Synthesis and conformational studies of peptidomimetics containing furanoid sugar amino acids and a sugar diacid

Furanoid sugar amino acids (1) were synthesized and used as dipeptide isosteres to induce interesting turn structures in small linear peptides. They belong to a new variety of designed hybrid structures that carry both amino and carboxyl groups on rigid furanose sugar rings. Four such molecules, 6-amino-2,5-anhydro-6-deoxy-D-gluconic acid (3, Gaa) and its mannonic (4, Maa), idonic (5, Iaa), and a 3,4-dideoxyidonic (6, ddIaa) congeners were synthesized. The synthesis followed a novel reaction path in which an intramolecular 5-exo S(N)2 opening of the hexose-derived terminal aziridine ring in 2 by the gamma-benzyloxy oxygen with concomitant debenzylation occurred during pyridinium dichromate oxidation of the primary delta-hydroxyl group to carboxyl function, leading to the formation of furanoid sugar amino acid frameworks in a single step. Incorporation of these furanoid sugar amino acids into Leu-enkephalin replacing its Gly-Gly portion gave analogues 8-11. Detailed structural analysis of these molecules by circular dichroism (CD) and various NMR techniques in combination with constrained molecular dynamics (MD) simulations revealed that two of these analogues, 8a and 10a, have folded conformations composed of an unusual nine-membered pseudo beta-turn-like structure with a strong intramolecular H-bond between LeuNH --> sugarC3-OH. This, in turn, brings the two aromatic rings of Tyr and Phe in close proximity, a prerequisite for biological activities of opioid peptides. The analgesic activities of 8a,b determined by mouse hot-plate and tail-clip methods were similar to that of Leu-enkephalin methyl ester. The syn disposition of the beta-hydroxycarboxyl motif on the sugar rings appears to be the driving force to nucleate the observed turn structures in some of these molecules (8 and 10). Repetition of the motif on both sides of a furanose ring resulted in a novel molecular design of sugar diacid, 2,5-anhydro-D-idaric acid (7, Idac). Bidirectional elongation of the diacid moieties of 7 with identical peptide strands led to the formation of a C2-symmetric reverse-turn mimetic 12 which displayed a very ordered structure consisting of identical intramolecular H-bonds at two ends between LeuNH --> sugar-OH, the same as in 8 and 10.     

Aminoglycoside antibiotics may interfere with microbial amino sugar analysis

The amino sugars (e.g., glucosamine, galactosamine, mannosamine, muramic acid) in soils are frequently employed as biomarkers of microbial residues. The analysis of amino sugars in environmental matrices, however, is expected to be more complicated than their determination in isolated microbial cells. In this study, we employed a widely used protocol for amino sugar analysis, and found that some aminoglycoside antibiotics interfere with amino sugar quantification in vitro. The method converts the aminoglycosides to compounds that coelute with the aldononitrile acetate derivatives of the amino sugars. Specifically, streptomycin significantly interferes with muramic acid analysis, and kanamycin, tobramycin and amikacin hamper glucosamine measurement. Mass spectrometry confirmed that the interfering compounds from aminoglycosides are not actually genuine microbial amino sugar monomers (bacterial muramic acid or fungal glucosamine), and are most likely to be N-methyl glucosamine or 3-amino-3-deoxy-glucopyranose. In contrast to their effects on muramic acid and glucosamine analyses, aminoglycosides do not interfere with galactosamine and mannosamine quantification. The few data that exist on the environmental occurrence of aminoglycoside antibiotics suggest they occur at only trace levels. Our findings may have implications for the qualitative and quantitative validity of results from amino sugar assays in some context. Application of the aldononitrile acetate derivatization method to samples (especially in selective microbial cultures using aminoglycosides as inhibitors) requires that potential interference be evaluated.     

Cyclic homooligomers of furanoid sugar amino acids

Cyclic homooligomers of mannose-derived furanoid sugar amino acid 1 [H-Maa(Bn(2))-OH] were synthesized by using BOP reagent in the presence of DIPEA under dilute conditions that converted the sugar amino acid monomer directly into its cyclic homooligomers 3a and 4a. The glucose-based sugar amino acid 2 [H-Gaa(Bn(2))-OH] under the same reaction conditions gave a bicyclic lactam 5a as the major product. Cyclic homooligomers of 2 were prepared by cyclizing their linear precursors 6 and 7 leading to the formation of cyclic peptides 8a and 9a, respectively. Conformational analysis by NMR and constrained MD studies revealed that all the cyclic products, 3, 4, 8, and 9, had symmetrical structures. The deprotected cyclic trimer of Maa 3b displayed a conformation in which all the C=O and the N-H bonds of the molecule point in opposite directions. In the deprotected cyclic tetramer of Maa 4b, the COs and NHs were in the plane of the ring with the former pointing to outside and the latter inside the ring. The structure of the cyclic Gaa dimer 8b displayed an unusual six-membered intramolecular hydrogen bond between NH(i)() --> C3-O(i)()(-)(1) and a syn orientation between the C2-H and CO. In this molecule, the C2-hydrogens and the COs can be seen on one side of the ring while the NHs point to the other side. Addition of the bicyclic lactam 5b resulted in the influx of Na(+) ions across the lipid bilayer leading to the dissipation of valinomycin-mediated K(+) diffusion potential.     

Synthesis and structural studies of peptides containing a mannose-derived furanoid sugar amino acid

A mannose-derived furanoid sugar amino acid (Maa) induced helical turns in peptides having repeat units of Maa(Bn(2))-Phe-Leu, which aggregated into head-to-tail duplexes in the longer oligomers.     

Synthesis and structural studies of peptides containing a glucose-derived furanoid sugar amino acid

Conformational analysis of peptides containing a glucose-derived furanoid sugar amino acid (Gaa) by detailed NMR and constrained MD studies revealed that peptides with repeating Gaa-Leu-Val units had conformational signatures very similar to those of linear homooligomers of Gaa.     

Synthesis and NMR studies of cyclopeptides containing a sugar amino Acid

[structure: see text] Cyclopeptides containing Glucuronic acid methylamine (Gum) alternating with Gly, L-Ala, D-Ala, L-Phe, D-Phe, L-Lys, or D-Lys were synthesized by a combination of solid-phase synthesis and solution chemistry. A more effective pathway to synthesize the sugar amino acid Gum in higher yields and in a shorter period of time was developed. Gum is employed in the benzylated and deprotected form. The cyclopeptides were characterized by NMR and the structure of one deprotected cyclic peptide solved.     

Synthesis of orthogonally protected cyclic homooligomers from sugar amino acids

Two new families of orthogonally protected cyclic homooligomers with two to four sugar units were synthesized from pyranoid sugar amino acids. Cyclic oligomers composed of amide-linked sugar amino acids (1-3) were prepared by cyclization of linear oligomers of the novel orthogonally protected pyranoid sugar amino acid 12 using a solution-phase coupling method. These orthogonally protected cyclic molecules can be selectively or fully deprotected, affording the macrocycles ready to further functionalization. The straightforward reduction of the amide bonds in the cyclic oligomers 1-3 gave the corresponding amine-linked macrocycles 4-6. This kind of amine-linked carbohydrate-based cyclic oligomer has never been reported before. These flexible molecular receptors could be studied as molecular hosts for molecular, cationic, and anionic recognition. Conformational analysis by molecular modeling (AM1) showed that all of the deprotected cyclic trimers and tetramers preferred a (4)C(1) chair conformation with oxygen atoms of the sugar ring located on the interior of the cavity and the secondary hydroxyl groups outward. In the amide-linked macrocycles, all of the amide bonds are in s-trans conformation. The estimated size of the internal cavity is about 4.5 A for the cyclic trimer and 6.9 A for the cyclic tetramer. The amine-linked macrocycles displayed similar conformational behavior with a slight decrease in internal cavity.     

Alkylated sugar amino acids: a new entry toward highly functionalized dipeptide isosters

[reaction: see text] Novel highly functionalized dipeptide isosters are synthesized via a diastereoselective alkyl/arylation protocol of a glucose-derived (R)-tert-butanesulfinylimine. One of these novel sugar amino acid derivatives, a D-Ala-Ser/Thr isostere, was applied in a peptide synthesis protocol to afford a cyclic tetramer.     

Pyranocyclopropanyl sugar amino acids,a new class of constrained (di)peptide isosteres

The design, synthesis and application of oxabicyclo[4.1.0]heptane amino acids as conformationally restricted sugar amino acid dipeptide isosteres are reported.     

Synthesis of cyclic oligomers of a modified sugar amino acid utilising dynamic combinatorial chemistry

Dynamic combinatorial chemistry has been utilised for the rapid synthesis of a library of cyclic oligomers based on a modified furanoid sugar amino acid repeat unit.     

Synthesis of a C3-symmetric macrocycle with alternating sugar amino acid and tyrosine residues

A C₃-symmetric macrocycle with alternating sugar amino acid and tyrosine residues was synthesized in seven steps from tyrosine tert-butyl ester and a sugar amino acid precursor derived from D-glucosamine. An Fmoc-protected D-glucosamine derivative was oxidized at C-6 to give the sugar amino acid, which was immediately coupled to tyrosine tert-butyl ester to produce an orthogonally protected building block. This building block was subsequently elongated to the trimer via the dimer, and finally cyclized to give the C₃-symmetric macrocycle.     

Synthesis of alkylated sugar amino acids: conformationally restricted L-Xaa-L-Ser/Thr mimics

Two synthetic strategies for the generation of delta-substituted pyranoid sugar amino acids (SAAs) are evaluated. The first employs chiral nonracemic tert-butane sulfinamides as key reagents. Regardless of the stereochemistry of the applied sulfinamide, the product formed has a stereochemistry resembling that of a d amino acid at C7. Direct Grignard reaction on formyl-tetra-O-benzyl-beta-D-C-glucopyranoside in the second strategy and subsequent Mitsunobu inversion, yields the l,l-dipeptide isosters.     

A practical synthesis of gramicidin s and sugar amino Acid containing analogues

A practical gram-scale and high-yielding synthesis of the antimicrobial peptide gramicidin S is presented. An Fmoc-based solid-phase peptide synthesis protocol is employed for the generation of the linear decapeptide precursor, which is cyclized in solution to afford the target compound. The versatility of our method is demonstrated by the construction of eight gramicidin S analogues (15a-h) having nonproteinogenic sugar amino acid residues (4-7) incorporated in the turn regions.     

Synthesis, conformational analysis and biological studies of cyclic cationic antimicrobial peptides containing sugar amino acids

Sugar amino acid based 24-membered macrocyclic C2-symmetric cationic peptides were designed and synthesized. The cationic group was introduced in the sugar amino acids. The conformation of these cyclic compounds was ascertained through NMR techniques, which proved they were amphipathic in nature. All the compounds were bacteriolytic, showed good activity against the Gr(+ve) and Gr(-ve) bacteria, and exhibited low hemolytic activity.     

A compendium of sugar amino acids (SAA): scaffolds,peptide- and glyco-mimetics

Sugar amino acids (SAAs) are carbohydrate derivatives bearing both amino and carboxylic acid functionalities. SAAs are very versatile conformationally biased building blocks amenable to serve as glyco- or peptidomimetics. The stereochemical arrangement of the substituents of the sugar ring, its ring-size as well as the presence of additional functional groups provides a plethora of possible combinations. In this overview the structures of oxygen heterocylic SAAs that have been reported thus far are provided, having 3, 4, 5, 6-membered rings as well as several bicyclic counterparts.     

An unusual reverse turn structure adopted by a furanoid sugar amino acid incorporated in gramicidin S

A new reverse turn, replacing one of the native type II' beta-turns in the cyclic peptide antibiotic gramicidin S, induced by a furanoid sugar amino acid is revealed. The C3-hydroxyl function plays a pivotal role by acting as a H-bond acceptor, consequently flipping the amide bond between residues i and i + 1, as was established by NMR and X-ray crystallographic analysis.     

Self-assembly of cyclic homo- and hetero-beta-peptides with cis- furanoid sugar amino acid and beta-hGly as building blocks

The design, synthesis and characterization of a new class of peptide nanotubes, self-assembled from cyclic homo- and hetero-beta-peptides based on cis-furanoid sugar amino acid and beta-hGly residues are described; these results represent the expansion of the conformational pool of cis beta-sugar amino acids in the design of peptide nanotubes.     

Analysis of the amino acid and sugar composition of streptococcal cell walls by gas chromatography-mass spectrometry

A procedure for determining the amino acid and sugar composition of streptococcal peptidoglycan-polysaccharide complexes by capillary gas chromatography-mass spectrometry (GC-MS) was established. Amino acids are analysed as butyl heptafluorobutyl derivatives and sugars as alditol acetates. These two different groups of compounds are derivatized independently but chromatography in both cases utilizes the same OV-1701 fused-silica capillary column which simplifies GC-MS analysis. The butyl heptafluorobutyl procedure incorporates new pre- and post-derivatization clean-up steps. Additionally, selected-ion monitoring MS allows amino acids to be readily analysed without interference from background noise.     

Preparation of some carbon-14-labeled amino acids by radiophotosynthesis using sugar beet plant

Radiophotosynthesis has been applied as a means for preparing a number of carbon-14-labeled amino acids using sugar beet plants grown under controlled environmental conditions. The apparatus used, conditions of operation, isolation of products, and their analysis and radiometry have been discussed in detail. From the results obtained, it can be inferred that a number of labeled amino acids could be separated from the leaves of sugar beet including [¹⁴C]alanine, γ-[¹⁴C]aminobutyric acid, [¹⁴C]glutamic acid, [¹⁴C]aspartic acid, [¹⁴C]serine, [¹⁴C]valine, and [¹⁴C]aspargine with specific activities of several microcuries per millimole. The specific activities of the products could be significantly increased by increasing the specific activity of the radioactive carbonate initially used.     

Towards the synthesis of sugar amino acid containing antimicrobial noncytotoxic CAP conjugates with gold nanoparticles and a mechanistic study of cell disruption

Cationic antimicrobial peptides are potent inhibitors of growth of a broad spectrum of micro-organisms but often have large cytotoxic effects. We prepared some novel sugar amino acid containing cyclic cationic peptides and their Au nanoparticle attached counterparts and studied their antimicrobial activities and cytotoxic behaviour, including an investigation of the mechanism of the cytotoxicity.