Cyclic octapeptides containing thiazole. Effect of stereochemistry and degree of flexibility on calcium binding properties

Abstract

Solution conformation and calcium binding properties have been investigated for the two cyclic octapeptides cyclo(-D-Thr-D-Val(Thz)-Ile-)₂ (4) and cyclo(-Thr-Gly(Thz)-Ile-Ser-Gly(Thz)-Ile-) (5) and the results are compared to those for the cyclic octapeptides previously studied; ascidiacyclamide (1), patellamide D (2), cyclo(-Thr-D-Val(Thz)-Ile-)₂ (3), and cyclo(-Thr-D-Val-αAbu-Ile-)₂ (6). Both 4 and 5 contain two heterocyclic thiazole ring constraints but the latter has a larger degree of flexibility as a consequence of the glycine residues within the cyclic framework. The solution conformation of 4 and 5 was determined from ¹H NMR spectra and found to be a “twisted figure of eight” similar to that for 2. Complexation studies using ¹H NMR and CD spectroscopy yielded 1 ∶ 1 calcium–peptide binding constants (logK) for the two peptides (2.3 (4) and 5.7 (5)). For 5 the magnitude of the binding constant was verified by a competition titration using CD. The different calcium-binding affinities of 3 (logK = 4.0) and 4 is attributed to the stereochemistry of the threonine residue. The magnitude of the binding constant for 5 compared to 3 and 4 (all peptides containing two thiazole ring constrains) demonstrates that the increase in flexibility of the cyclic peptide has a dramatic effect on the Ca²⁺ binding ability. The affinity for Ca²⁺ thus decreases in the order (6 ∼ 5 > 3 > 2 ∼ 1 > 4). The number of carbonyl donors available on each peptide has only a limited effect on calcium binding. The most important factor is the flexibility, which allows for a conformation of the peptide capable of binding calcium efficiently.