New Sulfur-Containing Polyarsenicals from the New Caledonian Sponge Echinochalina bargibanti

Arsenicin A (C₃H₆As₄O₃) was isolated from the New Caledonian poecilosclerid sponge Echinochalina bargibanti, and described as the first natural organic polyarsenic compound. Further bioguided fractionation of the extracts of this sponge led us to isolate the first sulfur-containing organic polyarsenicals ever found in Nature. These metabolites, called arsenicin B and arsenicin C, are built on a noradamantane-type framework that is characterized by an unusual As⁻As bonding. Extensive NMR measurements, in combination with mass spectra, enabled the assignment of the structure for arsenicin B (C₃H₆As₄S₂) as 2. The scarcity of arsenicin C and its intrinsic chemical instability only allowed the collection of partial spectral data, which prevented the full structural definition. After the extensive computational testing of several putative structures, structure 3 was inferred for arsenicin C (C₃H₆As₄OS) by comparing the experimental and density functional theory (DFT)-calculated ¹H and ¹³C NMR spectra. Finally, the absolute configurations of 2 and 3 were determined with a combined use of experimental and time-dependent (TD)-DFT calculated electronic circular dichroism (ECD) spectra and observed specific rotations. These findings pose great challenges for the investigation of the biosynthesis of these metabolites and the cycle of arsenic in Nature. Arsenicins B and C showed strong antimicrobial activities, especially against S. aureus, which is comparable to the reference compound gentamycin.

Peniginsengins B⁻E, New Farnesylcyclohexenones from the Deep Sea-Derived Fungus Penicillium sp. YPGA11

Chemical examination of the EtOAc extract of the deep sea-derived fungus Penicillium sp. YPGA11 resulted in the isolation of four new farnesylcyclohexenones, peniginsengins B–E (1–4), and a known analog peniginsengin A (5). The structures of compounds 1–4 were determined on the basis of comprehensive analyses of the nuclear magnetic resonance (NMR) and mass spectroscopy (MS) data, and the absolute configurations of 1, 2, and 4 were determined by comparisons of experimental electronic circular dichroism (ECD) with calculated ECD spectra. Compounds 1–5, characterized by a highly oxygenated 1-methylcyclohexene unit and a (4E,8E)-4,8-dimethyldeca-4,8-dienoic acid side chain, are rarely found in nature. Compounds 2–4 exhibited antibacterial activity against Staphylococcus aureus.

Sorbicillasins A⁻B and Scirpyrone K from a Deep-Sea-Derived Fungus, Phialocephala sp. FL30r

Two new nitrogen-containing sorbicillinoids named sorbicillasins A and B (1 and 2) and a new 3,4,6-trisubstituted α-pyrone derivative, scirpyrone K (3), together with two known biosynthetically related polyketides (4⁻5), were isolated from the deep-sea-derived fungus Phialocephala sp. FL30r by using the OSMAC (one strain-many compounds) method. The structures of 1⁻3, including absolute configurations, were deduced based on MS, NMR, and time-dependent density functional theory (TD-DFT) calculations of specific ECD (electronic circular dichroism) spectra. Compounds 1 and 2 possessed a novel hexahydropyrimido[2,1-a] isoindole moiety, and compound 3 exhibited weak radical scavenging activity against DPPH (2,2-diphenyl-1-picrylhydrazyl) with an IC50 value of 27.9 μM.

Enantiomeric impurity analysis using circular dichroism spectroscopy with United States Pharmacopeia liquid chromatographic methods

Over 300 chiral drug substances lack official United States Pharmacopeia (USP) methods for the enantiomeric purity determination. Because enantiomeric analysis typically requires specialized methods for each drug compound, developing protocols for each of these 300+ substances would be an expensive and laborious endeavor. Alternatively, if a detector capable of determining the enantiomeric composition without chiral separation could be used with certain drug compounds, this could be implemented relatively rapidly into official testing monographs. Circular dichroism (CD) detection following HPLC (HPLC-CD) has been proposed for this purpose but studies performed thus far have not prioritized its compatibility with validated regulatory methods. In this study, HPLC-CD was evaluated for enantiomeric purity determinations of 13 drug substances using HPLC methods consistent with assay protocols described in United States Pharmacopeia (USP) monographs. Of these selected substances, three (sitagliptin, timolol, and levalbuterol) showed no CD activity and one other (levofloxacin) could not be analyzed due to incompatibility of the mobile phase with the CD detector. For the remaining 9 substances, method validation was performed to determine the linearity, accuracy, precision and limits of quantitation of enantiomer impurities, which was compared to limits established by USP. It was found that enantiomeric impurities for four substances (pramipexole, levocetirizine, (S)-citalopram, and tolterodine) could be quantitatively determined at levels suitable to USP specifications. This analysis demonstrated that HPLC-CD does provide an effective enantiomeric characterization strategy for compatible chiral compounds, and can be implemented quickly and economically compared to traditional column-dependent chiral separation or derivatization methods.

G-Quadruplex Secondary Structure Obtained from Circular Dichroism Spectroscopy

A curated library of circular dichroism spectra of 23 G-quadruplexes of known structure was built and analyzed. The goal of this study was to use this reference library to develop an algorithm to derive quantitative estimates of the secondary structure content of quadruplexes from their experimental CD spectra. Principal component analysis and singular value decomposition were used to characterize the reference spectral library. CD spectra were successfully fit to obtain estimates of the amounts of base steps in anti-anti, syn-anti or anti-syn conformations, in diagonal or lateral loops, or in other conformations. The results show that CD spectra of nucleic acids can be analyzed to obtain quantitative structural information about secondary structure content in an analogous way to methods used to analyze protein CD spectra.

Co-aggregation of pro-inflammatory S100A9 with α-synuclein in Parkinson's disease: ex vivo and in vitro studies

BACKGROUND

Chronic neuroinflammation is a hallmark of Parkinson's disease (PD) pathophysiology, associated with increased levels of pro-inflammatory factors in PD brain tissues. The pro-inflammatory mediator and highly amyloidogenic protein S100A9 is involved in the amyloid-neuroinflammatory cascade in Alzheimer's disease. This is the first report on the co-aggregation of α-synuclein (α-syn) and S100A9 both in vitro and ex vivo in PD brain.

METHODS

Single and sequential immunohistochemistry, immunofluorescence, scanning electron and atomic force (AFM) microscopies were used to analyze the ex vivo PD brain tissues for S100A9 and α-syn location and aggregation. In vitro studies revealing S100A9 and α-syn interaction and co-aggregation were conducted by NMR, circular dichroism, Thioflavin-T fluorescence, AFM, and surface plasmon resonance methods.

RESULTS

Co-localized and co-aggregated S100A9 and α-syn were found in 20% Lewy bodies and 77% neuronal cells in the substantia nigra; both proteins were also observed in Lewy bodies in PD frontal lobe (Braak stages 4-6). Lewy bodies were characterized by ca. 10-23 μm outer diameter, with S100A9 and α-syn being co-localized in the same lamellar structures. S100A9 was also detected in neurons and blood vessels of the aged patients without PD, but in much lesser extent. In vitro S100A9 and α-syn were shown to interact with each other via the α-syn C-terminus with an apparent dissociation constant of ca. 5 μM. Their co-aggregation occurred significantly faster and led to formation of larger amyloid aggregates than the self-assembly of individual proteins. S100A9 amyloid oligomers were more toxic than those of α-syn, while co-aggregation of both proteins mitigated the cytotoxicity of S100A9 oligomers.

CONCLUSIONS

We suggest that sustained neuroinflammation promoting the spread of amyloidogenic S100A9 in the brain tissues may trigger the amyloid cascade involving α-syn and S100A9 and leading to PD, similar to the effect of S100A9 and Aβ co-aggregation in Alzheimer's disease. The finding of S100A9 involvement in PD may open a new avenue for therapeutic interventions targeting S100A9 and preventing its amyloid self-assembly in affected brain tissues.

Graphene-metal hybrid metamaterials for strong and tunable circular dichroism generation

A strong and dynamically controlled circular dichroism (CD) effect has aroused great attention due to its desirable applications in modern chemistry and life sciences. In this Letter, we propose a graphene-metal hybrid chiral metamaterial to generate mid-infrared CD with an intensity of more than 10%, which can be actively controlled over a wide wavelength range. In addition to the strong tunability, the CD signal intensity of our nanostructure is drastically larger than that of the purely graphene-based chiroptical nanostructures. Our design offers a new strategy for developing tunable chiral metadevices, which could be used in various applications, such as biochemical detection and information processing.

Structural and Dynamical Characterization of DNA and RNA Quadruplexes Obtained from the GGGGCC and GGGCCT Hexanucleotide Repeats Associated with C9FTD/ALS and SCA36 Diseases

A (GGGGCC) hexanucleotide repeat (HR) expansion in the C9ORF72 gene has been considered the major cause behind both frontotemporal dementia and amyotrophic lateral sclerosis, while a (GGGCCT) is associated with spinocerebellar ataxia 36. Recent experiments involving NMR, CD, optical melting and 1D 1H NMR spectroscopy, suggest that the r(GGGGCC) HR can adopt a hairpin structure with G-G mismatches in equilibrium with a G-quadruplex structure. G-Quadruplexes have also been identified for d(GGGGCC). As these experiments lack molecular resolution, we have used molecular dynamics microsecond simulations to obtain a structural characterization of the G-quadruplexes associated with both HRs. All DNA G-quadruplexes, parallel or antiparallel, with or without loops are stable, while only parallel and one antiparallel (stabilized by diagonal loops) RNA G-quadruplexes are stable. It is known that antiparallel G-quadruplexes require alternating guanines to be in a syn conformation that is hindered by the C3'-endo pucker preferred by RNA. Initial RNA antiparallel quadruplexes built with C2'-endo sugars evolve such that the transition (C2'-endo)-to-(C3'-endo) triggers unwinding and buckling of the flat G-tetrads, resulting in the unfolding of the RNA antiparallel quadruplex. Finally, a parallel G-quadruplex stabilizes an adjacent C-tetrad in both DNA and RNA (thus effectively becoming a mixed quadruplex of 5 layers). The C-tetrad is stabilized by the stacking interactions with the preceding G-tetrad, by cyclical hydrogen bonds C(N4)-(O2), and by an ion between the G-tetrad and the C-tetrad. In addition, antiparallel DNA G-quadruplexes also stabilize flat C-layers at the ends of the quadruplexes.

Advancements of two dimensional correlation spectroscopy in protein researches

The developments of two-dimensional correlation spectroscopy (2DCOS) applications in protein studies are discussed, especially for the past two decades. The powerful utilities of 2DCOS combined with various analytical techniques in protein studies are summarized. The emphasis is on the vibration spectroscopic techniques including IR, NIR, Raman and optical activity (ROA), as well as vibration circular dichroism (VCD) and fluorescence spectroscopy. In addition, some new developments, such as hetero-spectral 2DCOS, moving-window correlation, and model based correlation, are also reviewed for their utility in the investigation of the secondary structure, denaturation, folding and unfolding changes of protein. Finally, the new possibility and challenges of 2DCOS in protein research are highlighted as well.

Physicochemical stability and antioxidant activity of soy protein/pectin/tea polyphenol ternary nanoparticles obtained by photocatalysis

The ternary nanoparticles were fabricated by soy protein, pectin and tea polyphenol through photocatalysis. The particulate characteristics, including particle size, polydispersity index, and zeta potential were monitored for ternary nanoparticles formed under different photocatalysis time. Photocatalysis was favorable to form ternary nanoparticles with moderate particle size (310-370 nm), uniform distribution, spherical shape, and improved antioxidant activity. It was found that the fluorescence intensity of soy protein decreased with the increase in photocatalysis time in the ternary nanoparticles. Far-UV circular dichroism results indicated that increasing photocatalysis time could alter the secondary structure of soy protein with an increase in the proportion of β-sheet and β-turn structure at the cost of unordered coil and α-helix structure. According to FT-IR results, photocatalysis time could also modulate the conjugation between pectin and soy protein. In addition, photocatalysis could increase the binding affinities among the components, leading to better environmental stability of the ternary nanoparticles. The ternary nanoparticles in this study could be used as a good alternative to understand and consequently improve the physicochemical stability in food, pharmaceutical, and cosmetic matrices.

Bioassay- and Chemistry-Guided Isolation of Scalemic Caged Prenylxanthones from the Leaves of Garcinia bracteata

With bioassay- and chemistry-guided fractionation, seven new caged prenylxanthones including two scalemic mixtures, epiisobractatin (1), 13-hydroxyisobractatin (2), 13-hydroxyepiisobractatin (3), 8-methoxy-8,8a-dihydrobractatin (4), 8-ethoxy-8,8a-dihydrobractatin (5), garcibracteatone (6), and 8-methoxy-8,8a-dihydroneobractiatin (7), and the eight known compounds 8-15 were isolated from the leaves of Garcinia bracteata. The structures were unambiguously elucidated through analysis of spectroscopic data. The 2D structures and relative configurations of 1 and 5 were confirmed by X-ray crystallographic analysis. The separation of the enantiomers of 1-5 was accomplished by chiral-phase HPLC. The absolute configurations of the enantiomers of 1 and 5 were assigned by comparison of the experimental and calculated electronic circular dichroism (ECD) spectra. The absolute configurations of the related compounds were determined via comparisons of their ECD data with those of the enantiomers of 1 and 5, respectively. Notably, compound 7, with a neo caged skeleton, is the first representative of a novel type of caged xanthone lacking a Δ^8(8a) double bond. The isolated compounds exhibited significant cell growth inhibitory activities in vitro against human leukemic HL-60 and K562 cell lines, with GI₅₀ values ranging from 0.2 to 8.8 μM. A preliminary structure-activity relationship is discussed.

Hepatoprotective Dibenzocyclooctadiene and Tetrahydrobenzocyclooctabenzofuranone Lignans from Kadsura longipedunculata

Five new dibenzocyclooctadiene lignans, longipedlignans A-E (1-5), five new tetrahydrobenzocyclooctabenzofuranones (6-10), and 18 known analogues (11-28) were isolated from the roots of Kadsura longipedunculata. Compounds 6-10 are new spirobenzofuranoid-dibenzocyclooctadiene-type lignans. Their structures and absolute configurations were established using a combination of MS, NMR, and electronic circular dichroism data. Spirobenzofuranoids 6 and 15 showed moderate hepatoprotective activity against N-acetyl- p-aminophenol-induced toxicity in HepG2 cells with cell survival rates at 10 μM of 52.2% and 50.2%, respectively.

Methodology for the Absolute Configuration Determination of Epoxythymols Using the Constituents of Ageratina glabrata

A methodology to determine the enantiomeric excess and the absolute configuration (AC) of natural epoxythymols was developed and tested using five constituents of Ageratina glabrata. The methodology is based on enantiomeric purity determination employing 1,1'-bi-2-naphthol (BINOL) as a chiral solvating agent combined with vibrational circular dichroism (VCD) measurements and calculations. The conformational searching included an extensive Monte Carlo protocol that considered the rotational barriers to cover the whole conformational spaces. (+)-(8S)-10-Benzoyloxy-6-hydroxy-8,9-epoxythymol isobutyrate (1), (+)-(8S)-10-acetoxy-6-methoxy-8,9-epoxythymol isobutyrate (4), and (+)-(8S)-10-benzoyloxy-6-methoxy-8,9-epoxythymol isobutyrate (5) were isolated as enantiomerically pure constituents, while 10-isobutyryloxy-8,9-epoxythymol isobutyrate (2) was obtained as a 75:25 (8S)/(8R) scalemic mixture. In the case of 10-benzoyloxy-8,9-epoxythymol isobutyrate (3), the BINOL methodology revealed a 56:44 scalemic mixture and the VCD measurement was beyond the limit of sensitivity since the enantiomeric excess is only 12%. The racemization process of epoxythymol derivatives was studied using compound 1 and allowed the clarification of some stereochemical aspects of epoxythymol derivatives since their ACs have been scarcely analyzed and a particular behavior in their specific rotations was detected. In more than 30 oxygenated thymol derivatives, including some epoxythymols, the reported specific rotation values fluctuate from -1.6 to +1.4 passing through zero, suggesting the presence of scalemic and close to racemic mixtures, since enantiomerically pure natural constituents showed positive or negative specific rotations greater than 10 units.

Twenty-Nine New Limonoids with Skeletal Diversity from the Mangrove Plant, Xylocarpus moluccensis

Twenty-nine new limonoids—named xylomolins A₁–A₇, B₁–B₂, C₁–C₂, D–F, G₁–G₅, H–I, J₁–J₂, K₁–K₂, L₁–L₂, and M–N, were isolated from the seeds of the mangrove plant, Xylocarpus moluccensis. Compounds 1–13 are mexicanolides with one double bond or two conjugated double bonds, while 14 belongs to a small group of mexicanolides with an oxygen bridge between C1 and C8. Compounds 15–19 are khayanolides containing a Δ^8,14 double bond, whereas 20 and 21 are rare khayanolides containing a Δ^14,15 double bond and Δ^8,9, Δ^14,15 conjugated double bonds, respectively. Compounds 22 and 23 are unusual limonoids possessing a (Z)-bicyclo[5.2.1]dec-3-en-8-one motif, while 24 and 25 are 30-ketophragmalins with Δ^8,9, Δ^14,15 conjugated double bonds. Compounds 26 and 27 are phragmalin 8,9,30-ortho esters, whereas 28 and 29 are azadirone and andirobin derivatives, respectively. The structures of these compounds, including absolute configurations of 15–19, 21–23, and 26, were established by HRESIMS, extensive 1D and 2D NMR investigations, and the comparison of experimental electronic circular dichroism (ECD) spectra. The absolute configuration of 1 was unequivocally established by single-crystal X-ray diffraction analysis, obtained with Cu Kα radiation. The diverse cyclization patterns of 1–29 reveal the strong flexibility of skeletal plasticity in the limonoid biosynthesis of X. moluccensis. Compound 23 exhibited weak antitumor activity against human triple-negative breast MD-MBA-231 cancer cells with an IC₅₀ value of 37.7 μM. Anti-HIV activities of 1, 3, 8, 10, 11, 14, 20, 23–25, and 27 were tested in vitro. However, no compounds showed potent inhibitory activity.

Lipid-Lowering Polyketides from the Fungus Penicillium Steckii HDN13-279

Seven new polyketides, named tanzawaic acids R–X (1–6, 11), along with seven known analogues (7–10 and 12–14), were isolated from Penicillium steckii HDN13-279. Their structures, including the absolute configurations, were elucidated by NMR, MS, X-ray diffraction, circular dichroism (CD) analyses and chemical derivatization. Five compounds (2, 3, 6, 10 and 12) significantly decreased the oleic acid (OA)-elicited lipid accumulation in HepG2 liver cells at the concentration of 10 μM, among which, four compounds (3, 6, 10 and 12) significantly decreased intracellular total cholesterol (TC) levels and three Compounds (3, 6, and 10) significantly decreased intracellular triglyceride (TG) levels. Moreover, the TG-lowering capacities of compounds 6 and 10 were comparable with those of simvastatin, with the TG levels being nearly equal to blank control. This is the first report on the lipid-lowering activity of tanzawaic acid derivatives.

The role of d-allo-isoleucine in the deposition of the anti-Leishmania peptide bombinin H4 as revealed by 31P solid-state NMR, VCD spectroscopy, and MD simulation

Bombinin H4 is an antimicrobial peptide that was isolated from the toad Bombina variegata. Bombinin H family peptides are active against gram-positive, gram-negative bacteria, and fungi as well as the parasite Leishmania. Among them, bombinin H4 (H4), which contains d-allo-isoleucine (d-allo-Ile) as the second residue in its sequence, is the most active, and its l-isomer is bombinin H2 (H2). H4 has a significantly lower LC50 than H2 against Leishmania. However, the atomic-level mechanism of the membrane interaction and higher activity of H4 has not been clarified. In this work, we investigated the behavior of the conformations and interactions of H2 and H4 with the Leishmania membrane using ³¹P solid-state nuclear magnetic resonance (NMR), vibrational circular dichroism (VCD) spectroscopy, and molecular dynamics (MD) simulations. The generation of isotropic ³¹P NMR signals depending on the peptide concentration indicated the abilities of H2 and H4 to exert antimicrobial activity via membrane disruption. The VCD experiment and density functional theory calculation confirmed the different stability and conformations of the N-termini of H2 and H4. MD simulations revealed that the N-terminus of H4 is more stable than that of H2 in the membrane, in line with the VCD experiment data. VCD and MD analyses demonstrated that the first l-Ile and second d-allo-Ile of H4 tend to take a cis conformation. These residues function as an anchor and facilitate the easy winding of the helical conformation of H4 in the membrane. It may assist to quickly reach to the threshold concentration of H4 on the Leishmania membrane. This article is part of a Special Issue entitled: d-Amino acids: biology in the mirror, edited by Dr. Loredano Pollegioni, Dr. Jean-Pierre Mothet and Dr. Molla Gianluca.

A Tri-O-Bridged Diels-Alder Adduct from Cortex Mori Radicis

Sanggenon X, an unusual tri-O-bridged Diels-Alder adduct, was isolated from Cortex Mori Radicis. Its structure was established by spectroscopic analysis, including NMR and HR-MS (High Resolution Mass Spectrometry). Sanggenon X contained three O-bridged rings, where the oxygenated bridgeheads were all quaternary carbons. Chemical methylation was carried out to deduce the linkages of the three O-bridges. The absolute configuration was determined by calculating the ECD (Electronic Circular Dichroism) using the TDDFT (Time-Dependent Density Functional Theory) method. Sanggenon X showed significant antioxidant activity against Fe²⁺-Cys-induced lipid peroxidation in rat liver microsomes, and was as effective as the positive control, curcumin.

Liposomal Circular Dichroism (L-CD) of Arenoyl Derivatives of Sphingolipids. Amplification of Cotton Effects in Ordered Lipid Bilayers

Liposomal circular dichroism (L-CD) of acyclic amino alcohols exhibit amplification of Cotton effects when measured in highly uniform, unilamellar liposomes. The effect is likely due to intermolecular associations-H-aggregates-that self-assemble spontaneously within the lipid bilayer, and persists over long time scales. L-CD spectra of N,O,O'-tri-(6'methoxy-2'-naphthoyl)-d-erythro-sphingosine, or the corresponding dihydro-derivative (sphinganine), shows ~10-fold amplification of magnitudes of Cotton effects over conventional CD spectra recorded in isotropic solution.

Cofibrillization of Pathogenic and Functional Amyloid Proteins with Gold Nanoparticles against Amyloidogenesis

Biomimetic nanocomposites and scaffolds hold the key to a wide range of biomedical applications. Here we show, for the first time, a facile scheme of cofibrillizing pathogenic and functional amyloid fibrils via gold nanoparticles (AuNPs) and their applications against amyloidogenesis. This scheme was realized by β-sheet stacking between human islet amyloid polypeptide (IAPP) and the β-lactoglobulin "corona" of the AuNPs, as revealed by transmission electron microscopy, 3D atomic force microscopy, circular dichroism spectroscopy, and molecular dynamics simulations. The biomimetic AuNPs eliminated IAPP toxicity, enabled X-ray destruction of IAPP amyloids, and allowed dark-field imaging of pathogenic amyloids and their immunogenic response by human T cells. In addition to providing a viable new nanotechnology against amyloidogenesis, this study has implications for understanding the in vivo cross-talk between amyloid proteins of different pathologies.

Microindolinone A, a Novel 4,5,6,7-Tetrahydroindole, from the Deep-Sea-Derived Actinomycete Microbacterium sp. MCCC 1A11207

A novel indole, microindolinone A (1), was isolated from a deep-sea-derived actinomycete Microbacterium sp. MCCC 1A11207, together with 18 known compounds (2-19). By detailed analysis of the ¹H, ¹³C, HSQC, COSY, HMBC, high resolution electron spray ionization mass spectrum (HRESIMS), and circular dichroism (CD) data, the absolute configuration of 1 was elucidated as 5R-hydroxy-4,5,6,7-tetrahydroindole-4-one. It is noteworthy that 1 is the second example of a saturated indole isolated from nature.

Polarization resolved second harmonic microscopy

Second harmonic (SH) microscopy has proven to be a powerful imaging modality over the past years due to its intrinsic advantages as a multiphoton process with endogenous contrast specificity, which allows pinhole-less optical sectioning, non-invasive observation, deep tissue penetration, and the possibility of easier signal detection at visible wavelengths. Depending on the relative orientation between the polarization of the incoming light and the second-order susceptibility of non-centrosymmetric structures, SH microscopy provides the unique capacity to probe the absolute molecular structure of a broad variety of biological tissues without the necessity for additional labeling. In addition, SH microscopy, when working with polarimetry, provides clear and in-depth insights on the details of molecular orientation and structural symmetry. In this review, the working principles of the polarization resolving techniques and the corresponding implements of SH microscopy are elucidated, with focus on Stokes vector based polarimetry. An overview of the advancements on SH anisotropy measurements are also presented. Specifically, the recent progresses on the following three topics in polarization resolved SH microscopy will be elucidated, which include Stokes vector resolving for imaging molecular structure and orientation, 3-D structural chirality by SH circular dichroism, and correlation with fluorescence lifetime imaging (FLIM) for in vivo wound healing diagnosis. The potentials and challenges for future researches in exploring complex biological tissues are also discussed.

Photoluminescent Peptide-Based Nanostructures as FRET Donor for Fluorophore Dye

A great interest has been recently generated by the discovery that peptide-based nanostructures (NSs) endowed with cross-β structure may show interesting photoluminescent (PL) properties. It was shown that NSs formed by PEGylated hexaphenylalanine (PEG₈ -F6, PEG=polyethylene glycol) are able to emit at 460 nm when excited at 370 or 410 nm. Here, the possibility to transfer the fluorescence of these PEG₈ -F6-based NSs by foster resonance electron transfer (FRET) phenomenon to a fluorescent dye was explored. To achieve this aim, the 4-chloro-7-nitrobenzofurazan (NBD) dye was encapsulated in these NSs. Structural data in solution and in solid state, obtained by a variety of techniques (circular dichroism, Fourier-transform infrared spectroscopy, wide-angle X-ray scattering, and small-angle X-ray scattering), indicated that the organization of the peptide spine of PEG₈ -F6 NS, which consists of anti-parallel β-sheets separated by a dry interface made of interacting phenylalanine side chains, was maintained upon NBD encapsulation. The spectroscopic characterization of these NSs clearly showed a red-shift of the emission fluorescence peak both in solution and in solid state. This shift from 460 to 530 nm indicated that a FRET phenomenon from the peptide-based to the fluorophore-encapsulated NS occurred. FRET could also be detected in the PEG₈ -F6 conjugate, in which the NBD was covalently bound to the amine of the compound. On the basis of these results, it is suggested that the red-shift of the intrinsic PL of NSs may be exploited in the bio-imaging field.

A Vibrational Circular Dichroism Microsampling Accessory: Mapping Enhanced Vibrational Circular Dichroism in Amyloid Fibril Films

We report the first vibrational circular dichroism (VCD) measurement of spatial heterogeneity in a sample using infrared (IR) microsampling. Vibrational circular dichroism spectra are typically measured using a standard IR cell with an IR beam diameter of 10 mm or greater making it impossible to investigate the spatial heterogeneity of a solid film sample. We have constructed a VCD sampling assembly with either 3 mm or 1 mm spatial resolution. An XY-translation stage was used to measure spectra at different spatial locations producing IR and VCD maps of the sample. In addition, a rotating sample stage was employed using a dual photoelastic modulator (PEM) setup to suppress artifacts due to linear birefringence in solid-phase or film samples. Infrared and VCD mapping of an insulin fibril film has been carried out at both 3 and 1 mm spatial resolution, and lysozyme films were mapped at 1 mm resolution. The IR spectra of different spots vary in intensity due primarily to sample thickness. The changes in the VCD intensity across the map largely correlate to corresponding changes in the IR map. Closer inspection of the insulin map revealed changes in the relative intensities of the VCD spectra not present in the parent IR spectra, which indicated differences in the degree of supramolecular chirality of the fibrils in the various spatial regions. For lysozyme films, in addition to different degrees of supramolecular chirality, reversal of the net fibril chirality was observed. The large signal-to-noise ratio observed at 1 mm resolution implies the feasibility of further increasing the spatial resolution by one or two orders of magnitude for protein fibril film samples.

Vibrational Circular Dichroism: Recent Advances for the Assignment of the Absolute Configuration of Natural Products

Vibrational circular dichroism (VCD) emerged during the last decade as a reliable tool for the absolute configuration (AC) determination of organic compounds. The principles, instrumentation, and methodology applied prior to early 2013 were recently reviewed by us. Since VCD is a very dynamic field, the aim of this review is to update VCD advances for the AC assignment of terpenoids, aromatic compounds, alkaloids, and other natural products for the 2013-2014 period, when VCD was applied to the AC assignment of some 70 natural products. In addition, although discovered in 2012, a brief introduction to the VCD exciton coupling approach and its applications in natural products AC assignment is presented.

Optimization of the Number of Considered Conformers for the Absolute Configuration Determination of Catechin and Epicatechin Peracetates by VCD

Epimeric catechin and epicatechin peracetates offer the possibility to explore the number of conformers needed for the absolute configuration determination without affecting the reliability of the assignment. Comparisons between experimental and DFT B3PW91/DGDZVP calculated curves showed that, in molecules.where the conformational flexibility generates a large number of conformers, it is possible to significantly reduce the number of conformers needed to generate VCD results that provide secure absolute configuration assignment.