Comprehensive Structural Analysis of the Bitter Components in Beer by the HPLC-Assisted Crystalline Sponge Method

Trans-iso-α-acid is one of the main contributors to the bitter taste of fresh beer and is known to transform into various derivatives during beer aging. However, structural characterization of the derivatives has been a challenging task because of the formation of too many components. Herein, we report that most of the transformation products of trans-iso-α-acid, isolated in this study in only small quantities by HPLC, can be structurally analyzed with the crystalline sponge method. Thirteen compounds, including eight that were previously unreported, have been successfully isolated and analyzed with complete assignment of their absolute configuration. This provides an improved understanding of the chemical transformations that occur during beer aging.

Crystalline sponge affinity screening: A fast tool for soaking condition optimization without the need of X-ray diffraction analysis

Structural elucidation of small molecules only available in low quantity (nanogram) is one of the big advantages of the crystalline sponge method. The optimization of various soaking parameters is crucial for effective analyte absorption and repetitive positioning in the pores of the crystal. Time-consuming X-ray diffraction measurements are necessary for data collection and confirmation of successful guest inclusion. In this work, we report a screening method to select optimal soaking conditions without the need of single-crystal X-ray diffraction analysis for individual compounds and mixtures. 14 substances were chosen as test compounds. Parallel guest soaking of individual compounds and mixtures was conducted using various soaking conditions. After evaporation of solvent, excessive material was removed, and guest molecules released through dissolution of the framework. Liquid chromatography-tandem mass spectrometry allowed the estimation of analyte trapped in the pores and the selection of optimal soaking condition dependent on the highest amount of analyte to crystal size (affinity factor). The tool allowed subsequent crystallographic analysis of ten compounds with minimal experiment time. Additionally, a study to examine the lower limit of detection of the crystalline sponge method was conducted. Determination of two target analytes was possible using only 5 ng of sample. Our study shows the potential of an affinity screening to prioritize soaking parameters by estimation of the guest concentration in a single crystal for one or multiple target compounds within a short period of time.

Crystalline Sponge Method: X-ray Structure Analysis of Small Molecules by Post-Orientation within Porous Crystals-Principle and Proof-of-Concept Studies

This Review discusses, along with the historical background, the principles as well as proof-of-concept studies of the crystalline sponge (CS) method, a new single-crystal X-ray diffraction (SCXRD) method for the analysis of the structures of small molecules without sample crystallization. The method uses single-crystalline porous coordination networks (crystalline sponges) that can absorb small guest molecules within their pores. The absorbed guest molecules are ordered in the pores through molecular recognition and become observable by conventional SCXRD analysis. The complex {[(ZnI₂ )₃ (tpt)₂ ]⋅x(solvent)}_n (tpt=tris(4-pyridyl)-1,3,5-triazine) was first proposed as a crystalline sponge and has been most generally used. Crystalline sponges developed later are also discussed here. The principle of the CS method can be described as "post-crystallization" of the absorbed guest, whose ordering is templated by the pre-latticed cavities. The method has been widely applied to synthetic chemistry as well as natural product studies, for which proof-of-concept examples will be shown here.

An Enantioselective e-Nose: An Array of Nanoporous Homochiral MOF Films for Stereospecific Sensing of Chiral Odors

Chirality is essential in nature and often pivotal for biological information transfer, for example, via odor messenger molecules. While the human nose can distinguish the enantiomers of many chiral odors, the technical realization by an artificial sensor or an electronic nose, e-nose, remains a challenge. Herein, we present an array of six sensors coated with nanoporous metal-organic framework (MOF) films of different homochiral and achiral structures, working as an enantioselective e-nose. While the achiral-MOF-film sensors show identical responses for both isomers of one chiral odor molecule, the responses of the homochiral MOF films differ for different enantiomers. By machine learning algorithms, the combined array data allow the stereoselective identification of all compounds, here tested for five pairs of chiral odor molecules. We foresee the chiral-MOF-e-nose, able to enantioselectively detect and discriminate chiral odors, to be a powerful approach towards advanced odor sensing.