Understanding Photophysical Effects of Cucurbituril Encapsulation: A Model Study with Acridine Orange in the Gas Phase

Dispersion Interactions in Condensed Phases and inside Molecular Containers

ConspectusThe past decade has seen significant progress in the understanding and appreciation of the importance of London dispersion interactions (LDIs) in supramolecular systems and solutions. The Slater-Kirkwood formula relates LDIs to the molecular polarizabilities of the two interacting molecular species (α) and their interaction distance (a dependence of R-6). When advancing arguments related to intermolecular interactions, it is frequently assumed that molecules with larger molecular polarizabilities are more amenable to larger LDIs. However, arguments related to molecular polarizabilities are not always transferable to the condensed phase. In fact, the underlying bulk and molecular polarizabilities of common solvents show opposing trends. The intuitive concept that aromatic molecules are more polarizable than saturated hydrocarbons and that perfluorinated molecules are less polarizable than saturated hydrocarbons applies to the condensed phase only. When treating association phenomena in solution, where LDIs are generally very attenuated, the use of bulk polarizabilities is recommended, which are experimentally accessible through either refractive index measurements or suitable solvatochromic probes. Such probes can also be used to assess polarizabilities inside molecular container compounds, such as cucurbit[n]urils (CBn), cyclodextrins, calixarenes, and hemicarcerands. These macrocyclic cavities can have extreme microenvironments. For example, the inner concave phase of CB7 has been shown to be weakly polarizable, falling in between the gas phase and perfluorohexane; those of β-cyclodextrin and p-sulfonatocalix[4]arene have been found to be similarly polarizable as water and alkanes, respectively, and the inside of hemicarcerands displays a very large bulk polarizability, exceeding that of diiodomethane. CBn compounds are privileged molecular container compounds, which we exemplify in this Account through case studies. (1) CBn macrocycles are prime water-soluble receptors for hydrocarbons, allowing for the reduction of the binding free energies to two components: the hydrophobic effect and dispersion interactions. To understand hydrocarbon binding, we initiated the HYDROPHOBE challenge, which revealed the shortcomings of both quantum-chemical and molecular dynamics approaches. (2) The smallest CBn receptor, CB5, is uniquely suited to bind the entire noble gas series, where hydrophobic effects and dispersion interactions operate in opposite directions. CB5 was revaled to be a unique synthetic receptor for noble gases, with the dominant driving force being the recovery of the cavitation energies for the hydration of noble gases in aqueous solution. Computational methods that encounter challenges in predicting hydrocarbon affinities and trends for CB6 and CB7 perform well for noble gases binding to CB5. (3) The larger homologue, CB8, allows one to set up intermolecular interaction chambers by the encapsulation of a (first) aromatic guest, thereby tuning LDIs inside the receptor cavity. In this manner, CB8 can be modulated to preferentially bind unsaturated and aromatic rather than saturated hydrocarbons, while the unmodified cavities of the smaller macrocycles CB6 and CB7 show selective binding of saturated hydrocarbons. (4) The (charged) host-guest complexes of CBn hosts are sufficiently stable in the gas phase, allowing for the study of the influence of LDIs on inner-phase chemical reactions. These studies are particularly interesting for the theoretical analysis of isolated host-guest LDIs, as experimental and computational data are directly comparable in the gas phase due to the absence of the solvation effect.

Robust Fluorescence Collection Module for Wide-Bore Ion Cyclotron Resonance Mass Spectrometers

Mass spectrometers are at the heart of the most powerful toolboxes available to scientists when studying molecular structure, conformation, and dynamics in controlled molecular environments. Improved molecular characterization brought about by the implementation of new orthogonal methods into mass spectrometry-enabled analyses opens deeper insight into the complex interplay of forces that underlie chemistry. Here, we detail how one can add fluorescence detection to commercial ultrahigh-resolution Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometers without adverse effects to its preexisting analytical tools. This advance enables measurements based on fluorescence detection, such as Förster resonance energy transfer (FRET), to be used in conjunction with other MS/MS techniques to probe the conformation and dynamics of large biomolecules, such as proteins and their complexes, in the highly controlled environment of a Penning trap.

Rational design of a meso phosphate-substituted pyronin as a type I photosensitizer for photodynamic therapy

Type I photodynamic therapy (PDT) with less oxygen consumption shows great potential to overcome the malignant hypoxia in solid tumors. Herein, a novel meso phosphate-substituted pyronin PY-P and its nanoparticles (PY-P NPs) were prepared as an efficient type I organic photosensitizer. The in vivo data prove that PY-P NPs have outstanding low dark toxicity but high photocytotoxicity under hypoxia (<1% O₂).

Cucurbiturils mimicked by low polarizability solvents with pre-formed cavities: an empirical model to predict hydrocarbon selectivity

Relative binding affinities of a series of nine rigid hydrocarbons towards the cavity formed by a portion of the inner wall of cucurbit[8]uril (CB[8]) and a positive auxiliary guest were determined by competitive ¹⁹F NMR titrations in deuterium oxide. The corresponding free binding energies were corrected by the hydrocarbon computed solvation energies to obtain their free energies of transfer from the gas phase to the CB[8]/auxiliary guest cavity. These energies correlate linearly with the hydrocarbon static polarizabilities, thereby suggesting that the selectivity is driven, perhaps exclusively, by dispersive interactions between the hydrocarbons and the tailor-made cavity, regardless of the degree of unsaturation of the guests. The free energies of transfer also correlate linearly with the energy released upon introduction of the hydrocarbon into a pre-formed cavity extruded from a solvent (benzene) selected to mimic the polarity and polarizability of the CB[8]/auxiliary probe cavity – and this, with a unity slope. Among other features, this empirical model also accurately predicts the relative binding affinities of various rigid hydrocarbons to CB[6] and CB[7], as well as noble gases to CB[5], when the macrocycles are mimicked with pre-formed cavities in perfluorohexane or perfluorohexane/benzene mixtures, both being notoriously non-polar and non-polarizable environments.

Mimicking cucurbiturils with low polarizability solvents and pre-formed cavities allows the in silico prediction of their selectivities towards hydrocarbons and noble gases in aqueous solution.

Gas-Phase Fluorescence of Proflavine Reveals Two Close-Lying, Brightly Emitting States

Surprising excitation-dependent, dual emission from a small organic model fluorophore is reported. Gas-phase fluorescence spectra of proflavine (a diaminoacridine) ions reveal two long-lived emitting states, with distinct bands separated by just 1700 cm^-1. The relative intensities of these two bands depend on the excitation wavelength. Time-dependent density functional theory (TD-DFT) calculations support the existence of two close-lying singlet electronic states, with excitation into S₂ predicted to be >1000-fold more likely than into S₁. These data strongly suggest that internal conversion (IC) rates are suppressed relative to solvated proflavine, and that IC is competitive with intramolecular vibrational relaxation (IVR). This work offers an in-depth assessment of the gas-phase photophysics of a simple fluorophore that could open a new pathway to understanding dual emission in fluorophores.

Breaking the Brightness Barrier: Design and Characterization of a Selected-Ion Fluorescence Measurement Setup with High Optical Detection Efficiency

A quadrupole ion trap (QIT) mass spectrometer has been modified and coupled with tunable laser excitation and highly sensitive fluorescence detection systems to perform fluorescence studies on mass-selected ions. Gaseous ions, generated using nanoelectrospray ionization (nano-ESI), are trapped in the QIT that allows optical access for laser irradiation. The emitted fluorescence is collected from a 5.0 mm diameter hole drilled into the ring electrode of the QIT and is directed toward the detection setup. Due to the small inner diameter (7.07 mm) of the ring electrode and a relatively large opening for fluorescence collection, a fluorescence collection efficiency of 2.3% is achieved. After some losses in transmission, around 1.8% of the emitted fluorescence reaches the detectors, more than any other similar instrument reported in the literature. This improved fluorescence collection translates to a much shorter measurement time for a fluorescence signal. Another key feature of this setup is the ability to perform a variety of fluorescence experiments on trapped ions including excitation and emission spectroscopy, lifetime measurement, and ion imaging. The capabilities of the instrument are demonstrated by measuring fluorescence spectra of dyes and biomolecules labeled with dyes in a range of different excitation and emission wavelengths, quantum yields, m/z, and different polarities. A fluorescence lifetime measurement and ion image of trapped rhodamine 6G cations are also shown. With a wide array of functionality and high fluorescence detection performance, this setup provides an opportunity to study biomolecular structures and photophysics of fluorophores in well-controlled environments.

TURBOMOLE: Modular program suite for ab initio quantum-chemical and condensed-matter simulations

TURBOMOLE is a collaborative, multi-national software development project aiming to provide highly efficient and stable computational tools for quantum chemical simulations of molecules, clusters, periodic systems, and solutions. The TURBOMOLE software suite is optimized for widely available, inexpensive, and resource-efficient hardware such as multi-core workstations and small computer clusters. TURBOMOLE specializes in electronic structure methods with outstanding accuracy-cost ratio, such as density functional theory including local hybrids and the random phase approximation (RPA), GW-Bethe-Salpeter methods, second-order Møller-Plesset theory, and explicitly correlated coupled-cluster methods. TURBOMOLE is based on Gaussian basis sets and has been pivotal for the development of many fast and low-scaling algorithms in the past three decades, such as integral-direct methods, fast multipole methods, the resolution-of-the-identity approximation, imaginary frequency integration, Laplace transform, and pair natural orbital methods. This review focuses on recent additions to TURBOMOLE's functionality, including excited-state methods, RPA and Green's function methods, relativistic approaches, high-order molecular properties, solvation effects, and periodic systems. A variety of illustrative applications along with accuracy and timing data are discussed. Moreover, available interfaces to users as well as other software are summarized. TURBOMOLE's current licensing, distribution, and support model are discussed, and an overview of TURBOMOLE's development workflow is provided. Challenges such as communication and outreach, software infrastructure, and funding are highlighted.

Standard Redox Potentials of Fe(III) Aqua Complexes Included into the Cavities of Cucurbit[ n]urils ( n = 6-8): A DFT Forecast

An approach for estimating at the DFT level of the standard redox potentials of the inclusion compounds based on Fe(III) and Fe(II) aqua complexes inside the cavities of cucurbit[ n]urils ( n = 6-8) has been proposed. These inclusion compounds were established to have compositions which can be described by the formulas [Fe(H₂O)₆]^3+/2+@CB[6] and [Fe(H₂O)₆·4H₂O]^3+/2+@CB[7,8]. Redox potentials E⁰ relative to the standard hydrogen electrode for the half-reaction Fe(III)/Fe(II) in the CB[ n] cavities calculated at the PBE/TZVP level within the molecular-continuum solvation model are 1.607, 0.949, and 0.847 V for n = 6, 7, and 8, respectively. The obtained values indicate a relative increase of the oxidative ability of Fe(III) aqua-ions in the cavities of the examined CB[ n], especially in CB[6], compared to the calculated value ( E⁰ = 0.786 V) for the same half-reaction in the bulk of aqueous solution. Possible causes of the detected trend are discussed. The calculations also showed that the Fe(III) aqua complex inside the CB[6] changes its magnetic properties, transforming into a low-spin state with a total spin S = 1/2, whereas for all other systems high-spin states in accord with the classical ligand field theory are realized.

The effect of methylation on the intrinsic photophysical properties of simple rhodamines

Gas-phase studies of progressively methylated rhodamines display unexpected photophysical trends that are obscured in solution, revealing key solvent effects.

Protomers of DNA-binding dye fluoresce different colours: intrinsic photophysics of Hoechst 33258

A new form of DNA-binder Hoechst 33258 is stabilised upon desolvation. Altered optical properties include a distinct green fluorescence.

Ion mobility action spectroscopy of flavin dianions reveals deprotomer-dependent photochemistry

Photo-induced proton transfer, deprotomer-dependent photochemistry, and intramolecular charge transfer in flavin anions are investigated using action spectroscopy.

The intrinsic optical properties and photochemistry of flavin adenine dinucleotide (FAD) dianions are investigated using a combination of tandem ion mobility spectrometry and action spectroscopy. Two principal isomers are observed, the more stable form being deprotonated on the isoalloxazine group and a phosphate (N-3,PO₄ deprotomer), and the other on the two phosphates (PO₄,PO₄ deprotomer). Ion mobility data and electronic action spectra suggest that photo-induced proton transfer occurs from the isoalloxazine group to a phosphate group, converting the PO₄,PO₄ deprotomer to the N-3,PO₄ deprotomer. Comparisons of the isomer selective action spectra of FAD dianions and flavin monoanions with solution spectra and gas-phase photodissociation action spectra suggests that solvation shifts the electronic absorption of the deprotonated isoalloxazine group to higher energy. This is interpreted as evidence for significant charge transfer in the lowest optical transition of deprotonated isoalloxazine. Overall, this work demonstrates that the site of deprotonation of flavin anions strongly affects their electronic absorptions and photochemistry.

Photochemical behavior of biosupramolecular assemblies of photosensitizers, cucurbit[n]urils and albumins

Biosupramolecular assemblies combining cucurbit[n]urils (CB[n]s) and proteins for the targeted delivery of drugs have the potential to improve the photoactivity of photosensitizers used in the photodynamic therapy of cancer. Understanding the complexity of these systems and how it affects the properties of photosensitizers is the focus of this work. We used acridine orange (AO⁺) as a model photosensitizer and compared it with methylene blue (MB⁺) and a cationic porphyrin (TMPyP⁴⁺). Encapsulation of the photosensitizers into CB[n]s (n = 7, 8) modified their photoactivity. In particular, for AO⁺, the photo-oxidation of HSA was enhanced in the presence of CB[7]; meanwhile it was decreased when included into CB[8]. Accordingly, peroxide generation and protein fragmentation were also increased when AO⁺ was encapsulated into CB[7]. The triplet excited state lifetimes of all the photosensitizers were lengthened by their encapsulation into CB[n]s, while the singlet oxygen quantum yield was enhanced only for AO⁺ and TMPyP⁴⁺, but it decreased for MB⁺. The results obtained in this work prompt the necessity of further investigating these kinds of hybrid assemblies as drug delivery systems because of their possible applications in biomedicine.

Bioinspired Molecular Lantern: Tuning the Firefly Oxyluciferin Emission with Host-Guest Chemistry

Fireflies generate flashes of visible light via luciferase-catalyzed chemiexcitation of the substrate (luciferin) to the first excited state of the emitter (oxyluciferin). Microenvironment effects are often invoked to explain the effects of the luciferase active pocket on the emission; however, the exceedingly complex spectrochemistry and synthetic burdens have precluded elucidation of the nature of these interactions. To decipher the effects of microenvironment on the light emission, here the hydrophobic interior of cucurbit[7]uril (CB7) is used to mimic the nonpolar active pocket of luciferase. The hydrophobic interior of CB7 induces shifts of the ground-state pKas by 1.9-2.5 units to higher values. Upon sequestration, the emission maxima of neutral firefly oxyluciferin and its conjugate monodeprotonated base are blue-shifted by 40 and 39 nm, respectively, resulting in visual color changes of the emitted light.

A cylindrical quadrupole ion trap in combination with an electrospray ion source for gas-phase luminescence and absorption spectroscopy

A relatively simple setup for collection and detection of light emitted from isolated photo-excited molecular ions has been constructed. It benefits from a high collection efficiency of photons, which is accomplished by using a cylindrical ion trap where one end-cap electrode is a mesh grid combined with an aspheric condenser lens. The geometry permits nearly 10% of the emitted light to be collected and, after transmission losses, approximately 5% to be delivered to the entrance of a grating spectrometer equipped with a detector array. The high collection efficiency enables the use of pulsed tunable lasers with low repetition rates (e.g., 20 Hz) instead of continuous wave (cw) lasers or very high repetition rate (e.g., MHz) lasers that are typically used as light sources for gas-phase fluorescence experiments on molecular ions. A hole has been drilled in the cylinder electrode so that a light pulse can interact with the ion cloud in the center of the trap. Simulations indicate that these modifications to the trap do not significantly affect the storage capability and the overall shape of the ion cloud. The overlap between the ion cloud and the laser light is basically 100%, and experimentally >50% of negatively charged chromophore ions are routinely photodepleted. The performance of the setup is illustrated based on fluorescence spectra of several laser dyes, and the quality of these spectra is comparable to those reported by other groups. Finally, by replacing the optical system with a channeltron detector, we demonstrate that the setup can also be used for gas-phase action spectroscopy where either depletion or fragmentation is monitored to provide an indirect measurement on the absorption spectrum of the ion.

Supramolecular Adducts of Cucurbit[7]uril and Amino Acids in the Gas Phase

The complexation of the macrocyclic cavitand cucurbit[7]uril (Q7) with a series of amino acids (AA) with different side chains (Asp, Asn, Gln, Ser, Ala, Val, and Ile) is investigated by ESI-MS techniques. The 1:1 [Q7 + AA + 2H](2+) adducts are observed as the base peak when equimolar Q7:AA solutions are electrosprayed, whereas the 1:2 [Q7 + 2AA + 2H](2+) dications are dominant when an excess of the amino acid is used. A combination of ion mobility mass spectrometry (IM-MS) and DFT calculations of the 1:1 [Q7 + AA + 2H](2+) (AA = Tyr, Val, and Ser) adducts is also reported and proven to be unsuccessful at discriminating between exclusion or inclusion-type conformations in the gas phase. Collision induced dissociation (CID) revealed that the preferred dissociation pathways of the 1:1 [Q7 + AA + 2H](2+) dications are strongly influenced by the identity of the amino acid side chain, whereas ion molecule reactions towards N-butylmethylamine displayed a common reactivity pattern comprising AA displacement. Special emphasis is given on the differences between the gas-phase behavior of the supramolecular adducts with amino acids (AA = Asp, Asn, Gln, Ser, Ala, Val, and Ile) and those featuring basic (Lys and Arg) and aromatic (Tyr and Phe) side chains.

Time-resolved fluorescence anisotropy as a tool to study guest–cucurbit[n]uril–protein ternary supramolecular interactions

A combined time-resolved fluorescence and anisotropy analysis was used to study guest–cucurbit[n]uril–protein ternary supramolecular interactions in a model system.

Cucurbiturils: from synthesis to high-affinity binding and catalysis

Major developments in the synthesis of cucurbiturils and applications related to their high-affinity binding and catalysis have recently taken place.

Absorption and Emission Spectra of a Flexible Dye in Solution: a Computational Time-Dependent Approach

The spectroscopic properties of the organic chromophore 4-naphthoyloxy-1-methoxy-2,2,6,6-tetramethylpiperidine (NfO-TEMPO-Me) in toluene solution are explored through an integrated computational strategy combining a classical dynamic sampling with a quantum mechanical description within the framework of the time-dependent density functional theory (TDDFT) approach. The atomistic simulations are based on an accurately parametrized force field, specifically designed to represent the conformational behavior of the molecule in its ground and bright excited states, whereas TDDFT calculations are performed through a selected combination of hybrid functionals and basis sets to obtain optical spectra closely matching the experimental findings. Solvent effects, crucial to obtain good accuracy, are taken into account through explicit molecules and polarizable continuum descriptions. Although, in the case of toluene, specific solvation is not fundamental, the detailed conformational sampling in solution has confirmed the importance of a dynamic description of the molecular geometry for a reliable description of the photophysical properties of the dye. The agreement between theoretical and experimental data is established and a robust protocol for the prediction of the optical behaviour of flexible fluorophores in solution is set.