The resulting 3Car state is highly spin polarized. Its EPR spectrum consists of emission and absorption lines, the position AZD9291 chemical structure of which is determined by the zero-field-splitting (ZFS). Although this state is short-lived, it can be studied by pulse ENDOR if the pulse sequence is completed before the triplet decays to the singlet ground state (Niklas et al. 2007). Highly resolved Q-band Davies ENDOR spectra were obtained for magnetic field positions corresponding to the canonical orientations of the ZFS tensor (Fig. 8). For the
triplet state (S = 1), the ENDOR frequencies occur at \( \nu_\textENDOR = |\nu_\textn – M_s\;a|\) where M S = ±1,0. This makes the ENDOR spectrum asymmetric with respect to ν n and allows the direct determination of the signs of the HFI constants relative to the sign of the ZFS parameter D. For the studied system, a negative D value was deduced from the analysis of the ENDOR spectra. Fig. 8 Top: Field-swept
echo EPR at Q-band of the short-lived photoinduced spin-polarized triplet state of the carotenoid peridinin in the PCP (peridinin–chlorophyll–protein) antenna of A. carterae. Middle: Davies ENDOR experiment MLN2238 manufacturer at Q-band using orientational selection in the EPR with respect to the ZFS tensor axes (positions ZI and ZII). Note that lines with positive HFI constants appear on the high (low) frequency side of the spectrum and with negative signs on the low (high) frequency side, for the EPR field position ZI (ZII). Thus, magnitude and signs of the couplings are directly available from the spectrum. For the peridinin triplet, at least 12 1H HFI constants
were obtained. From the assigned couplings, the spin density distribution in the molecule can be constructed and compared with that obtained from DFT calculations. Bottom: Molecular structure of peridinin including axis system: For details see Niklas et al. (2007) Totally nine groups of nonequivalent protons were identified and tentatively assigned to molecular positions PLEK2 based on the comparison of the measured and DFT-calculated HFI tensors. The number of identified protons approximately equals the number of protons in the conjugated part of the peridinin, which confirms that the triplet is localized on one specific peridinin molecule at low temperatures. Limitations and perspectives of ENDOR spectroscopy For CW ENDOR, the major limitation is caused by the need of tuning spin-lattice relaxation rates of electrons and nuclei. For this reason, the CW ENDOR signal usually can be obtained only in a limited temperature range. Besides, at a given temperature the ENDOR lines belonging to some nuclei in a specific sample may disappear, while the lines belonging to other nuclei are still present with good signal-to-noise ratio. This may lead to mTOR activity misinterpretations of ENDOR spectra. The problem can partially be solved by using Special TRIPLE spectroscopy.