Both types (alpha-helices and beta-barrels) show high neighborhood correlation which limits the number of different topologies. Also in both types , all hydrogen bonding donor and acceptors of the polypeptide backbone are completely compensated and burried, while the nonpolar side chains point to the membrane. The alpha helices run generally perpendicular to the membrane plane andconnections are formed between neighboring helix ends. All beta barrels contain meandering antiparallel sheets,the topologies of which depend merely on the strand number.

The presently known sizes range from small eight stranded to large twenty-two stranded beta barrels existing as monomers and oligomers. The smallest barrels form inverse micelles , medium ranged barrels form more or less specific pores for nutrient uptake where as the largest barrels occur in active Fe+2 transporters. Beta barrel construction is described by the number of strands and the shear number which is the measure for the inclination of the beta strands against the barrel axis.