Using heteronuclear NMR spectroscopy, we studied
the solution structure and dynamics of bovine β-lactoglobulin
A at pH 2.0 and 45 °C, where the protein exists as
a monomeric native state. The monomeric NMR structure,
comprising an eight-stranded continuous antiparallel β-barrel
and one major α-helix, is similar to the X-ray dimeric
structure obtained at pH 6.2, including βI-strand
that forms the dimer interface and loop EF that serves
as a lid of the interior hydrophobic hole. {1H}-15N
NOE revealed that βF, βG,
and βH strands buried under the major α-helix
are rigid on a pico- to nanosecond time scale and also
emphasized rapid fluctuations of loops and the N- and C-terminal
regions.