New lamellarin alkaloids from the Indian ascidian Didemnum obscurum and their antioxidant properties.

Three new lamellarin alkaloids, lamellarins gamma (1), alpha (2), and epsilon (3), along with eight known lamellarin alkaloids, lamellarins M (4), K (5), K-diacetate (6), K-triacetate (7), U (8), I (9), C-diacetate (10), and X-triacetate (11), have been isolated from the Indian ascidian Didemnum obscurum. The structures of 1-11 were established using standard spectroscopic techniques. The structure of lamellarin K-triacetate (7) was further confirmed by X-ray crystallographic analysis. The antioxidant properties of lamellarin gamma, lamellarin gamma-monoacetate, lamellarins K, U, and I, and lamellarin C-diacetate were evaluated.

NMR study of the peptide present in the principal neutralizing determinant (PND) of HIV-1 envelope glycoprotein gp120.

The peptide sequence Gly-Pro-Gly-Arg-Ala-Phe (GPGRAF) is present in many principal neutralizing determinants (PND) of the human immunodeficiency virus type-1 (HIV-1). It has been shown that peptides from the PND sequence contain a significant beta turn in the conserved Gly-Pro-Gly-Arg sequence. In order to find out whether or not the smaller subunits also contain this turn, we have studied the NMR of a hexapeptide [GPGPRAF, peptide (I)], a heptapeptide Gly-Pro-Gly-Arg-Ala-Phe-Cys [GPGRAFC, peptide (II)] and a dodecapeptide [GPGRAFGPGRAF, peptide (III)], retaining the side chain protecting groups. Although the majority of conformations for these peptides are disordered, there is a considerable propensity of structures with beta turn in the GPGR sequence. While peptide (I) and peptide (III) seem to have both type I and type II beta turn conformations, peptide (II) shows a propensity of only type II beta turn. The nascent structures obtained in these peptides may get stabilized as the receptor binding conformation in the presence of the receptors, thus playing a significant role in vaccine development against HIV.