A comparison between poly-α, l-ornithine and poly-α, l-lysine in solution: The effect of a CH 2 group in the side chain on the conformation of poly-α-amino acids

Abstract

1. 1. The helix content of samples of poly-α, l-ornithine was compared with that of poly-α, l-lysine at 25° in dilute solution (up to 1.2 g/100 ml). Both water and 2-chloroethanol-water mixtures (3:1, v/v) were used as solvents. At high degrees of protonation of the side-chain amino groups, the helix content was effectively zero in all cases, as estimated by the optical rotatory dispersion parameter b o of Moffitt's equation in the range 300–600 mμ. At low degrees of protonation, in aqueous alkali, polyornithine of DP = 100 was only about 20% helical while low molecular weight polylysine of DP = 20–25 had about 40% helix content under the same conditions, a judged by the parameter b o . In alkaline 2-chloroethanol-water both poly-amino acids had a high helix content. Hydrophobic interactions between adjacent side chains on the helix due to the additional CH 2 group in polylysine are considered to stabilize polylysine relative to polyornithine in aqueous medium. 2. 2. Complexes between poly-α, l-ornithine (6–10 −3 monoM; DP = 100 ) and ferriprotoporphyrin IX (6·10 −5–7·10 −5 M) in aqueous solution at pH 11 and at 25° were identified by light-absorption spectra measured in the range 370–630 mμ. Differences from previously investigated polylysine complexes in their light absorption, rate of formation and pH-stability can be explained by the differences in initial helix content of the respective poly-amino acids.