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The crystal structures of the isovaline-containing dipeptides, Boc-D-Iva-L-Pro-NHBzl 4 and Boc-L-Iva-L-Pro-NHBzl 5 were determined by X-ray diffraction. The diastereoisomeric peptides adopt intramolecular hydrogen-bonded β-turn conformations closely similar to each other (4:φIva-51°, ψIva-38°, φPro-70° and ψPro-17° and 5: φIva-53°, ψIva-35°, φPro-72° and ψPro-14°). The Pro ring of each peptide is in Cγ-exo conformation. These conformations are essentially the same as those in the reported crystal structures of the Aib-L-Pro sequence possessing an NH group directly attached to the carbonyl of the L-Pro, indicating that replacement of either one of the two methyl groups of the Aib moiety with an ethyl group does not cause any significant change in the β-turn conformation of the Aib-L-Pro sequence in the crystalline state.CD spectral analysis of the terminal chromophoric group-carrying peptides Dnp-Gly-X-L-Pro-Gly-pNA (X = Aib 6 and D/L-Iva 7/8) has shown that these three tetrapeptides in CHCl3 and THF solutions also adopt a β-turn-type conformation. CD spectra of glycolic acid residue-containing analogues in place of the fourth Gly residue revealed a lack of β-turn tendency in these analogues, indicating the importance of intramolecular hydrogen bonding for the β-turn conformation of the central dipeptide moieties. The results are consistent with the reported unturned crystal structures of Aib-L-Pro arid D/L-Iva-L-Pro sequence-containing peptides lacking the NH group which directly follows the Pro residue available for intramolecular hydrogen bonding.

Seven novel cycloartane-type triterpenes were isolated from Combretum quadrangulare , and their structures were elucidated on the basis of spectral analysis. All these compounds were tested for their cytotoxicity against murine colon 26-L5 carcinoma cells. Methyl quadrangularate B ( 2 ) and methyl quadrangularate D ( 4 ) exhibited potent cytotoxicity having ED 50 values 9.54 and 5.42 μM, respectively.