Donatella Nava

Separation and identification of carotenoids in bird's plumage by high-performance liquid chromatography-diode-array detection

The coloured feathers of Carduelis spinus (Siskin), C. flammea (Redpoll), Serinus serinus (Serin), Loxia curvirostra (Crossbill), Pinicola enucleator (Grossbeak), Carpodacus roseus (Pallas Rosefinch) and Pyrrhula pyrrhula (Bullfinch) have been extracted with a new procedure using mild conditions (a few minutes at room temperature). After the separation of melanines and proteins, the extracts were analyzed by HPLC-MS and HPLC-UV-Vis. The main components of the pigments were identified in all the species examined; moreover, UV-Vis and MS data were collected also for the minor components. These data suggest that minor components are generally cis isomers accompanying the predominant all-trans isomers.

1H‐Azepine‐4‐amino‐4‐carboxylic Acid: A New α,α‐Disubstituted Ornithine Analogue Capable of Inducing Helix Conformations in Short Ala‐Aib Pentapeptides

A very efficient synthesis of orthogonally protected 1H-azepine-4-amino-4-carboxylic acid, abbreviated as Azn, a conformationally restricted analogue of ornithine, was realized. It was obtained on a gram scale in good overall yield in five steps, three of which did not require isolation of the intermediates, starting from the readily available 1-amino-4-oxo-cyclohexane-4-carboxylic acid. Both enantiomers were used for the preparation of pentapeptide models containing Ala, Aib, and Azn. Conformational studies using both spectroscopic techniques (NMR, CD) and molecular dynamics on model 5-mer peptides showed that the (R)-Azn isomer possesses a marked helicogenic effect.

Carbocyclic serine analogues: regio- and diastereoselective syntheses of new 1-amino-2,5-dihydroxycyclohexanecarboxylic acids

Abstract Spirooxazolones 3 , obtained by Diels–Alder reaction between oxazolone 1 and dienes 2 , are the key starting materials for the preparation of β-hydroxycyclohexenamino acid derivatives 4 – 6 . The regio- and diastereoselective functionalization of cyclohexyl ring with a second hydroxy group to give the new 1-amino-2,5-dihydroxycyclohexanecarboxylic acids 11 , 19 and the 2,4-dihydroxy derivative 20 was achieved when starting from compounds 4 – 6 . In fact, the iodo-oxazination reaction on compounds 4 , followed by reduction of the iodine atom, led to the dihydroxyamino acids 11 in which the cis relationship exists between the two hydroxy groups. The iodo-lactonization reaction, followed by reduction of the iodine atom, allowed for the formation of the trans dihydroxy derivatives 19 starting from the acids 5 .

Model peptides containing the 3-sulfanyl-norbornene amino acid, a conformationally constrained cysteine analogue effective inducer of 310-helix secondary structures

The properties of the constrained tetrasubstituted 3-sulfanylnorbornene amino acid (NRB), when inserted in Ala-Aib model peptides, were extensively studied. The conformational behaviour of these models was evaluated by theoretical calculations, spectroscopic analyses and by X-ray crystallography. Taken together, our data confirm that both (R,R,R,S)- and (S,S,S,R)-NRB enantiomers possess a strong helicogenic effect when inserted in short Ala-Aib sequences, suggesting that the rigid norbornane core has a positive effect on the ability to stabilize helical secondary structures. This information will be essential for future applications in the rational design of conformationally stable peptides targeted on protein–protein interaction (PPI) surfaces.

Tandem Tetrahydroisoquinoline-4-carboxylic Acid/β-Alanine as a New Construct Able to Induce a Flexible Turn

Tetrahydroisoquinoline-4-carboxylic acid, a constrained β2 -amino acid named β-TIC, was synthesised for the first time in enantiopure form. The biocatalytic route applied herein represents one of the few successful examples of enzymatic resolution of β2 -amino acids. Model tetrapeptides, namely, Fmoc-l-Ala-β-TIC-β-Ala-l-Val-OBn (Fmoc=fluorenylmethyloxycarbonyl, Bn=benzyl), containing both isomers of β-TIC, were prepared. Both computational and NMR spectroscopy studies were performed. A reverse-turn conformation was observed in the case of (R)-β-TIC enantiomer that was obtained in 99 % enantiomeric excess by enzymatic resolution. The β-TIC/β-Ala construct represents the first example of a flexible turn mimetic containing a cyclic and an acyclic β-amino acid. Furthermore, the presence of an aromatic ring of β-TIC could facilitate non-covalent interactions to increase the potential of this scaffold for the preparation of protein-protein interaction modulators.

Semisynthesis of New D-seco-C-nor-Taxane Derivatives Containing a Polyfunctionalized Furanosyl or Cyclopentenyl or Cyclopentyl C-Ring

The synthesis of new D-seco-C-nor-taxane derivatives in which the D-ring has been deleted and the C-ring has been transformed into a new pentatomic ring, i.e., the polyfunctionalized tetrahydrofuranosyl and cyclopentenyl or cyclopentyl ring, was performed starting from baccatin III derivatives. The synthetic strategy adopted took advantage of the oxetane ring opening and disconnection of the C4-C5 bond, followed by an intramolecular condensation. The formation of furanosyl or cyclopentyl rings is strictly dependent on the presence of unprotected or protected oxygen at C-7 in the starting material. The reactions proceeded with good diastereoselectivity with control of the stereochemistry of one or two stereocenters.

Structural elucidation of the Rifaximin Ph. Eur. Impurity H

Rifaximin, a semisynthetic, rifamycin-based non-systemic antibiotic is used in the treatment of acute and chronic gastrointestinal disorders. The aim of this study was the elucidation of the molecular structure of the 802 Dalton impurity which was found in Rifaximin industrial batches and reported with an erroneous structure in European Pharmacopoeia 6.5 (2009) [7] monograph as Rifaximin Impurity H. This impurity was isolated from Rifaximin by preparative HPLC and purified by column chromatography. The molecular structure was evidenced by means of (1)H and (13)C NMR spectroscopy, mass spectrometry and FT-IR.

Synthesis and biological evaluation of new natural phenolic 2E, 4E, 6E octatrienoic esters

In the present study the esterification of the OH groups of resveratrol, caffeic acid, ferulic acid, and β‐sitosterol with an antioxidant polyconjugated fatty acid, (2E,4E,6E)‐octa‐2,4,6‐trienoic acid, was achieved. As the selective esterification of OH groups of natural compounds can affect their biological activity, a selective esterification of resveratrol and caffeic acid was performed by an enzymatic approach. The new resulting compounds were characterized spectroscopically (FT‐IR, NMR mono, and bidimensional techniques); when necessary the experimental data were integrated by quantum chemical calculations. The antioxidant, anti‐inflammatory and proliferative activity was evaluated. The good results encourage the use of these molecules as antioxidant and/or anti‐inflammatory agents in dermocosmetic application.