Peptides segmentation complications

In view of these possible complications, the chemical synthesis of peptides is generally performed in the C-to-N direction by a stepwise elongation approach using preferentially A -alkoxycarbonyl-protected amino acid derivatives to produce the target polypeptide chain in solution and on solid supports directly. Specially designed intermediate segments, usually... 

One issue that complicates efforts to develop a model for the mechanism by which peptide ion channels work is the width of the bilayer that must be spanned by the transmembrane helix. Vodyanoy and Hall (251) measured the dielectric thickness of monoglyceride bilayers and found it to vary from 3.1 to 2.0 nm as the fatty acid chain length went from C22 to C14. The crystal structure (252) of the photosynthetic reaction center indicates a 3.0-3.1-nm hydrophobic zone perpendicular to the membrane that is smaller than might be expected for C18 lipids but is in accord with estimates of dielectric thickness as well as recent physical studies (253). With the assumption that a transmembrane segment must span this thickness to give a stable pore structure, a rise per residue of 1.5 A can be estimated for an a-helix, and 20 amino acid residues would be required. Peptides shorter than 20 residues, such as mastoparan and emerimicin, are capable of ion channel formation,... 

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