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 ch loroform and N,N-dimethylformamide (DMF). The peptide formed a gel in both chloroform and DMF at high concentrations. Scanning electron microscopic (S EM) studies of the peptide gel in CHCl3 and DMF revealed the presence of fi brils of 100 nm in diameter. In the gel formed in DMF, this fundamental uni t was found to undergo assembly into loosely placed fibrils. In chloroform, periodic sieves of varied diameter (between 600 and 1800 nm) are found bet ween the fibers. Peptide 1 also forms micellar aggregates in chloroform. Th e critical micellar concentration (cmc) of the peptide in chloroform was de termined by the 8-anilino-1- naphthalenesulfonic acid (ANS) fluorescence te chnique and induced circular dichroic (ICD) spectral technique using ANS. T he 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-ce tylpyridinium chloride (CPC) as external fluorescent probe and quencher, re spectively. The aggregation number of the peptide was found to be 13 +/- 1 in N,N-dimethylformamide as obtained by a gel permeation chromatographic (G PC) technique. The Fourier transform infrared (FTIR) spectrum of the peptid e shows no free NH in the aggregate, implying that intermolecular hydrogen bond formation is the main driving force for aggregate formation. The circu lar dichroism (CD) spectra of the peptide gels formed in CHCl3 and DMF show the presence of a parallel beta-sheet conformation. Thermodynamic paramete rs, such as Delta G(m)degrees", Delta H(m)degrees, and Delta S(m)degrees ap pear to substantiate the hydrogen bonded beta-sheet-like aggregate formatio n in the peptide.