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.     

Is there a relationship between branched amino acid pool size and cell cycle inhibition in roots treated with imidazolinone herbicides?

Excised pea root tips were cultured in White's medium for 24 h and then treated for 12 h with one of the imidazolinone herbicides at 0.2, 2, 20, or 200 M. Pursuit and Assert were almost ineffective in inhibiting the mitotic index (MI), except at the highest concentrations. Arsenal (ARS) and Scepter both showed good inhibition with 20 M by 8 h. Adding all three branched amino acids (BAA) (VAL, ILE, and LEU) at 0.1 mM blocked herbicide action. Treatment with the BAAs singly had no protective effect. Experiments were performed to determine the BAA pool size and MI after an 8-h treatment with ARS at 2 and 200 M and Chlorsulfuron (CS), a sulfonylurea herbicide, at 28 nM. Both CS and ARS at 200 M inhibited the MI to almost 0 by 8 h. ARS at 2 M inhibited the MI by about 40%. The BAA pool size in all three treatments was reduced by approximately 50%, whether the MI was totally blocked or not. The 1-mm root tips had a greater amount of VAL than did the mature portions of the roots, whereas ILE and LEU were slightly less in the root tip. Other soluble amino acids did not show consistent differences between herbicide-treated roots and controls. The implications of the pool size reduction, in instances where the MI was not totally inhibited, is discussed in light of new data from other laboratories on the mode of action of the imidazolinone herbicides.