The increasing problem of multidrug resistance (MDR) in bacteria calls for discovery of new molecules and diagnostic methodologies that are effective against a wide range of microbial pathogens. We have studied the role of alexidine dihydrochloride (alex) as a bioaffinity ligand against lipopolysaccharide (LPS), a pathogen-associated surface marker universally present on all Gram-negative bacteria. While the activity of alex against bacteria is biologically known, little information exists on its mechanism of action or binding stoichiometry. We have used nuclear magnetic resonance (NMR), fluorescence, and surface plasmon resonance (SPR) spectroscopies to probe the binding characteristics of alex and LPS molecules. Our results indicate that LPS:alex stoichiometry lies between 1:2 and 1:4 and has a dissociation constant ( K) of 38 μM that is mediated through electrostatic interactions between the negatively charged phosphate groups present on LPS and the positively charged guanidinium groups present in alex. Further, molecular dynamics (MD) simulations performed to determine the conformational interaction between the two molecules show good agreement with the experimental results, which substantiate the potential of alex molecule for LPS neutralization and hence, development of efficient in vitro diagnostic assays.
Nuclear magnetic resonance (NMR) is a powerful tool for structural and dynamical studies of molecules. Although widely applicable, the search for novel spectral editing methods that facilitate spectral assignment of peaks in high-resolution NMR is highly desirable. Earlier, the sensitivity of lifetime of spin states (spin-lattice relaxation time, T1) and coherences (spin-spin relaxation time, T2) to the immediate environment was utilized for spectral editing in solution NMR. Long-lived states (LLS) and coherences (LLCs) were recently uncovered to have longer and more domain sensitive lifetime than other type of states and coherences. Herein, this longevity and increased sensitivity of LLS and LLC lifetime is utilized for more enhanced dispersion in relaxation editing in NMR. The generality of the method as a powerful tool in spectral editing is confirmed with molecules containing a mixture of strongly and weakly coupled spin systems and finally with metabolomic mixture. Extension to insensitive nuclei enhanced by polarization transfer (INEPT), correlation spectroscopy (COSY), and heteronuclear single quantum coherence (HSQC) are also demonstrated.
In this paper, we presented a new design strategy for a peptide-based chiral supramolecular assembly. A series of aryl linked peptides 1a-1f were designed and synthesized. The bis-urea peptides 1a-1c self-assembled into a helical supramolecular arrangement resembling Trp zipper (Trpzip) structures present in proteins. Interestingly, a dihydrogenphosphate anion, upon binding to the assembly, could invert the chirality of the supramolecular assembly which could be reverted to the original by the addition of water. This chiroptical behavior can be repeated several times. Microscopy analysis showed that the supramolecular helices were assembled to form spheres. In addition to that, we also found that the handedness of supramolecular chirality is dependent on the position of Trp residues on the aromatic scaffold. Both left and right handed helical supramolecular arrangements were obtained by placing l-Trp residues at different positions on the aromatic core. The unprecedented Trpzip in these designed small peptidomimetics will stimulate more work in the area of peptide-based assemblies.
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