Isolated and multiple beta-turns with proline in the third position

We report the observation of a number of unexpected isolated beta -turns wi th a proline in the third position. These beta -turns are mainly, either ty pe VIa1, type VIb or type IV although 3 examples of type I, 4 examples of t ype II' and 5 examples of type VIa2 beta -turns were also observed in a rep resentative protein data set, Nearly a third of these turns are located in the loop regions and the rest are situated at ends of either helices or bet a -strands, The formation of double beta -turns, where proline in the third position in the first beta -turn corresponds to proline in second position in the second overlapping beta -turn has earlier been suggested to stabili ze the otherwise unfavourable proline in the third position. We observed a number of multiple beta -turns, mainly of the type VIb or type IV, with pro line in the third position that did not correspond to proline in second pos ition in the overlapping beta -turn, Although these beta -turns may also be stabilized due to the formation of multiple turns, the isolated beta -turn s with proline residue in the third position are mainly surrounded by water molecules which are likely to stabilize these turns. The phi, psi dihedral angle values for proline residue in isolated and multiple beta -turns show distinct preferences in the Ramachandran plot.