Investigating starch gelatinization through Stokes vector resolved second harmonic generation microscopy

Spatially Resolved Chiroptical Spectroscopies Emphasizing Recent Applications to Thin Films of Chiral Organic Dyes

Instrumental techniques able to identify and structurally characterize the aggregation states in thin films of chiral organic π-conjugated materials, from the first-order supramolecular arrangement up to the microscopic and mesoscopic scale, are very helpful for clarifying structure-property relationships. Chiroptical imaging is currently gaining a central role, for its ability of mapping local supramolecular structures in thin films. The present review gives an overview of electronic circular dichroism imaging (ECDi), circularly polarized luminescence imaging (CPLi), and vibrational circular dichroism imaging (VCDi), with a focus on their applications on thin films of chiral organic dyes as case studies.

Multiphoton microscopy is a nondestructive label-free approach to investigate the 3D structure of gas cell walls in bread dough

During the different steps of bread-making, changes in the microstructure of the dough, particularly in the gas cell walls (GCW), have a major influence on the final bread crumb texture. Investigation of the spatial conformation of GCWs is still a challenge because it requires both high resolutions and 3D depth imaging. The originality of the present work lies in the use of label-free non-destructive multiphoton microscopy (NLOM) to image the 3D structure of GCWs, shedding light on their behavior and organization in wheat bread dough. We demonstrated that second and third harmonic generation (SHG, THG) allow imaging, respectively, of starch granules and interfaces in bread dough, while the gluten matrix was detected via two-photon excitation fluorescence (TPEF). Last, a distinction between the gluten network and starch granules was achieved using gluten endogenous fluorescence (EF) imaging, while the position, size, and 3D orientation of starch granules in GCWs were determined from harmonic imaging, made possible by the acquisition of backward and forward SHG with linear polarization. These innovative experiments highlight the strengths of NLOM for a label-free characterization of bread dough microstructure for the first time, in order to understand the role of starch granules in dough stabilization.

Revealing the Structural Organization of Gamma-irradiated Starch Granules Using Polarization-resolved Second Harmonic Generation Microscopy

Starch is a semi-crystalline macromolecule with the presence of amorphous and crystalline components. The amorphous amylose and crystalline amylopectin regions in starch granules are susceptible to certain physical modifications, such as gamma irradiation. Polarization-resolved second harmonic generation (P-SHG) microscopy in conjunction with SHG-circular dichroism (CD) was used to assess the three-dimensional molecular order and inherent chirality of starch granules and their reaction to different dosages of gamma irradiation. For the first time, the relationship between starch achirality (χ21/χ16 and χ22/χ16) and chirality (χ14/χ16) determining susceptibility tensor ratios has been elucidated. The results showed that changes in the structure and orientation of long-chain amylopectin were supported by the decrease in the SHG anisotropy factor and the χ22/χ16 ratio. Furthermore, SHG-CD illustrated the molecular tilt angle by revealing the arrangement of amylopectin molecules pointing either upward or downward owing to molecular polarity.

Investigation into the structure of crystalline maltodextrin particles by second harmonic generation microscopy

Crystalline maltodextrin particles (CMPs) were investigated using polarization-sensitive second harmonic generation (PSHG) microscopy to determine changes in their crystalline organization due to crystal type (A- and B-type) and hydration for application as starch model systems. Optimization of their synthesis resulted in intense SHG emission, exceeding maize starch granules. PSHG data showed that CMPs have a radial macrostructure with respect to their nucleation regions, fitted ρ values of 2-6, and some similar hydration variations, mimicking starch granules and validating that CMPs may be used as a model system for improved understanding of the SHG properties and applications of starch granules.

Second harmonic generation microscopy: a powerful tool for bio-imaging

Second harmonic generation (SHG) microscopy is an important optical imaging technique in a variety of applications. This article describes the history and physical principles of SHG microscopy and its more advanced variants, as well as their strengths and weaknesses in biomedical applications. It also provides an overview of SHG and advanced SHG imaging in neuroscience and microtubule imaging and how these methods can aid in understanding microtubule formation, structuration, and involvement in neuronal function. Finally, we offer a perspective on the future of these methods and how technological advancements can help make SHG microscopy a more widely adopted imaging technique.

Advanced microscopy techniques for revealing molecular structure of starch granules

Starch is a major source of our daily diet and it is important to understand the molecular structure that plays a significant role in its wide number of applications. In this review article, microscopic structures of starch granules from potato, corn, rice canna, tania, wheat, sweet potato, and cassava are revealed using advanced microscopic techniques. Optical microscopy depicts the size and shape, polarization microscopy shows the anisotropy properties of starch granules, scanning electron microscopy (SEM) displays surface topography, and confocal microscopy is used to observe the three-dimensional internal structure of starch granules. The crystallinity of starch granules is revealed by second harmonic generation (SHG) microscopy and atomic force microscopy (AFM) provides mechanical properties including strength, texture, and elasticity. These properties play an important role in understanding the stability of starch granules under various processing conditions like heating, enzyme degradation, and hydration and determining its applications in various industries such as food packaging and textile industries.

Microscopic and spectroscopic characterization of rice and corn starch

Starch granules from rice and corn were isolated, and their molecular mechanism on interaction with α-amylase was characterized through biochemical test, microscopic imaging, and spectroscopic measurements. The micro-scale structure of starch granules were observed under an optical microscope and their average size was in the range 1-100 μm. The surface topological structures of starch with micro-holes due to the effect of α- amylase were also visualized under scanning electron microscope. The crystallinity was confirmed by X-ray diffraction patterns as well as second-harmonic generation microscopy. The change in chemical bonds before and after hydrolysis of the starch granules by α- amylase was determined by Fourier transform infrared spectroscopy. Combination of microscopy and spectroscopy techniques relates structural and chemical features that explain starch enzymatic hydrolysis which will provide a valid basis for future studies in food science and insights into the energy transformation dynamics.

Characterization of Pancreatic Cancer Tissue Using Multiphoton Excitation Fluorescence and Polarization-Sensitive Harmonic Generation Microscopy

Thin tissue sections of normal and tumorous pancreatic tissues stained with hematoxylin and eosin were investigated using multiphoton excitation fluorescence (MPF), second harmonic generation (SHG), and third harmonic generation (THG) microscopies. The cytoplasm, connective tissue, collagen and extracellular structures are visualized with MPF due to the eosin stain, whereas collagen is imaged with endogenous SHG contrast that does not require staining. Cellular structures, including membranous interfaces and nuclear components, are seen with THG due to the aggregation of hematoxylin dye. Changes in the collagen ultrastructure in pancreatic cancer were investigated by a polarization-sensitive SHG microscopy technique, polarization-in, polarization-out (PIPO) SHG. This involves measuring the orientation of the linear polarization of the SHG signal as a function of the linear polarization orientation of the incident laser radiation. From the PIPO SHG data, the second-order non-linear optical susceptibility ratio, χ(2) zzz '/χ(2) zxx ', was obtained that serves as a structural parameter for characterizing the tissue. Furthermore, by assuming C6 symmetry, an additional second-order non-linear optical susceptibility ratio, χ(2) xyz '/χ(2) zxx ', was obtained, which is a measure of the chirality of the collagen fibers. Statistically-significant differences in the χ(2) zzz '/χ(2) zxx ' values were found between tumor and normal pancreatic tissues in periductal, lobular, and parenchymal regions, whereas statistically-significant differences in the full width at half maximum (FWHM) of χ(2) xyz '/χ(2) zxx ' occurrence histograms were found between tumor and normal pancreatic tissues in periductal and parenchymal regions. Additionally, the PIPO SHG data were used to determine the degree of linear polarization (DOLP) of the SHG signal, which indicates the relative linear depolarization of the signal. Statistically-significant differences in DOLP values were found between tumor and normal pancreatic tissues in periductal and parenchymal regions. Hence, the differences observed in the χ(2) zzz '/χ(2) zxx ' values, the FWHM of χ(2) xyz '/χ(2) zxx ' values and the DOLP values could potentially be used to aid pathologists in diagnosing pancreatic cancer.

Evaluation of structural and molecular variation of starch granules during the gelatinization process by using the rapid Mueller matrix imaging polarimetry system

Starch is an essential and widely distributed natural material, but its detailed conformation and thermal transition properties are not yet well understood. We present a rapid Mueller matrix imaging system to explore the structural characteristics of starch granules by using 16 measurements with different incoming and outgoing polarizations. Due to the minimum rotation of the optical elements and the self-calibration ability of this system, the full Mueller matrix images can be accurately obtained within ten-odd seconds. Both structural and molecular features of the starch granule were investigated in the static state and gelatinization process by means of multiple optical characteristics deduced from the Mueller matrix. The experimental results for the structural changes during the gelatinization were close to other nonlinear optical approaches; moreover, the crystallinity and optical rotation of the starch granule are also determined through the use of this approach.

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.