beta-Sheet modeling by helical surfaces

We present a topological description of a beta-sheet in terms of a piece of helical surface. It requires only two easy-to-handle parameters: the twist , i,e, the turn of the helical surface per residue, and the coiling, which is a curvature along the strands or in the direction perpendicular to the s trands of the sheet. This method applies fairly well to three- and four-str and sheets, forming a too limited structure to be able to build a barrel. F rom an analysis of beta-sheets derived from a structural database, we show that this picture can even be reduced to the use of one main value, the twi st angle. The dependence of beta-sheet twisting on the number of strands in a sheet, and also on the length and direction of strands, has been demonst rated. The applications of such a description may include the rapid modelin g of 3D structures.