  We have shown recently that the notion of poking pairwise interactions along
a chain provides a unifying framework for understanding the formation of both
secondary and the tertiary protein structure based on symmetry and geometry.
$\alpha$-helices and $\beta$-sheets are found to be special geometries that
have systematic poking contacts in a repetitive manner with the contacts being
local along the $\alpha$-helix and non-local along a pair of adjacent strands
within a $\beta$-sheet. Pairwise poking interactions also govern tertiary
structure formation, but they are weaker and there are no special geometrical
constraints as in secondary structure formation. Here we demonstrate that
protein turns, the most prevalent non-repetitive structural element in
proteins, are instances of local (as in $\alpha$-helices) and isolated
(non-repetitive) poking pairwise contacts for which the geometrical constraints
are partially relaxed. This simple and purely geometrical definition of protein
turns (also sometimes known as reverse turns, $\beta$-turns, $\beta$-bends,
hairpin bends, $3_{10}$ bends, kinks, widgets, ...) provides a simple framework
for unifying them. We present the results of a systematic analysis and identify
their structural classes as well as their respective amino acid preferences.
