Four broad-spectrum, 11 and 12 residue, novel antimicrobial peptides have been isolated from the adrenalinestimulated skin secretions of the Indian frog Rana tigerina. Sequences of these peptides have been determined by automated Edman degradation, by mass spectral analysis and confirmed by chemical synthesis. These peptides, which we have named as tigerinins, are characterized by an intramolecular disulfide bridge between two cysteine residues forming a nonapeptide ring. This feature is not found in other amphibian peptides. Conformational analysis indicate that the peptides tend to form ␤-turn structures. The peptides are cationic and exert their activity by permeabilizing bacterial membranes. Tigerinins represent the smallest, nonhelical, cationic antimicrobial peptides from amphibians.Antimicrobial peptides constitute a very important component of the innate immune system in organisms across the evolutionary scale (1-8). Amphibians being the first group of organisms forming a connecting link between land and water are forced to adopt and survive in a variety of conditions laden with pathogenic microbes. Thereby, they are endowed with an excellent chemical defense system composed of pharmacological and antimicrobial peptides (9). Bombinins were the first antimicrobial peptides characterized from the skin of Bombina variegata in 1969 (10). The discovery of magainins from the skin secretions of Xenopus laevis in 1987 (11) triggered extensive search and characterization of antimicrobial peptides from amphibians (12)(13)(14). Antimicrobial peptides from genus Rana share an interesting structural motif composed of a disulfidebridged cationic heptapeptide segment at the COOH-terminal end. Peptides with this motif include brevinins and esculentins which are composed of 24 and 46 amino acids, respectively (14). The primary structures of large number of peptides belonging to this family have been determined. Another group of short peptides composed of 13 residues called temporins, which do not contain this COOH-terminal ring, have also been characterized from frogs of genus Rana (15). However, considering the large variety of amphibian species in nature, antimicrobial peptides from only a small number of them have been characterized, that too only with respect to primary structure. Also, studies directed toward determining structure-function relationships have been confined to magainins (16) and dermaseptins (17, 18). Hence, characterizing host-defense peptides from other species would be of interest and could conceivably result in the identification of new structural motifs which would be useful in designing peptides for therapeutic applications. Rana tigerina is the predominant species of frogs found in India (19). The skin of these frogs have been used traditionally by some tribal communities to heal both open and burn wounds and the antimicrobial components could possibly contribute to the wound healing process (20). In this study, we have described the isolation and characterization of antimicrobial peptides from...

Secondary structural conformation of hybrid oligo-peptides comprised of 1 : 1 alternating Nucleoside Derived beta-Amino acid (NDA) and l-amino acid residues has been reported. The studies reveal that the NDA residues organize the heterogeneous backbone featuring the surface properties of both nucleic acids and peptides, to adopt a novel 11/8-helical fold.

Hybrid peptidic oligomers comprising natural and unnatural amino acid residues that can exhibit biomolecular folding and hydrogen-bonding mimicry have attracted considerable interest in recent years. While a variety of hybrid peptidic helices have been reported in the literature, other secondary structural patterns such as γ-turns and ribbons have not been well explored so far. The present work reports the design of novel periodic γ-turns in the oligomers of 1:1 natural-α/unnatural trans-β-norborenene (TNAA) amino acid residues. Through DFT, NMR, and MD studies, it is convincingly shown that, in the mixed conformational pool, the heterogeneous backbone of the hybrid peptides preferentially adopt periodic 8-membered (pseudo γ-turn)/7-membered (inverse γ-turn) hydrogen bonds in both polar and non-polar solvent media. It is observed that the stereochemistry and local conformational preference of the β-amino acid building blocks have a profound influence on accessing the specific secondary fold. These findings may be of significant relevance for the development of molecular scaffolds that facilitate desired positioning of functional side-chains.

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