Folding versatility of the C-terminal tetrapeptide amide sequence of the lipopeptaibol antibiotics trichodecenin and trichogin

An X-ray diffraction analysis of Z-L-Leu-Aib-Gly-L-Ile-L-Leu-OMe, containing the Na-acylated tetrapeptide amide sequence -L-Leu-Aib-Gly-L-Ile-, showed that in the crystal state the carbonyl group preceding the L-Leu(1) residue acts as the acceptor of two C=O...H-N intramolecular II-bonds, which give rise to an -L-Leu(1)-Aib(2)- type-III' beta-turn and an -L-Leu(1)-Aib(2)-Gly(3)-L-Ile(4)- pi-turn, respectively A second (type-I') beta-turn encompasses the -Aib(2)-Gly(3)-sequence. This is the third type of folding motif known for that tetrapeptide sequence, considering also those already published for the C-terminal segment of the lipopeptaibol antibiotics trichodecenin I and trichogin A IV.