Cationic peptides stability

Recently the use of another bifunctional reagent, glutaraldehyde, has been described for the stabilization of DNA complexes with cationic peptide CWK18 [104]. The authors of this paper, however, limited the study to the protective effects toward nuclease degradation. 

The improved DNA binding and condensation provided by amino acids such as tryptophan suggests that the inclusion of hydrophobic interactions within DNA complexes may be beneficial. Peptides with moities that provide cooperative hydrophobic behavior of alkyl chains of cationic lipids would improve the stability of the peptide-based DNA delivery systems. Two general classes of lipopeptide analogs of Tyr-Lys-Ala-Lysn-Trp-Lys peptides have been prepared by including a hydrophobic anchor. The general structures are N, N-dialkyl-Gly-Tyr-Lys-Ala-Lysn-Trp-Lys and Na,Ne-diacyl-Lys-Lysn-Trp-Lys. These peptides differ from the parent structures in that they self-associate to form micelles in aqueous solutions. The inclusion of dialkyl or diacyl chains in the cationic peptides improves the peptide ability to bind DNA and reduces aggregation of the complexes in ionic media. 

Cationic polymers, such as poly(L-lysine) (PEL), polyethylenimine (PEI), chitosan, polyamidoamine (PAMAM) dendrimers, poly(2-dimethylamino) ethyl methacrylate, and polyphosphoesters, condense DNA to form compacted polyplexes. ° The size and the stability of polyplexes depend on the ratio of cations vs. anions, temperature, ionic strength, and the solvent. Stability of polyplexes can be enhanced by conjugating PEG to the polycations or by using PEG-containing block or graft polymers that form micelles. Small cationic peptides are also able to condense DNA, however, six-consecutive-cations is the minimal requirement to achieve this effectively. 

Different amphipathic helical peptides can be membrane-stabilizing or lytic to membranes depending on the structure of the helix, i ich in turn determines the nature of its association with the membrane. Features of peptides that are responsible for their specific properties have been discussed [22]. The type of peptide is of importance for the functioning of the cells. It has been shown that a mixture of anionic amphiphilic peptides with charge-reversed cationic peptides did induce a rapid and efficient fusion of egg phosphatidylcholine vesicles, but no fusion was observed with each peptide alone. 

Initial studies of brain delivery based on the chimeric peptide strategy used the absorptive-mediated uptake of cationized albumin which was chemically coupled to the opioid peptide P-endorphin [80] or its metabohcaUy stabilized analogue [D-Ala ]P-endorphin. Tracer experiments in which the chimeric peptide was labelled in the endorphin moiety provided evidence of internalization by isolated brain capillaries and transport into brain tissue in vivo [81]. 

Small ACTH fragments related to ACTH-(4-10) have also been investigated for the presence of ordered structure. CD of ACTH--(5-10) in TFE showed a random structure (50) as was found with H-NMR for fragment 4-10 (51). The addition of anionic or cationic surfactants to an aqueous solution of ACTH-(4-11) dit not promote any a-helix or 3-form in this peptide (CD experiments S2). When ACTH-(1-14) and 1-10 were measured by CD and NMR respectively, indications for a helical or ordered structure were found (90, ). Thus it seems that the addition of the non-helix "prone" fragment 1-3 or 1-4 can promote the formation of a helical structure in the adjacent sequence. Arguments in favour of this come from the theoretical work of Argos and Palau (53) on amino acid distribution in protein secondary structures. They found that Ser and Thr frequently occur at the N-terminal helical position (cf. Ser in ACTH) to provide stability the position adjacent to the helical C-terminus is often occupied by Gly or Pro (adjacent toTrp in ACTH we have Gly ) acidic amino acid residues are frequently found at the helix N-terminus (cf. Glu in ACTH) and/or basic residues at the C-terminus (cf. Arg ). 

A reasonable mechanism for the iodine oxidation of 5-Trt cysteine peptides is given in Scheme 6. 45 Reaction of iodine with the divalent sulfur atom leads to the iodosulfonium ion 5 which is then transformed to the sulfenyl iodide 6 and the trityl cation. Sulfenyl iodides are also postulated as intermediates in the iodine oxidation of thiols to disulfides. The disulfide bond is then formed by disproportionation of two sulfenyl iodides or by reaction between the electrophilic sulfur atom of R -S-I and the nucleophilic S-atom of a second R -S-Trt molecule. The proposed mechanism suggests that any sulfur substitution (i.e., thiol protecting group) capable of forming a stabilized species on cleavage, such as the trityl cation, can be oxidatively cleaved by iodine. 

As discussed in Section 9.4.4, the incorporation of a metal ligand binding group in the side chain of peptidic structures can be used to induce and stabilize secondary structures and in the development metallopeptide molecular devices. Along these lines and based on the unique complexing ability toward metal cations of EDTA, several amino acids containing aminodiacetic adds have been reported. 

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