The Raman detection of peptide tyrosine phosphorylation

A Chemometric Approach to Assess the Rheological Properties of Durum Wheat Dough by Indirect FTIR Measurements

Rheological measurements and FTIR spectroscopy were used to characterize different doughs, obtained by commercial and monovarietal durum wheat flours (Cappelli and Karalis). Rheological frequency sweep tests were carried out, and the Weak Gel model, whose parameters may be related to gluten network extension and strength, was applied. IR analysis mainly focused on the Amide III band, revealing significant variations in the gluten network. Compared to the other varieties, Karalis semolina showed a higher amount of α-helices and a lower amount of β-sheets and random structures. Spectroscopic and rheological data were then correlated using Partial Least Squares regression (PLS) coupled with the Variable Importance in Projection (VIP) technique. The combined use of the techniques provided useful insights into the interplay among protein structures, gluten network features, and rheological properties. In detail, β-sheets and α-helices protein conformations were shown to significantly affect the gluten network's mechanical strength.

Early cardiac-chamber-specific fingerprints in heart failure with preserved ejection fraction detected by FTIR and Raman spectroscopic techniques

The pathophysiology of heart failure with preserved ejection fraction (HFpEF) is a matter of investigation and its diagnosis remains challenging. Although the mechanisms that are responsible for the development of HFpEF are not fully understood, it is well known that nearly 80% of patients with HFpEF have concomitant hypertension. We investigated whether early biochemical alterations were detectable during HFpEF progression in salt-induced hypertensive rats, using Fourier-transformed infrared (FTIR) and Raman spectroscopic techniques as a new diagnostic approach. Greater protein content and, specifically, greater collagen deposition were observed in the left atrium and right ventricle of hypertensive rats, together with altered metabolism of myocytes. Additionally, Raman spectra indicated a conformational change, or different degree of phosphorylation/methylation, in tyrosine-rich proteins. A correlation was found between tyrosine content and cardiac fibrosis of both right and left ventricles. Microcalcifications were detected in the left and right atria of control animals, with a progressive augmentation from six to 22 weeks. A further increase occurred in the left ventricle and right atrium of 22-week salt-fed animals, and a positive correlation was shown between the mineral deposits and the cardiac size of the left ventricle. Overall, FTIR and Raman techniques proved to be sensitive to early biochemical changes in HFpEF and preceded clinical humoral and imaging markers.

Towards Raman-Based Screening of Acute Lymphoblastic Leukemia-Type B (B-ALL) Subtypes

Simple Summary

Acute lymphoblastic leukemia (ALL) is the most common pediatric malignancy originating from abnormal lymphoid progenitor cells. Since ALL is genetically highly heterogenous, more sensitive and rapid methods for identifying the molecular subtype of ALL are still being searched, and Raman spectroscopy (RS) has a chance of becoming a valuable tool for this purpose. Herein, the RS was applied to analyze normal B cells and three subtypes of B-ALL, characterized by the presence of the product of gene fusion, i.e., BCR-ABL1, TEL-AML1, and TCF3-PBX1. The classification and discrimination of normal and neoplastic cells were carried out with the chemometric approach. Normal B cells were characterized mostly by bands assigned to nucleic acids and proteins, whereas three subtypes of ALL appeared to contain a higher lipid content. Spectral differences between particular ALL subtypes were modest. The results lead to the conclusion that RS has the potential as a diagnostic tool in clinical practice.

Abstract

Acute lymphoblastic leukemia (ALL) is the most common type of malignant neoplasms in the pediatric population. B-cell precursor ALLs (BCP-ALLs) are derived from the progenitors of B lymphocytes. Traditionally, risk factors stratifying therapy in ALL patients included age at diagnosis, initial leukocytosis, and the response to chemotherapy. Currently, treatment intensity is modified according to the presence of specific gene alterations in the leukemic genome. Raman imaging is a promising diagnostic tool, which enables the molecular characterization of cells and differentiation of subtypes of leukemia in clinical samples. This study aimed to characterize and distinguish cells isolated from the bone marrow of patients suffering from three subtypes of BCP-ALL, defined by gene rearrangements, i.e., BCR-ABL1 (Philadelphia-positive, t(9;22)), TEL-AML1 (t(12;21)) and TCF3-PBX1 (t(1;19)), using single-cell Raman imaging combined with multivariate statistical analysis. Spectra collected from clinical samples were compared with single-cell spectra of B-cells collected from healthy donors, constituting the control group. We demonstrated that Raman spectra of normal B cells strongly differ from spectra of their malignant counterparts, especially in the intensity of bands, which can be assigned to nucleic acids. We also showed that the identification of leukemia subtypes could be automated with the use of chemometric methods. Results prove the clinical suitability of Raman imaging for the identification of spectroscopic markers characterizing leukemia cells.

Chirality in peptide-based materials: From chirality effects to potential applications

Chirality is ubiquitous in nature with primary cellular functions that include construction of right-/left-handed helix and selective communications among diverse biomolecules. Of particularly intriguing are the chiral peptide-based materials that can be deliberately designed to change physicochemistry properties via tuning peptide sequences. Critically, understanding their chiral effects are fundamental for the development of novel materials in chemistry and biomedicine fields. Here, we review recent researches on chirality in peptide-based materials, summarizing relevant typical chiral effects towards recognition, amplification, and induction. Driven forces for the chiral discrimination in affinity interaction as well as the handedness preferences in supramolecular structure formation at both the macroscale and microscale are illustrated. The implementation of such chirality effects of artificial copolymers, assembled aggregates and their composites in the fields of bioseparation and bioenrichment, cell incubation, protein aggregation inhibitors, chiral smart gels, and bionic electro devices are also presented. At last, the challenges in these areas and possible directions are pointed out. The diversity of chiral roles in the origin of life and chirality design in different organic or composite systems as well as their applications in drug development and chirality detection in environmental protection are discussed.

Spectroscopic Evidence of the Salt-Induced Conformational Change around the Localized Electric Charges on the Protein Surface of Fibronectin Type III

The effect of salt on the electrostatic interaction of a protein is an important issue, because addition of salt affects protein stability and association/aggregation. Although adding salt is a generally recognized strategy to improve protein stability, this improvement does not necessarily occur. The lack of an effect upon the addition of salt was previously confirmed for the tenth fibronectin type III domain from human fibronectin (FN3) by thermal stability analysis. However, the detailed molecular mechanism is unknown. In the present study, by employing the negatively charged carboxyl triad on the surface of FN3 as a case study, the molecular mechanism of the inefficient NaCl effect on protein stability was experimentally addressed using spectroscopic methods. Complementary analysis using Raman spectroscopy and 8-anilino-1-naphthalenesulfonic acid fluorescence revealed the three-phase behavior of the salt-protein interaction between NaCl and FN3 over a wide salt concentration range from 100 mM to 4.0 M, suggesting that the Na⁺-specific binding to the negatively charged carboxyl triad causes a local conformational change around the binding site with an accompanying structural change in the overall protein, which contributes to the protein's structural destabilization. This spectroscopic evidence clarifies the molecular understanding of the inefficiency of salt to improve protein stability. The findings will inform the optimization of formulation conditions.

Aberrant Protein Phosphorylation in Cancer by Using Raman Biomarkers

(1) Background: Novel methods are required for analysing post-translational modifications of protein phosphorylation by visualizing biochemical landscapes of proteins in human normal and cancerous tissues and cells. (2) Methods: A label-free Raman method is presented for detecting spectral changes that arise in proteins due to phosphorylation in the tissue of human breasts, small intestines, and brain tumours, as well as in the normal human astrocytes and primary glioblastoma U-87 MG cell lines. Raman spectroscopy and Raman imaging are effective tools for monitoring and analysing the vibrations of functional groups involved in aberrant phosphorylation in cancer without any phosphorecognition of tag molecules. (3) Results: Our results based on 35 fresh human cancer and normal tissues prove that the aberrant tyrosine phosphorylation monitored by the unique spectral signatures of Raman vibrations is a universal characteristic in the metabolic regulation in different types of cancers. Overexpressed tyrosine phosphorylation in the human breast, small intestine and brain tissues and in the human primary glioblastoma U-87 MG cell line was monitored by using Raman biomarkers. (4) We showed that the bands at 1586 cm-1 and 829 cm-1, corresponding to phosphorylated tyrosine, play a pivotal role as a Raman biomarker of the phosphorylation status in aggressive cancers. We found that the best Raman biomarker of phosphorylation is the 1586/829 ratio showing the statistical significance at p Values of ≤ 0.05. (5) Conclusions: Raman spectroscopy and imaging have the potential to be used as screening functional assays to detect phosphorylated target proteins and will help researchers to understand the role of phosphorylation in cellular processes and cancer progression. The abnormal and excessive high level of tyrosine phosphorylation in cancer samples compared with normal samples was found in the cancerous human tissue of breasts, small intestines and brain tumours, as well as in the mitochondria and lipid droplets of the glioblastoma U-87 MG cell line. Detailed insights are presented into the intracellular oncogenic metabolic pathways mediated by phosphorylated tyrosine.

Combined Raman and polarization sensitive holographic imaging for a multimodal label-free assessment of human sperm function

Raman microspectroscopy (RM) and polarization sensitive digital holographic imaging (PSDHI) are valuable analytical tools in biological and medical research, allowing the detection of both biochemical and morphological variations of the sample without labels or long sample preparation. Here, using this multi-modal approach we analyze in vitro human sperm capacitation and the acrosome reaction induced by heparin. The multimodal microscopy provides morphofunctional information that can assess the sperms ability to respond to capacitation stimuli (sperm function). More precisely, the birefringence analysis in sperm cells can be used as an indicator of its structural normality. Indeed, digital holography applied for polarization imaging allows for revelation of the polarization state of the sample, showing a total birefringence of the sperm head in non-reacted spermatozoa, and a birefringence localized in the post-acrosomal region in reacted spermatozoa. Additionally, RM allows the detection and spectroscopic characterization of protein/lipid delocalization in the plasma and acrosomal membranes that can be used as valuable Raman biomarkers of sperm function. Interestingly, these spectral variations can be correlated with different time phases of the cell capacitation response. Although further experimentation is required, the proposed multimodal approach could represent a potential label-free diagnostic tool for use in reproductive medicine and the diagnosis of infertility.

Study on the pathological and biomedical characteristics of spinal cord injury by confocal Raman microspectral imaging

Confocal Raman microspectral imaging (CRMI) in combination with multivariate analysis was used to study pathological progression after spinal cord injury (SCI). By establishing moderate contusion in rat models, ex vivo longitudinal spinal cord tissue sections were prepared for microspectroscopic analysis. Comparative studies were then performed to determine the pathological distinctions among before injury (BI), one day post-injury (1 DPI), seven days post-injury (7 DPI), and 14 days post-injury (14 DPI) groups. Multivariate analysis algorithms, including K-mean cluster analysis (KCA) and principal component analysis (PCA), were conducted to highlight biochemical and structural variations after tissue damage. It is confirmed that typical spectral features and profiles can illustrate some fundamental and significant pathological processes post-injury, such as neuron apoptosis, hemorrhage, demyelination, and chondroitin sulfate proteoglycans (CSPGs) upregulation. Further, by establishing spectra-structure correlations, the reconstructed spectral images revealed some minute and important morphological characteristics following tissue injury, such as glial scar formation surrounding the cavity structure. The observed spectral phenomena also provide a detailed view on relevant pathobiological factors, which are involved in the spread of secondary damage after traumatic spinal cord injury. Our findings not only provide a spectral perspective to the well-known cellular mechanisms underlying SCI, but further provide a sound basis for developing real-time Raman methodologies to evaluate the prognostic factors and therapeutic results of SCI.

Drop-Coating Deposition Raman (DCDR) Spectroscopy as a Tool for Membrane Interaction Studies: Liposome-Porphyrin Complex

Drop-coating deposition Raman (DCDR) spectroscopy is based on the measurement of a sample that has been preconcentrated by being dried on a special hydrophobic plate. In addition to its higher sensitivity, the advantage of DCDR over the conventional Raman spectroscopy is the small sample volume needed, the lack of interference from solvents, and the capability of segregating any impurities present and separating components in more complex samples. In this study, DCDR spectroscopy was employed to investigate the complex of the cationic copper(II) 5,10,15,20-tetrakis(1-methyl-4-pyridyl) porphyrin (CuTMPyP) and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) liposomes. Drop-coating deposition Raman spectra were treated using factor analysis (FA), which led to the following conclusions: (i) the distribution of CuTMPyP in the complex is not homogenous, (ii) the DCDR technique segregates complexed and noncomplexed parts of the sample, (iii) the spectral changes caused by the drying process and by the interaction of CuTMPyP with the DPPC liposomes can be distinguished, and (iv) the porphyrin molecules interacting with DPPC affect both the order-disorder properties of the lipid chains and the lipid head.

Characterization of biofluids prepared by sessile drop formation

Sessile drop formation, also called drop deposition, has been studied as a potential medical diagnostic, but the effects of complex biofluid rheology on the final deposition pattern are not well understood. We studied two model biofluids, blood plasma and synovial fluid, when deposited onto slightly hydrophilic substrates forming a contact angle of 50-90°. Drops were imaged during the evaporation process and geometric properties of the drop, such as contact angle and drop height, were calculated from the images. The resulting dried biofluid drops were then examined using light microscopy and Raman spectroscopy to assess morphological and chemical composition of the dried drop. The effect of substrate contact angle (surface wetting) and fluid concentration was examined. We found that when biofluids are deposited onto slightly hydrophilic surfaces, with a contact angle of 50-90°, a ring-shaped deposit was formed. Analysis of the drying drop's geometric properties indicates that biofluid dynamics follow the piling model of drop formation, as proposed by Deegan et al. The final deposition pattern varied with substrate surface and concentration, as shown by light microscopy photos of dried drops. The chemical composition of the outer ring was minimally affected by substrate surface, but the spatial heterogeneity of protein distribution within the ring varied with concentration. These results indicate that biofluid drop deposition produces ring-shaped deposits which can be examined by multiple analytical techniques.

Clinical study of noninvasive in vivo melanoma and nonmelanoma skin cancers using multimodal spectral diagnosis

The goal of this study was to determine the diagnostic capability of a multimodal spectral diagnosis (SD) for in vivo noninvasive disease diagnosis of melanoma and nonmelanoma skin cancers. We acquired reflectance, fluorescence, and Raman spectra from 137 lesions in 76 patients using custom-built optical fiber-based clinical systems. Biopsies of lesions were classified using standard histopathology as malignant melanoma (MM), nonmelanoma pigmented lesion (PL), basal cell carcinoma (BCC), actinic keratosis (AK), and squamous cell carcinoma (SCC). Spectral data were analyzed using principal component analysis. Using multiple diagnostically relevant principal components, we built leave-one-out logistic regression classifiers. Classification results were compared with histopathology of the lesion. Sensitivity/specificity for classifying MM versus PL (12 versus 17 lesions) was 100%/100%, for SCC and BCC versus AK (57 versus 14 lesions) was 95%/71%, and for AK and SCC and BCC versus normal skin (71 versus 71 lesions) was 90%/85%. The best classification for nonmelanoma skin cancers required multiple modalities; however, the best melanoma classification occurred with Raman spectroscopy alone. The high diagnostic accuracy for classifying both melanoma and nonmelanoma skin cancer lesions demonstrates the potential for SD as a clinical diagnostic device.

Decavanadate, decaniobate, tungstate and molybdate interactions with sarcoplasmic reticulum Ca(2+)-ATPase: quercetin prevents cysteine oxidation by vanadate but does not reverse ATPase inhibition

Recently we demonstrated that the decavanadate (V(10)) ion is a stronger Ca(2+)-ATPase inhibitor than other oxometalates, such as the isoelectronic and isostructural decaniobate ion, and the tungstate and molybdate monomer ions, and that it binds to this protein with a 1 : 1 stoichiometry. The V(10) interaction is not affected by any of the protein conformations that occur during the process of calcium translocation (i.e. E1, E1P, E2 and E2P) (Fraqueza et al., J. Inorg. Biochem., 2012). In the present study, we further explore this subject, and we can now show that the decaniobate ion, [Nb(10) = Nb(10)O(28)](6-), is a useful tool in deducing the interaction and the non-competitive Ca(2+)-ATPase inhibition by the decavanadate ion [V(10) = V(10)O(28)](6-). Moreover, decavanadate and vanadate induce protein cysteine oxidation whereas no effects were detected for the decaniobate, tungstate or molybdate ions. The presence of the antioxidant quercetin prevents cysteine oxidation, but not ATPase inhibition, by vanadate or decavanadate. Definitive V(IV) EPR spectra were observed for decavanadate in the presence of sarcoplasmic reticulum Ca(2+)-ATPase, indicating a vanadate reduction at some stage of the protein interaction. Raman spectroscopy clearly shows that the protein conformation changes that are induced by V(10), Nb(10) and vanadate are different from the ones induced by molybdate and tungstate monomer ions. Here, Mo and W cause changes similar to those by phosphate, yielding changes similar to the E1P protein conformation. The putative reduction of vanadium(V) to vanadium(IV) and the non-competitive binding of the V(10) and Nb(10) decametalates may explain the differences in the Raman spectra compared to those seen in the presence of molybdate or tungstate. Putting it all together, we suggest that the ability of V(10) to inhibit the Ca(2+)-ATPase may be at least in part due to the process of vanadate reduction and associated protein cysteine oxidation. These results contribute to the understanding and application of these families of mono- and polyoxometalates as effective modulators of many biological processes, particularly those associated with calcium homeostasis.

Casting new physicochemical light on the fundamental biological processes in single living cells by using Raman microspectroscopy

This Personal Account highlights the capabilities of spontaneous Raman microspectroscopy for studying fundamental biological processes in a single living cell. Raman microspectroscopy provides time- and space-resolved vibrational Raman spectra that contain detailed information on the structure and dynamics of biomolecules in a cell. By using yeast as a model system, we have made great progress in the development of this methodology. The results that we have obtained so far are described herein with an emphasis placed on how three cellular processes, that is, cell-division, respiration, and cell-death, are traced and elucidated with the use of time- and space-resolved structural information that is extracted from the Raman spectra. For cell-division, compositional- and structural changes of various biomolecules are observed during the course of the whole cell cycle. For respiration, the redox state of mitochondrial cytochromes, which is inferred from the resonance Raman bands of cytochromes, is used to evaluate the respiration activity of a single cell, as well as that of isolated mitochondrial particles. Special reference is made to the "Raman spectroscopic signature of life", which is a characteristic Raman band at 1602 cm(-1) that is found in yeast cells. This signature reflects the cellular metabolic activity and may serve as a measure of mitochondrial dysfunction. For cell-death, "cross-talk" between the mitochondria and the vacuole in a dying cell is suggested.

Disentangling dynamic changes of multiple cellular components during the yeast cell cycle by in vivo multivariate Raman imaging

Cellular processes are intrinsically complex and dynamic, in which a myriad of cellular components including nucleic acids, proteins, membranes, and organelles are involved and undergo spatiotemporal changes. Label-free Raman imaging has proven powerful for studying such dynamic behaviors in vivo and at the molecular level. To construct Raman images, univariate data analysis has been commonly employed, but it cannot be free from uncertainties due to severely overlapped spectral information. Here, we demonstrate multivariate curve resolution analysis for time-lapse Raman imaging of a single dividing yeast cell. A four-dimensional (spectral variable, spatial positions in the two-dimensional image plane, and time sequence) Raman data "hypercube" is unfolded to a two-way array and then analyzed globally using multivariate curve resolution. The multivariate Raman imaging thus accomplished successfully disentangles dynamic changes of both concentrations and distributions of major cellular components (lipids, proteins, and polysaccharides) during the cell cycle of the yeast cell. The results show a drastic decrease in the amount of lipids by ~50% after cell division and uncover a protein-associated component that has not been detected with previous univariate approaches.

Cell kinase activity assay based on surface enhanced Raman spectroscopy

Kinases control many important aspects of cell behavior, such as signal transduction, growth/differentiation, and tumorogenesis. Current methods for assessing kinase activity often require specific antibodies, and/or radioactive labeling. Here we demonstrated a novel detection method to assess kinase activity based on surface enhanced Raman spectroscopy (SERS). Raman signal was obtained after amplification by silver nanoparticles. The sensitivity of this method was comparable to fluorescence measurement of peptide concentration. When purified kinase enzyme was used, the detection limit was comparable to conventional radio-labeling method. We further demonstrated the feasibility to measure kinase activity in crude cell lysate. We suggested this SERS-based kinase activity assay could be a new tool for biomedical research and application.

Ratiometric Raman spectroscopy for quantification of protein oxidative damage

A novel ratiometric Raman spectroscopic (RMRS) method has been developed for quantitative determination of protein carbonyl levels. Oxidized bovine serum albumin (BSA) and oxidized lysozyme were used as model proteins to demonstrate this method. The technique involves conjugation of protein carbonyls with dinitrophenyl hydrazine (DNPH), followed by drop coating deposition Raman spectral acquisition (DCDR). The RMRS method is easy to implement because it requires only one conjugation reaction, uses a single spectral acquisition, and does not require sample calibration. Characteristic peaks from both protein and DNPH moieties are obtained in a single spectral acquisition, allowing the protein carbonyl level to be calculated from the peak intensity ratio. Detection sensitivity for the RMRS method is approximately 0.33 pmol carbonyl per measurement. Fluorescence and/or immunoassay-based techniques only detect a signal from the labeling molecule and, thus, yield no structural or quantitative information for the modified protein, whereas the RMRS technique allows protein identification and protein carbonyl quantification in a single experiment.

Plasma-enhanced synthesis of thin fluoropolymer layers with low Raman and fluorescence backgrounds

Radio-frequency (RF) plasma enhanced chemical vapor deposition (PECVD) provides a promising way to deposit extremely hydrophobic, highly adherent nanometer- to micrometer-thick films with thermal stability, a low coefficient of friction, a low dielectric constant, and a low value of surface energy. We describe the synthesis of these fluorinated thin films using hexafluoropropene as starting material and discuss their properties. These coatings, applied to stainless steel, provide ideal substrates for Raman spectroscopy, when extremely low backgrounds are required. Raman spectroscopy measurements of a low-concentration protein film are used to demonstrate sensitivity and level of detectability.

Non-resonance micro-Raman spectroscopic studies on crystalline domoic acid and its aqueous solutions

Domoic acid (DA) is a neurotoxin naturally present in the marine ecosystem. Since DA's toxicity has been explained by its molecular structure and particularly because of its ethylenic double bond, spectroscopic investigation of this molecule is of importance. We carried out Raman spectroscopy on crystalline DA and on DA in aqueous solutions (28,000-25 ng DA/mL) and assigned Raman modes in comparison with the Raman spectra of its substructures. Noise-free, clear Raman signal from the solutions containing low concentrations of DA were obtained by applying the drop coating deposition Raman (DCDR) technique. Raman spectra reveal that crystalline DA exists in the zwitterionic form. The Raman spectra of the DA aqueous solutions were analysed in the light of their pH whereas the variation in the spectra was attributed to the hydration, the degree of protonation and crystallinity of the solid film. We show that DCDR can be applied for the rapid detection of domoic acid down to 25 ng DA/mL (0.025 ppm).

Analysis of insulin amyloid fibrils by Raman spectroscopy

The formation of amyloid fibrils from insulin is investigated using drop-coating-deposition-Raman (DCDR) difference spectroscopy and atomic force microscopy (AFM). Fibrils formed using various co-solvents and heating cycles are found to induce the appearance of Raman difference peaks in the amide I (approximately 1675 cm(-1)), amide III (approximately 1220 cm(-1)), and peptide backbone (approximately 1010 cm(-1)), consistent with an increase in beta-sheet content. Comparisons of results obtained from fibrils in either H2O or D2O suggest that the NH/ND stretch bands (at approximately 3300 cm(-1)/ approximately 2400 cm(-1)) are also enhanced in intensity upon fibril formation. If there is any water trapped in the core of the fibrils its OH/OD Raman intensity is too small to be detected in the presence of the stronger NH/ND bands which appear in the same region. AFM is used to confirm the formation of fibrils of about 5 nm diameter (and various lengths).

Validation of the drop coating deposition Raman method for protein analysis

Drop coating deposition Raman (DCDR) spectroscopy is critically evaluated to establish the limits to which it may be used to detect changes in protein conformation, binding, and purity. Difference spectroscopy is used to evaluate the reproducibility of the DCDR spectra under various experimental conditions. The results indicate (i) the absence of thermal/photochemical laser damage induced by the Raman excitation laser under typical DCDR data collection conditions, (ii) the reproducibility of DCDR spectra from samples with different volumes or concentrations, (iii) the water content of DCDR protein deposits and associated spectral signatures, and (iv) the degree of similarity between solution Raman spectra and DCDR spectra.

Detection of amino acid and peptide phosphate protonation using Raman spectroscopy

Raman spectra of phosphorylated amino acids and peptides undergo pH-dependent changes attributed to protonation of -OPO(3)(2-) (dibasic) to -OPO(3)H(-) (monobasic). Bands at approximately 980 and 1080cm(-1) in solution Raman spectra of phosphoserine and phosphothreonine are assigned to the monobasic and dibasic phosphate groups, respectively. Calibrated Raman peak area ratio measurements, performed as a function of pH, are used to determine the corresponding pKa values of 5.6 (phosphoserine) and 5.9 (phosphothreonine). In peptides, the phosphate Raman bands are difficult to distinguish due to interference from other neighboring bands (particularly those derived from aromatic amino acid residues) as well as the relatively low solubility of peptides. Nevertheless, drop coating deposition Raman (DCDR) spectra obtained from 100-microM peptide solutions reveal pH-dependent second derivative features at approximately 980 and 1080cm(-1), which are indicative of phosphate protonation.

Identification of insulin variants using Raman spectroscopy

Drop coating deposition Raman (DCDR) spectroscopy is used to obtain high-quality normal Raman spectra from small volumes (10 microl) of dilute insulin solutions (3-400 microM) for spectral identification and chromatographic detection. The results are used to demonstrate the spectroscopic classification (identification) of three natural insulin variants-human, bovine, and porcine-that differ by between one and three amino acid residues. DCDR measurements were performed on solutions obtained from reverse phase high-performance liquid chromatography (RP-HPLC) eluent fractions, either before or after lyophilization. Classification is demonstrated using replicate DCDR measurements, followed by normalized Savitsky-Golay second derivative preprocessing and partial least squares training with either leave-one-out or batch-to-batch testing.