Positive Dichroism Research Articles

26.4: Invited Paper: Preparation and Spectral quality of Naphthalene Derivatives Dichroic Liquid Crystal Dyes

A series of new dyes based on 4‐bromo‐1, 8‐naphthaliminde had been designed, synthesized and evaluated. Spectroscopic tests of those compounds exhibited that these dyes emitted yellow‐green fluorescence at 490‐520 nm and had high quantum yields 0.76. Those dyes suggested positive fluorescent dichroism, when dispersed in LC E7, SA and SF which were up to 0.51, 0.41, respectively. The dye and LC E7 were mixtured and then did the photoelectric test. Mass spectrometry, NMR and melting point were characterized. Ultraviolet and fluorescence spectra were tested in the isotropic solvents, and the absorption and emission spectra in the anisotropic liquid crystal.

Circular Dichroism in Cholesteric Liquid Crystals

The circular dichroism in cholesteric liquid crystals (CLCs) has been studied in the absorption bands of dissolved dichroic dyes possessing both the positive and negative linear dichroism. The circular dichroism is studied depending on the position of CLC selective reflection band with respect to the dye absorption band. The induced circular dichroism due to dye absorption is compared to intrinsic CLC circular dichroism due to the light diffraction from the periodic chiral structure.

Circularly polarized light to study linear magneto-optics for ferrofluids: θ-scan technique

Circularly polarized light can be divided into two vertically linearly polarized light beams with ±π/2 phase differences. In the presence of an external magnetic field, when circularly polarized light travels through a ferrofluid film, whose thickness is no more than that of λ/4 plate, magneto-optical, magnetic birefringence and dichroism effects cause the transmitted light to behave as elliptically polarized light. Using angular scan by a continuously rotating polarizer as analyzer, the angular (θ) distribution curve of relative intensity (T) corresponding to elliptically polarized light can be measured. From the T − θ curve having ellipsometry, the parameters such as the ratio of short to long axis, and angular orientation of the long axis to the vertical field direction can be obtained. Thus, magnetic birefringence and dichroism can be probed simultaneously by measuring magneto-optical, positive or negative birefringence and dichroism features from the transmission mode. The proposed method is called θ-scan technique, and can accurately determine sample stability, magnetic field direction, and cancel intrinsic light source ellipticity. This study may be helpful to further research done to ferrofluids and other similar colloidal samples with anisotropic optics.

Circular dichroism of chiral Majorana states.

Background: Majorana states in condensed matter devices may be of a localized nature, such as in hybrid semiconductor/superconductor nanowires, or chirally propagating along the edges such as in hybrid 2D quantum-anomalous Hall/superconductor structures.Results: We calculate the circular dichroism due to chiral Majorana states in a hybrid structure made of a quantum-anomalous Hall insulator and a superconductor. The optical absorption of chiral Majorana states is characterized by equally spaced absorption peaks of both positive and negative dichroism. In the limit of a very long structure (a 2D ribbon) peaks of a single sign are favored.Conclusion: Circular-dichroism spectroscopy of chiral Majorana states is suggested as a relevant probe for these peculiar states of topological matter.

Contributions of Hydroxyethyl Groups to the DNA Binding Affinities of Anthracene Probes

Contributions of hydroxyethyl functions to the DNA binding affinities of substituted anthracenes are evaluated by calorimetry and spectroscopy. Isothermal titration calorimetry indicated that binding of the ligands to calf thymus DNA (5 mM Tris buffer, 50 mM NaCl, pH 7.2, 25 degrees C) is exothermic. The binding constants increased from 1.5 x 10(4) to 1.7 x 10(6) M(-1) as a function of increase in the number of hydroxyethyl functions (0-4). DNA binding was accompanied by red-shifted absorption (approximately 630 cm(-1)), strong hypochromism (>65%), positive induced-circular dichroism bands, and negative linear dichroism signals. DNA binding, in general, increased the helix stabilities to a significant extent (DeltaT(m) approximately 7 degrees C, DeltaDeltaH approximately 3 kcal/mol, DeltaDeltaS approximately 6-20 cal/K.mol). The binding constants showed a strong correlation with the number of hydroxyethyl groups present on the anthracene ring system. Analysis of the binding data using the hydrophobicity parameter (Log P) showed a poor correlation between the binding affinity and hydrophobicity. This observation was also supported by a comparison of the affinities of probes carrying N-ethyl (Kb = 0.8 x 10(5) M(-1)) versus N-hydroxyethyl side chains (Kb = 5.5 x 10(5) M(-1)). These are the very first examples of a strong quantitative correlation between the DNA binding affinity of a probe and the number of hydroxyethyl groups present on the probe. These quantitative findings are useful in the rational design of new ligands for high-affinity binding to DNA.

Transition Metal Dithiolene Near-IR Dyes and Their Applications in Liquid Crystal Devices

Numerous commercial and military applications exist for guest–host liquid crystal (LC) devices operating in the near- to mid-IR region. Progress in this area has been hindered by the severe lack of near-IR dyes with good solubility in the LC host, low impact on the inherent order of the LC phase, good thermal and chemical stability, and a large absorbance maximum tunable by structural modification over a broad range of the near-IR region. Transition metal complexes based on nickel, palladium, or platinum dithiolene cores show substantial promise in meeting these requirements. These new dye complexes are extraordinarily stable, possess liquid crystalline phases in their own right with the proper terminal functional groups, and can have melting points below room temperature. The latter property is especially significant for producing liquid crystal/dye mixtures with both high dye concentration and good resistance to phase separation. Because transition metal dithiolenes are zerovalent, they can exhibit high solubility in LC hosts (up to 10 wt%). The λmax in these materials can range from 600 to 1500 nm, depending on structure. Depending on their overall molecular geometry and the choice of terminal functional groups, transition metal dithiolenes that show either positive or negative dichroism from the same basic core structure can be readily synthesized. When enantiomerically enriched terminal substituents are employed, nickel dithiolenes can induce a chiral mesophase in a nonchiral nematic host. This finding opens the possibility of generating novel LC mixtures with two degrees of tunability—an electronic absorbance band tunable by synthesis, and a selective reflection band tunable by temperature or applied electric field. In this paper, we overview our past and present activities in the design and synthesis of transition metal dithiolene dyes, show some specific applications examples for these materials as near-IR dyes in LC electro-optical devices, and present our most recent results in the computational modeling of physical and optical properties of this interesting class of organometallic optical materials.

Molecular arrangement of rhodamine 6G cations in the films of layered silicates: the effect of the layer charge

Montmorillonite nanocor saturated with Li+ cations was used for the synthesis of reduced charge montmorillonites (RCMs). The films of RCMs with intercalated rhodamine 6G (R6G) were prepared by adsorption of the dye on montmorillonite film deposited on a quartz slide. Penetration of the dye into the interlayer spaces of silicates was monitored by elemental analysis and X-ray diffraction measurements of the prepared R6G/silicate films. Polarized UV-Vis spectroscopy was used for the characterization of the molecular orientation of dye cations on silicate surface. There were several species of R6G cations in the interlayer spaces of the inorganic host, which was easily resolvable by the polarized spectra. The cations forming H-aggregates and absorbing light at low wavelengths were oriented in nearly perpendicular fashion, which was observed as a strongly positive dichroism, i.e., the increase of the dichroic ratio with film tilting. Adsorbed isolated dye cations were inclined at low angles with respect to the plane of the silicate surface. The layer charge of the inorganic host affected only partially the orientation of dye cations in the H-aggregates, but rather it controlled the extent of aggregation and in this way the number of nearly perpendicularly oriented cations.

Electron microscopic determination of silver incorporation in collagen fibers as a model of organic-metal chiral supramolecular structure with optical anisotropic properties

The high positive linear dichroism (LD) exhibited by silver-impregnated collagen fibers are thought to be caused by an ordered binding of silver nanoparticles to the collagen fibers. A correlation between LD, established by image analysis, and the size of silver particles as measured by electron microscopy was investigated in the same sections of silver-impregnated tendon collagen fibers. Silver particles were found to be periodically ordered along the collagen fibers, with some preference for the 67nm collagen period and collagen a bands. The silver particle diameter for a sample of n=279 was 18.51nm on average, with 95% of the values ranging between 17.90 and 19.14nm. The finding that interference colors are correlated to abnormal dispersion of birefringence characteristics supports the concept that collagen fibers induce an ordered binding of silver distributed along the fibers.

Spectroscopic characterization of the spinach Lhcb4 protein (CP29), a minor light-harvesting complex of photosystem II.

A spectroscopic characterization is presented of the minor photosystem II chlorophyll a/b-binding protein CP29 (or the Lhcb4 protein) from spinach, prepared by a modified form of a published protocol [Henrysson, T., Schroder, W. P., Spangfort, M. & Akerlund, H.-E. (1989) Biochim. Biophys. Acta 977, 301-308]. The isolation procedure represents a quicker, cheaper means of isolating this minor antenna protein to an equally high level of purity to that published previously. The pigment-binding protein shows similarities to other related light-harvesting complexes (LHCs), including the bulk complex LHCIIb but more particularly another minor antenna protein CP26 (Lhcb5). It is also, in the main, similar to other preparations of CP29, although some significant differences are discussed. In common with CP26, the protein binds about six chlorophyll a and two chlorophyll b molecules. Two chlorophyll b absorption bands are present at 638 and 650 nm and they are somewhat more pronounced than in a recent report [Giuffra, E., Zucchelli, G., Sandonà, D., Croce, R., Cugini, D., Garlaschi, F.M., Bassi, R. & Jennings, R.C. (1997) Biochem. 36, 12984-12993]. The bands give rise to positive and negative linear dichroism, respectively; both show negative CD bands (cf. bands with similar properties at 637 and 650 nm in CP26). Chlorophyll a absorption is dominated by a large contribution at 674 nm which also shows similarities to the major band in LHCIIb and CP26, while (as for CP26) a reduction in absorption around 670 nm is observed relative to the bulk complex. Principal differences from LHCIIb and CP26, and from other CP29 preparations, occur in the carotenoid region.

Orbital polarization inNiFe2O4measured byNi−2px-ray magnetic circular dichroism

We report a comprehensive study of the x-ray magnetic circular dichroism (XMCD) at the Ni ${L}_{2,3}$ edges of ${\mathrm{NiFe}}_{2}{\mathrm{O}}_{4}$ (trevorite), a ferrimagnetic compound which belongs to the class of strongly correlated $3d$ systems. The measured XMCD line shape is in good agreement with full-multiplet calculations for a ${3d}^{8}$ ground state including an octahedral crystal field of $10Dq=1.2$ eV. Using the XMCD sum rules, we find for the nickel an orbital to spin magnetic moment ratio of $L/S=0.27\ifmmode\pm\else\textpm\fi{}0.07,$ which means that the orbital contribution to the magnetic moment is 12\ifmmode\pm\else\textpm\fi{}2 %. The size of the error bar is not due to the neglect of band structure and hybridization, but due to Coster-Kronig transitions which give a transfer of spectral weight in the dichroism near the ${L}_{2}$ edge. Using the sum rules for the isotropic x-ray absorption spectrum, we obtain the expectation value of the spin-orbit interaction, which can be converted into an $L/S$ ratio of 0.34\ifmmode\pm\else\textpm\fi{}0.11. All sum-rule results were corrected for the influence of core-valence exchange interaction, which gives rise to jj mixing between the two absorption edges. The correction is done by comparing the calculated sum-rule results with the correct ground-state values calculated as a function of crystal-field strength. The 2-eV shoulder of the ${L}_{3}$ line shows a strong positive dichroism, which can be attributed to a spin-flip state. From the overall agreement between the experimental and theoretical results, we conclude that a localized model provides a good description for the orbital magnetization.

Photoregulated Liquid Crystal Alignment on Photoreactive Side-Chain Liquid-Crystalline Polymer

We describe the anisotropic photoreaction of photoreactive side-chain liquid-crystalline polymers (SLCPs), containing biphenyl mesogen and photoreactive cinnamoyl group, by irradiation with a linearly polarized (LP) UV light, and the alignment behavior of the low-molecular-weight (LMW) liquid crystal (LC) on them. The photo-crosslinked SLCP thin film photoreacted at room temperature showed very small negative dichroism, while that photoreacted in the LC temperature range of the film exhibited positive dichroism. The photoreacted SLCP film can align LMW-LC director \ ilden both parallel and perpendicular to the incident \\hatE vector of the LP-UV light. The alignment direction of the LMW-LC was controlled by the amount of photoreaction of the cinnamoyl group.

Characterisation of LHC II in the aggregated state by linear and circular dichroism spectroscopy

Absorption, linear dichroism and circular dichroism spectroscopy at 293 and 77 K have been used in order to further explore the process of energy quenching in LHC IIb, the main light-harvesting complex of photosystem II. Upon aggregation there was an enhancement of linear dichroism bands in the Q y absorption region of chlorophyll b. The absorption spectrum at 77 K of aggregates revealed new bands around 656 nm and 680 nm, characterised by positive linear dichroism and negative circular dichroism signals. In the circular dichroism spectrum of aggregates a characteristic change was seen in the carotenoid and chlorophyll b regions, an increase of the chlorophyll a transition at 438 nm (−) and decrease of the red most negative band at around 677 nm. The amplitude of this band was in a tight correlation with a fluorescence quenching occurring upon LHC II aggregation. A new transition appeared at 505 nm with positive linear dichroism signal. It is suggested that protein aggregation causes a change in conformation and association of some chlorophyll a, chlorophyll b and xanthophyll molecules. These features of the linear dichroism spectrum of the aggregates were also detected for thylakoids in which they were particularly enhanced at low pH, suggesting that at least part of the light harvesting complex in the thylakoid membrane is in an aggregated form and the extent of aggregation in vivo can be controlled by the thylakoid pH gradient.

Macrodipoles Unusual electric properties of biological macromolecules

The wide range of different effects induced by electric fields in biological macromolecules is clearly due to the unusual quality and quantity of their electric parameters. A general concept for a quantitative description of the polarizability of macromolecules remains to be established. In the case of DNA, experimental data indicate the existence of an effective polarization length N p ; at chain lengths N < N p the polarizability increases with N 2, whereas saturation is approached at N ≥ N p . The polarization length decreases with increasing ionic strength in close analogy to the Debye length, but is ∼10 times larger than the Debye length. The dynamics of DNA polarization at high field strengths has been observed in the ns time range and is consistent with biased field iduced ion dissociation. In the range of chain lengths from ∼400 to ∼850 base pairs DNA molecules exhibit permanent dipole moments, which are in a preferentially perpendicular direction to the end-to-end-vector, leading to a positive electric dichroism. These results are consistent with a “frozen” ensemble of bent DNA configurations and provide evidence for the existence of slow, non-elastic bending transitions. The electric parameters of proteins are usually dominated by a permanent anisotropy of the charge distribution, corresponding to permanent dipole moments of the order of several hundred Debye up to about 1500 Debye. Relatively small dipole moments of protein monomers add up to millions of Debye, when these proteins are in a vectorial organization in membrane patches, as found for bacteriorhodopsin and Na + K + -ATPase . In these cases the dipole vector may support vectorial ion transport. It is remarkable that the dipole moments of proteins usually show a relatively small dependence on the salt concentration; a rational for these observations is provided by a dipole potential at the plane of shear for rotational diffusion, which is defined in close analogy to the ζ-potential for translational diffusion. Symmetry breaking leading to huge electric dipole moments may be expected for mixed lipid vesicles: according to model calculations the phase separation of lipid components with and without net charges may lead to very high dipole moments; the expectation has been verified experimentally for vesicles containing DMPA and DMPC. The state of these systems should be extremely sensitive to electric fields. In summary, there is an unusual wide variation of electric parameters associated with biological macromolecules and with biomolecular assemblies, which is the basis for the complexity of different phenomena induced by electric fields in biological systems.

Electric linear dichroism as a new tool to study sequence preference in drug binding to DNA

An original approach using electric linear dichroism (ELD) and natural DNAs and polynucleotides of differing base composition has been developed with the aim to investigate the sequence-dependent recognition of DNA by drugs. Both intercalators and minor groove binders have been studied as well as certain hybrid molecules. The results indicate that the orientation of drugs upon binding to nucleic acids can change markedly according to the target sequence. Among the intercalators tested, only actinomycin D and hycanthone show a clear preference for GC- and AT -rich sequences, respectively. For minor groove binders, the linear dichroism showing a strong dependence on base composition of the DNA and polynucleotides is most pronounced. Netropsin and distamycin bind to DNA with a marked AT specificity. Hoechst 33258, berenil and DAPI exhibit positive and negative dichroism signals at AT and GC sites respectively, suggesting that at least two types of drug-DNA interaction are involved depending on the AT GC content of the DNA. Further investigations using polynucleotides with inosine substituted for guanosine, and competition experiments with intercalative drugs suggest that Hoechst 33258, berenil and DAPI interact with GC sequences via a non-classical intercalation process.

Electric parameters of Na +/K +-ATPase by measurements of the fluorescence-detected electric dichroism

The electric parameters of Na +/K +-ATPase labeled by FITC have been characterized by measurements of the fluorescence-detected electric dichroism. The fluorescence emission was measured with polarizers at the magic angle and the light for excitation was usually polarized parallel to the field vector. The FITC-Na +/K +-ATPase preparations exhibit a negative electric dichroism at field strengths up to about 600 V/cm and a positive dichroism at higher field strengths. Pulse reversal experiments reveal a dominant permanent electric moment at low electric field strengths and an increasing contribution from an induced electric moment at higher field strengths. The dichroism rise curves and the transients upon pulse reversal show two relaxation processes with opposite amplitudes, whereas the dichroism decay curves in most cases can be represented by single exponentials at a reasonable accuracy. The amplitude A 2 associated with the slower of the rise processes is dominant at low field strengths and also approaches saturation already at low field strengths. The dependence of A 2 on the electric field strength is consistent with the orientation function for permanent dipoles and cannot be represented by the orientation function for induced dipoles. The fitted permanent dipole moment is in the range of 3.5 · 10 −24 Cm [1 · 10 6 D] and shows only a relatively small decrease with increasing ionic strength. The stationary values of the electric dichroism up to field strengths E ≤ 800 V/cm can be represented with high accuracy by an orientation function for disk-shaped particles with a permanent moment along the particle symmetry axis and an induced moment along the semi-major axis. The permanent electric moment determined according to this function is consistent with the one obtained from the amplitudes A 2. In summary, our measurements indicate that Na +/K +-ATPase is associated with a large permanent electric moment directed perpendicular to the membrane plane. The dipole moment per ATPase monomer unit is estimated to be 1.4 · 10 −27Cm [430 ± 50 D].

DNA double helices with positive electric dichroism and permanent dipole moments: Non-symmetric charge distributions and “frozen” configurations

The stationary electric dichroism of DNA restriction fragments with chain lengths of 399, 587 and 859 base pairs (bp) is shown to be positive at low electric field strengths in the range up to about 1 kV/cm; the dichroism is changed to values with the usual negative sign at higher electric field strengths. A positive dichroism at low field strengths has been observed in buffers with Na +-cations of various ionic strengths from 1 to 11 m M. Experiments with reversal of the electric field vector indicate that the alignment is induced by a permanent electric moment at low field strengths; when the field strength is increased, the contribution from an induced dipole moment increases and becomes dominant at high field strengths. Thus, the permanent dipole mechanism is prevailing in the range with positive values of the dichroism found at low electric fields, whereas the induced dipole mechanism is dominant in the range with negative values of the stationary dichroism found at high electric fields. In contrast to these results obtained in buffers containing only monovalent ions, the electric dichroism is negative in the whole accessible range of electric field strengths, when the buffer contained 100 μ M Mg 2+. The stationary values of the dichroism are negative under all tested conditions of buffer concentrations and electric field strengths for DNA double helices with 95, 179, 256, 4361 and 48502 bp. The positive dichroism found in the intermediate range of chain lengths is reduced and finally changed to the standard negative dichroism by UV irradiation. Positive values of the linear dichroism are predicted by Monte Carlo simulations for ensembles of wormlike chains with the charge density and optical parameters of DNA double helices, when the polarizability along the chain axis is negligible and when the internal mobility is frozen. The positive dichroism is due to the fact that virtually all DNA fragments are bent by thermal agitation; most of these bent DNA's are asymmetric and, thus, are associated with permanent dipole moments; the majority of dipole vectors in a given range of chain lengths is perpendicular to the end-to-end vector and, thus, leads to positive values of the linear dichroism at low degrees of bending. The model predicts a chain length dependence consistent with the experimental one due to a superposition of chain length dependences of the dipole moment and of the dichroism. The change from positive to negative values of the linear dichroism at increasing field strengths suggests some increase of the polarizability with the field strength, but may also be partly due to field induced stretching. The absence of the special effect in the presence of Mg 2+ indicates a particularly high compensation of the phosphate charges by this ion. The time constants of electrooptical rise curves observed in the range with the positive stationary dichroism are rather close to those of the decay curves. It is shown by Brownian dynamics simulations that this result is consistent with the existence of a permanent dipole moment. The deviation from the standard expected for permanent dipoles is due to the fact that in the present case the dipole vectors are mainly perpendicular to the long axis of the molecules. The decrease of the positive dichroism by UV irradiation is consistent with the interpretation of the data by a frozen ensemble of configurations and indicates that photoproducts introduce flexible joints into the DNA chains.

Arylazopyrimidines as dichroic dyes for liquid crystals

Substituted 5-arylazopyrimidines and 2-arylazopyrimidines have been synthesized and investigated as dichroic dyes with positive dichroism, for liquid crystals. A promising series of 2-aryl-5-arylazopyrimidine dyes has been found, and a method is proposed for their synthesis by condensation of 2-aryl-5-aminopyrimidines with nitrosobenzenes in a superbasic medium.

Homology in Trivaline Complex Formation with dsDNA and ssRNA

It has been shown by the equilibrium dialysis that at a polyU concentration above the "critical" one, the complete polymer saturation with trivaline reaches approximately 0.7 trivaline molecules per one phosphate group. i.e. at these conditions peptide dimer occupies on polyU a site of three bases (phosphates) in length. The trivaline complex with polyU at a concentration lower than the "critical" one does not reveal any noticeable fluorescence, but has rather significant positive linear dichroism at 265 and 330 nm. The trivaline-nucleic acids complex has a significant fluorescence at any dsDNA concentration while with polyU it is only so at a concentration above the "critical" one. Electron microscopy has shown that at a rather high concentration of dsDNA molecules in solution a "biduplex" structure undergoes an additional stage of compaction, during which the extended particles more than 30 nm in diameter are formed.

Drug-DNA sequence-dependent interactions analysed by electric linear dichroism.

The interactions between 20 drugs and a variety of synthetic DNA polymers and natural DNAs were studied by electric linear dichroism (ELD). All compounds tested, including several clinically used antitumour agents, are thought to exert their biological activities mainly by virtue of their abilities to bind to DNA. The selected drugs include intercalating agents with fused and unfused aromatic structures and several groove binders. To examine the role of base composition and base sequence in the binding of these drugs to DNA, ELD experiments were carried out with natural DNAs of widely differing base composition as well as with polynucleotides containing defined alternating and non-alternating repeating sequences, poly(dA).poly(dT), poly(dA-dT).poly(dA-dT),poly(dG).poly(dC) and poly(dG-dC).poly(dG-dC). Among intercalating agents, actinomycin D was found to be by far the most GC-selective. GC selectivity was also observed with an amsacrine-4-carboxamide derivative and to a lesser extent with methylene blue. In contrast, the binding of amsacrine and 9-aminoacridine was practically unaffected by varying the GC content of the DNAs. Ethidium bromide, proflavine, mitoxantrone, daunomycin and an ellipticine derivative were found to bind best to alternating purine-pyrimidine sequences regardless of their nature. ELD measurements provided evidence for non-specific intercalation of amiloride. A significant AT selectivity was observed with hycanthone and lucanthone. The triphenyl methane dye methyl green was found to exhibit positive and negative dichroism signals at AT and GC sites, respectively, showing that the mode of binding of a drug can change markedly with the DNA base composition. Among minor groove binders, the N-methylpyrrole carboxamide-containing antibiotics netropsin and distamycin bound to DNA with very pronounced AT specificity, as expected. More interestingly the dye Hoechst 33258, berenil and a thiazole-containing lexitropsin elicited negative reduced dichroism in the presence of GC-rich DNA which is totally inconsistent with a groove binding process. We postulate that these three drugs share with the trypanocide 4',6-diamidino-2-phenylindole (DAPI) the property of intercalating at GC-rich sites and binding to the minor groove of DNA at other sites. Replacement of guanines by inosines (i.e., removal of the protruding exocyclic C-2 amino group of guanine) restored minor groove binding of DAPI, Hoechst 33258 and berenil. Thus there are several cases where the mode of binding to DNA is directly dependent on the base composition of the polymer. Consequently the ELD technique appears uniquely valuable as a means of investigating the possibility of sequence-dependent recognition of DNA by drugs.

Loss of Infrared Linear Dichroism in Collagen Fibers as a Measure of Deterioration in Skin and Semi-Tanned Leather Artifacts

ABSTRACTPolarized radiation is used in a Fourier transform infrared (FTIR) microspectroscopic technique to detect molecular conformational change in collagen fibers resulting from deterioration. Absorbance spectra of single fibers are produced with polarization oriented first parallel (A‖) and then perpendicular (A⊥) to the fiber axis. The amide Ill band (1230- 1300 cm−1) shows strong positive dichroism, with up to six times greater absorbance occurring with parallel polarization. Losses of dichroism result from breakdown in the molecular structure of collagen. Integrations of the amide III bands are used in ratios, A‖/A⊥;, which provide a measure of the degree of molecular order. Ratios for degraded and undegraded material are compared to produce semi-quantitative information about deterioration.