To test the effect of NHÀCS groups (Scheme 1) on the stability of b-peptide secondary structures, we have synthesized three b-thiohexapeptide analogues of H-(b-HVal-b-HAla-b-HLeu) 2 -OH (1) with one, two, and three CS groups in the N-terminal positions (cf. 2 ± 4 and model in Fig. ). The first CS group was introduced selectively by treatment with Lawesson reagent of Boc-b-dipeptide esters (6 and 8). A series of fragment-coupling steps (with reagents as for the corresponding sulfur-free building blocks) and another thionation reaction led to the title compounds with a CS group in residues 1, 1, and 3, as well as 1, 2, and 3 of the b-hexapeptide (Schemes 2 and 3). The sulfur derivatives, especially those with three CS groups, were much more soluble in organic media than the sulfur-free analogues (b 1000-fold in CHCl 3 ; Table ). The UV and CD spectra (in CHCl 3 , MeOH, and H 2 O ) of the new compounds were recorded and compared with those of the parent b-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 ( Dl ca. 20 nm, amplitude up to 121000) of the pp* CS band near 270 nm with the b-trithiohexapeptide (with and without terminal protecting groups), and strong, so-called primary solvent effects, in the CD spectra. The CD spectrum of the b-dithiohexapeptide 3 undergoes drastic changes upon irradiation with 266-nm laser light of a MeOH solution ( Fig. ). The NMR structure in CD 3 OH of the unprotected b-trithiohexapeptide 4 was determined to be an ( M )-3 14 -helix ( Fig. ), very similar to that of the non-thionated analogue (cf. 1). NMR and mass spectra of the b-hexapeptides with CS and with CO groups are compared ( Figs. and).