Stereochemical Assignment of the Pyochelins

Abstract

The synthesis of pyochelin has been much improved but gives four stereoisomers. The stereochemistry of the two naturally occurring pyochelins I and II has been assigned, based on (1) the similarity of the NMR spectra of pyochelin I methyl ester to those for 4-methylpyochelin I methyl ester, whose X-ray structure has been determined and relative stereochemistry assigned; (2) comparison of the rotation of the natural material to that of pyochelin I methyl ester synthesized from IV-methyl-L-cysteine methyl ester, which is not expected to epimerize during the synthesis and thus assigns pyochelin I methyl ester as 4'R,2''R,4"R; and (3) the known facile epimerization at C-2" of 2"-substituted thiazolidine-4-carboxylic acids, which assigns the other (unfavored) naturally occurring isomer, pyochelin II, as 4'R,2"S,4"R. The remaining two synthetic products, neopyochelin I methyl ester and neopyochelin II methyl ester, were assigned the stereochemistry 4'S,2"S,4"R and 4'S,2"R,4"R, respectively, based on (1) the use of IV-methyl-L-cysteine methyl ester in their synthesis, which establishes C-4" as R; (2) the known instability at C-2", which favors neopyochelin II (2"R) over neopyochelin I (2"S); and (3) the requirement of nonidentity with pyochelins I and II, which requires the S configuration at C-4'.Pyochelin (la, Scheme 1) is a unique iron-chelating siderophore isolated from Pseudomonas aeruginosa.2,3 As produced in nature pyochelin exists as a mixture of two interconvertible isomers, pyochelins I and II, whose absolute and relative configurations at the three chiral centers have not previously been assigned. The proposed structure of pyochelin has been corroborated by total synthesis4 and further spectroscopic studies,5 67and three mutasynthetic analogues were recently isolated from a salicylic acid idiotrophic mutant (Salphenotype).6,7 We have found that the free carboxylic acids cannot be isolated in pure form due to their rapid interconversion, but the methyl esters (lb) are more stable configurationally and can be easily purified. We report here the assignment of the absolute stereochemistry of the natural and synthetic isomers as well as the mutasynthetic analogues of pyochelin, based on spectral data and an X-ray crystal structure. We also report a significant improvement in the synthetic approach to the compounds, one that provides four of the eight possible stereoisomers with the ring system of pyochelin.1 ResultsSynthesis of Pyochelin. The previous synthetic approach to the pyochelins (Scheme 1) proceeded in 8% overall yield from salicylonitrile (2), with the critical step

Stereospecificity of the Siderophore Pyochelin Outer Membrane Transporters in Fluorescent Pseudomonads

“…The four other diastereoisomers, NeoPchI/II and ENeoPchI/II, have so far never been isolated from any siderophore-producing strain. In the presence of a metal ion, PchII isomerizes into PchI (24,36), and NeoPchII isomerizes into NeoPchI. This suggests a metal-driven template effect giving a cis H2Љ-H4Љ relationship that is necessary for optimal ferric ion tetracoordination by the siderophore.…”

“…The purification procedure used did not separate the diastereoisomer couples. However, in the presence of a metal ion (Fe 3ϩ and Zn 2ϩ ), PchII isomerizes into PchI (35,36) by epimerization of the C2Љ carbon atom (Fig. 1).…”

Ferripyochelin uptake genes are involved in pyochelin-mediated signalling in Pseudomonas aeruginosa

“…Dha and salicylate were quantified at 256 and 237 nm, respectively. Pyochelin, which exists as a mixture of two interconvertible isomers, pyochelin I and pyochelin II (Rinehart et al, 1995), was quantified at 258 nm and 254 nm, respectively.…”

“…In this type of regulation, also known as siderophore-mediated signalling , perception of the siderophore can occur either at the cell surface, in the periplasm, or in the cytoplasm; examples are provided by pyoverdine-, enterobactin-and pyochelinmediated signalling in the Gram-negative bacterium Pseudomonas aeruginosa. This opportunistic human pathogen produces two siderophores, pyoverdine and pyochelin (Cox, 1980;Cox & Adams, 1985;Meyer & Abdallah, 1978;Rinehart et al, 1995), but can promote iron uptake also with a variety of heterologous siderophores of fungal and bacterial origin (Poole & McKay, 2003). Pyoverdine is perceived at the cell surface, where interaction of the ferripyoverdine complex with the outer-membrane receptor FpvA transmits a signal to the anti-sigma factor FpvR.…”

Stereospecific recognition of pyochelin and enantio-pyochelin by the PchR proteins in fluorescent pseudomonads

“…Pyochelin contains three chiral centres (C49, C20 and C40) and exists in solution as a mixture of two interconvertible stereoisomers ( Fig. 1): pyochelin I (49R, 20R, 40R) and pyochelin II (49R, 20S, 40R) (Ankenbauer et al, 1988;Rinehart et al, 1995) although only pyochelin I has been observed to bind ferric iron (Schlegel et al, 2004(Schlegel et al, , 2006. Upon iron chelation, pyochelin is transported back into the cell via a specific outer-membrane receptor, FptA, in conjunction with an inner-membrane permease, FptX (Ankenbauer & Quan, 1994;Ó Cuiv et al, 2004;Michel et al, 2007).…”