Self-assembly of a peptide Boc-(IIe)5-OMe in chloroform and N,N-dimethylformamide

Ordered aggregation of pentapeptide Boc-(Ile)(5)-OMe (1) was observed in chloroform and N,N-dimethylformamide (DMF). The peptide formed a gel in both chloroform and DMF at high concentrations. Scanning electron microscopic (SEM) studies of the peptide gel in CHCl3 and DMF revealed the presence of fibrils of 100 nm in diameter. In the gel formed in DMF, this fundamental unit was found to undergo assembly into loosely placed fibrils. In chloroform, periodic sieves of varied diameter (between 600 and 1800 nm) are found between the fibers. Peptide 1 also forms micellar aggregates in chloroform. The critical micellar concentration (cmc) of the peptide in chloroform was determined by the 8-anilino-1- naphthalenesulfonic acid (ANS) fluorescence technique and induced circular dichroic (ICD) spectral technique using ANS. The aggregation number of the peptide in chloroform was found to be 6 +/- 1, as determined by a static fluorescence quenching method using ANS and N-cetylpyridinium chloride (CPC) as external fluorescent probe and quencher, respectively. The aggregation number of the peptide was found to be 13 +/- 1 in N,N-dimethylformamide as obtained by a gel permeation chromatographic (GPC) technique. The Fourier transform infrared (FTIR) spectrum of the peptide shows no free NH in the aggregate, implying that intermolecular hydrogen bond formation is the main driving force for aggregate formation. The circular dichroism (CD) spectra of the peptide gels formed in CHCl3 and DMF show the presence of a parallel beta-sheet conformation. Thermodynamic parameters, such as Delta G(m)degrees, Delta H(m)degrees, and Delta S(m)degrees appear to substantiate the hydrogen bonded beta-sheet-like aggregate formation in the peptide.