Synthesis and protonation behavior of a water-soluble N-fused porphyrin: Conjugation with an oligoarginine by click chemistry Yoshiya Ikawa a,b , Hiroyuki Harada a , Motoki Toganoh a , Hiroyuki Furuta a, * a Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University, 744 Moto-oka, Nishi-Ku Fukuoka 819-0395, Japan b PRESTO, Precursory Research for Embryonic Science and Technology, Japan Science and Technology Agency, Tokyo 102-0075, Japan article info Article history: Received 31 December 2008 Revised 7 March 2009 Accepted 13 March 2009 Available online 21 March 2009 Keywords: N-Fused porphyrin Water-soluble porphyrin Oligoarginine Click chemistry Protonation abstract A water-soluble derivative of N-fused porphyrin (NFP) possessing a nona-arginine (R9) peptide tail was synthesized for the first time by a Cu(I)-catalyzed azide-alkyne ‘click’ reaction. In aqueous solution, at pH 8, the conjugated molecule (NFP-R9) exists as free base and protonated below pH < 6.5 to form mono- protonated species dominantly, and diprotonated one below pH < 2.3, while such clear two-step proton- ation behavior was not observed in the DMF solution. Ó 2009 Elsevier Ltd. All rights reserved. In recent years, application of near-infrared (NIR) light has been expanding in various fields involving material science, 1 informa- tion technology, 2 and also biomedical science/technology. 3 In the biomedical fields, NIR has been recognized as wavelength most suitable for diagnostic and therapeutic applications because inter- ference by water and bio(macro)molecules can be minimized at NIR region. 4 However, only limited numbers of organic compounds applicable for NIR science/technology have been reported to date, 5 presumably because of the difficulty in satisfying stability and syn- thetic accessibility required for practical use. N-Fused porphyrin (NFP) is a porphyrin analogue with an 18p aromaticity derived from a tetrapyrrolic macrocycle (Chart 1). 6a A notable feature of NFP is its optical property that absorbs NIR light over 1000 nm. 6b Such unusual long wavelength absorption of NFP is supposed to originate from its characteristic tri-pentacylic unit, by which a HOMO–LUMO gap of the macrocycle would become smaller than ordinary porphyrins. 6b NFP can be readily prepared in two steps from a porphyrin isomer, N-confused porphyrin (NCP, Chart 1), 6b which is synthesized by a one-pot condensation of pyrrole and aromatic aldehyde. 7 Owing to these features, NFP is expected as a promising candidate applicable to NIR biosci- ence/technology. However, due to the lack of water-soluble deriv- ative, the property of NFP in aqueous media has been totally veiled. Herein, we report a first synthesis of a water-soluble derivative of NFP, which possesses a hydrophilic nona-arginine peptide tail (R9), and its protonation behavior in aqueous solution. Synthesis of the title conjugated molecule NFP-R9 consists of three parts. The first part covers a selective introduction of an ethy- nylaryl group to the C(21) position of NFP (Scheme 1). 6c At first, N- confused tetraphenylporphyrin (1) was converted to the C(21)- brominated N-fused tetraphenylporphyrin (2) in two steps. 6b Suc- cessively, 2 was treated with 4-(triisopropylsilylethynyl)phenylbo- ronic acid in the presence of Pd(PPh 3 ) 4 and Cs 2 CO 3 , affording 3 in 68% yield. 6c Triisopropylsilyl group of 3 was then removed by treating with tetrabutylammonium fluoride (TBAF) to afford 4 in 99% yield (see Supplementary data online). In the second part, a nona-arginine peptide (R9) possessing an azidoglycine moiety at the N-terminus was synthesized by Fmoc- solid phase synthesis (Scheme 2). 8 Starting from fluoren-9-ylmeth- 0960-894X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.bmcl.2009.03.066 * Corresponding author. Tel./fax: +81 92 802 2865. E-mail address: hfuruta@cstf.kyushu-u.ac.jp (H. Furuta). N NH N N HN N NH N Porphyrin N-Fused Porphyrin (NFP) HN N NH N N-Confused Porphyrin (NCP) 21 21 19 20 23 22 7 8 10 12 13 14 15 16 17 18 24 20 2 3 5 18 17 16 15 14 13 12 10 8 7 6 5 4 9 22 3 2 1 4 6 9 11 19 24 23 11 1 Chart 1. Basic structures of porphyrin (left), N-confused porphyrin (middle), and N- Fused porphyrin (right). Bioorganic & Medicinal Chemistry Letters 19 (2009) 2448–2452 Contents lists available at ScienceDirect Bioorganic & Medicinal Chemistry Letters journal homepage: www.elsevier.com/locate/bmcl