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Fig. 2. CD spectra in water of (⋅⋅⋅⋅⋅⋅) TPP-apidaecin (10 -4 M) and (⎯⎯) TPP-[Leu 21 ]magainin (10 -5 M) in the absorption regions of the amide bond (left) and the Soret band(right).peptides: magainin presents a disordered conformation in aqueous environment butassumes a prevalently amphipathic helical structure in organic solvents; apidaecin, becauseof the abundance of Pro residues, adopts a prevalently extend conformation in differentenvironments. In the far-UV region (Figure 2, left), the CD spectrum of TPP-apidaecin inwater is similar to that of the parent peptide and it is characterized by a broad negativeminimum around 200 nm, indicative of extended and prevalently disordered structures. Onthe contrary, in the same solvent, the spectrum of the TPP-[Leu 21 ]magainin differssubstantially from that of the peptide and exhibits a positive maximum around 190 nm andtwo negative minima near 208 and 222 nm. This pattern, typical of peptides which adopt anα-helical structure, suggests that the porphyrin system induces the peptide backbone tofold, at least partially, into an amphiphatic α-helix. Moreover, split Cotton effects appear inthe porphyrin Soret band region (Figure 2, right) and they could indicate the appearance ofaggregation phenomena, in particular in the case of TPP-[Leu 21 ]magainin.The bactericidal activity under light activation of the porphyrin-peptide conjugateswas preliminarily tested for TPP-apidaecin [4]. At low concentration (1.5 – 15 μM) theconjugate was able to reduce the survival of E. coli cells by 3-4 log 10 and it remainedphotoactive also against hard-to-treat P. aeruginosa bacteria, although at higherconcentration (60 μM). Under similar condition, the photosensitizer alone was onlyphotoactive against S. aureus, but at higher concentration than the conjugate. In conclusion,the antibacterial activity of the conjugate was higher than that of the individual componentsand TPP-apidaecin exhibited a broader spectrum activity.AcknowledgmentsThis work was supported by the University of Padova (PRAT 2009).References1. Hamblin, M.R., Hasan, T. Photochem. Photobiol. Sci. 3, 436-450 (2004).2. Hancock, R.W., Sahl, H.-G. Nature Biotechnology 24, 1551-1557 (2006).3. Lindsey, J.S., Schreiman, I.C., Hsu, H.C., Kearney, P.C., Marguerettaz, A.M. J. Org. Chem. 52,827-836 (1987).4. Dosselli, R., Gobbo, M., Bolognini, E., Campestrini, S., Reddi, E. ACS Med. Chem. Lett. 1, 35-38(2010).385