Hydrazino peptides

Since this postsynthetic, chemoselective lipidation procedure bypasses all the difficulties encountered during preparative HPLC purification of lipopeptides, the a-hydrazinoacetyl peptides were further developed into a strategy of lipidation by chemoselective hydrazone formation between lipophilic glyoxaldehyde vectors and prepurified hydrazino peptides (Scheme 10).11241 The hydrazino handle can be located at the N-termini or even at preselected lysine side-chain positions. Various lipophilic vectors were proposed containing one or more fatty acids coupled via amides to spacers which in turn are linked to glyoxylic acid via an amide bond.1124-1271... 

Marraud et alJ31 demonstrated that incorporation of an A-hydroxy amide into model dipeptides induced a y-turn-like structure 4, as do hydrazino peptides 5 (Scheme 5). 

Figure 43. Hydrazino peptide backbones and a diagram showing the many possible intrastrand H-bond interactions in this backbone.

Hydrazino peptides, peptides composed of a-hydrazino acids, H2N-NR-CH2-COOH and H2N-NH-CHR-COOH, respectively. They are structurally related to /3-peptides because they also contain an additional skeleton atom between the amino and carboxy groups [A. Aubry et al., Int. J. Pept. Protein Res. 1994, 43, 305 R. Gunther,... 

Hydrazines and hydrazides are acylated by the reagent. The mono-Boc derivatives of hydrazine or phenylhydrazine, obtained in isopropanol, react further with B0C2O in benzene or dioxane to give the (Boc-NR)2 diacylated derivatives. Regioselective AT-protection of ethyl hydrazinoacetate is performed at 0 °C and the product used for the synthesis of hydrazino peptides (eq 13). ... 

Another structurally simple modification involves replacement of a Lys residue with Lys(NH2) (6.83). Here, the amino group of lysine is replaced with a hydrazino group, which is less-basic by ca. 3 pKa units. This modification allows model peptides to be stabilized against endopeptidases such as trypsin and thrombin [207], Thus, the peptides Tyr-Gly-Xaa-Gly-Tyr-Ala-NH2 with Xaa = Lys or Arg were very rapidly hydrolyzed by trypsin (half-... 

More recently, a new chelation method based on the technetium chelator, HYNIC, was developed by Laverman et al. (36). HYNIC is well known for its use in labeling peptides and proteins with high efficiency and excellent stability (37). A-hydroxysuccinimidyl hydrazino nicotinate hydrochloride was conjugated to the free amino group of distearoylpho-sphatidyl-ethanolamine (DSPE) and subsequently incorporated in the lipid bilayer during the liposome preparation. 

The treatment of a N-substituted hydrazine hydrobromide successively with the a,a-dicyano-epoxide 91 and with TV-methyl hydrazine results in the racemic a-hydrazino hydrazide 92 (Scheme 29)/111 Repeating the above two reactions allows the elongation of 92 into the racemic (hydrazide) peptide 93/1121 A derived procedure, with ClCH2COCl instead of 91, is applied to the synthesis of the first (hydrazide) cyclotripeptide 94 (R1 = 4-C1C6H4 or 4-MeQlLi) (Scheme 30), by coupling of the N-terminal chloroacetyl and C-terminal hydrazine groups/113 It has been later extended to the synthesis of linear (hydrazide) polypeptides/114 It is noteworthy that the a-carbon is racemic in all of the above cases. 

The amidoxy, or ip[C0-NH-0], link 6 (R1 = H) results from N-acylation of an a-aminoxy acid. a-Aminoxy acids have been isolated from natural sources and some of them present antibacterial properties as such, or when coupled to an a-amino acid, as in malioxamycin H-L-Vali )[C0-NH-0]D-Asp-0Hj115 117l The a-aminoxy acids are homologous to the a-hy-drazino acids with the advantage that there is no problem of regioselectivity for N-acylation. Despite the easy preparation of a-aminoxy acids,1118-1201 the number of (amidoxy) peptides reported in the literature is rather limited, with no examples of solid-phase synthesis. Recently, it has been observed that the a-aminoxy and a-hydrazino acid units induce quite similar local folded structures.1 21-123 ... 

Tc(V) Hydrazino Nicotinamide (HYNIC) Derivatives. The introduction of the Tc(V)-HYNIC system (Schwartz et al. 1991) represents a milestone in the development of Tc-99m radiopharmaceuticals. Particularly, peptides have been labeled with very high specific activity (Edwards et al. 1999 a Harris et al. 1999 Rose et al. 1998). Since the HYNIC linker occupies only one coordination site, coligands such as tricine, ethylenediamine dia-cetic acid (EDDA), etc., may complete the coordination sphere of the metal (Babich et al. 2000 Edwards et al. 1999 b Ono et al. 2000) (Fig. 2.3.4). 

Ono M, Arano Y, Mukai T, et al. (2001). Plasma protein binding of Tc-labeled hydrazino nicotinamide derivatized polypeptides and peptides. Nucl. Med. Biol. 28(2) 155-164. 

Babich J W, Fischman A J (1995). Effect of co-ligand on the biodistribntion of Tc-labeled hydrazino nicotinic acid derivatized chemotactic peptides. Nucl. Med. Biol. 22 25-30. 

The method described by Bonnet et al. [20] offers an effective approach to conjugate synthetic peptides prepared by solid-phase peptide synthesis (SPPS) to liposomes. The hydrazino functionality is easily introduced to the synthetic peptide on resin by the use of A, A A -tri(rert-butyloxycarbonyl)-hydrazino acetic acid, which is fuUy compatible with SPPS synthesis [18, 19]. Furthermore, hydrazone formation occurs spontaneously without the need of a catalyst. Thus, this method is one of the most effective for the functionalization of liposomes with targeting ligands when it is possible to introduce a hydrazino group into the ligand. However, this is unfortunately problematic for antibodies and other complex ligands. 

Linkers cleavable by nucleophiles include the safety-catch linker [13, 14], hydrazino benzoyl linker [15, 16] and 4-hydroxymethylbenzoic acid (HMBA) [14, 17] (Table 1). All three linkers can provide fully protected peptides, and as the HMBA and hydrazinobenzoyl linkers are acid-stable, the peptide side-chains can be deprotected prior to release of the peptide. The HMBA linker is a versatile but less often used linker for Fmoc-SPPS. Varying the nucleophile employed in the cleavage cocktail a wide range of C-terminal modified peptides can be obtained primary amide (NH3 in methanol) [18,19], hydrazides (5 % NH2NH2 in DMF) [20], acids (0.1 M aqueous NaOH) [21], and methyl esters (5 % DIEA, 5 % MeOH in DMF at 50 °C) [22] (Fig. 1). 

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