Intramolecular H-bonds and thioamide rotational isomerism in thiopeptides

Mono- and dithionated N-acyl amino acid and dipeptide N′-methylamides were synthesized using Lawesson's reagent and 5-thioacetyl thioglycolic acid. The conformation of the thionated models was characterized by IR, ¹³C, and ¹H NMR spectroscopy, including NOE experiments. The formation of —C=S…H—N—C=X (X = O or S) intramolecular H-bonds of the type 2 → 2. 1 → 3 and 1 → 4 was evidenced by the characteristic shifts of the IR stretching frequencies of the NH group. Act-Pro-NHCH₃ (4) and Act-Prot-NHCH₃ (5) were found to be present as mixtures of rotational isomers about the CS—N bond. ¹³C chemical shifts of the γ- and β-carbons of the proline ring elucidated the conformation (Z or E) of the tertiary thioamide group. Our results suggest that the conformation of thiopeptides is determined by two factors: 1) the H-bond donating and accepting ability of the thioamide group and 2) the repulsion between the thiocarbonyl sulfur atom and the side chain groups of the neighbouring amino acid residues.     

Is an amphiphilic region responsible for the haemolytic activity of Bacillus thuringiensis toxin?†

The amino acid sequence of the 27 kDa protein responsible for the haemolytic activity of Bacillus thuringiensis subsp. israelensis toxin has been analysed by secondary structure prediction, helical wheel/net diagrams and molecular mechanics calculations. We found that segment 116–126 presumably forms a strongly amphiphilic α-helix. This is supported by the findings that the synthesized segment 116-126 (a) has a significant α-helical content in water, and (b) displays an in vitro haemolytic activity comparable to that of bee venom peptide melittin. As segment 116-126 is present in the haemolyzing, but not present in the non-haemolyzing proteins from B. thuringiensis toxins, we suggest that this segment is responsible for the lytic potential of the B. thuringiensis subsp. israelensis protein.