A comparison is made between the structures and calcium binding properties of four cyclic octapeptides that differ in the number of heterocyclic thiazole and oxazoline ring constraints. The conformations of the naturally occurring cyclic octapeptides ascidiacyclamide 1 and patellamide D 2, which each contain two oxazoline and two thiazole rings, are compared by ¹H NMR spectroscopy with the analogues cyclo(Thr-d-Val(Thz)-Ile)2 3 with just two thiazoles, and cyclo(Thr-d-Val-αAbu-Ile)2 4, with no 5-membered rings. The conformations observed in the solid state for ascidiacyclamide (â saddleâ ) and patellamide D (â twisted figure of eightâ ) were retained in solution, whilst peptide 3 was found to have a â chairâ shape and peptide 4 displayed a range of conformations. The solid state structure of 4 revealed that the peptide takes a relatively planar conformation with a number of transannular hydrogen bonds, which are apparently retained in solution. Complexation studies utilising ¹H NMR and CD spectroscopy yielded 1â ¶1 calciumâ peptide binding constants (log Kâ ) for the four peptides (2.9 (1), 2.8 (2), 4.0 (3) and 5.5 (4)) as well as a 1â ¶2 metalâ peptide binding constant for 3 (log Kâ =â 4.5). The affinity for Ca²⁺ thus decreases with increasing number of 5-membered ring constraints in the macrocycle (4â >â 3â >â 2â â â 1).