Phenylethanethiolate monolayer-protected Au-38 and Au-140 nanoclusters were modified by ligand place exchange with a series of thiolated peptides. The peptides were homooligomers based on the alpha-aminoisobutyiric acid unit. The effects of changing the peptide concentration and the peptide length in the capping monolayer were studied by differential pulse voltammetry. The results showed that the redox behavior of the nanoparticles can be affected very significantly by such modifications. For example, the first oxidation peak of Au-38, a cluster displaying molecule-like behavior, could be shifted positively by as much as 0.7-0.8 V. Detectable redox shifts were noted even when one single oriented peptide was in the Au-140 monolayer. These effects were attributed to the molecular dipole moments of the peptide ligands.