RESUMO
A cyclic peptide containing thiazole and oxazole moieties was isolated from Nostoc 31 and its structure determined by chemical degradation detailed NMR and mass spectroscopic analyses. The compound is stereochemically pure and closely related to nostocyclamide in which D-valine is replaced by D-methionine. Therefore, it differs from tenuecyclamide C reported to contain L-methionine. It shows allelopathic activity against Anabaena 7120, but is inactive against grazers.
Assuntos
Cianobactérias/química , Peptídeos Cíclicos/química , Peptídeos Cíclicos/isolamento & purificação , Tiazóis/química , Anabaena/efeitos dos fármacos , Espectroscopia de Ressonância Magnética , Conformação Molecular , Peptídeos Cíclicos/farmacologiaRESUMO
The ability of (S)-alpha-methylproline (alpha-MePro) to stabilise reverse-turn conformations in the peptide hormone bradykinin (BK = Arg1-Pro2-Pro3-Gly4-Phe5-Ser6-Pro7-Phe8-Arg9) has been investigated. Two BK analogues containing alpha-MePro at position 3 or position 7 were synthesised and their conformations in aqueous solution investigated by NMR spectroscopy. Whereas BK is largely disordered on the NMR time scale both analogues showed ROE connectivities in 2D-ROESY spectra indicative of reverse-turn conformations at both Pro2-Phe5 and Ser6-Arg9, whose formation appears to be cooperative. Some potential applications of alpha-MePro as a reverse-turn mimetic in the construction of synthetic peptide libraries is discussed.
Assuntos
Bradicinina/química , Prolina/análogos & derivados , Sequência de Aminoácidos , Bradicinina/metabolismo , Espectroscopia de Ressonância Magnética , Conformação Molecular , Dados de Sequência Molecular , Prolina/química , TemperaturaRESUMO
The 15N chemical shifts and 15N, 1H spin coupling constants were determined in the title compounds using the INEPT pulse sequence and assigned with the aid of selective proton decoupling. The delta/15N/ and J/N, H/ values are discussed in terms of involvement of the imidazole ring created by ethenobridging in the electronic structure of the whole molecule. Both spectral parameters indicate that the diligant nitrogen in this ring is the primary site of protonation in these modified nucleosides. It is concluded that 15N NMR of nucleoside bases can be largely a complementary method to 1H and 13C NMR studies and, in addition, can serve as a direct probe for studies of nitrogen environment in oligomeric fragments of nucleic acids even at moderately strong magnetic fields due to the higher spectral dispersion compared with 1H and 13C NMR spectra.
