KCS

A better understanding of chiro-optical properties manifested by all living organisms is critical for studies of the molecular basis of biological activity. Particularly, the time-resolved circular dichroism (CD) spectroscopy can provide crucial information on the structural changes undergone in biomolecules in the process of their function. However, the CD is a notoriously weak effect, which renders the time-resolved CD spectroscopy experimentally difficult. Moreover, time-resolved vibrational CD (VCD) experiment has never been carried out before. Recently, we have developed a femtosecond optical activity free-induction-decay (OA FID) measurement technique utilizing a combination of cross-polarization geometry and heterodyned spectral interferometric detection. Direct heterodyne-detection of the phase and amplitude of OA FID field enabled us to overcome the weak-signal and nonzero achiral background problems of the conventional differential CD measurement method. Consequently, we could detect much weaker vibrational OA (VOA) FID fields of small chiral organic molecules in comparison with the electronic one and retrieve their individual VCD and VORD spectra simultaneously via the Fourier transform spectral interferometry (FTSI). In addition, we found that the present method has a highly phase-sensitive detection capability of separating the chiral and achiral FID signals. Combined with pump-probe experiment, our method, which is a CD analog of pulsed NMR, will be a valuable tool for observing the ultrafast chirality changes of biomolecules and therefore allow us to study their dynamics in aqueous solution at physiological conditions.