Amino adds conformational properties

The first family is represented by peptides based on a, 3-dehydro a-amino adds in which a C=C bond confers rigidity and electronic Y-conjugation on the system (Section 11.1). Peptides rich in these amino acids are widespread in nature, particularly as antibiotics, and possess interesting conformational properties. 1-11 In addition to the E and Z configurational isomers, occurring when two different atoms (substituents) on CP are present, fully-extended, characteristically flat conformations or p-tums and 310/a-helices may be induced by u,p-dehydro a-amino acids. 

Both in the case of sensory rhodopsin in humans and of bacteriorhodopsin (a heptahelical membrane protein in halobacteria which is not coupled to a G protein) translocation of a Schiff-base proton is the essential step in making the protein functional (reviewed in ref 58). In rhodopsin the conversion of the inactive AH state to the AHI state that binds to the G protein is coupled to proton transfer from the Schiff base to the counterion, Glul 13, and proton uptake from the medium to the highly conserved Glul34, which serves as proton acceptor. Based on that similarity, one could consider sensory rhodopsin as an incomplete proton pump. Furthermore, a property shared by all G-protein-coupled receptors is a triplet, formed by residues 134-136 in rhodopsin, consisting of Glu-Arg-Tyr. The consequences of mutational replacement of Glul34 supports the notion that the state of protonation of this amino add is crudal for activity, and that its protonation triggers the conformational transition of the receptor from the inactive to the active state. 

The detailed information on the backbone protection of IFN and NEM-IFN provided by HDX MS measurements allows the conformational properties of intact and alkylated forms of the protein to be compared directly [14,16]. Perhaps the most important conclusion derived from the HDX MS work is the dramatic destabilization of helix D (residues 91-106) by the PTM event which affects a remote site in the amino add... 

The above conaderations ate described in fiiller detail in previous papers [6,7] and indicate that macromolecules assuming a single chirality conformation can show chiroptical properties characteristic of the conformation itself. Moreover, if chromophores are present in the side chains specific chiroptical properties can arise from dipole-dipole dectronic interactions among these chromophores disposed along the chirally arranged backbone. This situation is cleariy shown in poly-a-amino adds, in which spedfic and typical chiroptical properties are assodated with spedfic and typcal conformations (a-helix, -structures, random coil) [8]. 

Cyclic peptides are important biologically active compounds that combine peptide properties with conformational bias and often improved serum stability and membrane penetration [1-3]. Depsipeptides are similar compounds, but they contain as part of their backbone one or more hydroxylated amino acid residues that add ester or lactone moieties to the peptide (Fig. 1) [4-6]. Cyclo- and... 

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