text: In biology literature, the term topology is also used to refer to mutual orientation of regular secondary structures, such as alpha-helices and beta strands in protein structure [http://www.diss.fu-berlin.de/diss/servlets/MCRFileNodeServlet/FUDISS_derivate_000000003407/08_kapitel3.pdf?hosts=]. For example, two adjacent interacting alpha-helices or beta-strands can go in the same or in opposite directions. Topology diagrams of different proteins with known three-dimensional structure are provided by PDBsum ([http://www.ebi.ac.uk/thornton-srv/databases/cgi-bin/pdbsum/GetPage.pl?pdbcode=1qjp&template=protein.html&r=wiring&l=1&chain=A an example]).