Aggregation-Induced Resonance Raman Optical Activity (AIRROA): A New Mechanism for Chirality Enhancement

Self-stacked small molecules for ultrasensitive, substrate-free Raman imaging in vivo

Raman spectroscopy using surface-enhanced Raman scattering (SERS) nanoprobes represents an ultrasensitive and high-precision technique for in vivo imaging. Clinical translation of SERS nanoprobes has been hampered by biosafety concerns about the metal substrates used to enhance Raman signals. We report a set of small molecules with bis-thienyl-substituted benzobisthiadiazole structures that enhance Raman signal through self-stacking rather than external substrates. In our technique, called stacking-induced charge transfer-enhanced Raman scattering (SICTERS), the self-stacked small molecules form an ordered spatial arrangement that enables three-dimensional charge transfer between neighboring molecules. The Raman scattering cross-section of SICTERS nanoprobes is 1350 times higher than that of conventional SERS gold nanoprobes of similar particle size. SICTERS outperforms SERS in terms of in vivo imaging sensitivity, resolution and depth. SICTERS is capable of noninvasive Raman imaging of blood and lymphatic vasculatures, which has not been achieved by SERS. SICTERS represents an alternative technique to enhance Raman scattering for guiding the design of ultrasensitive substrate-free Raman imaging probes.

Glutathione and its structural modifications recognized by Raman Optical Activity and Circularly Polarized Luminescence

Raman Optical Activity combined with Circularly Polarized Luminescence (ROA-CPL) was used in the spectral recognition of glutathione peptide (GSH) and its model post-translational modifications (PTMs). We demonstrate the potential of ROA spectroscopy and CPL probes (EuCl3, Na3[Eu(DPA)3], NaEuEDTA) in the study of unmodified peptide, i.e. GSH, and its derivatives, i.e. glutathione oxidized (GSSG), S-acetylglutathione (GSAc) and S-nitrosoglutathione (GSNO). ROA spectral features of GSH, GSSG, and GSAc were determined along with thier changes upon the different pH conditions. Apart from the ROA, induced CPL signals of Eu(III) probes also proved to be sensitive to the structural modifications of GSH-based model PTMs, enabling their spectral recognition, especially by the NaEuEDTA probe.

Organization of Carotenoid Aggregates in Membranes Studied Selectively using Resonance Raman Optical Activity

In living organisms, carotenoids are incorporated in biomembranes, remarkably modulating their mechanical characteristics, fluidity, and permeability. Significant resonance enhancement of Raman optical activity (ROA) signals of carotenoid chiral aggregates makes resonance ROA (RROA), a highly selective tool to study exclusively carotenoid assemblies in model membranes. Hence, RROA is combined with electronic circular dichroism (ECD), dynamic light scattering (DLS), molecular dynamics, and quantum-chemical calculations to shed new light on the carotenoid aggregation in dipalmitoylphosphatidylcholine (DPPC) liposomes. Using representative members of the carotenoid family: apolar α-carotene and more polar fucoxanthin and zeaxanthin, the authors demonstrate that the stability of carotenoid aggregates is directly linked with their orientation in membranes and the monomer structures inside the assemblies. In particular, polyene chain distortion of α-carotene molecules is an important feature of J-aggregates that show increased orientational freedom and stability inside liposomes compared to H-assemblies of more polar xanthophylls. In light of these results, RROA emerges as a new tool to study active compounds and drugs embedded in membranes.

Induced Chirality in Canthaxanthin Aggregates Reveals Multiple Levels of Supramolecular Organization

Carotenoids tend to form supramolecular aggregates via non-covalent interactions where the chirality of individual molecules is amplified to the macroscopic level. We show that this can also be achieved for non-chiral carotenoid monomers interacting with polysaccharides. The chirality induction in canthaxanthin (CAX), caused by heparin (HP) and hyaluronic acid (HA), was monitored by chiroptical spectroscopy. Electronic circular dichroism (ECD) and Raman optical activity (ROA) spectra indicated the presence of multiple carotenoid formations, such as H- and J-type aggregates. This is consistent with molecular dynamics (MD) and density functional theory (DFT) simulations of the supramolecular structures and their spectroscopic response.

Why Does One Measure Resonance Raman Optical Activity? A Unique Case of Measurements under Strong Resonance versus Far-from-Resonance Conditions

Raman optical activity (ROA) spectroscopy exhibits significant potential in the study of (bio)molecules as it encodes information on their molecular structure, chirality, and conformations. Furthermore, the method reveals details on excited electronic states when applied under resonance conditions. Here, we present a combined study of the far from resonance (FFR)-ROA and resonance ROA (RROA) of a single relatively small molecular system. Notably, this study is the first to employ the density functional theory (DFT) analysis of both FFR-ROA and RROA spectra. This is illustrated for cobalamin derivatives using near-infrared and visible light excitation. Although the commonly observed monosignate RROA spectra lose additional information visible in bisignate nonresonance ROA spectra, the RROA technique acts as a complement to nonresonance ROA spectroscopy. In particular, the combination of these methods integrated with DFT calculations can reveal a complete spectral picture of the structural and conformational differences between tested compounds.

Efficient delivery of carotenoids to adipocytes with albumin

Carotenoids are very effectively delivered by albumin to adipocytes. The uptake of carotenoids to the cells occurs in the form of self-aggregates that localize in the vicinity of the adipocyte membrane, as shown by high spatial resolution Raman spectroscopy. The binding of carotenoids to albumin and the mechanism of their transport were elucidated with the help of chiroptical spectroscopies, in tandem with molecular docking and molecular dynamics simulations. In particular, apart from the recognized high affinity pocket of albumin that binds a carotenoid monomer in domain I, we have identified a hydrophobic periphery area in domain IIIB that loosely bounds the self-aggregated carotenoid in aqueous media and enables its easy detachment in hydrophobic environments. This explains the effectiveness of albumins as nanocarriers of carotenoids to adipocytes in vitro.

Discovery of ultra-weakly coupled β-carotene J-aggregates by machine learning

Carotenoid aggregates are omnipresent in natural world and can be synthesized in hydrophilic environments. Despite different types of carotenoid aggregates have been reported hitherto, the way to predict the formation of carotenoid aggregates, i.e. H- or J-aggregates, is still challenging. Here, for the first time, we established machine learning models that can predict the formation behavior of carotenoid aggregates. The models are trained based on a database containing different types of carotenoid aggregates reported in the literatures. With the help of these machine learning models, we found a series of unknown types of β-carotene J-aggregates. These novel aggregates are ultra-weakly coupled and have absorption bands up to 700 nm, different from all the carotenoid aggregates reported previously. Our work demonstrates that the machine learning is a powerful tool to predict the formation behavior of carotenoid aggregates and can further lead into the discovery of new carotenoid aggregates for different applications.

Raman spectroscopy and Raman optical activity of blood plasma for differential diagnosis of gastrointestinal cancers

Improving the early diagnosis of gastrointestinal cancers is a crucial step in reducing their mortality. Given the non-specificity of the initial symptoms, the ability of any diagnostic method to differentiate between various types of gastrointestinal cancers also needs to be addressed. To detect disease-specific alterations in biomolecular structure and composition of the blood plasma, we have implemented an approach combining Raman spectroscopy and its conformation-sensitive polarized version, Raman optical activity, to analyze blood plasma samples of patients suffering from three different types of gastrointestinal cancer - hepatocellular, colorectal and pancreatic. First, we aimed to discriminate any type of gastrointestinal cancer from healthy control individuals; inthenext step, the focus was on differentiating among the three cancer types studied. The more straightforward of the two statistical approaches tested, the combination of linear discriminant analysis and principal component analysis applied to the entire spectral dataset, allowed the discrimination of cancer and control samples with 87% accuracy. The three gastrointestinal cancers were classified with an overall accuracy of 76%. The second method, the linear discriminant analysis applied to a selection of spectral bands, yielded even higher values. Cancer and control samples were distinguished with 89% accuracy and hepatocellular, colorectal and pancreatic cancer with an overall accuracy of 87%. The results obtained in our study suggest that the proposed approach may become a disease-specific diagnostic tool in daily clinical practice.

Comparative study of Raman spectroscopy techniques in blood plasma-based clinical diagnostics: A demonstration on Alzheimer's disease

Nowadays, there are still many diseases with limited or no reliable methods of early diagnosis. A popular approach in clinical diagnostic research is Raman spectroscopy, as a relatively simple, cost-effective, and high-throughput method for searching for disease-specific alterations in the composition of blood plasma. However, the high variability of the experimental designs, targeted diseases, or statistical processing in the individual studies makes it challenging to compare and compile the results to critically assess the applicability of Raman spectroscopy in real clinical practice. This study aimed to compare data from a single series of blood plasma samples of patients with Alzheimer's disease and non-demented elderly controls obtained by four different techniques/experimental setups - Raman spectroscopy with excitation at 532 and 785 nm, Raman optical activity, and surface-enhanced Raman scattering spectroscopy. The obtained results showed that the spectra from each Raman spectroscopy technique contain different information about biomolecules of blood plasma or their conformation and may, therefore, offer diverse points of view on underlying biochemical processes of the disease. The classification models based on the datasets generated by the three non-chiroptical variants of Raman spectroscopy exhibited comparable diagnostic performance, all reaching an accuracy close to or equal to 80%. Raman optical activity achieved only 60% classification accuracy, suggesting its limited applicability in the specific case of Alzheimer's disease diagnostics. The described differences in the outputs of the four utilized techniques/setups of Raman spectroscopy imply that their choice may crucially affect the acquired results and thus should be approached carefully concerning the specific purpose.

Exceptionally enhanced Raman optical activity (ROA) of amyloid fibrils and their prefibrillar states

Amyloid fibrils form remarkable, multi-layered chiral supramolecular architectures. The proximity of interacting oscillators in the chiral fibril supramolecules is responsible for the unusual sensitivity of vibrational circular dichroism (VCD) for fibril formation. Surprisingly, up to now, such characteristics have not been shown for ROA, although it displays the same vibrational markers of fibrils as VCD, including the amide I band. Here, we report an exceptionally large enhancement of the ROA signal detected for mature amyloid fibrils and their prefibrillar states. Remarkably, the same ROA signal has been obtained for fibrils of homologous lysozymes and the dissimilar protein, insulin, indicating a possible common enhanced ROA spectrum, analogous to that for VCD for all amyloid fibrils investigated to date. The ROA signal is observed at earlier stages of fibril formation than VCD and provides access to a considerably broader range of vibrations. Further studies are necessary to verify the applicability of ROA for the analysis of amyloid fibrils.

Photoactive Yellow Protein Adsorption at Hydrated Polyethyleneimine and Poly-l-Glutamic Acid Interfaces

Chiral and achiral vibrational sum-frequency generation (VSFG) spectroscopy was performed in the 1400-1700 and 2800-3800 cm^-1 range to study the interfacial structure of photoactive yellow protein (PYP) adsorbed on polyethyleneimine (PEI) and poly-l-glutamic acid (PGA) surfaces. Nanometer-thick polyelectrolyte layers served as the substrate for PYP adsorption, with 6.5-pair layers providing the most homogeneous surfaces. When the topmost material was PGA, it acquired a random coil structure with a small number of β₂-fibrils. Upon adsorption on oppositely charged surfaces, PYP yielded similar achiral spectra. However, the VSFG signal intensity increased for PGA surfaces with a concomitant redshift of the chiral C^α-H and N-H stretching bands, suggesting increased adsorption for PGA compared to PEI. At low wavenumbers, both the backbone and the side chains of PYP induced drastic changes to all measured chiral and achiral VSFG spectra. Decreasing ambient humidity led to the loss of tertiary structure with a re-orientation of α-helixes, evidenced by a strongly blue-shifted chiral amide I band of the β-sheet structure with a shoulder at 1654 cm^-1. Our observations indicate that chiral VSFG spectroscopy is not only capable of determining the main type of secondary structure of PYP, i.e., β-scaffold, but is also sensitive to tertiary protein structure.

Formation of carotenoid supramolecular aggregates in nanocarriers monitored via aggregation-sensitive chiroptical output of enantiopure (3S,3'S)-astaxanthin

The aggregation-sensitive chiroptical (ECD and RROA) output, provided by enantiopure (3S,3'S)-astaxanthin, was used to investigate and control the assembling processes of the carotenoid in Pluronic F-127 nanoparticles. The process of carotenoid J-aggregation inside nanocarriers is interfered with by the formation of kinetically stabilized H1 self-assemblies outside the micelles. Nanocarriers with encapsulated stable J-aggregates provide controlled release of carotenoid molecules to primary murine adipocytes.

Resolving Resonant Electronic States in Chiral Metal Complexes by Raman Optical Activity Spectroscopy

Chiral metal complexes exhibit rich photophysical properties and are important for applications ranging from biosensing to photocatalysis. We present a combined experimental and computational approach leading to information about energies and transition moments of excited electronic states, documented on two chiral metal complexes. The experimental protocol for measurement of the resonance Raman optical activity comprises multiple techniques, i.e., absorption, circular dichroism, and polarized and differential Raman scattering. An accurate formula for subtraction of the interfering circular dichroism/polarized Raman scattering effect is given. An analysis of the spectra based on density functional theory calculations unveils the geometric and electronic structures of the molecules. Such insight into molecular electronic states of chromophores may be useful for understanding and tuning photochemical properties of metal-containing complexes, biomolecules, and supramolecules.

3D Boranil Complexes with Aggregation-Amplified Circularly Polarized Luminescence

The development of small organic CPL-active molecules with large luminescent dissymmetry factors is highly demanded due to their promising applications in chiroptical devices and sensors. This work describes the design and synthesis of a new family of CPL-active BF₂ complexes, (R_p)/(S_p)-3a-3e, which were constructed by fusing a N̂O-chelated BF₂ complex with [2.2]paracyclophane. These complexes display aggregation-amplified CPL with moderate dissymmetry factors values and moderate quantum yields both in solution and in the solid state. In addition, these photophysical properties were rationalized via X-ray diffraction and TD-DFT calculations.

Measurement and Theory of Resonance Raman Optical Activity for Gases, Liquids, and Aggregates. What It Tells about Molecules

Resonance Raman Optical Activity (RROA) appeared as a natural extension of the nonresonance branch. It combines the structural sensitivity of chiroptical spectroscopy with the signal enhancement coming from the resonance of molecular electronic transitions with the excitation laser light. However, the idea has been hampered by many technical and theoretical problems that are being clarified only in recent years. We provide the theoretical basis and several examples documenting the problems, achievements, and potential of RROA, in particular in biomolecular studies.

Chiral recognition via a stereodynamic vanadium probe using the electronic circular dichroism effect in differential Raman scattering

Intermolecular interactions sensitive to chirality occur in many biological events. We report a complex formation between a versatile vanadium-based probe and a chiral co-ligand monitored via the combination of electronic circular dichroism (ECD) and Raman scattering. This "ECD-Raman" effect was discovered relatively recently and can be measured using a Raman optical activity (ROA) spectrometer. Simulated spectra based on experimental ECD and degree of circularity (DOC) values agree with the observed ones. Sensitive recognition of the chiral enantiopure co-ligand is thus enabled by a combination of resonance of the excitation light with the diastereoisomeric complex, co-ligand complexation, circular dichroism, and polarized Raman scattering from the achiral solvent. Relatively dilute solutions could be detected (10^-4 mol dm^-3), about 1000× less than is necessary for conventional ROA detection of the pure co-ligand and comparable to concentrations needed for conventional ECD spectroscopy. The results thus show that differential ECD-Raman measurements can be conveniently used to monitor molecular interactions and molecular spectroscopic properties.

Conformational Analysis of a Retinal Schiff Base Chromophore in Proteorhodopsin by Raman Optical Activity

Raman optical activity (ROA) spectroscopy was used to study the conformation of the retinal Schiff base chromophore in green-light-absorbing proteorhodopsin, which is a globally distributed light-driven proton pump of aquatic bacteria. The ROA spectrum consisted mostly of the negative vibrational bands of the chromophore, while the hydrogen out-of-plane mode (at 960 cm^-1) appeared as the sole positive band. This distinct spectral feature was not explained by the twisted structure of the retinal Schiff base but was reproduced by the structural model in which the polyene chain on the β-ionone ring side was bent out-of-plane. The bent chromophore structure potentially couples with proton pumping through the motion of the sixth helix in contact with the β-ionone ring.

Recognition of the True and False Resonance Raman Optical Activity

Resonance Raman optical activity (RROA) possesses all aspects of a sensitive tool for molecular detection, but its measurement remains challenging. We demonstrate that reliable recording of RROA of chiral colorful compounds is possible, but only after considering the effect of the electronic circular dichroism (ECD) on the ROA spectra induced by the dissolved chiral compound. We show RROA for a number of model vitamin B12 derivatives that are chemically similar but exhibit distinctively different spectroscopic behavior. The ECD/ROA effect is proportional to the concentration and dependent on the optical pathlength of the light propagating through the sample. It can severely alter relative band intensities and signs in the natural RROA spectra. The spectra analyses are supported by computational modeling based on density functional theory. Neglecting the ECD effect during ROA measurement can lead to misinterpretation of the recorded spectra and erroneous conclusions about the molecular structure.

Raman and ROA analyses of twisted anthracenes: connecting vibrational and electronic/photonic structures

In this article the Raman and Raman Optical Activity (ROA) spectra of a series of enantiomeric twisted anthracenes are presented. The evolution of their vibrational spectra is understood in the context of the variation of π-electron delocalization as a result of the twisting imparted by the belt structure and in terms of the modulation of the resonance Raman/ROA effects which are photonic properties also tuned by anthracene twisting. The Raman/ROA vibrational spectra are simulated by several theoretical approaches to account for their vibrational and electronic properties including the theoretical evaluation of resonance effects. We furthermore incorporate a vibrational and ROA activity dissection analysis as provided in the Pyvib2 program valid to establish correlations among vibrational modes of different molecules with different electronic structures and equivalent vibrational dynamics. This paper is one of the very first attempts to use ROA spectroscopy in π-conjugated molecules with twisted and helical morphologies that contrast with the well-known cases of ROA studies of chiral helicenes in which the impact of π-electron delocalization in the electronic/photonic/vibrational (Raman/ROA) spectra is negligible.

All-experimental analysis of doubly resonant sum-frequency generation spectra: Application to aggregated rhodamine films

A new method is proposed to analyze Doubly Resonant infrared-visible Sum-Frequency Generation (DR-SFG) spectra. Based on the transform technique, this approach is free from assumptions about vibronic modes, energies, or line widths and accurately captures through the overlap spectral function all required aspects of the vibronic structure from simple experimental linear absorption spectra. Details and implementation of the method are provided along with three examples treating rhodamine thin films about one monolayer thick. The technique leads to a perfect agreement between experiment and simulations of the visible DR-SFG line shapes, even in the case of complex intermolecular interactions resulting from J-aggregated chromophores in heterogeneous films. For films with mixed H- and J-aggregates, separation of their responses shows that the J-aggregate DR-SFG response is dominant. Our analysis also accounts for the unexplained results published in the early times of DR-SFG experiments.

Enhancing Optical Activities of Benzimidazole Derivatives through Coassembly for High-Efficiency Synthesis of Chiroptical Nanomaterials and Accurate ee % Detection of Natural Acids

Developing efficient protocols to enhance the optical activities of chiral self-assemblies is a key to realizing their chiroptical functions such as chiral sensing and displays. Here, we have reported a coassembly protocol to efficiently boost the chiroptical responses, whereby the synthesis of chiroptical nanomaterials and highly accurate detection of enantiomeric excess (ee %) were achieved. A series of benzimidazole derivatives with different topologies underwent spontaneous aggregation and symmetry breaking in solution, generating silent Cotton effects, yet exclusive weak left-handed circularly polarized luminescence (CPL). The coassembly with natural hydroxyl acids via complementary H bonds afforded chiral nanostructures with emerged Cotton effects and enhanced CPL. Dissymmetry g-factors were dramatically boosted (g_lum from 1 × 10^-3 to 5.5 × 10^-2, g_abs from 0 to 6.7 × 10^-3). In addition, proof of concept of recognition and detection of natural chiral molecules was realized with high accuracy.

Two Spectroscopies in One: Interference of Circular Dichroism and Raman Optical Activity

Previously, we and other laboratories have reported an unusual and strong Raman optical activity (ROA) induced in solvents by chiral dyes. Various theories of the phenomenon appeared, but they were not capable of explaining fully the observed ROA band signs and intensities. In this work, an analysis based both on the light scattering theory and dedicated experiments provides a more complete understanding. For example, double-cell magnetic circular dichroism and magnetic ROA experiments with copper-porphyrin complex show that the induced chirality is observed without any contact of the solvents with the complex. The results thus indicate that a combination of electronic circular dichroism (ECD) with the polarized Raman scattering is responsible for the effect. The degree of circularity of solvent vibrational bands is a principal molecular property participating in the event. The insight and the possibility to predict the chirality transfer promise future applications in spectroscopy, chemical analysis and polarized imaging.

Chiroptical Properties in Thin Films of π-Conjugated Systems

Chiral π-conjugated molecules provide new materials with outstanding features for current and perspective applications, especially in the field of optoelectronic devices. In thin films, processes such as charge conduction, light absorption, and emission are governed not only by the structure of the individual molecules but also by their supramolecular structures and intermolecular interactions to a large extent. Electronic circular dichroism, ECD, and its emission counterpart, circularly polarized luminescence, CPL, provide tools for studying aggregated states and the key properties to be sought for designing innovative devices. In this review, we shall present a comprehensive coverage of chiroptical properties measured on thin films of organic π-conjugated molecules. In the first part, we shall discuss some general concepts of ECD, CPL, and other chiroptical spectroscopies, with a focus on their applications to thin film samples. In the following, we will overview the existing literature on chiral π-conjugated systems whose thin films have been characterized by ECD and/or CPL, as well other chiroptical spectroscopies. Special emphasis will be put on systems with large dissymmetry factors (g_abs and g_lum) and on the application of ECD and CPL to derive structural information on aggregated states.

Optical purity, enantiomeric excess and the Horeau effect

The optical purity of an enantiomeric mixture deduced from specific rotation measurements was found by Horeau to be different from its enantiomeric excess, which came to be known as the Horeau effect. This observation had important implications in the practical use of specific rotations and has led to investigations on homochiral and heterochiral aggregation processes. In this review, dedicated to the Horeau principle, the theoretical basis for the observance of the Horeau effect and a survey of the specific rotation studies investigating the Horeau effect are provided, and possible future investigations are suggested.

Resonance Raman Optical Activity Shows Unusual Structural Sensitivity for Systems in Resonance with Multiple Excited States: Vitamin B12 Case

In this work, cobalamins with different upper axial substituents and a cobalamin derivative with a ring modification were studied using chiroptical spectroscopies, in particular resonance Raman optical activity (RROA), to shed light on the influence of structural modifications on RROA spectra in these strongly chiral systems in resonance with multiple excited states at 532 nm excitation. We have demonstrated that for these unique systems RROA possesses augmented structural specificity, surpassing resonance Raman spectroscopy and enabling at the same time measurement of cobalamins at fairy low concentrations of ∼10^-5 mol dm^-3. The enhanced structural specificity of RROA is a result of bisignate spectra due to resonance via more than one electronic state. The observation of increased structural capability of RROA for cobalamins opens a new perspective for studying chiral properties of other biological systems incorporating d-metal ions.

Chiral Amplification in Nature: Studying Cell-Extracted Chiral Carotenoid Microcrystals via the Resonance Raman Optical Activity of Model Systems

Carotenoid microcrystals, extracted from cells of carrot roots and consisting of 95 % of achiral β-carotene, exhibit a very intense chiroptical (ECD and ROA) signal. The preferential chirality of crystalline aggregates that consist mostly of achiral building blocks is a newly observed phenomenon in nature, and may be related to asymmetric information transfer from the chiral seeds (small amount of α-carotene or lutein) present in carrot cells. To confirm this hypothesis, we synthesized several model aggregates from various achiral and chiral carotenoids. Because of the sergeant-and-soldier behavior, a small number of chiral sergeants (α-carotene or astaxanthin) force the achiral soldier molecules (β- or 11,11'-[D₂ ]-β-carotene) to jointly form supramolecular assemblies of induced chirality. The chiral amplification observed in these model systems confirmed that chiral microcrystals appearing in nature might consist predominantly of achiral building blocks and their supramolecular chirality might result from the co-crystallization of chiral and achiral analogues.

Structure of supramolecular astaxanthin aggregates revealed by molecular dynamics and electronic circular dichroism spectroscopy

Biomolecular aggregation is omnipresent in nature and important for metabolic processes or in medical treatment; however, the phenomenon is rather difficult to predict or understand on the basis of computational models. Recently, we found that electronic circular dichroism (ECD) spectroscopy and closely related resonance Raman optical activity (RROA) are extremely sensitive to the aggregation mechanism and structure of the astaxanthin dye. In the present study, molecular dynamics (MD) and quantum chemical (QC) computations (ZIndo/S, TDDFT) are used to link the aggregate structure with ECD spectral shapes. Realistic absorption and ECD intensities were obtained and the simulations reproduced many trends observed experimentally, such as the prevalent sign pattern and dependence of the aggregate structure on the solvent type. The computationally cheaper ZIndo/S method provided results very similar to those obtained by TDDFT. In the future, the accuracy of the combined MD/QC methodology of spectra interpretation should be improved to provide more detailed information on astaxanthin aggregates and similar macromolecular systems.

The Importance of Solvent Effects on the Mechanism of the Pfeiffer Effect

The Pfeiffer effect is observed when an optically active compound such as an amino acid is introduced to a solution containing a labile racemic metal complex, and an equilibrium shift is obtained. The "perturbation" results in an excess of one enantiomer over the other. The shift is a result of a preferential outer sphere interaction between the introduced chiral species and one enantiomeric form (Λ or Δ) of a labile metal complex. Speculations regarding the mechanism of the Pfeiffer effect have attributed observations to a singular factor such as pH, solvent polarity, or numerous other intermolecular interactions. Through the use of the lanthanide(III) complexes [Tb(DPA)3]3- and [Eu(DPA)3]3- (where DPA = 2,6-pyridinedicarboxylate) and the amino acids l-serine and l-proline; it is becoming clear that the mechanism is not so simply described as per the preliminary findings that are discussed in this study. It appears that the true mechanism is far more complicated than the attribute just a singular factor. This work attempts to shine light on the fact that understanding the behavior of the solvent environment may hypothetically be the key to offering a more detailed description of the mechanism.

Pre-resonance enhancement of exceptional intensity in Aggregation-Induced Raman Optical Activity (AIROA) spectra of lutein derivatives

Recently reported new phenomenon of Aggregation-Induced Raman Optical Activity is demonstrated here for the first time in the pre-resonance conditions for lutein diacetate and 3'-epi-lutein supramolecular self-assembles. We demonstrate that minor alterations in the lutein structure (e.g. acetylation of hydroxyl groups or different configuration at one of the chiral center) can lead to definitely different spectral profiles and optical properties due to formation of aggregates of different structure and type. Lutein forms only H-aggregates, lutein diacetate only J-aggregates, while 3'-epi-lutein can occur in both forms simultaneously. Variety of aggregates' structures is so large that not only the type of aggregation is different, but also their chirality. It is remarkable that even in the pre-resonance conditions, aggregation of lutein derivatives can lead to the intense ROA signal, and moreover, 3'-epi-lutein demonstrated the highest resonance ROA CID ratio that has ever been reported.

Aggregation-Induced Resonance Raman Optical Activity (AIRROA) and Time-Dependent Helicity Switching of Astaxanthin Supramolecular Assemblies

New methods for enhancing the Raman optical activity (ROA) signal are desirable due to the low efficiency of ROA, demanding otherwise high sample concentrations, high laser powers, and/or long acquisition times. Previously, we have demonstrated a new phenomenon, aggregation-induced resonance ROA (AIRROA), that produces significant enhancement of the ROA signal provided that the excitation wavelength coincides with the absorption of the measured species and that the electronic circular dichroism (ECD) signal in the range of this absorption is nonzero. In this work, analyzing three very different supramolecular astaxanthin aggregates (H1, H2, and J), we confirm the phenomenon and demonstrate that aggregation itself is not enough to enhance the ROA signal and that the above-mentioned conditions are necessary for induction of the resonance ROA effect. Additionally, by analyzing the changes in the ECD spectra of the H1 assembly, we demonstrate that the supramolecular helicity sign switches with time, which is dependent on the prevalence of kinetic or thermodynamic stabilization of the obtained aggregates.