Vibrational analysis of peptides, polypeptides, and proteins. V. Normal vibrations of beta-turns.

T h e normal modes have been calculated for p-turns of types I, 11, 111, 1’, 11’, and 111’. The complete se t of frequencies is given for the first three structures; only the amide I, 11, and I l l modes are given for the latter three structures. Calculations have been done for structures with standard dihedral angles, as well as for structures whose dihedral angles differ from these by amounts found in protein structures. The force field was that refined in our previous work on polypeptides. Transition dipole coupling was included, and is crucial to predicting frequency splittings in the amide I and amide I1 modes. T h e results show that in the amide I region, P-turn frequencies can overlap with those of the cu-helix and @-sheet structures, and therefore caution must be exercised in the interpretation of protein bands in this region. The amide I11 modes of 0turns are predicted a t significantly higher frequencies than those of (?-helix and 0-sheet structures, and this region therefore provides the hest possibility of identifying 0turn structures. Amide V frequencies of fl-turns may also he distinctive for such structures.