beta-Thiopeptides: Synthesis, NMR solution structure, CD spectra, and photochemistry

To test the effect of NH-C=S groups (Scheme 1) on the stability of beta-pep tide secondary structures, we have synthesized three beta-thiohexapeptide a nalogues of H-(beta-HVal-beta-HAla-beta-HLeu)(2)-OH (1) with one, two, and three C=S groups in the N-terminal positions (cf. 2-4 and model in Fig. 1). The first C=S group was introduced selectively by treatment with Lawesson reagent of Boc-beta-dipeptide esters (6 and 8). A series of fragment-coupli ng steps(with reagents as for the corresponding sulfur-free building blocks ) and another thionation reaction led to the title compounds with a C=S gro up in residues 1, 1, and 3, as well as 1, 2, and 3 of the beta-hexapeptide (Schemes 2 and 3). The sulfur derivatives, especially those with three C=S groups, were much more soluble in organic media than the sulfur-free analog ues (> 1000-fold in CHCl3; Table 1). The UV and CD spectra (in CHCl3, MeOH, and H2O) of the new compounds were recorded and compared with those of the parent beta-hexapeptide 1 (Figs. 2-4); they indicate the presence of more than one secondary structure under the various conditions. Most striking is a pronounced exciton splitting (Delta lambda ca. 20 nm, amplitude up to +1 21000) of the pi pi*(C=S) band near 270 nm with the beta-trithiohexapeptide (with and without terminal protecting groups),and strong, so-called 'prima ry solvent effects', in the CD spectra. The CD spectrum of the beta-dithioh exapeptide 3 undergoes drastic changes upon irradiation with 266-nm laser l ight of a MeOH solution (Fig. 5). The NMR structure in CD3OH of the unprote cted beta-trithiohexapeptide 4 was determined to be an (M)-3(14)-helix (Fig . 7), very similar to that of the non-thionated analogue (cf. 1). NMR and m ass spectra of the beta-hexapeptides with C=S and with C=O groups are compa red (Figs. 6 and 8).