The square conformation of phenylglycine-incorporated ascidiacyclamide is stabilized by CH/π interactions between amino acid side chains

We designed a phenylglycine (Phg)-incorporated ascidiacyclamide (ASC) analogue, cyclo(-Phg-oxazoline-d-Val-thiazole-Ile-oxazoline-d-Val-thiazole- ([Phg]ASC), with the aim of stabilizing the square conformation of ASC through interactions between amino acid side chains. X-ray diffraction analysis showed that [Phg]ASC has a square structure, similar to ASC, in which the sec-butyl group of Ile and the benzene ring of Phg are in close proximity. Consistent with that finding, 1H NMR experiments revealed significant high-field shifts in the sec-butyl group of Ile, which suggests a potential for CH/π interactions between the sec-butyl group of Ile and the benzene ring of Phg. The CD spectra of [Phg]ASC were less affected by TFE titration or increasing temperature than those of ASC. In addition, [Phg]ASC showed approximately three times greater toxicity toward HL-60 cells than ASC. Thus the potently cytotoxic conformation of [Phg]ASC may be stabilized by CH/π interactions between the side chains of the Ile and Phg residues.