Five concomitant polymorphs of a green fluorescent protein chromophore (GFPc) analogue: understanding variations in photoluminescence with π-stacking interactions.

The synthetically modified green fluorescent protein chromophore analogue 3,4,5-trimethoxybenzylidene imidazolinone (1) yielded five polymorphs (I, II, III, IV, V) concomitantly irrespective of the solvent used for crystallization. The pentamorphic modification of 1 is solely due to the interplay of iso-energetic weak intermolecular interactions in molecular associations as well as the conformational flexibility offered by a C-C single bond, which connects the electron-deficient moiety imidazolinone with the electron-rich trimethoxybenzylidene group. A common structural feature observed in all the polymorphs is the formation of a `zero-dimensional' centrosymmetric dimeric unit through a short and linear C-H...O hydrogen bond engaging phenyl C-H and imidazolinone carbonyl oxygen. However, the networking of these dimeric units showed a subtle difference in all the polymorphs. The 2D isostructurality was observed between polymorphs I, II and III, while the other two polymorphs IV and V revealed only `zero-dimensional' isostructurality. The different fluorescence emissions of Form I (blue) and Forms II to V (yellow) were attributed to the differences in π-stacking interactions. It shows that one can modulate the photophysical properties of these smart materials by slightly altering their crystal structure. Such an approach will aid in developing new multi-colour organic fluorescent materials of varying crystal structures for live-cell imaging and fluorescent sensing applications.

Mechanical-Bending-Induced Fluorescence Enhancement in Plastically Flexible Crystals of a GFP Chromophore Analogue.

Single crystals of optoelectronic materials that respond to external stimuli, such as mechanical, light, or heat, are immensely attractive for next generation smart materials. Here we report single crystals of a green fluorescent protein (GFP) chromophore analogue with irreversible mechanical bending and associated unusual enhancement of the fluorescence, which is attributed to the strained molecular packing in the perturbed region. Soft crystalline materials with such fluorescence intensity modulations occurring in response to mechanical stimuli under ambient pressure conditions will have potential implications for the design of technologically relevant tunable fluorescent materials.

Preferential Enrichment of DL-Leucine Using Cocrystal Formation With Oxalic Acid Under Nonequilibrium Crystallization Conditions.

By utilizing the preferential enrichment (PE) technique, we achieved an improved enantiomeric resolution of DL-leucine (Leu) using a 1:1 cocrystal (DL-) of DL-Leu and oxalic acid. The crystal structure analysis of DL- indicated the occurrence of a novel type of phase transition and subsequent preferential redissolution of one enantiomer from the resulting crystals into solution.

Highly efficient chiral resolution of DL-arginine by cocrystal formation followed by recrystallization under preferential-enrichment conditions.

An excellent chiral symmetry-breaking spontaneous enantiomeric resolution phenomenon, denoted preferential enrichment, was observed on recrystallization of the 1:1 cocrystal of dl-arginine and fumaric acid, which is classified as a racemic compound crystal with a high eutectic ee value (>95 %), under non-equilibrium crystallization conditions. On the basis of temperature-controlled video microscopy and in situ time-resolved solid-state (13) C NMR spectroscopic studies on the crystallization process, a new mechanism of phase transition that can induce preferential enrichment is proposed.

Visible-light-induced photodimerization of a photoactive yellow protein (PYP) chromophore model in a single crystal.

The chromophore of the photoactive yellow protein (PYP), the photoreceptor in the photomotility of the bacterium Halorhodospira halophila, is a deprotonated para-coumaric thioester linked to the side residue of a cysteine residue. The photophysics of the PYP chromophore is conveniently modeled with para-hydroxycinnamic thiophenyl esters. Herein, we report the first direct evidence, obtained with X-ray diffraction, of photodimerization of a para-hydroxycinnamic thiophenyl ester in single crystalline state. This result represents the first direct observation of [2+2] dimerization of a model PYP chromophore, and demonstrates that even very weak light in the visible region is capable of inducing parallel radical reactions in PYP from the excited state of the chromophore, in addition to the main reaction pathway (trans-cis isomerization). This PYP model system adds an interesting example to the known solid-state photodimerizations, because unlike the anhydrous crystal (which is not capable of sustaining the stress and disintegrates in the course of photodimerization), a single water molecule "dilutes" the structure to the extent sufficient for single-crystal-to-single-crystal reaction.

Occurrence of spontaneous resolution of ketoprofen with a racemic crystal structure by simple crystallization under nonequilibrium preferential enrichment conditions.

Nearly racemic ketoprofen, which satisfies the requirements for the occurrence of preferential enrichment, was spontaneously resolved into the two enantiomers by simple crystallization under nonequilibrium conditions using high concentrations.