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Cellular membranes contain a multi-layered sheet of lipids and protein molecules that is hydrophilic on both surfaces and extremely hydrophobic at the core. Embedded proteins, then, need to have hydrophilic surface regions surrounding a hydrophobic membrane-spanning segment.

Alpha helices can contain long hydrophobic regions because their main-chain amino and carboxy groups are hydrogen-bonded. Beta sheets, in contrast, have edges with exposed, main-chain, polar amino and carboxy groups.

beta sheet, showing exposed main chain polar groups at the edges

Beta sheet, showing exposed main chain polar groups at the edges. H-bonds between sheets are indicated by magenta lines.

As a consequence, most membrane-spanning proteins contain alpha helices in their membrane-spanning sections. Porins are an exception because their membrane-spanning, closed beta-barrels ensure that all hydrophilic main chain atoms are neutralized, i.e. their beta sheets contain no exposed edges.

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