“…N-methylation i) strongly affects the bioavailability of (cyclic) peptides [10e] ii)reduces the flexibility of cyclic peptides [125] iii)shields externally oriented NH groups and thus increasing lipophilicity [99,111] iv) in aseries of two different types of di-, tri-, and tetra-N-methylated cyclic alanines with the general backbone structure cyclo(-d-Ala-Ala 5 ), only the di-and one tetra-N-methylated peptide were highly permeable in Caco-2 cell tests [126] v) an increasedp opulation of cis-peptide bonds (type VI b-turns) is linked with astrong effect on bioavailability [10d, 52,99,111] lipophilic side chains i) high Leu content in CsA (17) [127] and in peptides that permeate by passive diffusion, such as c(L*l*LLp*Y) (9), [99b] poly-Leu (8) [110] or ahighly N-methylated lipophilic decapeptide (7) [97] ii)one of the most used methods for enhancing lipophilicity iii)insanguinamide A, exchange of Ala with tert-Butyl-Gly improved the bioavailability by shielding an amide bond, resulting in danamide F (12) [112] iv) b-branching of aliphatica mino acids only slightly affects the bioavailability of sanguinamide A [128] v) examples of very nonpolar bioactive peptides are griselimycin derivative (21,F= 89%) [129] and baceridin [130] (not yet determined) vi)l ipoamino acids (LAAs [131] )show enhanced serum stability,plasma half-life, [132] and increased permeability; [133] see also properties of lipidation [128] cis-peptide bond i) present in the backbone structure of many bioavailable cyclic peptides, e.g. CsA (17), danamide F( 12), tri-N-methylated Veber-Hirschmann peptide (18), c(*aAAA*AA) ( 15), and c(*a*AA*A*AA) ( 16) ii)often linked with the presence of asecondary amide and results in atype VI b-turn [134] iii)inarecent study,the presence of N-methylated cis-peptide bonds was correlatedw ith promotion of active transport [101b] [52, 101b] Angewandte Chemie Reviews depsipeptide i) high frequency in natural cyclic peptides, such as valinomycin, [135] griselimycin (21), [129]…”