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Table 1. Gold nanoparticles obtained by ligand-exchange reactionLigandLigand/Ph(CH 2 ) 2 SH Ratio Usedfor the Exchange ReactionExtent of Exchange(“death reaction”)Percentage ofExchange1 1.72 16-17 ~90%1 0.7 5-6 ~30%1 0.2 3-4 ~20%2 0.7 7-8 ~40%observed that irradiation at 366 nm leads to formation of a mixture of the cis and transisomers with, under our experimental conditions, a cis to trans ratio of about 70% (Trtprotected1, Figure 2B) and 40% (2). Upon trans→cis transition, the peptide 3 10 -helicalstructure remained intact, as indicated by FT-IR, but we observed noticeable changes in the1 H NMR spectrum, including peaks of the peptide moiety (CH 3 groups of Aib). Thermaldecay experiments at r.t. showed that the spontaneous cis to trans conversion (in the dark)has a t 1/2 of 29.6 hours for Trt-protected 1.Different amounts of 1 or 2 were used to carry out the ligand-exchange reaction. Thepercentage of substitution was determined by destruction of the so-obtained MPCs withiodine, and calculation of the ratio between the resulting oxidized phenylethanethiol andexogenous ligand by 1 H NMR (“death reaction”) (Table 1).We also carried an analytical HPLC separation of the MPCs containing 90% of 1,whereas the MPCs with 20 and 30% of 1 showed a single broad peak. Irradiation of thesubstituted MPCs also led to spectral changes related to trans→cis isomerization. Underotherwise identical conditions, the decrease of the intensity of the main band was much lesspronounced than for the free ligand (Figure 2C). We also observed that the decrease wasless marked for more substituted MPCs. The cis→trans thermal decay was about 3 timesfaster than observed for the free ligand. These observations are in agreement with stericalhindrance affecting the isomerization process when the ligands are in the MPC monolayer.FT-IR analysis of N-H stretchregion (Amide A) showed thepresence of inter- and intramolecularH-bonds as well as freeN-H groups (Figure 3). Whereasthe stretch frequency of the freeN-H groups does not changesignificantly, the band of intramolecularH-bonds red-shiftswhen the ligand is in the MPCmonolayer. This is attributed toenhancement of the stiffness ofthe 3 10 -helical structure when thepeptide ligand is in the monolayer.Further shift takes place uponirradiation causing trans→cisisomerization, but the effectFig.3. FT-IR analysis of the substituted MPCs.vanishes when the nanoparticle isheavily substituted (90%).Differential Pulse Voltammetry (DPV) showed that for the MPCs with 20% and 30%of 1 as well as 40% of 2 the two main oxidation peaks are located at more positivepotentials and are more separated with respect to the corresponding oxidation peaks of thenative nanoparticle, Au 25 (SCH 2 CH 2 Ph) 18 . The positive shift is in line with the expectedpeptide dipole-moment effect on the charging behavior. In particular, the potential shift ismore evident for the 30% substituted MPC.References1. Murray, R.W. Chem. Rev. 108, 2688-2720 (2008).2. Antonello, S., et al. J. Am. Chem. Soc. 129, 9836-9837 (2007).3. Fabris, L., et al. J. Am. Chem. Soc. 128, 326-336 (2006).4. Holms, A.H., et al. Langmuir 22, 10584-10589 (2006).617