The structure and dynamics of the gastrointestinal peptide hormone motilin, consisting of 22 amino acid residues, have been studied in the presence of isotropic q = 0.5 phospholipid bicelles. The NMR solution structure of the peptide in acidic bicelle solution was determined from 203 NOE-derived distance constraints and six backbone torsion angle constraints. Dynamic properties for the <sup>13</sup>C<sub>α</sub>→<sup>1</sup>H vector in Leu-10 were determined for motilin specifically labeled with <sup>13</sup>C at this position by analysis of multiple-field relaxation data. The structure reveals an ordered alpha-helical conformation between Glu-9 and Lys-20. The N-terminus is also well structured with a turn resembling that of a classical beta-turn. The <sup>13</sup>C dynamics clearly show that motilin tumbles slowly in solution, with a correlation time characteristic of a large object.<ref name="pmid12495026"/>
