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Figure 11.7 (A) Synchronous and (B) asynchronous 2D IR correlation maps in the
1400-1000 cm-1 region of the oligomerization reaction at 220 °C. (Reproduced with permission from Ref. No. 4. Copyright (2002) American Chemical Society.)
and 270 °C. Compared with the synchronous map at 220 °C (Figure 11.6(A)), the number of autopeaks increases at 270 °C (Figure 11.7(A)), i.e. new peaks emerge at 1262, 1115, and 1065 cm-1. These new peaks have negative correlation with the first two peaks, and thus are attributed to the bands of BHET. Another peak at 1032 cm-1 appears, and has positive cross peaks with peaks at 1233 and 1090 cm-1. Amari and Ozaki attributed the band at 1032 cm-1 to the C-O stretching mode of free EG based on the spectrum in Figure 11.5. Figure 11.8 illustrates the slice spectra extracted from the synchronous and asynchronous 2D correlation spectra at 1032 cm-1 shown in Figure 11.7. It can be seen from the synchronous slice spectrum that positive cross peaks develop at (1032, 880) and (1032, 861cm-1). Note that this synchronous slice spectrum is similar to the spectrum of EG (Figure 11.5), and thus it was concluded that the 1032 cm-1 peak is due to EG. The asynchronous slice spectrum shows the development of a cross peak at (1032, 870 cm-1). The 870 cm-1 band is assigned to the benzene ring vibration mode.
The asynchronous correlation spectra at 270 and 220 °C are very similar to each other. The 1032 cm-1 band has cross peaks with those at 1233 and 1090 cm-1, which are assigned to the oligomer C-O stretching modes, but does not show cross peaks with the BHET bands (1262, 1115, and 1065 cm-1). This means that the intensity increase of free EG band at 1032 cm-1 and the decrease of BHET bands proceed in phase. Therefore, it is very likely that the intensity decrease of the BHET bands is at least partly due to the relative concentration decrease of BHET upon the formation of free EG. This is probably the reason why the bands from BHET and oligomers have cross peaks, or in other words, change out of phase with each other.
Thus, IR spectra obtained for the in situ monitoring of the initial oligomerization of BHET were investigated by using 2D correlation spectroscopy. Careful
2D Correlation Spectroscopy and Chemical Reactions
Figure 11.8 Slice spectra of the synchronous and asynchronous 2D IR correlation spectra at 1032 cm-1 of the oligomerization reaction at 220 °C. (Reproduced with permission from Ref. No. 4. Copyright (2002) American Chemical Society.)
inspection of the 2D IR spectra revealed that the small intensity changes in the 1000-800 cm-1 region are due to evolving free EG. Amari and Ozaki investigated the same reaction by using 2D NIR and 2D NIR-IR heterocorrelation spectroscopy. The 2D NIR-IR study is outlined in Chapter 14. Sasic et al. analyzed the same IR data by means of sample-sample and variable-variable 2D correlation spectroscopy.6 The sample-sample correlation analysis reveals the correlations among the concentration features of the components, and the variable-variable correlation analysis elucidates the relations among the spectral features.
11.2 HYDROGEN - DEUTERIUM EXCHANGE OF HUMAN SERUM ALBUMIN
The hydrogen-deuterium (H/D) exchange of the amide protons is very useful in the investigation of the secondary structure of proteins. Since the amide protons associated with each conformation are not exchanged simultaneously, the contributions from different secondary structures to the amide bands can be separated by using the H/D exchange. Deuterium exchange rates are also a powerful probe for amide structure, providing information about solvent accessibility and hydrogen bond
Hydrogen-Deuterium Exchange of Human Serum Albumin
stability of amide protons. The 2D correlation analysis has provided deeper insight into the mechanism of the H/D exchange in the amide protons of different secondary structures.2 3 For example, Nabet and Pezolet reported a 2D IR correlation spectroscopy study of the H/D exchange of myoglobin (Mb) earlier.2 They showed that the use of two different exchange time domains is very efficient to separate the fast kinetics from the slow ones. Wu et al. combined 2D correlation spectroscopy and PCA to analyze the kinetics of H/D exchange of human serum albumin (HSA).3 They measured IR spectra of HSA as a function of time after dissolving it into D2O to investigate the secondary structure and kinetics of H/D exchange.3 2D IR spectra in the Amide I and Amide II (and IT) regions were generated from time-dependent spectral variations in different exposure time domains of HSA in the D2O solution. The synchronous and asynchronous spectra in each time domain provided a clear separation of amide bands due to the different secondary structures. The asynchronous spectrum also showed the specific sequence of the secondary structures exposed to the H/D exchange. PCA was used to select the appropriate time domains for the calculation of 2D correlation spectra.