Spectroscopic, potentiometric and theoretical studies of novel imino-phenolate chelators for Fe(III)

Advances in the Synthesis of Enterobactin, Artificial Analogues, and Enterobactin-Derived Antimicrobial Drug Conjugates and Imaging Tools for Infection Diagnosis

Iron is an essential growth factor for bacteria, but although highly abundant in nature, its bioavailability during infection in the human host or the environment is limited. Therefore, bacteria produce and secrete siderophores to ensure their supply of iron. The triscatecholate siderophore enterobactin and its glycosylated derivatives, the salmochelins, play a crucial role for iron acquisition in several bacteria. As these compounds can serve as carrier molecules for the design of antimicrobial siderophore drug conjugates as well as siderophore-derived tool compounds for the detection of infections with bacteria, their synthesis and the design of artificial analogues is of interest. In this review, we give an overview on the synthesis of enterobactin, biomimetic as well as totally artificial analogues, and related drug-conjugates covering up to 12/2021.

1 Introduction

2 Antibiotic Crisis and Sideromycins as Natural Templates for New Antimicrobial Drugs

3 Biosynthesis of Enterobactin, Salmochelins, and Microcins

4 Total Synthesis of Enterobactin and Salmochelins

5 Chemoenzymatic Semi-synthesis of Salmochelins and Microcin E492m Derivatives

6 Synthesis of Biomimetic Enterobactin Derivatives with Natural Tris-lactone Backbone

7 Synthesis of Artificial Enterobactin Derivatives without Tris-lactone Backbone

8 Conclusions

Cation-induced ring-opening and oxidation reaction of photoreluctant spirooxazine-quinolizinium conjugates

Two new spiroindolinonaphthoxazine derivatives with an electron-accepting styrylquinolizinium or styrylcoralyne unit, respectively, were synthesized, and the influence of such an arylvinyl substituent on the chemical and photochemical properties of the compounds was investigated. Specifically, these spirooxazines turned out to be resistant towards the photoinduced merocyanine formation, and the irradiation with light mainly led to photodegradation of the substrates. However, it was shown by colorimetric and fluorimetric screening assays as well as by detailed NMR spectroscopic and mass spectrometric studies that the addition of particular metal ions (Cu2+, Fe3+, and to a certain extent Hg2+) initially induced a ring-opening reaction that was irreversibly followed by a fast ring closure-deprotonation-oxidation sequence to give styryl-substituted naphthoxazole derivatives as the products quantitatively. For the quinolizinium-substituted spirooxazine derivative, the formation of the respective oxidation product caused the development of a broad absorption band between 425 nm and 500 nm and a new emission band at λfl = 628 nm, so that it may be employed as a selective chemosensor or chemodosimeter for the colorimetric and fluorimetric detection of Cu2+ and Fe3+.

Bacterial Cellulose Supported Gold Nanoparticles with Excellent Catalytic Properties

Amidoxime surface functionalized bacterial cellulose (AOBC) has been successfully prepared by a simple two-step method without obviously changing the morphology of bacterial cellulose. AOBC has been used as the reducing agent and carrier for the synthesis of gold nanoparticles (AuNPs) that distributed homogeneously on bacterial cellulose surface. Higher content in amidoxime groups in AOBC is beneficial for the synthesis of AuNPs with smaller and more uniform size. The AuNPs/AOBC nanohybrids have excellent catalytic activity for reduction of 4-nitrophenol (4-NP) by using NaBH4. It was found that catalytic activity of AuNPs/AOBC first increases with increasing NaBH4 concentration and temperature, and then leveled off at NaBH4 concentration above 238 mM and temperature above 50 °C. Moreover, AuNPs with smaller size have higher catalytic activity. The highest apparent turnover frequency of AuNPs/AOBC is 1190 h(-1). The high catalytic activity is due to the high affinity of 4-NP with AuNPs/AOBC and the reduced product 4-aminophenol has good solubility in water in the presence of AuNPs/AOBC. The catalytic stability of the AuNPs/AOBC was estimated by filling a fluid column contained AuNPs/AOBC and used for continuously catalysis of the reduction of 4-NP by using NaBH4. The column works well without detection of 4-NP in the eluent after running for more than two months, and it is still running. This work provides an excellent catalyst based on bacterial cellulose stabilized AuNPs and has promising applications in industry.

TAME5OX, abiotic siderophore analogue to enterobactin involving 8-hydroxyquinoline subunits: thermodynamic and photophysical studies

The synthesis, thermodynamic and photophysical properties of trivalent metal complexes of biomimetic nonadentate analogue, 5,5'-(2-(((8-hydroxyquinolin-5-yl)methylamino)methyl)-2-methylpropane-1,3-diyl)bis(azanediyl)bis(methylene)diquinolin-8-ol (TAME5OX), have been described. Combination of absorption and emission spectrophotometry, potentiometry, electrospray mass spectrometry, IR, and theoretical investigation were used to fully characterize metal (Fe(+3), Al(+3) and Cr(+3)) chelates of TAME5OX. In solution, TAME5OX forms protonated complexes [M(H3L)](3+) below pH 3.4, which consecutively deprotonates through one to three-proton processes with rise of pH. The formation constants (Logβ11n) of neutral complexes formed at or above physiological pH, have been determined to be 30.18, 23.27 and 22.02 with pM values of 31.16, 18.07 and 18.12 for Fe(+3), Al(+3) and Cr(+3) ions, respectively, calculated at pH 7.4, indicating TAME5OX is a powerful among synthetic metal chelator. The results clearly demonstrate that the ligand in a tripodal orchestration firmly binds these ions over wide pH range and forms distorted octahedral complexes. The binding and the coordination event could be monitored from absorption and fluorescence spectroscopy. The high thermodynamic stability in water at physiological pH of ferric complex of TAME5OX indicates that these complexes are resistant to hydrolysis and therefore are well suited for the development of device for applications as probes. The ligand displays high sensitive fluorescence enhancement to Al(3+) at pH 7.4, in water. Moreover, TAME5OX can distinguish Al(3+) from Fe(3+) and Cr(3+) via two different sensing mechanisms: photoinduced electron transfer (PET) for Al(3+) and internal charge transfer (ICT) for Fe(3+) and Cr(3+). Density functional theory was employed for optimization and evaluation of vibrational modes, NBO analysis, excitation and emission properties of the different species of metal complexes observed by solution studies.