Effect of asymmetric modification on the conformation of ascidiacyclamide analogs.

Abstract: Ascidiacyclamide (ASC), cyclo(-Ile[sup 1]-Oxz[sup 2]-d-Val[sup 3]-Thz[sup 4]- )[sub 2] (Oxz=oxazoline and Thz=thiazole) has a C[sub 2]-symmetric sequence, and the relationships between its conformation and symmetry have been studied. In a previous study, we performed asymmetric modifications in which an Ile residue was replaced by Gly, Leu or Phe to disturb the symmetry [Doi et al. (1999) Biopolymers49, 459–469]. In this study, the modifications were extended. The Ile[sup 1] residue was replaced by Gly, Ala, aminoisobutyric acid (Aib), Val, Leu, Phe or d-Ile, and the d-Val[sup 3] residue was replaced by Val. The structures of these analogs were analyzed by X-ray diffraction, [sup 1]H NMR and CD techniques. X-Ray diffraction analyses revealed that the [Ala[sup 1]], [Aib[sup 1]] and [Phe[sup 1]]ASC analogs are folded, whereas [Val[sup 1]]ASC has a square form. These structures are the first examples of folded structures for ASC analogs in the crystal state and are similar to the previously reported structures of [Gly[sup 1]] and [Phe[sup 1]]ASC in solution. The resonances of amide NH and Thz CH protons linearly shift with temperature changes; in particular, those of [Aib[sup 1]], [d-Ile[sup 1]] and [Val[sup 3]]ASCs exhibited a large temperature dependence. DMSO titration caused nonlinear shifts of proton resonances for all analogs and largely affected [d-Ile[sup 1]] and [Val[sup 3]]ASCs. A similar tendency was observed upon the addition of acetone to peptide solutions. Regarding peptide concentration changes, amide NH and Thz CH protons of [Gly[sup 1]]ASC showed a relatively large dependence. CD spectra of these analogs indicated approximately two patterns in MeCN solution, which were related to the crystal structures. However, all spectra showed a similar positive Cotton effect in TFE solution, except that of [Val[sup 3]]ASC. In the cytotoxicity test using P388 cells, [Val[sup 1]]ASC exhibited the strongest activity, whereas the epimers of ASC ([d-Ile[sup 1]] and [Val[sup 3]]ASCs), showed fairly moderate activities. [ABSTRACT FROM AUTHOR]