SUPPORTING INFORMATION. Solution Measurements. The CD curves shown in Figures S1 and S2 are typical of short peptides folded in a 310-helix conformation,1,2 as expected for SSA4WA and A8Pyr peptides, owing to the high content of Cα -disubstituted amino acids in their primary sequence.3
Figure S1. Electronic far-UV CD spectra of A8Pyr in acetonitrile: 1 · 10-4 M.
Figure S2. Electronic far-UV CD spectra of SSA4WA in ethanol: 1 · 10-3M (a); 5 · 10-4 M (b); 1 · 10-4 M (c); 5 · 10-5 M (d).
STM Measurements. STM imaging of the gold substrate under ultra-high vacuum conditions (Figure S3) showed wide atomically flat terraces, separated by steps about 0.2 nm high. These steps correspond to the Au(111) monoatomic step (0.24 nm). Analysis of the STM images also allowed us to estimate the
average roughness of the flat terraces on the gold surface (0.04 ± 0.01 nm). The gold surface roughness (R) was evaluated by taking the root mean square of the vertical position (z):
R=
1 N
∑
N
i =1
( z i − < z >) 2
where N is the number of data points and is the mean z-value.
Figure S3. STM images of the gold surface before peptide deposition. 1 µm · 1 µm (Vsample = 0.22 V; I tunnel = 0.8 nA).
STM images of the A8Pyr/SSA4WA mixed SAM acquired on a surface area of 1 µm x 1 µm, with a stepwise increase of bias voltage from 1.0 to 3.8 V are shown in Figure S4. Many bright spots are clearly observed at highly positive bias voltages [Figure S4 (d)].
Figure S4. STM images of the bicomponent SAM obtained with stepwise increase of bias voltage from (a) 1.0 V, (b)1.2 V, (c) 1.8 V to (d) 3.8 V. The statistical analysis performed on figure 8, has been obtained by using the flooding analysis of the WSxM 5.0 program,4 setting a minum height of 2.7 nm. The obtained histogram shows the statistical dots height and their relative area, as compared to the surrounding matrix. The most probable dots height has been obtained at 0.2-0.3 nm, which corresponds to the height experimentally obtained for the dots of figure 8, while the area occupied by them was the 33% of the total. A geometrical analysis of the dot diameters was performed on Figure 8. It shows that most of the bright spots has a diameter of 1 nm (Figure S5). The large spots are interpreted as a bundle of small dots, so as peptide domains. They are very few as compared to the 0.5-1 nm diameter dots.
Figure S5. Geometrical analysis of the dots diameter performed on Figure 8.
Cyclic Voltammetry. Cyclic voltammetry experiments have been performed in a 0.50 mM ferricyanide solution, before and after the SAM removal, and are shown in Figure S6. After applying a -1.5 V potential for three minutes, a signal comparable to the bare gold electrode was obtained, confirming the complete removal of the bicomponent SAM
Figure S6. Cyclic voltammetry experiments in a 0.50 mM K3[Fe(CN)6] aqueous solution: (a) bare gold electrode; (b) gold electrode modified by the mixed peptide SAM (c) gold electrode after the SAM removal. Sweep rate: 50 mV/s.
REFERENCES 1. Toniolo, C.; Polese, A.; Formaggio, F.; Crisma, M.; Kamphuis, J. J. Am. Chem. Soc. 1996, 118, 2744-2745. 2. Toniolo, C.; Formaggio, F.; Tognon, S.; Broxterman, Q. B.; Kaptein, B.; Huang, R.; Setnicka,V.; Keiderling, T-A.; McColl, H. I.; Hecht, L.; Barron, L.D. Biopolymers 2004, 75, 32-45. 3. Toniolo, C.; Crisma, M.; Formaggio, F.; Peggion, C. Biopolymers (Pept. Sci.) 2001, 60, 396-419. 4. Horchas, I., R. Fernández, R.; Gómez-Rodríguez, J. M.; Colchero, J.; Gómez-Herrero, J.; Baro, A. M. Rev. Sci. Instrum. 2007, 78, 013705 (8 pages).
