Complete sequence-specific, proton-resonance assignments have been determin
ed for the calcium phosphate-stabilizing tryptic peptide beta -casein-(1-25
) containing the phosphorylated sequence motif Ser(P)(17)-Ser(P)-Ser(P)-Glu
-Glu(21). Spectra of the peptide have been recorded, in separate experiment
s, in the presence of excess ammonium ions, sodium ions and calcium ions, a
nd of the dephosphorylated peptide in the presence of excess sodium ions. W
e observed significant changes to chemical shifts for backbone and side-cha
in resonances that were dependent upon the nature of the cation present. Me
dium-range nuclear Overhauser effect (nOe) enhancements, characteristic of
small structured regions in the peptide, were observed and also found to be
cation dependent. The secondary structure of the peptide was characterized
by sequential and medium-range (i, i+2/3/4, which denotes an interaction b
etween residue i and residue i + 2, i+3 or i + 4 in the peptide) nOe connec
tivities, and Ha chemical shifts. Four structured regions were identified i
n the calcium-bound peptide: residues Arg(1) to Glu(4) were involved in a l
oop-type structure, and residues Val(8) to Glu(11), Ser(P)(17) to Glu(20) a
nd Glu(21) to Thr(24) were implicated in p-turn conformations. Comparison o
f the patterns of medium-range nOe connectivities in beta -casein-(1-25) wi
th those in alpha (s1)-casein-(59-79) suggest that the two peptides have di
stinctly different conformations in the presence of calcium ions, despite h
aving a high degree of sequential and functional similarity.