Excited-State Proton Transfer Reactions of 10-Hydroxycamptothecin 1 Kyril M. Solntsev,* ,² Erica N. Sullivan, ²,‡ Laren M. Tolbert,* ,² Shay Ashkenazi, § Pavel Leiderman, § and Dan Huppert § Contribution from the School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia, 30332-0400, and Raymond and BeVerly Sackler Faculty of Exact Sciences, School of Chemistry, Tel AViV UniVersity, Tel AViV 69978, Israel Received April 15, 2004; E-mail: solntsev@chemistry.gatech.edu; tolbert@chemistry.gatech.edu Abstract: Time-resolved and steady-state emission characterization of 10-hydroxycamptothecin reveals a rich but less complex proton-transfer behavior than its parent hydroxyquinoline. The electronic effect of the additional electron-withdrawing ring makes the excited-state both less basic and more acidic than the parent and adds to the class of high-acidity excited-state proton donors in photochemistry and photobiology. 1. Introduction Camptothecin (CPT) is a pentacyclic alkaloid first isolated from extracts of the Chinese tree Camptotheca acuminata. This brightly fluorescent compound was found to be a potent inhibitor of the growth of leukemia cell by a unique mechanism: inhibition of DNA topoisomerase I. 2a,b A Mannich derivative, topotecan (TPT), is now used in the treatment of ovarian cancer. The natural fluorescent properties of CPT and its derivatives have been exploited to monitor its concentration in living cells. 2 Significant attention has been paid to the equilibrium between the lactone and its ring-opened carboxylate form, which influences its antitumor ability. Among the wide variety of CPT derivatives, two moderately water-soluble compounds, 10-hydroxycamptothecin (10-CPT) and 7-ethyl-10-hydroxycamptothecin (SN-38), as well as TPT, contain the 6-hydroxyquinoline (6HQ) subunit (see Figure 1). Hydroxyquinoline derivatives are known to be both strong photoacids and strong photobases, and therefore, they undergo efficient tautomerization in a wide pH range resulting in weak tautomer (zwitterion) emission. 3,4 Because of our interest in the acid-base properties of photoexcited molecules 5 and, in par- ticular, because of the relative paucity of such examples in the biological literature, we were particularly interested in reports of anomalous behavior that might involve excited-state proto- tropic processes. Proton transfer, both in ground and excited states, plays a key role in many biological processes. It is not surprising that the functioning of many biological objects depends on the pH of the media within and outside them. The problem of determination of static pH in this medium as well as proton translocation is being successfully solved using various artificial pH probes. 6 On the other hand, less is known about excited-state proton transfer (ESPT) observed in naturally occurring systems. It is interesting to note that the overall action of light on several such systems is completely different, though ESPT is an important step in all of them: Upon irradiation, several photoproteins such as Green Fluorescent Protein 7 or obelin 8 demonstrate bioluminescence; a transmembrane protein bacteriorhodopsin acts as a light-driven proton pump in convert- ² Georgia Institute of Technology. ‡ Current address: Materials and Processes Branch, NASA-Johnson Space Center, Houston, TX 77058. § Tel Aviv University. (1) This is part of our series “Photochemistry of ‘super’ photoacids”. For a previous article, see: Clower, C.; Solntsev, K. M.; Kowalik, J.; Tolbert, L. M.; Huppert, D. J. Phys. Chem. A 2002, 106, 3114-3122. (2) (a) Camptothecins: New Anticancer Agents; Potmesil, M., Pinedo, H., Eds.; CRC Press: Boca Raton, FL, 1995. (b) Garcia-Carbonero, R.; Supko, J. G. Clin. Cancer Res. 2002, 8, 641-661. (c) Croce, A. C.; Bottiroli, G.; Supino, R.; Favini, E.; Zuco, V.; Zunino, F. Biochem. Pharm. 2004, 67, 1035-1045. (3) Bardez, E.; Chaˆtelain, A.; Larrey, B.; Valeur, B. J. Phys. Chem. 1994, 98, 2357-2366. (4) (a) Kim, T. G.; Kim, Y.; Jang, D.-J. J. Phys. Chem. A 2001, 105, 4328- 4332. (b) Poizat, O.; Bardez, E.; Buntinx, G.; Alain, V. J. Phys. Chem. A 2004, 108, 1873-1880. (5) Tolbert, L. M.; Solntsev, K. M. Acc. Chem. Res. 2002, 35, 19-27. (6) Handbook of Fluorescent Probes and Research Products, 9th ed.; Haugh- land, R. P., Ed.; Molecular Probes: Eugene, OR, 2002; Ch. 21. (7) Zimmer, M. Chem. ReV. 2002, 102, 759-781. (8) Malikova, N. P.; Stepanyuk, G. A.; Frank, L. A.; Markova, S. V.; Vysotski, E. S.; Lee, J. FEBS Lett. 2003, 554, 184-188. Figure 1. CPT and its hydroxyderivatives with the conventional ring numbering system. Published on Web 09/10/2004 10.1021/ja047821e CCC: $27.50 © 2004 American Chemical Society J. AM. CHEM. SOC. 2004, 126, 12701-12708 9 12701