Tunable recognition of the steroid α-face by adjacent π-electron density

Abstract

We report a previously unknown recognition motif between the α-face of the steroid hydrocarbon backbone and π-electron-rich aromatic substrates. Our study is based on a systematic and comparative analysis of the solid-state complexation of four steroids with 24 aromatic molecules. By using the solid state as a medium for complexation, we circumvent solubility and solvent competition problems that are inherent to the liquid phase. Characterization is performed using powder and single crystal X-ray diffraction, infrared solid-state spectroscopy and is complemented by a comprehensive cocrystal structure prediction methodology that surpasses earlier computational approaches in terms of realism and complexity. Our combined experimental and theoretical approach reveals that the α⋯π stacking is of electrostatic origin and is highly dependent on the steroid backbone's unsaturated and conjugated character. We demonstrate that the α⋯π stacking interaction can drive the assembly of molecules, in particular progesterone, into solid-state complexes without the need for additional strong interactions. It results in a marked difference in the solid-state complexation propensities of different steroids with aromatic molecules, suggesting a strong dependence of the steroid-binding affinity and even physicochemical properties on the steroid's A-ring structure. Hence, the hydrocarbon part of the steroid is a potentially important variable in structure-activity relationships for establishing the binding and signaling properties of steroids, and in the manufacture of pharmaceutical cocrystals.steroids | molecular recognition | mechanosynthesis | crystal structure prediction A wide variety of tools, including site-directed mutagenesis (1), binding and inhibition screening (2-4), computational modeling (5), and protein crystallography (6, 7) are commonly used in studying the interaction of biologically important molecules, such as steroids, with their respective receptor binding domains (8). Molecular recognition can also be effectively probed using the considerably simpler and inexpensive methodology of forming crystalline molecular complexes (9) (multicomponent crystals, also known as cocrystals). Similarly to binding on synthetic model receptors (10, 11), solid-state complexation with small molecules (cocrystallization) offers the possibility to separately screen and deconvolute a far larger space of molecular recognition motifs (12) that collectively account for the biological activity. However, formation of solid-state complexes from solution is not a reliable measure of molecular affinity due to the vexatious problems of solubility and solvent competition. Mechanosynthesis, in the form of liquid-assisted grinding (LAG) (13), avoids these adverse effects and offers the possibility to systematically explore molecular recognition in the solid state (14,15).The rationalization of statistical data from structure-activity binding (16) or cocrystallization studies (17) is often based on qualitative and intuitive arguments and an establ...

Shape and Polarity in Co-crystal Formation: Database Analysis and Experimental Validation

“…Specifically, liquid-assisted grinding experiments involving four steroids as target molecules and 24 small planar molecules as co-crystal formers showed surprising results (Figure 5.7). 29 The shapes and polarities of the steroids progesterone, pregnenolone, b-estradiol and estrone are similar (Table 5.4). Based on descriptor cut-offs, all four steroids would be expected to cocrystallise with at least 16 of the 24 co-formers.…”

“…While puzzling, these differences indicate that specific intermolecular interactions are at work. 29 The hydrogen atoms pointing outwards from the a-face of progesterone ( Figure 5.8(a), below the molecule) generate a flat, positively polarised surface. This surface interacts favourably with the negatively polarised p-electron cloud of aromatic compounds, leading to the formation of co-crystals involving singly or doubly stacked steroid and coformer molecules ( Figure 5.8(b) and (c)).…”

Preparation of Progesterone Co-Crystals Based on Crystal Engineering Strategies

“…The formation of co-crystals can improve properties of pharmaceuticals [9]. Karamertzanis has investigated the α···π stacking interaction of different steroids with aromatic molecules and has indicated that the steroid-binding affinity depends on the steroid backbone and especially the A-ring structure [10] (Figure 1). However, the solid-state complexation of PROG and aromatic molecules has never been systematically studied.…”

“…Although several PROG co-crystals have been reported in past years, a detailed and reliable structure-activity correlation is still yet to be established between the co-crystal structure and its mechanical properties [10]. As part of an ongoing study in crystal engineering, we investigated a series of co-crystals formed by PROG and 2-chloro-4-nitroaniline (CNA), PROG and 2,5-dihydroxybenzoic acid (DHB), and PROG and 4,4′-biphenol (DOD), see in Scheme 1.…”

Synthesis and antifungal activity of functionalized 2,3-spirostane isomers

“…Further, the reaction occurs from the α-face of steroid, due to β-face steric hindrance, and therefore the only isolated product is the keto ester 5 (Scheme 1). The preference for α-face substitutions in steroid reactions has been well documented [34,35]. We further used this knowledge to perform the stereoselective sodium borohydride reduction of keto ester 5, a reaction that produced only the 3β-hydroxy isomer 7 in 90% isolated yield (Scheme 1).…”