Solution structure of the isolated ribonuclease C-terminal 112-124 fragment

The conformational properties of the ribonuclease C-terminal 112-124 fragment have been studied by CD and 1H- and 13C-NMR in an attempt to determine whether native secondary structure elements other than alpha-helices have stability enough to be detected when isolated in aqueous solution. Only sequential alpha N and intraresidue NOE cross-peaks are observed in the NOESY spectra, a fact which points towards an essentially extended polypeptidic chain. Observed spectral variations with temperature, pH and urea addition allowed the identification of two non-random regions within the chain. The first one is located within residues 119-121, the same region where a native salt bridge (H119...D121) exists in the native protein, and the stability of that structure is affected by the protonation state of carboxylate groups. The second one involves the S123 and V124 residues at the C-terminal end. No signs of the native 112-115 beta-turn were detected which suggests that, in contrast to alpha-helices, long range interactions may be needed to stabilize these secondary structure elements.