The peptide Pro130–Thr–Ser–Thr–Pro–Thr–Ile–Glu–Ala–Val–Glu140–Ser–Thr–Val–Ala–Thr–Leu–Glu–Ala–Ser–Pro150–Glu–Val–Ile, which corresponds to residues 130–153 of κ-casein B, was synthesized and the conformation
of the peptide in solution investigated by circular dichroism (CD) spectroscopy,
structure prediction algorithms and 1H-nuclear magnetic resonance
spectroscopy. In a solution containing
the structure-enhancing solvent trifluoroethanol the CD spectrum was typical of a
peptide in the α-helical conformation and nuclear magnetic resonance
showed that the amino acids between Ile136
and Ser149 (κ-casein numbering) were predominantly
in the α-helical conformation, but that Pro130 to Thr135
and Pro150 to Ile153 were not.
In addition, Thr133–Pro134 and
Ser149–Pro150
were primarily in the trans conformation,
the residues from Thr131 to Thr135
were in unordered structures and the residues from
Glu151 to Ile153
were in an extended conformation. Residues Glu137
to Glu140 and Thr145 to Ala148
also displayed some 310-helix character. When the peptide was
dissolved in 10 mM-cetyltrimethylammonium chloride solution at pH 6,
the CD spectra indicated that the proportion of helical structure was comparable to
that of the peptide in trifluoroethanol solution (400 ml/l), whereas when
the peptide was dissolved in buffer alone or in 10 mM-SDS solution, the
CD spectra were consistent
with a low helical content. Acidification of these solutions to
pH 2·85 resulted in a
slight increase in the helical content of the peptide in buffer and more markedly in
buffer containing SDS. When the peptide was in 5 mM-CaCl2
solution at neutral pH,
the CD spectrum indicated that some ordered structure was present. Taken together
these results indicate that the ionizable residues Glu137,
Glu140, Glu147 and Glu151
could be important in determining the stability of the putative helix. The structure
predictions found that the sequence from Glu137 to Pro150
would be more likely to be
in a helical than any other conformation in the intact bovine protein, but that pig,
sheep and goat κ-caseins did not give a prediction of a strongly
helical region in this part of the molecule.