Length of the apolar core of a transmembrane helix
|15 - 30 amino acid residues
|von Heijne G. Membrane protein assembly: rules of the game. Bioessays. 1995 Jan17(1):25-30. p.27 left column top paragraphPubMed ID7702590
|p.27 left column top paragraph:"The most obvious characteristic of a transmembrane helix is that it is apolar, i.e. composed of mostly apolar amino acids such as leucine, valine, isoleucine and phenylalanine. The length of the apolar core of a transmembrane helix is typically 15-30 residues, although the polar ends of the helix may extend quite far outside the bilayer."