Types of Compounds. Ionic compounds are compounds composed of ions, charged particles that form when an atom (or group of atoms, in the case of polyatomic ions) gains or loses electrons. 1  A cation is a positively charged ion. 2  An anion is a negatively charged ion.
Porins are members of an extended family of transmembrane proteins found in the outer membrane of gram negative bacteria such as E. coli, or in the outer membranes of mitochondria in eukaryotes. The general porins provide a simple diffusion pathway across the membrane for molecules less than 1 kDa with little substrate selectivity.
In some porins, e.g. E. coli OmpF, the N-and C-terminals link via a salt bridge to form a pseudo-cyclic peptide. The strands are arranged as a simple meander, in which adjacent strands are nearest neighbours in the sequence, unlike the Greek key arrangement that's common in soluble proteins. Each beta strand is inclined about 45 o to the barrel axis