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Proceedings of the 31 st European Peptide SymposiumMichal Lebl, Morten Meldal, Knud J. Jensen, Thomas Hoeg-Jensen (Editors)European Peptide Society, 2010Optical Spectroscopy and Conformational Analysis of PeptideAggregates. The Role of Aromatic Interactions andConformational FlexibilityMario Caruso 1 , Ernesto Placidi 2 , Emanuela Gatto 1 , Lorenzo Stella 1 ,Antonio Palleschi 1 , Gianfranco Bocchinfuso 1 , Fernando Formaggio 3 ,Claudio Toniolo 3 , and Mariano Venanzi 11 Department of Chemical Sciences and Technologies, University of Rome "Tor Vergata",Rome, 00133, Italy; 2 CNR, Department of Physics, University of Rome "Tor Vergata",Rome, 00133, Italy; 3 ICB, Padova Unit, CNR, Department of Chemistry,University of Padova, Padova, 35131, ItalyIntroductionPeptide aggregation is the basic process underlying a number of phenomena like sparklingof neuropathologies, formation of peptide films, and structuration of mesoscopic structureslike fibrils or dendrimers. Experimental evidence indicates that aggregation is determinedby a complex interplay of intermolecular interactions and structural effects, with a specificrole played by aromatic residues.Results and DiscussionIn this study, the aggregation propensities of Aib ( -aminoisobutyric acid) homooligopeptides,of general formula Z-Aib n -N (Z, benzyloxycarbonyl, N, naphthyl), with n=6,12 and 15, were investigated by optical spectroscopy and atomic force microscopytechniques. From structural studies in solution and in the crystal state, Aib homo-peptidesare well-known to populate 3 10 - or -helical structures depending, among other factors, onthe length of the peptide chain [1,2].The functionalization of the peptide scaffold with an N group allowed for opticalspectroscopy techniques to be applied, particularly steady-state and time-resolvedfluorescence. In methanol solution, all peptides investigated were found to exhibit emissionspectra typical of the isolated N chromophores (Figure 1a). On the other hand, in the 30:70(v/v) water/methanol mixture, while the shortest peptide shows an emission spectrumindicative of a monomeric species, the Z-Aib 15 -N spectrum is characterized by a broademission (Figure 1b), typical of excimer-like species, i.e. excited-state homo-dimericcomplexes requiring the two interacting groups be located at short distances. This findingstrongly suggests that hydrophobic effects are mainly responsible for the formation of thefluorescent aggregated species. A high fluorescence quantum yield is characteristic ofJ-type aggregates, i.e. stacked arrays of aromatic groups characterized by a slip angle0 < /2.Fluorescence (a.u.)126Z-Aib6N 100% metOHZ-Aib12N 100% metOHZ-Aib15N 100% metOHaFluorescence (a.u.)612Z-Aib6N 30% H2OZ-Aib12N 30% H2OZ-Aib15N 30% H2O15b15300 350 400 450 500(nm)300 350 400 450 500Fig. 1. Emission spectra of Z-Aib n -N (n=6,12,15) in methanol (a) and in the 30:70 (v/v)water/methanol solvent mixture (b).(nm)582