To study the effects of a point mutation found in
Pelizaeus-Merzbacher disease (PMD) on the physicochemical and structural properties of the extracellular loop 4 of the
myelin proteolipid protein (PLP), we synthesized the
peptide PLP(181-230)Pro215 and one mutant PLP(181-230)Ser215 with regioselective formation of the two disulphide bridges Cys200-Cys219 and Cys183-Cys227. As conventional
amino acid building blocks failed to give crude
peptides of good quality we had to optimize the synthesis by introducing
pseudoproline dipeptide building blocks during the
peptide elongation. In
peptide Pro215 the first bridge Cys200-Cys219 was obtained after air oxidation, but in
peptide Ser215 because of aggregation,
dimethyl sulfoxide (
DMSO) oxidation had to be used. The second bridge Cys183-Cys227 was obtained by
iodine oxidation of both Cys (acetamidomethyl, Acm)-protected
peptides. The secondary structures of the parent and mutant loops were analysed by circular dichroism (CD) in the presence of
trifluoroethanol (
TFE) and
sodium dodecyl sulphate (SDS) as a membrane mimetic. Analysis of the spectra showed that the content of alpha-helix and beta-sheet varied differently for both
peptides in
TFE and SDS solutions, demonstrating the sensitivity of their conformation to the environment and the differences in their secondary structure. The ability of both
peptides to insert into the SDS
micelles was assayed by intrinsic
tryptophan fluorescence.