Binding and Action of Triphenylphosphonium Analog of Chloramphenicol upon the Bacterial Ribosome

Abstract

Chloramphenicol (CHL) is a ribosome-targeting antibiotic that binds to the peptidyl transferase center (PTC) of the bacterial ribosome and inhibits peptide bond formation. As an approach for modifying and potentially improving the properties of this inhibitor, we explored ribosome binding and inhibitory properties of a semi-synthetic triphenylphosphonium analog of CHL—CAM-C4-TPP. Our data demonstrate that this compound exhibits a ~5-fold stronger affinity for the bacterial ribosome and higher potency as an in vitro protein synthesis inhibitor compared to CHL. The X-ray crystal structure of the Thermus thermophilus 70S ribosome in complex with CAM-C4-TPP reveals that, while its amphenicol moiety binds at the PTC in a fashion identical to CHL, the C4-TPP tail adopts an extended propeller-like conformation within the ribosome exit tunnel where it establishes multiple hydrophobic Van der Waals interactions with the rRNA. The synthesized compound represents a promising chemical scaffold for further development by medicinal chemists because it simultaneously targets the two key functional centers of the bacterial ribosome—PTC and peptide exit tunnel.

Triphenilphosphonium Analogs of Chloramphenicol as Dual-Acting Antimicrobial and Antiproliferating Agents

“…We have recently synthesized and extensively characterized the first semi-synthetic TPP analogs of CHL, CAM-C4-TPP [ 51 ]. In the current study, we continued our research on the TPP analogs of CHL, synthesized two more of them, CAM-C10-TPP and CAM-C14-TPP, and explored their inhibitory properties for the potential development of novel antimicrobials, as well as antiproliferative agents.…”

“…We chose TPP as such a group because its positive charge, delocalized over the relatively large hydrophobic surface of the benzene rings, can provide non-specific interactions with negatively charged phosphates of the 23S rRNA, and its three phenyl rings may be available for stacking with nucleobases. In the case of CAM-C4-TPP [ 51 ], we showed how this analog binds to the bacterial ribosome and inhibits bacterial protein synthesis, both in vitro and in vivo. Moreover, according to our rational design hypothesis, the TPP group in these compounds should allow CAM-Cn-TPPs into bacterial cells, since the TPP itself is known to be a membrane-penetrating cation [ 13 ].…”

“…As for the interactions with the ribosome, we expected that linkers of variable lengths connecting the two terminal parts of these compounds (CAM and TPP) would ensure the optimal binding of CAM-Cn-TPPs to the ribosome and enable similar non-specific interactions with rRNA nucleotides at different depths of the NPET, as observed in the X-ray crystal structure of the Thermus thermophilus 70S ribosome in the complex with the CAM-C4-TPP [ 51 ]. The choice of linker length was also based on the data on the inhibition of bacterial growth by alkyl-TPPs, where it was shown that the toxic effect on different bacterial species increases with increasing lipophilicity, and this effect is related to the different permeability of bacterial coats for alkyl-TPPs [ 17 ].…”

“…The synthesis of CAM-C10-TPP and CAM-C14-TPP was performed by the acylation of chloramphenicol amine (CAM) with carboxyl derivatives of TPP using succinimide ester ( Figure 1 ), which is similar to the synthesis of CAM-C4-TPP [ 51 ]. The chemical structures of the obtained CAM-Cn-TPP molecules were confirmed by mass-spectrometric analysis, as well as by 1H-, 13C-, and 31P-NMR.…”

“…In the current study, we continued our research on the synthesis and exploration of semi-synthetic TPP analogs of CHL [ 51 ], i.e., CAM-Cn-TPPs, with the goal of obtaining a new group of CHL derivatives with potentially improved properties. To this end, the dichloromethyl group of the parent CHL compound was replaced with alkyl(triphenyl)phosphonium residues, resulting in CAM-Cn-TPP molecules ( Figure 1 ).…”