Evidence Of Protein Collective Motions On The Picosecond Time Scale

We investigate the presence of structural collective motions on a picosecond time scale for the heme protein, cytochrome c, as a function of oxidation and hydration, using terahertz (THz) time-domain spectroscopy and molecular dynamics simulations. The THz response dramatically increases with oxidation, with the largest increase for lowest hydrations and highest frequencies. For both oxidation states the THz response rapidly increases with hydration saturating above ~25% (g H2O/g protein). Quasi-harmonic vibrational modes and dipole-dipole correlation functions are calculated from molecular dynamics trajectories. The collective mode density of states alone reproduces the measured hydration dependence providing strong evidence of the existence of these motions. The large oxidation dependence is reproduced only by the dipole-dipole correlation function, indicating the contrast arises from diffusive motions consistent with structural changes occurring in the vicinity of a buried internal water molecule

Terahertz Time-Domain Spectroscopy of Four Hydroxycinnamic Acid Derivatives

The well-resolved absorption spectra of the hydroxycinnamic acid (HCA) derivatives, caffeic acid, ferulic acid, sinapic acid and chlorogenic acid, were measured over the frequency region from 0.3 to 2.0 THz at 294 K with terahertz time-domain spectroscopy (THz-TDS). Theoretical calculation was applied to assist the analysis and assignment of the individual THz absorption spectra of the HCA derivatives with density functional theory (DFT). The distinctive spectral features were originated from the collective motion of molecules held together by hydrogen bonds. The real and imaginary parts of dielectric function of the four HCA derivatives were also obtained

Optical out-of-plane spin polarization and charge conductivities in spin-orbit-coupled systems in the presence of an in-plane magnetic field

Out-of-plane spin and charge responses to the terahertz field for a clean two-dimensional electron gas with a Rashba spin-orbit interaction in the presence of an in-plane magnetic field are studied. We show that the characteristic optical spectral behavior is remarkably different from that of the system in the absence of in-plane magnetic fields. It is found that the optical spin polarization normal to the plane is nonzero even for this clean system, in sharp contrast to the static case. Due to the combined effect of spin-orbit coupling and in-plane magnetic field, both diagonal and off-diagonal components of optical charge conductivity tensor are nonvanishing. It is indicated that one can control the spin polarization and the optical current by adjusting the optical frequency. In addition, the out-of-plane spin polarization and conductivities strongly rely on the direction of the external magnetic field. Nevertheless, they meet different angle-dependent relations. This dynamical out-of-plane spin polarization could be measured by the time-resolved Kerr rotation technique. Copyright EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg 2010