do not preclude binding to the adjacent pyrimidine ring nitrogens for the corresponding Ru(I1) complexes. An N-7 coordinated complex of guanine is also reported. These complexes exhibit strong li- gand to metal charge transfer bands in the visible and near-ultraviolet. The cytidine and adenosine complexes are remarkably acidic (having pK, values of 3.15 and 3.64, respectively) with displacement of a proton from the exocyclic amine. The acidity of these complexes is attributed to the close juxtaposition of the Ru(II1) to the ionizable proton and to intramolecular hydrogen bonding between a coordinated ammine and an adjacent nitrogen following proton loss. Values of pK, for the Ru(I1) com- plexes have also been measured. Cyclic voltammetry studies indicate that the cytidine and adenosine complexes dissociate or isomerize following reduction to the Ru(I1) state. Implications of the chemistry of these complexes regarding their formation and behavior in biochemical systems are discussed. Introduction The effect of transition metals on nucleotides, nucleic acids, and their constituent bases has implications relating to (1) heavy metal toxicity,1,2 (2) the efficacy of transition metal antit~mor~-~ agents, and (3) the use of metals as labels in structure studies by x-ray or electron microscopy technique^.^-^ Chemical mechanisms bearing on the toxicity of heavy metals have been the focus of considerable investigation in recent years. In particular, the discovery by Rosenberg* that platinum ammines are effective as anticancer drugs has provoked the search for similar antitumor agents based on platinum and other transition metals. Many of these drugs appear to interfere with the replication of DNA by binding directly to the cellular DNA.9 Since they are not selective for tumor cells, the plati- num drugs in use at the present time show a number of unde- sirable side effects. It is hoped that by using other metal ions and by paying careful attention to their chemical properties, more selective and less toxic agents can be developed. Ru- thenium ammine complexes are particularly attractive in this regard since (1) a reasonable amount of chemical information concerning them already exists,l0I1 (2) their redox properties and reaction rates can be varied by chemical modification of the ligands,12 and (3) several ruthenium complexes have been shown to inhibit cellular DNA synthesis in vitro at a level similar to that of cis-Cl2(NH3)2Pt1I l 3 9 l 4 and have also shown antitumor activity in animal studies.I3 Ruthenium(I1) ammine complexes generally undergo substitution reactions more rapidly than the corresponding Ru(II1) ions10s11$15 and the reduction potentials of many Ru(II1) ammine complexes are such that they may be reduced by a variety of reductants found in biological system^.'^^^* On the other hand, many Ru(I1) ammine complexes can be oxi- dized fairly rapidly by oxygen.16 It should be possible to take advantage of these properties to design Ru(II1) ammine or amine complexes which would be activated by cellular re- duction. Owing to the differences in oxygen content often present between normal and solid tumor cells,17 the Ru(II)/ Ru(II1) ratio should be higher in solid tumors than in the surrounding tissue. A higher rate of metal binding to DNA is therefore expected in solid tumor cells, thereby providing a possible means for selective tumor toxicity. Compounds containing the pentaammineruthenium group are presently under investigation and have been shown to in- hibit cellular DNA synthesis.13aSince these complexes do not 0002-7863/78/1500-5068$01 .OO/O have cis leaving groups, it is unlikely that they act entirely analogously to the platinum agents. Nevertheless, it is probable that they function by binding to cellular DNA and provide a convenient system for study. In order to test the approach of cellular activation of the metal complex by reduction and to understand the binding of this class of compounds to nucleic acids we have been studying the interactions of (NH3)5RuI1 and with DNA. The (NH~)~Ru(OH~)~+ ion appears to bind rapidly to DNA and subsequent aerial oxidation of these solutions yields colored prod~cts.~~?~~ The present investigation was undertaken in an effort to understand (1) the interactions of the pentaam- mineruthenium group with adenine and cytosine ligands and (2) the origin of the bands in the visible spectra of [(NH3)5 Ru~~'],-DNA in order to determine if they could be used to assess the type and degree of metal binding to DNA. Moreover, the systematic study of a firmly bound metal ion on the phys- ical and chemical properties of the constituent bases of nucleic acids will provide basic information necessary not only to un- derstand the mechanism of metal-containing drugs, but also in designing base-specific labels to aid in the sequencing of nucleic acids by electron microscopy. Experimental Section Chemicals and Reagents. Chloropentaammineruthenium(lI1) chloride was prepared by the method of Vogt, Katz, and Wiberly20 and recrystallized from 0.1 m hcl. Cytidine, adenine, and adenosine (Aldrich), deoxyguanosine (ICN), 1-methylcytidine (Sigma), and tubercidin (Calbiochem) were used without further purification. Standard acid, base, electrolyte solutions, and ion exchange resins were prepared according to previously reported methods.21 Equipment. Spectra were recorded on a Cary Model 14 spectro- photometer. Electrochemical measurements were made on a cyclic voltammetry apparatus constructed in this laboratory based on Analog Devices 514K operational amplifiers for the potentiostat, current, and voltage follower circuits. Platinum button or disk electrodes (Beck- man) or a hanging drop mercury electrode (Brinkmann) were used as the indicator electrode against a standard calomel reference elec- trode. The (NH~),RU"~-~~ couple gave an Ef of 49 mV (corrected vs. SHE) on this system. Measurements of pH were made with a Me- trohm combination glass electrode. Atomic absorption measurements were made on a Perkin-Elmer Model 300 atomic absorption spec- trophotometer, using a Perkin-Elmer Ru-Pt combination hollow cathode lamp. Synthesis of Compounds. The cytidine complex was prepared by (method I) allowing an argon-purged solution of chloropentaammi- 0 1978 American Chemical Society