Higher Energy Conformers Research Articles

Conformational cycle of a protease-containing ABC transporter in lipid nanodiscs reveals the mechanism of cargo-protein coupling

Protease-containing ABC transporters (PCATs) couple the energy of ATP hydrolysis to the processing and export of diverse cargo proteins across cell membranes to mediate antimicrobial resistance and quorum sensing. Here, we combine biochemical analysis, single particle cryoEM, and DEER spectroscopy in lipid bilayers along with computational analysis to illuminate the structural and energetic underpinnings of coupled cargo protein export. Our integrated investigation uncovers competitive interplay between nucleotides and cargo protein binding that ensures the latter’s orderly processing and subsequent transport. The energetics of cryoEM structures in lipid bilayers are congruent with the inferred mechanism from ATP turnover analysis and reveal a snapshot of a high-energy outward-facing conformation that provides an exit pathway into the lipid bilayer and/or the extracellular side. DEER investigation of the core ABC transporter suggests evolutionary tuning of the energetic landscape to fulfill the function of substrate processing prior to export.

AlphaFold predictions of fold-switched conformations are driven by structure memorization

Recent work suggests that AlphaFold (AF)–a deep learning-based model that can accurately infer protein structure from sequence–may discern important features of folded protein energy landscapes, defined by the diversity and frequency of different conformations in the folded state. Here, we test the limits of its predictive power on fold-switching proteins, which assume two structures with regions of distinct secondary and/or tertiary structure. We find that (1) AF is a weak predictor of fold switching and (2) some of its successes result from memorization of training-set structures rather than learned protein energetics. Combining >280,000 models from several implementations of AF2 and AF3, a 35% success rate was achieved for fold switchers likely in AF’s training sets. AF2’s confidence metrics selected against models consistent with experimentally determined fold-switching structures and failed to discriminate between low and high energy conformations. Further, AF captured only one out of seven experimentally confirmed fold switchers outside of its training sets despite extensive sampling of an additional ~280,000 models. Several observations indicate that AF2 has memorized structural information during training, and AF3 misassigns coevolutionary restraints. These limitations constrain the scope of successful predictions, highlighting the need for physically based methods that readily predict multiple protein conformations.

Dataset for quantum-mechanical exploration of conformers and solvent effects in large drug-like molecules

We here introduce the Aquamarine (AQM) dataset, an extensive quantum-mechanical (QM) dataset that contains the structural and electronic information of 59,783 low-and high-energy conformers of 1,653 molecules with a total number of atoms ranging from 2 to 92 (mean: 50.9), and containing up to 54 (mean: 28.2) non-hydrogen atoms. To gain insights into the solvent effects as well as collective dispersion interactions for drug-like molecules, we have performed QM calculations supplemented with a treatment of many-body dispersion (MBD) interactions of structures and properties in the gas phase and implicit water. Thus, AQM contains over 40 global and local physicochemical properties (including ground-state and response properties) per conformer computed at the tightly converged PBE0+MBD level of theory for gas-phase molecules, whereas PBE0+MBD with the modified Poisson-Boltzmann (MPB) model of water was used for solvated molecules. By addressing both molecule-solvent and dispersion interactions, AQM dataset can serve as a challenging benchmark for state-of-the-art machine learning methods for property modeling and de novo generation of large (solvated) molecules with pharmaceutical and biological relevance.

An Eutectic Mixture in the Tetrabutylammonium Bromide-Octanol System: Macroscopic and Microscopic Points of View.

An eutectic mixture of tetrabutylammonium bromide and octanol in the molar ratio 1-10 exhibited a melting point of -17 °C. This system was investigated by means of infrared spectroscopy, in the liquid and in the solid state. Classical molecular dynamics was performed to study the fine details of the hydrogen bond interactions established in the mixture. Both octanol and the mixtures displayed an almost featureless far-infrared spectrum in the liquid state but it becomes highly structured in the solid phase. DFT calculations suggest that new vibrational modes appearing in the mixture at low temperatures may be related to the population of the higher energy conformers of the alcohol. Mid-infrared spectroscopy measurements evidenced no shift of the CH stretching bands in the mixture compared to the starting materials, while the OH stretching are blue shifted by a few cm-1. Consistently, molecular dynamics provides a picture of the mixture in which part of the hydrogen bonds (HB) of pure octanol is replaced by weaker HB formed with the Br anion. Due to these interactions the ionic couple becomes more separated. In agreement with this model, the lengths of all HB are much larger than those observed in mixtures containing acids reported in previous studies.

Anharmonic rotational and vibrational spectroscopic constants of NH2CH2OH

The recent experimental identification of aminomethanol (NH2CH2OH) and its synthesis under simulated interstellar conditions suggests that it may be formed, and thus observable, in the interstellar medium. To aid in its possible detection, this work presents high-level theoretical, anharmonic vibrational and rotational spectroscopic data for the three lowest-energy conformers of NH2CH2OH. All three conformers fall within 0.80 kcal mol−1 of each other at the CCSD(T)-F12b/cc-pCVTZ-F12 level, with an anharmonic zero-point vibrational energy correction. Additionally, all three conformers are shown to have multiple vibrational frequencies with intensities greater than 100 km mol−1. The most notable of these are the symmetric O-C-N stretch of the lower-energy C1 conformer at 976.4 cm−1, the C-O stretch of the Cs conformer at 964.9 cm−1, and the antisymmetric O-C-N stretch of the higher-energy C1 conformer at 1099.0 cm−1. While much of the vibrational spectra of the three overlap, these features should help to separate them. Finally, all three conformers have non-zero dipole moments on the order of 1 D, suggesting that they may be observable by rotational spectroscopy, as well. In all cases, the computational data provided herein will help to facilitate future experimental and observational studies on this key molecule in the formation of extraterrestrial amino acids.

Rotational spectrum, structure, and quadrupole coupling of cyclopropylchloromethyldifluorosilane.

Cyclopropylchloromethyldifluorosilane, c-C3H5SiF2CH2Cl, has been synthesized, and its rotational spectrum has been recorded by chirped-pulse Fourier transform microwave spectroscopy. The spectral analysis of several isotopologues indicates the presence of two distinct conformations in the free-jet expansion, which are interconvertible through a rotation of the chloromethyl group. A partial substitution structure is presented for the lower energy conformation and is compared to the equilibrium structure obtained from quantum chemical calculations. Additionally, the presence of the chlorine nucleus leads to the rotational transitions splitting into multiple hyperfine components and χaa, a measure of the electric field gradient along the a axis, is unusually small at merely +0.1393(73) MHz. Various common abinitio and density functional theory methods fail to predict good quadrupole coupling constants (in the principal axis system) that adequately reproduce the observed hyperfine splitting, although diagonalizing the quadrupole coupling tensor from the principal axis system into a nucleus-centered axis system reveals that, overall, these methods calculate reasonably the electric field gradient about the chlorine nucleus. Finally, a total of nine electric dipole forbidden, quadrupole allowed transitions are observed in the rotational spectra of the parent species of the higher energy conformation and the 37Cl isotopologue of the lower energy conformation. These include those of x-type (no change in parity of Ka or Kc), which, to our knowledge, is the first time such transitions have been observed in a chlorine-containing molecule.

Ligand-induced protein transition state stabilization switches the binding pathway from conformational selection to induced fit

Protein-ligand complex formation is fundamental to biological function. A central question is whether proteins spontaneously adopt binding-competent conformations to which ligands bind conformational selection (CS) or whether ligands induce the binding-competent conformation induced fit (IF). Here, we resolve the CS and IF binding pathways by characterizing protein conformational dynamics over a wide range of ligand concentrations using NMR relaxation dispersion. We determined the relative flux through the two pathways using a four-state binding model that includes both CS and IF. Experiments conducted without ligand show that galectin-3 exchanges between the ground-state conformation and a high-energy conformation similar to the ligand-bound conformation, demonstrating that CS is a plausible pathway. Near-identical crystal structures of the apo and ligand-bound states suggest that the high-energy conformation in solution corresponds to the apo crystal structure. Stepwise additions of the ligand lactose induce progressive changes in the relaxation dispersions that we fit collectively to the four-state model, yielding all microscopic rate constants and binding affinities. The ligand affinity is higher for the bound-like conformation than for the ground state, as expected for CS. Nonetheless, the IF pathway contributes greater than 70% of the total flux even at low ligand concentrations. The higher flux through the IF pathway is explained by considerably higher rates of exchange between the two protein conformations in the ligand-associated state. Thus, the ligand acts to decrease the activation barrier between protein conformations in a manner reciprocal to enzymatic transition-state stabilization of reactions involving ligand transformation.

Does Inter-Residue Hydrogen Bonding in β-(1→4)-Linked Disaccharides Influence Linkage Conformation in Aqueous Solution?

Two disaccharides, methyl β-d-galactopyranosyl-(1→4)-α-d-glucopyranoside (1) and methyl β-d-galactopyranosyl-(1→4)-3-deoxy-α-d-ribo-hexopyranoside (3), were prepared with selective 13C-enrichment to allow measurement of six trans-O-glycosidic J-couplings (2JCOC, 3JCOCH, and 3JCOCC) in each compound. Density functional theory (DFT) was used to parameterize Karplus-like equations that relate these J-couplings to either ϕ or ψ. MA'AT analysis was applied to both linkages to determine mean values of ϕ and ψ in each disaccharide and their associated circular standard deviations (CSDs). Results show that deoxygenation at C3 of 1 has little effect on both the mean values and librational motions of the linkage torsion angles. This finding implies that, if inter-residue hydrogen bonding between O3H and O5' of 1 is present in aqueous solution and persistent, it plays little if any role in dictating preferred linkage conformation. Hydrogen bonding may lower the energy of the preferred linkage geometry but does not determine it to any appreciable extent. Aqueous 1-μs MD simulation supports this conclusion and also indicates greater conformational flexibility in deoxydisaccharide 3 in terms of sampling several, conformationally distinct, higher-energy conformers in solution. The populations of these latter conformers are low (3-14%) and could not be validated by MA'AT analysis. If the MD model is correct, however, C3 deoxygenation does enable conformational sampling over a wider range of ϕ/ψ values, but linkage conformation in the predominant conformer is essentially identical in both 1 and 3.

Revisiting the vibrational spectrum of formic acid anhydride

The potential energy surface of formic acid anhydride has been investigated at highly accurate quantum chemical methods. The rotation of CHO group in global minimum conformer of formic acid anhydride can lead two different local minimum conformers. Both local minimum conformers are around three kcal/mol higher in energy than the global minimum conformer. One conformer is planar and another has two rotamers which rapidly interconvert to each others via planar transition state. In some earlier studies, this planar transition state has incorrectly assigned to be a local minimum structure. We calculated anharmonic vibrational frequencies for the global and local minimum energy conformers and compared the theoretical wavenumbers with experimental gas phase and argon matrix measurements. Our results suggest that some of the experimentally detected peaks are overtone and combination bands. In previous studies with harmonic calculations, those peaks have assigned to be fundamental bands with zero intensities. We confirmed that the higher energy conformer produced in argon matrix by ultra violet induced rotamerization of global minimum conformer belongs to the planar conformer.

Investing in entropy: the strategy of cucurbit[n]urils to accelerate the intramolecular Diels-Alder cycloaddition reaction of tertiary furfuryl amines.

Cucurbit[n]urils, renowned for their host-guest chemistry, are becoming versatile biomimetic receptors. Herein, we report that cucurbit[7]uril (CB[7]) accelerates the intramolecular Diels-Alder (IMDA) reaction for previously elusive and unreactive tertiary N-methyl-N-(homo)allyl-2-furfurylamines by up to 4 orders of magnitude under mild conditions. Using 1H NMR titrations and ITC experiments, we characterize the dissimilar thermodynamic and kinetic properties of the complexes. We also determine the activation parameters (ΔG ≠, ΔH ≠ and ΔS ≠) leading to the transition state of the IMDA reactions, both in the bulk and included in CB[7], to shed light on the key role of the receptor on the acceleration observed. CB[7] acts as an "entropy trap" utilizing guest binding to primarily pay the entropy penalty for reorganizing the substrate in a high-energy reactive conformation that resembles the geometry of the highly ordered transition state required for the IMDA reaction. This study underscores the potential of cucurbit[n]urils as artificial active sites, emulating specific aspects of enzymatic catalysis.

Polymorph transformation in a mixed-stacking nickel-dithiolene complex with the derivative of 4,4'-bipyridinium.

In this study, two polymorphs of the [1,1'-dibutyl-4,4'-bipyridinium][Ni(mnt)2] salt (1) were synthesized. The dark-green polymorph (designated as 1-g) was prepared under ambient conditions by the rapid precipitation method in aqueous solutions. Subsequently, the red polymorph (labeled as 1-r) was obtained by subjecting 1-g to ultrasonication in MeOH at room temperature. Microanalysis, infrared spectroscopy, thermogravimetry (TG), differential scanning calorimetry (DSC), and powder X-ray diffraction (PXRD) techniques were used to characterize the two polymorphs. Both 1-g and 1-r exhibit structural phase transitions: a reversible phase transition at ∼403 K (∼268 K) upon heating and 384 K (∼252 K) upon cooling for 1-g (1-r) within the temperature range below 473 K. Interestingly, on heating 1-r to 523 K, an irreversible phase transition occurred at about 494 K, resulting in the conversion of 1-r into 1-g. Relative to 1-r, 1-g represents a thermodynamically metastable phase wherein numerous high-energy conformations in butyl chains of cations are confined within the lattice owing to quick precipitation or rapid annealing from higher temperatures. Through variable-temperature single crystal and powder X-ray diffractions, UV-visible spectroscopy, dielectric spectroscopy, and DSC analyses, this study delves into the mechanism underlying phase transitions for each polymorph and the manual transformation between 1-g and 1-r as well.

Z-Form Adoption of Nucleic Acid is a Multi-Step Process Which Proceeds through a Melted Intermediate.

The left-handed Z-conformation of nucleic acids can be adopted by both DNA and RNA when bound by Zα domains found within a variety of innate immune response proteins. Zα domains stabilize this higher-energy conformation by making specific interactions with the unique geometry of Z-DNA/Z-RNA. However, the mechanism by which a right-handed helix contorts to become left-handed in the presence of proteins, including the intermediate steps involved, is poorly understood. Through a combination of nuclear magnetic resonance (NMR) and other biophysical measurements, we have determined that in the absence of Zα, under low salt conditions at room temperature, d(CpG) and r(CpG) constructs show no observable evidence of transient Z-conformations greater than 0.5% on either the intermediate or slow NMR time scales. At higher temperatures, we observed a transient unfolded intermediate. The ease of melting a nucleic acid duplex correlates with Z-form adoption rates in the presence of Zα. The largest contributing factor to the activation energies of Z-form adoption as calculated by Arrhenius plots is the ease of flipping the sugar pucker, as required for Z-DNA and Z-RNA. Together, these data validate the previously proposed "zipper model" for Z-form adoption in the presence of Zα. Overall, Z-conformations are more likely to be adopted by double-stranded DNA and RNA regions flanked by less stable regions and by RNAs experiencing torsional/mechanical stress.

Phenylpropiolic acid isolated in cryogenic nitrogen and xenon matrices: NIR and UV-induced study.

Phenylpropiolic acid (C6H5C≡CCOOH, PPA) isolated in nitrogen and xenon cryogenic matrices was studied by infrared spectroscopy. The experimental studies were complemented by a series of quantum chemical calculations carried out at the density functional theory (B3LYP) and MP2 levels of theory (with different basis sets). The calculations predicted the existence of two planar PPA conformers, differing in the arrangement of the carboxylic group. The higher-energy trans-PPA conformer has a negligible population in the gas phase at room temperature and was prepared in situ in the N2 cryomatrix through vibrationally-induced rotamerization of the lower-energy cis-PPA conformer, achieved using selective narrowband infrared excitation of the OH stretching coordinate of the latter species. Broadband UV (λ > 235nm) irradiation of matrix-isolated cis-PPA was also undertaken, leading to the observation of cis-PPA → trans-PPA isomerization. No other UV-induced photoreactions were observed. The in situ generated trans-PPA conformer was found to decay back to cis-PPA in the dark by tunneling, and its lifetimes under different experimental conditions were determined. The assignment of the infrared spectra of both conformers is presented, considerably extending the vibrational information available on this molecule.

Disallowed spots in protein structures

Ramachandran (ϕ, ψ) steric map was introduced in 1963 to describe available conformation space for protein structures. Subsequently, residues were observed in high-energy disallowed regions of the map. To unequivocally identify the locations of disallowed conformations of residues, we got 36 noise-free protein structures (resolution ≤1 Å, Rwork/Rfree ≤ 0.10). These stringent criteria were applied to rule out data or model errors or any crystallographic disorders. No disallowed conformation was found in the dataset. Further, we also examined disallowed conformations in a larger dataset (resolution ≤1.5 Å, devoid of any model errors, or disorders). The observed locations of disallowed residues are referred as disallowed spots. These spots include short loops of 3–5 residues, and locations where residues participate in disulfide bonding or intramolecular interactions or inter-molecular interactions with neighboring water, metals or ligands. Conformational sampling revealed that short loops in between secondary structures hardly have any opportunity to relieve from conformational strain. Residues involved in interactions, which provide energetic compensation for high-energy conformational states, were relieved from strain once the causative interaction was removed. The present study aims to identify disallowed spots in the native state of proteins, wherein residues are forced to be trapped in high-energy disallowed conformations. Moreover, it was also observed that pre-Pro, Ser, Asp, trans-Pro, Val, Asn & Gly have higher tendency to occur in disallowed conformation, which could be attributed to factors such as conformational restrictions, residue propensity of secondary structures and compensating sidechain and mainchain interactions, stabilizing turn-mimics.

Infrared Ion Spectroscopic Characterization of the Gaseous [Co(15-crown-5)(H2O)]2+ Complex.

We report fingerprint infrared multiple-photon dissociation spectra of the gaseous monohydrated coordination complex of cobalt(II) and the macrocycle 1,4,7,10,13-pentaoxacyclopentadecane (or 15-crown-5), [Co(15-crown-5)(H2O)]2+. The metal-ligand complexes are generated using electrospray ionization, and their IR action spectra are recorded in a quadrupole ion trap mass spectrometer using the free-electron laser FELIX. The electronic structure and chelation motif are derived from spectral comparison with computed vibrational spectra obtained at the density functional theory level. We focus here on the gas-phase structure, addressing the question of doublet versus quartet spin multiplicity and the chelation geometry. We conclude that the gas-phase complex adopts a quartet spin state, excluding contributions of doublet species, and that the chelation geometry is pseudo-octahedral with the six oxygen centers of 15-crown-5 and H2O coordinated to the metal ion. We also address the possible presence of higher-energy conformers based on the IR spectral evidence and calculated thermodynamics.

First Glycine Isomer Detected in the Interstellar Medium: Glycolamide (NH2C(O)CH2OH)

We report the first detection in the interstellar medium (ISM) of a C2H5O2N isomer: syn-glycolamide (NH2C(O)CH2OH). The exquisite sensitivity at sub-mK levels of an ultradeep spectral survey carried out with the Yebes 40 m and IRAM 30 m telescopes toward the G+0.693–0.027 molecular cloud has allowed us to unambiguously identify multiple transitions of this species. We derived a column density of (7.4 ± 0.7) × 1012 cm−2, which implies a molecular abundance with respect to H2 of 5.5 × 10−11. The other C2H5O2N isomers, including the higher-energy anti conformer of glycolamide and two conformers of glycine, were not detected. The upper limit derived for the abundance of glycine indicates that this amino acid is surely less abundant than its isomer glycolamide in the ISM. The abundances of the C2H5O2N isomers cannot be explained in terms of thermodynamic equilibrium; thus, chemical kinetics need to be invoked. While the low abundance of glycine might not be surprising, based on the relative low abundances of acids in the ISM compared to other compounds (e.g., alcohols, aldehydes, or amines), several chemical pathways can favor the formation of its isomer glycolamide. It can be formed through radical–radical reactions on the surface of dust grains. The abundances of these radicals can be significantly boosted in an environment affected by a strong ultraviolet field induced by cosmic rays, such as that expected in G+0.693–0.027. Therefore, as shown by several recent molecular detections toward this molecular cloud, it stands out as the best target to discover new species with carbon, oxygen, and nitrogen with increasing chemical complexity.

Accessing three-dimensional molecular diversity through benzylic C-H cross-coupling.

Pharmaceutical and agrochemical discovery efforts rely on robust methods for chemical synthesis that rapidly access diverse molecules1,2. Cross-coupling reactions are the most widely used synthetic methods3, but these methods typically form bonds to C(sp2)-hybridized carbon atoms (e.g., amide coupling, biaryl coupling) and lead to a prevalence of "flat" molecular structures with suboptimal physicochemical and topological properties4. Benzylic C(sp3)-H cross-coupling methods offer an appealing strategy to address this limitation by directly forming bonds to C(sp3)-hybridized carbon atoms, and emerging methods exhibit synthetic versatility that rivals conventional cross-coupling methods to access products with drug-like properties. Here, we use a virtual library of >350,000 benzylic ethers and ureas derived from benzylic C-H cross-coupling to test the widely held view that coupling at C(sp3)-hybridized carbon atoms affords products with improved three-dimensionality. The results show that the conformational rigidity of the benzylic scaffold strongly influences the product dimensionality. Products derived from flexible scaffolds often exhibit little or no improvement in three-dimensionality, unless they adopt higher energy conformations. This outcome introduces an important consideration when designing routes to topologically diverse molecular libraries. The concepts elaborated herein are validated experimentally through an informatics-guided synthesis of selected targets and the use of high-throughput experimentation to prepare a library of three-dimensional products that are broadly distributed across drug-like chemical space.

Rotational spectroscopic studies of para-nitrobenzoic acid, para-aminobenzoic acid, para-chlorobenzoic acid, and para-hydroxybenzoic acid

The rotational spectra of four substituted benzoic acids, i.e., para-aminobenzoic acid, para-nitrobenzoic acid, para-chlorobenzoic acid, and para-hydroxybenzoic acid were recorded with a chirp-pulse Fourier transform microwave spectrometer and analysed in terms of rotational constants and, where applicable, nuclear quadrupole coupling constants and tunnelling motions. In all instances, the experimentally identified conformer contains a carboxylic acid group in the cis-arrangement. In the case of para-hydroxybenzoic acid, spectra of two conformers were observed which differ in the orientation of the para-OH group. Quantum chemical calculations at the B3LYP-D3BJ/def2-TZVP and B2PLYP-D3BJ/cc-pCVTZ levels show that the trans-conformers are about 23 kJ mol−1 higher in energy than the global minimum cis-conformers. In general, the calculated rotational constants are in good agreement with the experimental derived ones. Agreement between calculated and experimental 14N and 35/37Cl nuclear quadrupole coupling constants is somewhat worse and necessitated the use of the B2PLYP functional in the case of para-nitrobenzoic acid to reduce the discrepancy. Conversion barriers between possible conformers were calculated to explain the absence of tunnelling splittings and/or higher energy conformers in the experimental spectra. An analysis of the electron density distribution was used to rationalize the calculated out-of-plane excursions of the carboxylic acid group in the trans-conformers.

On the Role of Molecular Conformation of the 8-Oxoguanine Lesion in Damaged DNA Processing by Polymerases.

A common and insidious DNA damage is 8-oxoguanine (8OG), bypassed with low catalytic efficiency and high error frequency by polymerases (Pols) during DNA replication. This is a fundamental process with far-reaching implications in cell function and diseases. However, the molecular determinants of how 8OG exactly affects the catalytic efficiency of Pols remain largely unclear. By examining ternary deoxycytidine triphosphate/DNA/Pol complexes containing the 8OG damage, we found that 8OG consistently adopts different conformations when bound to Pols, compared to when in isolated DNA. Equilibrium molecular dynamics and metadynamics free energy calculations quantified that 8OG is in the lowest energy conformation in isolated DNA. In contrast, 8OG adopts high-energy conformations often characterized by intramolecular steric repulsion when bound to Pols. We show that the 8OG conformation can be regulated by mutating Pol residues interacting with the 8OG phosphate group. These findings propose the 8OG conformation as a factor in Pol-mediated processing of damaged DNA.

Structure, Vibrational Spectra, and Cryogenic MatrixPhotochemistry of 6-Bromopyridine-2-carbaldehyde: From the Single Molecule of the Compound to the Neat Crystalline Material

6-Bromopyridine-2-carbaldehyde (abbreviated as BPCA) is used both as a building block in supramolecular chemistry and as a ligand for transition metal catalysts and luminescent complexes. In this study, the structure and vibrational spectra of BPCA were investigated in both the room temperature neat crystalline phase and for the compound isolated in cryogenic Ar, Kr and Xe matrices. The experimental studies were complemented by quantum chemical DFT(B3LYP)/6-311++G(d,p) calculations. For the crystalline compound, infrared and Raman spectra were obtained and interpreted. Comparison of the obtained infrared spectrum of the crystal with those obtained for the isolated molecules of BPCA in the studied cryomatrices helped to conclude that the intermolecular interactions in the crystal do not significantly perturb the intramolecular vibrational potential. Structural analysis further supports the existence of weak coupling between the intermolecular interactions and the structure of the constituting molecular units in crystalline state. The intermolecular interactions in the BPCA crystal were also evaluated by means of Hirshfeld analysis, which revealed that the most important interactions are weak and of the H…N, H…O, H…H, H…Br and Br…Br types. The conformer of BPCA present in the crystal was found to correspond to the most stable form of the isolated molecule (trans), which bears stabilizing C-H…O=C and C(=O)H…N interactions. This conformer was shown to be the single conformer present in the as-deposited cryogenic matrices prepared from the room temperature gaseous compound. Broadband UV irradiation of matrix-isolated BPCA (λ ≥ 235 nm) resulted in the conversion of the trans conformer into the higher-energy cis conformer, where repulsive C-H…H-C(=O) and C=OLP…LPN (where LP designates a lone electron pair) interactions are present, and decarbonylation of the compound with formation of 2-bromopyridine (plus CO). The decarbonylation reaction was found to be more efficient in the more polarizable Xe matrix, indicating stabilization of the radicals initially formed upon breaking of the C-C(HO) and C-H (aldehyde) bonds in this medium, and testifying the occurrence of the decarbonylation reaction with involvement of radical species. TD-DFT calculations were used to access the nature of the excited states associated with the observed UV-induced reactions. As a whole, this study provides fundamental data to understand the physicochemical behavior of the compound, bridging the properties of the isolated molecule to those of the neat crystalline com-pound. Such information is of fundamental importance for the understanding of the role of BPCA in supramolecular chemistry and to potentiate its applications in synthesis and as a ligand for transition metal catalysts and luminescent complexes.