Fourier transform infrared (FT-IR) spectra were measured for an aqueous solution (pD = 5.40) of defatted monomer bovine serum albumin (BSA) over a temperature range of 25-90 degrees C to investigate temperature-induced secondary structure and conformation changes. The curve fitting method combined with the Fourier self-deconvolution technique allowed us to explore details of the secondary structure and conformation changes in defatted BSA. Particularly striking in the FT-IR spectra was an observation of the formation of an irreversible intermolecular beta-sheet of BSA on heating above 70 degrees C. A band at 1630 cm(-1) in the spectra was assigned to short-segment chains connecting alpha-helical segments. The transition temperature for the short-segment chains connecting alpha-helical segments is lower by 17-18 degrees C, when compared to those of the alpha-helix, turn, and intermolecular beta-sheet structures of BSA, suggesting that the alpha-helix and turn structures of BSA are cooperatively denatured on heating. Moreover, the results give an important feature in heat-induced denaturation of BSA that the conformation changes occur twice around both 57 and 75 degrees C. The appearance of two peaks is interpreted by the collapse of the N-terminal BSA domain due to the crevice in the vicinity between domains I and II at low-temperature transition and by the change in cooperative unit composed of the other two BSA domains at high-temperature transition.

Heat-induced denaturation of ovalbumin in aqueous solutions has been investigated by generalized two-dimensional (2D) Fourier transform near-infrared (FT-NIR) correlation spectroscopy. New insight has been gained into hydration and the unfolding process of secondary structures of ovalbumin by studying temperature-dependent correlation patterns in 2D synchronous and asynchronous spectra, which are constructed from concentration-perturbed NIR spectra at different temperatures. The correlation patterns have provided information about the correlation and phase relationships between different absorption bands of ovalbumin and water. The hydration of ovalbumin is almost unchanged from 45 to 67 °C, where ovalbumin molecules are in a natively folded state. A sudden change in the hydration is detected in a narrow temperature range of 67−69 °C, where the unfolding of the ordered secondary structures starts. The hydration, again, remains nearly unchanged upon further heating to 80 °C, even though the unfolding process develops progressively until the denatured state. The sudden change in the hydration around 68 °C seems to be caused by the stabilization of slightly unstable hydrogen bonds in the folded state. The change may make the intramolecular hydrophobic cores of ovalbumin less condensed and more accessible to the solvent molecules. On the other hand, the development of unfolding from 69 to 80 °C results in band shifts for combination bands involving free NH stretching−amide II (amide A/II), intramolecular hydrogen-bonded NH stretching−amide II (amide B/II) of ovalbumin. The present experiment demonstrates that the generalized 2D NIR correlation spectroscopy is powerful in detecting subtle but valuable structural information about the protein denaturation in the aqueous solution.

This paper demonstrates the potential of generalized two-dimensional (2D) attenuated total reflection/infrared (ATR/IR) spectroscopy in studies of spectral variations in the amide I region of aqueous solutions of protein. Two examples of the 2D correlation analysis are discussed in this paper. The first is concerned with adsorption-dependent spectral changes of β-lactoglobulin (BLG) in solution. The second approach is dedicated to the concentration-dependent spectral changes. To generate the 2D correlation spectra, the original spectra have been subjected to pretreatment procedures consisting of ATR correction, subtraction of the spectrum of the buffer solution, smoothing, and normalization over the concentration. The adsorption-dependent 2D study shows that the interaction between the crystal surface and the protein molecules can be monitored successfully by the synchronous and asynchronous correlation spectra. This interaction is characterized by pronounced intensity changes at frequencies assigned to β-sheet elements buried in the hydrophobic core of the molecule. The concentration-dependent 2D correlation maps, which develop 10 times more intense features than the adsorption-dependent 2D maps, are concerned with changes in various secondary structure elements located in the hydrophilic parts facing the solvent. The present study has also aimed at expanding generalized 2D correlation spectroscopy to quantitative utilization. The quantitative analysis of the 2D maps reveals that the intensity changes observed for the series of aqueous solutions of BLG with different concentrations are predominantly composed of concentration-induced secondary structure changes even in the presence of the adsorption of protein molecules to the ATR crystal.

Attenuated total reflection (ATR)/infrared (IR) spectra were measured for human serum albumin (HSA) in aqueous solutions over a pH range of 5.0−3.2. Generalized two-dimensional (2D) correlation analysis was applied to the amide I region of the spectra to investigate pH-dependent changes in secondary structures and in hydrogen bondings of side chains of HSA. The synchronous and asynchronous 2D spectra were generated from the pH-dependent spectral variations for the three states of HSA, the N isomeric form (pH 5.0−4.4), the N−F transition (pH 4.6−3.8), and the F isomeric form (pH 3.8−3.0). The most interesting finding in the 2D spectra is identification of four bands at 1740, 1715, 1705, and 1696 cm-1 due to a CO stretching mode of free and hydrogen bonded (weak, medium, and strong) COOH groups of HSA. The 2D correlation analysis provides unambiguous evidence for the existence at least the four CO bands, demonstrating its powerful deconvolution ability. The asynchronous spectrum of the N form is characterized by a rather broad cross-peak centered at (1715, 1654) cm-1. The sign of the cross-peak indicates that protonation of COO- groups of glutamic (Glu) and aspartic (Asp) acid residues occurs at pH's higher than those of structural changes in the α-helices. In the N−F transition, it seems that the formation of free COOH groups and those with the hydrogen bonds of the medium strength occurs is linked with changes in secondary structures of HSA. The asynchronous spectrum indicates that the formation of the strongly hydrogen-bonded COOH groups upon the protonation of COO- groups plays a key role in the initiation of the N−F transition, where mainly α-helices undergo the conformational changes. The synchronous and asynchronous spectra of the F form show that β-strands and β-turns of HSA change significantly in this pH region.

Generalized two-dimensional (2D) correlation spectroscopy has been applied to analyze near-infrared (NIR) spectra of milk with different protein and fat concentrations. The NIR spectra of milk show rather poor signal-to-noise ratios compared with those of a protein or fat solution and have changing baselines from one spectrum to another. Poor signal-to-noise ratio and variations in baseline are common problems for NIR spectra of real-world samples. This study aims at expanding the utility of generalized 2D correlation spectroscopy to complicated multicomponent biological systems. In order to overcome the above two problems, we have employed multiplicative scatter correction (MSC) and smoothing as pretreatment procedures of the milk spectra selected for the calculation of 2D NIR correlation. 2D synchronous correlation spectra in the 2000–2400 nm region constructed from protein or fat concentration-dependent spectral changes of milk sharply enhance bands assignable to proteins or fats, respectively. It has been found that a power spectrum along the diagonal line in a synchronous spectrum very effectively shows the contribution of a particular component to the NIR spectra of milk. In fact, for example, the power spectrum for the fat concentration-dependent spectral changes of milk is very close to an NIR spectrum of fat itself. Two-dimensional asynchronous correlation spectra demonstrate the existence of bands that cannot be identified even by calculation of second derivatives and chemometrics analysis of the spectra. The asynchronous spectra also elucidate interaction between fats, proteins, and water.

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The present study has aimed at comparing three methods for analyzing near-infrared (NIR) spectra: conventional spectral analysis methods, chemometrics, and generalized two-dimensional (2D) correlation spectroscopy. In a comparison of these approaches, NIR spectra were measured for aqueous solutions of human serum albumin (HSA) with the concentration range of 0.5–5.0 wt %. Synchronous and asynchronous 2D correlation spectra were generated from the concentration-dependent NIR spectral variations of HSA in distilled water. The first and second loadings plots were calculated for principal component analysis (PCA) models based upon the above NIR spectral data. It was found that slice spectra calculated from the synchronous spectra are very close to the first loadings plots and that slice spectra from the asynchronous spectra have a close resemblance to the second loadings plots. The reasons why the synchronous and asynchronous spectra bear close resemblance to the first and second loadings plots for the PCA model, respectively, are discussed in the paper.

The highly enantioselective synthesis of functionalized helicenes and helicene-like molecules have been achieved via rhodium-catalyzed [2+2+2] cycloaddition reactions. The rhodium-catalyzed enantioselective intramolecular [2+2+2] cycloaddition of 2-naphthol-linked triynes afforded [7]helicene-like molecules in good yields and ee values. The more sterically encumbered reaction, the rhodium-catalyzed enantioselective double intramolecular [2+2+2] cycloaddition of a 2-naphthol-linked hexayne, also proceeded to give a [11]helicene-like molecule with high ee value, although the product yield was low. Not only intramolecular cycloaddition reactions but also intermolecular ones were accomplished by combinations of electron-rich tetraynes and electron-poor diynes to give [7]-and [9]helicene-like molecules and [7]helicenes in varying yields and ee values.