Shape Matters : Characterization of Weak Interactions and Macrocycles by Conformational Analysis

When studying conformational ensembles, it is most challenging to identify and characterize rapidly interconverting individual conformers. A precise description of the structural dynamics, however, rewards viable knowledge on conformation stabilizing forces as well as on the impact of external influences on the conformer composition. This thesis focusses on the solution NMR spectroscopic conformational analysis of flexible molecules with the aim to unveil the impact, strength and geometry of a single weak non-covalent interaction, a halogen bond. In addition, I investigated the impact of the solvent polarity on the conformational distribution of macrocyclic drugs. Weak interactions are difficult to characterize in solution with current techniques. Therefore, I designed a peptidic β-hairpin model system that serves as platform to probe a single weak halogen bond in solution. The presented strategy benefits from the cooperativity of non-covalent forces, from preorganization, and from the entropic advantage of studying an interaction in an intramolecular setting. A weak C−I···O halogen bond was characterized thermodynamically and geometrically using NMR-based variable temperature, NOE, scalar coupling and RDC analyses. Time-averaged NMR parameters were deconvoluted with NAMFIS and by Singular Value Decomposition as implemented into MSpin. Characterization of such a weak interaction (ΔΔG° < 0.9 kJ/mol) in dilute solution is remarkable. Making use of NMR-based ensemble analysis, I further studied the impact of solvent polarity on the conformational distribution of macrocyclic drugs that do not obey Lipinski’s Rule-of-5, yet experience good membrane permeability. I demonstrate experimentally that this class of compounds behaves as molecular chameleons by adjusting their conformation to shield or expose polar functionalities as an adaptation to the surrounding environment. Solution ensembles in D2O and CDCl3 mimicking the plasma/cytosol and cell membrane, respectively, were determined and thus revealed that the flexibility of studied macrocycles facilitates a major difference in size and polarity between different environments.  Overall, this thesis demonstrates the capabilities and precision of solution NMR spectroscopic conformation analysis techniques, and two possible applications of their use for solving scientific challenges of high relevance to medicinal and organic chemistry

Pushing the Limits of Characterising a Weak Halogen Bond in Solution

Detection and characterisation of very weak, non-covalent interactions in solution is inherently challenging. Low affinity, short complex lifetime and a constant battle against entropy brings even the most sensitive spectroscopic methods to their knees. Herein we introduce a strategy for the accurate experimental description of weak chemical forces in solution. Its scope is demonstrated by the detailed geometric and thermodynamic characterisation of the weak halogen bond of a non-fluorinated aryl iodide and an ether oxygen (0.6 kJ mol−1). Our approach makes use of the entropic advantage of studying a weak force intramolecularly, embedded into a cooperatively folding system, and of the combined use of NOE- and RDC-based ensemble analyses to accurately describe the orientation of the donor and acceptor sites. Thermodynamic constants (ΔG, ΔH and ΔS), describing the specific interaction, were derived from variable temperature chemical shift analysis. We present a methodology for the experimental investigation of remarkably weak halogen bonds and other related weak forces in solution, paving the way for their improved understanding and strategic use in chemistry and biology

Conformational Analysis of Rifampicin in Solution

Rifampicin is a macrocyclic drug used to treat bacterial infections.1 With a mass of 823 Da, rifampicin violates the Lipinki’s rule of five (Ro5),2 and therefore would be expected to have poor membrane permeability, and hence to not be applicable as an oral therapeutic agent. Despite this fact, rifampicin has proven to reach its target in a biological system and thus is able to pass multiple cell membranes without major problems. We hypothesize that the permeability of rifampicin may be explained by its molecular flexibility. We have therefore determined the conformational ensembles of rifampicin in aqueous and in chloroform solutions using the NMR Analysis of Molecular Flexibility in Solution (NAMFIS) approach.3 Comparing the ensembles present in environments possessing different polarities, we hypothesized that simultaneous aqueous solubility and membrane permeability of rifampicin may be explained by its ability to adjust its conformation to the molecular environment. In this presentation the ensemble analysis of rifampicin in polar and non-polar media will be disclosed, and the results will be discussed in relation to the above hypothesis on its permeability. We propose that this macrocycle folds into a conformation with its hydrophilic groups being better shielded from the hydrophobic membrane when it crosses a membrane, whereas it makes its polar functions solvent accessible in a polar environment (Fig. 1)

Conformational Analysis of Rifampicin in Solution

Rifampicin is a macrocyclic drug used to treat bacterial infections.1 With a mass of 823 Da, rifampicin violates the Lipinki’s rule of five (Ro5),2 and therefore would be expected to have poor membrane permeability, and hence to not be applicable as an oral therapeutic agent. Despite this fact, rifampicin has proven to reach its target in a biological system and thus is able to pass multiple cell membranes without major problems. We hypothesize that the permeability of rifampicin may be explained by its molecular flexibility. We have therefore determined the conformational ensembles of rifampicin in aqueous and in chloroform solutions using the NMR Analysis of Molecular Flexibility in Solution (NAMFIS) approach.3 Comparing the ensembles present in environments possessing different polarities, we hypothesized that simultaneous aqueous solubility and membrane permeability of rifampicin may be explained by its ability to adjust its conformation to the molecular environment. In this presentation the ensemble analysis of rifampicin in polar and non-polar media will be disclosed, and the results will be discussed in relation to the above hypothesis on its permeability. We propose that this macrocycle folds into a conformation with its hydrophilic groups being better shielded from the hydrophobic membrane when it crosses a membrane, whereas it makes its polar functions solvent accessible in a polar environment (Fig. 1)

Conformational Analysis of Rifampicin in Solution

Rifampicin is a macrocyclic drug used to treat bacterial infections.1 With a mass of 823 Da, rifampicin violates the Lipinki’s rule of five (Ro5),2 and therefore would be expected to have poor membrane permeability, and hence to not be applicable as an oral therapeutic agent. Despite this fact, rifampicin has proven to reach its target in a biological system and thus is able to pass multiple cell membranes without major problems. We hypothesize that the permeability of rifampicin may be explained by its molecular flexibility. We have therefore determined the conformational ensembles of rifampicin in aqueous and in chloroform solutions using the NMR Analysis of Molecular Flexibility in Solution (NAMFIS) approach.3 Comparing the ensembles present in environments possessing different polarities, we hypothesized that simultaneous aqueous solubility and membrane permeability of rifampicin may be explained by its ability to adjust its conformation to the molecular environment. In this presentation the ensemble analysis of rifampicin in polar and non-polar media will be disclosed, and the results will be discussed in relation to the above hypothesis on its permeability. We propose that this macrocycle folds into a conformation with its hydrophilic groups being better shielded from the hydrophobic membrane when it crosses a membrane, whereas it makes its polar functions solvent accessible in a polar environment (Fig. 1)

Employing complementary spectroscopies to study the conformations of an epimeric pair of side-chain stapled peptides in aqueous solution

Understanding the conformational preferences of free ligands in solution is often necessary to rationalize structure–activity relationships in drug discovery. Herein, we examine the conformational behavior of an epimeric pair of side-chain stapled peptides that inhibit the FAD dependent amine oxidase lysine specific demethylase 1 (LSD1). The peptides differ only at a single stereocenter, but display a major difference in binding affinity. Their Raman optical activity (ROA) spectra are most likely dominated by the C-terminus, obscuring the analysis of the epimeric macrocycle. By employing NMR spectroscopy, we show a difference in conformational behavior between the two compounds and that the LSD1 bound conformation of the most potent compound is present to a measurable extent in aqueous solution. In addition, we illustrate that Molecular Dynamics (MD) simulations produce ensembles that include the most important solution conformations, but that it remains problematic to identify relevant conformations with no a priori knowledge from the large conformational pool. Furthermore, this work highlights the importance of understanding the scope and limitations of the available techniques for conducting conformational analyses. It also emphasizes the importance of conformational selection of a flexible ligand in molecular recognition.J. Bogaerts and Y. Atilaw share first authorship</p