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JAST 2010 September;1(2):147-151.
Published online 2010 September 04.   doi:http://dx.doi.org/10.5355/JAST.2010.147
Copyright ⓒ 2010 Journal of Analytical Science & Technology
Multichannel array detection of vibrational optical activity free-induction-decay
Hanju Rhee1,2*, Seong-Soo Kim3, Minhaeng Cho1,3*
1Korea Basic Science Institute Seoul Center, Seoul 136-713, Republic of Korea
2GRAST, Chungnam National University, Daejon 305-764, Republic of Korea
3Department of Chemistry, Korea University, Seoul 136-701, Republic of Korea
*
Corresponding Author: Hanju Rhee, Minhaeng Cho ,Tel: 82-2-920-0741, Email: hjrhee@kbsi.re.kr
ABSTRACT
By introducing a multichannel array detector to vibrational optical activity free-induction-decay (VOA FID) measurement spectrometer, we successfully measured the vibrational circular dichroism (VCD) spectrum of (1S)-β-pinene. By virtue of its fast measurement capability without a large number of frequency scans, the data collection time needed to obatain a high-quality VCD spectrum can be substantially reduced down to less than a minute. We anticipate that this type of linear chiroptical spectrometer will be applicable to a variety of vibrational optical activity studies of biomolecules and drugs.
Keywords: Vibrational optical activity, Vibratinal circular dichroism, Spectral interferometry, Multichannel array detection
INTRODUCTION
Among a few different versions of optical activity measurement techniques, vibrational circular dichroism (VCD) spectroscopy has proven to be extremely useful to characterize structures of chiral molecules and also to investigate the structural conformations and their inhomogeneous distributions of peptides, proteins and DNAs, and so on [1-3]. Nevertheless, it has not been a popular method, in comparison to its electronic analog, because of its notoriously weak signal problem. Recently, however, a newly developed femtosecond vibrational optical activity free-induction-decay (VOA FID) measurement technique [4-7] has been paid a great deal of attention because such a novel technique overcomes the weak signal and non-zero background problems of the conventional approach and has opened up a new possibility of ultrafast time-resolved VCD experiment, which has been considered to be quite challenging.
In addition to an apparent advantage of its ultrafast time-resolution, another intriguing aspect of the VOA FID method is an excellent signal-to-noise ratio originating from the non-differential and heterodyned detection scheme. Usually, it takes a couple of hours to obtain a statistically meaningful spectrum with a commercial VCD spectrometer that relies on the conventional differential measurement scheme. In the VOA FID method, however, just a few tens of minutes are enough to get a single VCD spectrum of sufficient quality. Furthermore, it should be noted that such reduced data collection time was even achieved by scanning the monochromator, where the frequencyscanning and signal-settling times were the measurement rate-determining steps. In our previous measurements employing this frequency-scan mode [5,6], for example, the total number of spectral points collected was ~50 and each point was averaged with a 100 ms-time constant of the lock-in amplifier for a single scan and then such scan was repeated twenty times for further averaging. The effective measurement time per point is thus expected to be roughly a few seconds (100 msx20). That is to say, most of the measurement time is essentially dead time, which is unfortunately very long in comparison to the effective collection time. Consequently, such a frequency-scan mode measurement is not desirable.
In this regard, a multichannel array detector capable of recording the whole spectrum at once, in principle, without a monochromator frequency-scan would be of use to significantly reduce the data collection time by removing the unnecessary dead time. In addition, such a fast acquisition of the spectrum can minimize a noise factor caused by a long-term drift of the laser intensity. Here we will present a brief account on characteristics of the multichannel array detection technique and show a simple experimental result of the VCD measurement using this technique.
MATERIALS AND METHODS
Spectral interferometric measurement of VOA FID signal
In contrast to the conventional VCD technique measuring the absorbed intensity difference between left- and right-circularly polarized lights by the chiral sample, our approach is based on phase-and-amplitude measurement of the time-domain VOA FID signal electric field. To achieve such measurement, a combination of cross-polarization detection configuration (for VOA FID field-generation) [4] and heterodyned spectral interferometry (for fieldcharacterization) [5-8] is needed. Figure 1(a) sketches the corresponding VOA FID measurement setup based on a Mach-Zehnder interferometer. A femtosecond IR pulse is injected into the Mach-Zehnder interferometer and split into two pulses traveling in the two arms of the interferometer. One (signal arm) of them is used to generate the VOA FID signal from a chiral sample placed in between two crossed polarizers (P1,P2: homemade Brewster’s angle Ge polarizers [7,9]) and the other one (reference arm) as a strong reference pulse for heterodyne-detection with the VOA FID. The two timedomain optical fields are combined at the detector and the reference pulse (Er) precedes the VOA FID signal (Es) by a proper time delay (τd ~1 ps). Then, they interfere with each other in the frequency domain and the resultant heterodyned spectral interferogram Shet(ω) can finally be detected at a spectrometer.

Multichannel array detection
Figure 1(b) compares two different detection schemes (single and multichannel) measuring the spectral interferogram Shet(ω) formed by the two electric fields, Er(ω) and Es(ω), separated by the time delay τd. In the case of using a single element detector (upper), one has to scan the grating of the monochromator such that the detector reads the entire spectrum of interest (from red to blue part of Shet(ω). A large number of sequential frequency scans (N) thus increase the unnecessary dead time. In contrast, the multichannel array detector with N pixels (lower) can simultaneously record all the spectral components of Shet(ω) that are spatially dispersed by the grating. Therefore, no grating scan is necessary in an ideal case, where the array is dense enough to provide high spectral resolution and its size is sufficiently large to cover the target frequency window, and the measurement time can significantly be reduced to about 1/N times in comparison to that of the frequency scan mode.
Charge coupled device (CCD) is the most popular array detector used in the visible spectral region but there is no such type of detector in the mid IR range. Thus we utilized a multichannel pulse integrator system (IR-6416, Infrared System Development Corp.) instead, which is composed of two parts: one is the mercurycadmium-telluride (MCT) array detector (1x64 pixels) and the other a pulsed signal processing unit. The total interferometric signal reached at the spectrometer contains both the VOA FID and reference signals. Thus to remove the latter signal, an optical chopper synchronized with sub-harmonic (500 Hz) of the repetition rate of the laser (1 kHz) is inserted into the signal arm (Figure 1(a)). The 64 fast pulse integrators (< 2 kHz) connected to every pixel make it possible to process the pulse-to-pulse signals of the broad frequency component simultaneously. The on-and-off signals of the chopper can thus be recorded sequentially and then only the heterodyned term ontaining the chiral signal can be obtained by taking the difference between these neighboring signals. Since the number of pixels of the MCT array we have is not sufficient for getting a highly structured spectrum, three adjacent grating positions (3 nm interval) were minimally scanned to acquire sufficient data points and the resultant data sets were then rearranged for a proper data-ordering. The spectrometer fitted with the MCT array was calibrated by using four C-H stretch vibrational peaks in the FT-IR absorption spectrum of (1S)-β-pinene.
RESULTS AND DISCUSSIONS
To verify the experimental feasibility and fast measurement capability of the present multichannel array detection technique, we measured the C-H stretch-active VOA FID signal of (1S)-β-pinene. Figure 2(a) shows its heterodyned spectral interferograms obtained by using the MCT array detector. In the redlined spectral interferogram (without scanning), which is measured with the grating position of the monochromator fixed at 3420 nm, the frequency resolution appears to be insufficient due to the limited number of data points (N=64). This is because the pixel density of the array is not well matched with the spectral dispersion of the monochromator, which is basically determined by the focal length of the monochromator and/or the groove density of the grating. To increase the number of data points, three adjacent grating positions (3420, 3423, 3426 nm) were scanned and then the total 192 data points (N=192) were obtained. The resultant spectral interferogram (black line) becomes smooth and the highly oscillating interferometric pattern (blue square box), which didn’t appear in the spectral interferogram measured at the fixed mode (red line), is clearly observable in this case. Although such minimal 3-position scanning is somewhat disadvantageous in terms of the datacollection time, it is an inevitable process to obtain a more accurate and quantitatively reliable spectrum in our experimental condition.
The heterodyned spectral interferogram of the VOA FID signal can be converted into a VCD spectrum via standard Fourier transform spectral interferometric (FTSI) method [5-8]. Figure 2(b) shows the retrieved VCD spectrum (solid line) from the black-lined spectral interferogram shown in Figure 2(a). For the sake of comparison, the same VCD spectrum obtained by scanning the monochromator over the whole spectral range (2800~3000 cm-1) with a single MCT element detector is also plotted together (dashed line). It is found that the two VCD spectra show a similar quality but their data collection times are notably different. The former was less than 3 minutes whereas the latter took more than 20 minutes. It should be noted that the multichannel array detections at three different grating positions were performed and consequently the measurement time was taken three times more in the present case. Therefore, the data collection time could be further shortened down to less than one minute if one were able to avoid the unnecessary scanning process by adjusting some parameters such as the focal length of the monochromator and/or the pixel density of the array detector. Considering that with the commercial instrument, it takes multiple hours to obtain a meaningful VCD spectrum for a sample with similar rotational strength, we strongly believe that such a fast measurement of the weak VCD signal is a remarkable achievement and we anticipate that this technique will be applicable to a wide range of VCD measurement experiments.
CONCLUSIONS
We successfully measured the heterodyned VOA FID signal of (1S)-β-pinene by using the multichannel MCT array detector instead of the single element MCT. From the advantage of the multichannel detection scheme, the quite decent VCD spectrum could be obtained in less than 3 minutes. It is expected that the data collection time will be further reduced to a few seconds regime by using either a denser MCT array or a monochromator with longer focal length to obtain the entire spectrum indeed without any frequency-scanning.
Acknowledgement
This work was supported by KBSI grant T30401 to HR and MC and by NRF grant 20090078897 to MC.
FIGURES
Fig.1 Fig.1
(a) The VOA FID measurement setup based on the Mach-Zehnder interferometer. P0-2; Brewster’s angle Ge polarizers. The transmission axis of P1 (vertical) is perpendicular to those of P0 and P2 (horizontal). The delay time (τd) between the VOA FID signal (Es) and reference pulse (Er) is controlled by a delay stage. The heterodyned spectral interferometric signal (Shet(ω)) is finally recorded at a spectrometer. (b) Comparison between single (upper) and multichannel (lower) VOA FID detection schemes (dashed box region in (a)). In the single channel detection, the entire frequency components (red to blue) of the spectral interferogram Shet(ω) is sequentially recorded by scanning N grating positions. In contrast, the multichannel array detector with N pixels can measure N spectral components simultaneously without any grating scan in an ideal case.
Fig.2 Fig.2
(a) Heterodyned spectral interferograms of the C-H stretch-active VOA FID signal of (1S)-β-pinene obtained by using the multichannel MCT array detector with 64 pixels. The total number of data points (N) is equal to the array size (64 pixels) when the monochromator is fixed at 3420 nm (red line), whereas N=192 when 3 positions (3420, 3423, and 3426 nm) are scanned (black line). (b) A retrieved VCD spectrum (solid) from the spectral interferogram (N=192, black) shown in (a). For the sake of comparison, the same VCD spectrum obtained by using the single element detector (dashed) is also plotted together. The data collection time in the multichannel detection is substantially reduced to < 3 minutes shorter than that (> 20 minutes) of the single channel detection.
 
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