CD studies of synthetic polypeptides as histone models: Poly(L‐Arg‐L‐Ile‐Gly), poly(L‐Arg‐L‐Nva‐Gly), and poly(L‐Arg‐L‐Nle‐Gly)

Abstract

AbstractSequential polypeptides (L‐Arg‐X‐Gly)n were prepared as synthetic models of arginine‐rich histones to study their structure and their stereospecific interactions with DNA. In our previous work the conformational characteristics of poly(L‐Arg‐L‐Ala‐Gly), poly(L‐Arg‐L‐Val‐Gly), and poly(L‐Arg‐L‐Leu‐Gly) have already been analyzed. To obtain further insight into the influence of the X residue side chain on the conformation of the (L‐Arg‐X‐Gly)n polytripeptides, we now report their synthesis and cd properties when X represents the amino acid residues Ile, Nva, and Nle. The pentachlorophenyl active esters of the appropriate tripeptides were used to perform the polymerization, and the toluene‐4‐sulfonyl group was used to protect the arginine guanido group. CD spectroscopy showed that, in 100% trifluoroethanol, the degree of helical conformation increased in the order Ile → Nle → Nva. An equilibrium between β‐turn, α‐helix, and random‐coil conformers occurred in 100% hexafluoroisopropyl alcohol, while a rise in the temperature or the addition of water favored the α‐helix, the highest percentage of which was observed in a mixture of hexafluoroisopropyl alcohol: water (20 : 80) and in the order Ile → Nle → Nva. In aqueous solutions (at pH 7 and 12) the polymers behaved as a random coil, but they were forced to a less aperiodic structure, over a range of ionic strengths (0–0.5M NaF). A rise in temperature of up to 70°C in 100% trifluoroethanol resulted in a decrease of the α‐helix percentage of the polymers, while in aqueous solutions the aperiodic structure decreased with increasing temperature. This study proved the importance of the nature of the X residue (length, Cβ branching) in relation to the structural order of the sequential polypeptides. We concluded that the polymers prepared are suitable models for arginine‐rich histones.