Models of thioredoxin hairpin structures: conformational properties of beta-turn containing sequences.

Abstract

AbstractSequences 74–91 and 77–91 of E. coli thioredoxin, which according to x‐ray structure contain an irregular β‐turn, a hairpinlike structural element, have been synthesized and their conformational properties in solution have been investigated by means of CD spectroscopy. In addition, analogs of these sequences, containing the regular β‐turn element Gly‐Pro‐(Gly)2, have also been prepared and investigated. These are BOC‐Ile‐Gly‐Pro‐(Gly)2‐Val‐OMe (III) and BOC‐(Ile)3Gly‐Pro‐(Gly)2‐(Val)5‐OMe (IV) that on the basis of probability, should form hairpin structures stabilized by intramolecular interactions. While the natural sequences were shown to be unable to adopt structures characterized by an intrinsic conformational stability, the two analogs showed evidence of intramolecular folding in methanol and trifluoroethanol–water solution. In particular, the CD spectra are indicative of β‐structure. The most interesting case was observed for compound IV, as the highest degree of conformational order was present in solutions containing a large proportion of water. In addition, the formation of this structure took place in a highly cooperative manner. The results are utilized to discuss whether and to what extent conformationally stable folding peptide units of small size can be formed in aqueous solution.