Hanju Rhee

Femtosecond characterization of vibrational optical activity of chiral molecules

Optical activity is the result of chiral molecules interacting differently with left versus right circularly polarized light. Because of this intrinsic link to molecular structure, the determination of optical activity through circular dichroism (CD) spectroscopy has long served as a routine method for obtaining structural information about chemical and biological systems in condensed phases. A recent development is time-resolved CD spectroscopy, which can in principle map the structural changes associated with biomolecular function and thus lead to mechanistic insights into fundamental biological processes. But implementing time-resolved CD measurements is experimentally challenging because CD is a notoriously weak effect (a factor of 10-4–10-6 smaller than absorption). In fact, this problem has so far prevented time-resolved vibrational CD experiments. Here we show that vibrational CD spectroscopy with femtosecond time resolution can be realized when using heterodyned spectral interferometry to detect the phase and amplitude of the infrared optical activity free-induction-decay field in time (much like in a pulsed NMR experiment). We show that we can detect extremely weak signals in the presence of large achiral background contributions, by simultaneously measuring with a femtosecond laser pulse the vibrational CD and optical rotatory dispersion spectra of dissolved chiral limonene molecules. We have so far only targeted molecules in equilibrium, but it would be straightforward to extend the method for the observation of ultrafast structural changes such as those occurring during protein folding or asymmetric chemical reactions. That is, we should now be in a position to produce ‘molecular motion pictures’ of fundamental molecular processes from a chiral perspective.

Femtosecond spectral interferometry of optical activity: Theory

Optical activities such as circular dichroism (CD) and optical rotatory dispersion (ORD) are manifested by almost all natural products. However, the CD is an extremely weak effect so that time-resolved CD spectroscopy has been found to be experimentally difficult and even impossible for vibrational CD with current technology. Here, we show that the weak-signal and nonzero background problems can be overcome by heterodyned spectral interferometric detection of the phase and amplitude of optical activity free-induction-decay (OA FID) field. A detailed theoretical description and a cross-polarization scheme for selectively measuring the OA FID are presented and discussed. It is shown that the parallel and perpendicular electric fields when the solution sample contains chiral molecules are coupled to each other. Therefore, simultaneous spectral interferometric measurements of the parallel and perpendicular FID fields can provide the complex susceptibility, which is associated with the circular dichroism and optical rotatory dispersion as its imaginary and real parts, respectively. On the basis of the theoretical results, to examine its experimental possibility, we present numerical simulations for a model system. We anticipate the method discussed here to be a valuable tool for detecting electronic or vibrational optical activity in femtosecond time scale.

Phase sensitive detection of vibrational optical activity free-induction-decay: vibrational CD and ORD

Optical activity is manifested by chiral molecules including natural products and drugs, so that circular dichroism (CD) and optical rotatory dispersion (ORD) measurements can provide invaluable information on their chiro-optical properties and structures. It is experimentally demonstrated that heterodyne-detected Fourier-transform spectral interferometry with a femtosecond infrared pulse can be used to fully characterize the phase and amplitude of vibrational optical activity free-induction-decay field. The measured spectral interferograms are then converted to the linear optical activity susceptibility whose imaginary and real parts correspond to vibrational CD and ORD spectra. Unlike the conventional differential measurement technique, the present method based on a heterodyned interferometry is shown to be quite robust and stable. We anticipate that the present vibrational optical activity measurement technique will be of critical use in elucidating chiro-optical properties and structural changes in biomolecules.

Femtosecond Measurements of Vibrational Circular Dichroism and Optical Rotatory Dispersion Spectra

Vibrational optical activity (VOA) is a chiroptical property originating from nuclear vibrations of chiral molecules. Consequently it is highly sensitive to molecular conformations and provides valuable information on absolute configuration of chiral molecules such as proteins, nucleic acids, and asymmetrically synthesized drugs in condensed phases. Recently, we developed a novel vibrational optical activity free-inductiondecay (VOA FID) measurement technique, as an alternative to overcome the weak signal and non-zero background problems of traditional vibrational circular dichroism (VCD) measurement method. A combined cross-polarization and heterodyned detection scheme employed in such technique significantly boosts the chiral sensitivity. In addition, its potential capability of femtosecond time-resolution makes it possible to monitor the chirality change of molecular systems in ultrafast motion. A success of the VOA FID measurement strongly depends on optical perfectness of the cross-polarization configuration. Real polarizers, however, have finite (non-zero) extinction ratio (ER) and the achiral FID thus leaked through such a cross geometry could substantially contaminate the pure chiral signal. For typical chiral samples, differential absorbance DA (VCD) is about 10 4 times smaller than the absorbance itself. In this case, high quality polarizers with ER smaller than ~10 8 are needed to discriminate such a weak VOA signal from the large achiral background. Although the previous experimental setup enabled us to examine the C H stretching VCD-active modes (2830 ~3000 cm ) of organic chiral molecules, 10] it still poses a serious technical problem: the exceptional ER (~10 ) of the dichroic absorptive calcite polarizers used is achieved at only restricted IR frequency ranges. The spectral limitation imposed by the effective working frequency range of the calcite polarizer thus confines our scope to the C H stretching vibrations only and therefore precludes versatile applications to the studies of other vibrations. Indeed, the C=O stretching vibration (amide I mode, 1600~1700 cm ) of proteins and polypeptides has been more extensively studied to interrogate their secondary structures. 12–14] In this regard, the reflective Brewster’s angle germanium polarizer (BAGP) that was designed and characterized previously can be of critical use in a wide range of applications of the cross-polarization VOA FID measurement technique. BAGP consists of four germanium windows arranged in a chevrongeometry (Figure 1 a), where the incident beam is reflected four times at germanium windows at the Brewster’s angle (qB).

Broadband near UV to visible optical activity measurement using self-heterodyned method.

We demonstrate that broadband electronic optical activity can be measured with supercontinuum light pulse generated by a femtosecond pump (800 nm). It is the self-heterodyned detection technique that enables us to selectively measure the real (optical rotatory dispersion, ORD) or imaginary (circular dichroism, CD) part of the chiroptical susceptibility by controlling the incident polarization state. The single-shot-based measurement that is capable of correcting power fluctuations of the continuum light is realized by using a fast CCD detector and a polarizing beam splitter. Particularly, non-differential scheme used does not rely on any polarization-switching components. We anticipate that this broadband CD/ORD spectrometry with intrinsically ultrafast time-resolution will be applied to a variety of ultrafast chiroptical dynamics studies.

Photolytic Control and Infrared Probing of Amide I Mode in the Dipeptide Backbone-Caged with the 4,5-Dimethoxy-2-nitrobenzyl Group

Alanine dipeptide analog 1 backbone-caged with a photolabile linker, 4,5-dimethoxy-2-nitrobenzyl (DmNb), was synthesized. UV-pulse-induced photochemical reaction of 1 was monitored by Fourier transform IR absorption spectroscopy under a steady-state condition or in a fast-scan mode. Upon photolysis of 1, the amide I band is changed from a doublet to a singlet with concomitant line shape changes of several IR bands. The change of the amide I band is directly associated with the photocleavage of the covalent N-C bond connecting the backbone amide of 2 to DmNb. Therefore, IR spectroscopy is useful for directly probing the photocleavage of backbone-caged peptide 1 and the concurrent release of native peptide 2. In contrast, UV-vis spectroscopy probing the irradiation-induced structural change of the 2-nitrobenzyl moiety itself may not provide a clue directly relevant to the photocleavage of such N-C bond. Time-resolved IR spectra recorded in a fast-scan mode after pulsed UV irradiation of 1 reveal that such photocleavage occurs at least faster than a few seconds of our instrumental time resolution.

Selective Suppression of Stimulated Raman Scattering with Another Competing Stimulated Raman Scattering

A three-beam femtosecond stimulated Raman scattering (SRS) scheme is formulated and demonstrated to simultaneously induce two different SRS processes associated with Raman-active modes in the same molecule. Two SR gains involving a common pump pulse are coupled and compete: As one of the Stokes beam intensities increases, the other SRS is selectively suppressed. We provide theoretical description and experimental evidence that the selective suppression behavior is due to the limited number of pump photons used for both of the two SRS processes when an intense depletion beam induces one SRS process. The maximum suppression efficiency was ∼60% with our experimental setup, where the SR gain of the ring breathing mode of benzene is the target SRS signal, which is allowed to compete with another SRS process, induced by an intense depletion beam, of the CH stretching mode. We anticipate a potential of this new switching-off concept in super-resolution label-free microscopy.

Quantum Beats and Phase Shifts in Two-Dimensional Electronic Spectra of Zinc Naphthalocyanine Monomer and Aggregate

The origin of quantum coherence in two-dimensional (2D) electronic spectra of molecular aggregates and light-harvesting complexes still remains an open question. In particular, it could be challenging to distinguish between electronic and vibrational coherences for a coupled system, where both degrees of freedom can be simultaneously excited. In this Letter, we examine quantum beats in the 2D spectra of zinc naphthalocyanine (ZnNc) aggregate and monomer, and compare their characteristic features in terms of the frequency and relative phase of diagonal and off-diagonal amplitude oscillations. The long-lasting oscillating components (>1 ps) at 600-700 cm(-1) observed in both the aggregate and monomer are found to be attributed to the vibrational coherence. The wide phase variations of the 2D spectral amplitude oscillations are observed not just in the aggregate but also in the monomer state. This suggests that the unusual 90° phase shift may be attributed to neither quantum population-to-coherence transfer nor vibronic exciton coupling.

Multichannel array detection of vibrational optical activity free-induction-decay

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)ǰ -pinene. By virtue of its fast measurement capability without a large number of frequency scans, the data collection time needed to obtain 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.

Ultrafast chiroptical spectroscopy: Monitoring optical activity in quick time

Optical activity spectroscopy provides rich structural information of biologically important molecules in condensed phases. However, a few intrinsic problems of conventional method based on electric field intensity measurement scheme prohibited its extension to time domain technique. We have recently developed new types of optical activity spectroscopic methods capable of measuring chiroptical signals with femtosecond pulses. It is believed that these novel approaches will be applied to a variety of ultrafast chiroptical studies.