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We show that beta-hairpins can be divided into four classes, each with a number of members. Hairpins from a single class are readily interconverted by loss or gain of hydrogen bonds, but interconversion between classes requires complete unzipping and reformation of the entire beta-hairpin. Sibanda & Thornton [(1985) Nature (London) 316, 170-174] have classified beta-hairpins as either two-residue, three-residue, four-residue etc., loops. We point out that their nomenclature, by itself, gives rise to ambiguities, but that, if the class (one of the four mentioned above) is also specified, the description of beta-hairpins becomes straightforward. A range of proteins of known three-dimensional structure has been examined; it provides examples of hairpins of each of the four classes and give some indication of their frequency of occurrence. The distribution observed is substantially different from that described by Sibanda & Thornton [(1985) Nature (London) 316, 170-174].

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