Pressure-dependent changes in the structure of the melittin alpha-helix determined by NMR

A novel method is described, which uses changes in NMR chemical shifts to characterise the structural change in a protein with pressure. Melittin in methanol is a small alpha-helical protein, and its chemical shifts change linearly and reversibly with pressure between 1 and 2000 bar. An improved relationship between structure and HN shift has been calculated, and used to drive a molecular dynamics-based calculation of the change in structure. With pressure, the helix is compressed, with the H-O distance of the NH-O=C hydrogen bonds decreased by 0.021 +/- 0.039 A, leading to an overall compression along the entire helix of about 0.4 A, corresponding to a static compressibility of 6 x 10(-6) bar(-1). The backbone dihedral angles phi and psi are altered by no more than +/- 3 degrees for most residues with a negative correlation coefficient of -0.85 between phi(i) and psi(i - 1), indicating that the local conformation alters to maintain hydrogen bonds in good geometries. The method is shown to be capable of calculating structural change with high precision, and the results agree with structural changes determined using other methodologies.