Fibrils or Globules? Tuning the Morphology of Peptide Aggregates from Helical Building Blocks

The aggregation propensity of helical oligopeptides formed exclusively by the conformationally constrained alpha-aminoisobutyric acid (Aib or U in a three- or single-letter code, respectively) was studied in methanol and methanol/water solutions by spectroscopic methods (UV-vis absorption, steady-state and time-resolved fluorescence, and FT-IR absorption) and atomic force microscopy (AFM) imaging. The peptides investigated have the general formula UnN, where n = 6, 12, and 15 and N stands for a naphthyl chromophore introduced with the dual aim to serve as a spectroscopic probe and to analyze the effect of an extended aromatic group on the aggregation process. Experiments showed that the aggregation propensity in (70/30)(v/v) and (50/50)(v/v) methanol/water solutions increases with increasing the length of the peptide chain, i.e., U6N < U12N < U15N. When the peptides are immobilized on mica as a dried film, the interplay of aromatic aromatic and interhelix interactions, the latter becoming more and more important with the elongation of the peptide chain, governs the morphology of the resulting mesoscopic aggregates. AFM imaging revealed the formation of globular or fibrillar structures, the predominance of which is controlled by the helix length of the peptide building block.