H.J. Den Blanken

High-resolution optical absorption-difference spectra of the triplet state of the primary donor in isolated reaction centers of the photosynthetic bacteria Rhodopseudomonas sphaeroides R-26 and Rhodopseudomonas viridis measured with optically detected magnetic resonance at 1.2 K

Abstract We have recorded triplet optical absorption-difference spectra of the reaction center triplet state of isolated reaction centers from Rhodopseudomonas sphaeroides R-26 and Rps. viridis with optical absorption-detected electron spin resonance in zero magnetic field (ADMR) at 1.2 K. This technique is one to two orders of magnitude more sensitive than conventional flash absorption spectroscopy, and consequently allows a much higher spectral resolution. Besides the relatively broad bleachings and appearances found previously (see, e.g., Shuvalov V.A. and Parson W.W. (1981) Biochim. Biophys. Acta 638, 50–59) we have found strong, sharp oscillations in the wavelength regions 790–830 nm ( Rps. sphaeroides ) and 810–890 nm ( Rps. viridis ). For Rps. viridis these features are resolved into two band shifts (a blue shift at about 830 nm and a red shift at about 855 nm) and a strong, narrow absorption band at 838 nm. For Rps. sphaeroides R-26 the features are resolved into a red shift at about 810 nm and a strong absorption band at 807 nm. We conclude that the appearance of the absorption bands at 807 and 838 nm, respectively, is due to monomeric bacteriochlorophyll. Apparently, the exciton interaction between the pigments constituting the primary donor is much weaker in the triplet state than in the singlet state, and at low temperature the triplet is localized on one of the bacteriochlorophylls on an optical time scale. The fact that for Rps. sphaeroides the strong band shift and the monomeric band found at 1.2 K are absent at 293 K and very weak at 77 K indicates that these features are strongly temperature dependent. It seems, therefore, premature to ascribe the temperature dependence between 293 and 77 K of the intensity of the triplet absorption-difference spectrum at 810 nm (solely) to a delocalization of the triplet state on one of the accessory bacteriochlorophyll pigments.

High-resolution triplet-minus-singlet absorbance difference spectrum of photosystem II particles

The triplet state of the primary donor of photosystem II particles prepared from a mutant of Chlamydomonas reinhardtii has been studied at 1.2 K with absorbance‐detected ESR in zero‐magnetic field (ADMR). Two sets of resonances with slightly different zero‐field splitting parameters |D| and |E| were observed, |D| = 285.5, |E| = 38.8 and |D| = 288.8 × 10−4 cm−1, |E| = 42.2 × 10−4 cm−1, respectively. Both sets of |D| and |E| values are close to those found for PT‐700, as are the sublevel decay rates k x = 930 ± 40, k y = 1088 ± 50 and k z = 110 ± 5 s−1. The AMDR‐detected triplet‐minus‐singlet absorbance difference spectrum of PT‐680 is very similar to that of PT‐700 and closely resembles that of covalently connected Chl a dimers in vitro. We conclude that P‐680 is a Chl a dimer whose general structure is similar to that of P‐700.

High-resolution absorbance-difference spectra of the triplet state of the primary donor P-700 in Photosystem I subchloroplast particles measured with absorbance-detected magnetic resonance at 1.2 K. Evidence that P-700 is a dimeric chlorophyll complex

Abstract ESR transitions in zero field of the triplet state of the primary donor (P-700 T ) of Photosystem (PS) I subchloroplast particles of spinach have been measured by monitoring the absorbance at 697 nm at 1.2 K (absorbance-detected magnetic resonance (ADMR) spectra). Using the ADMR technique, we have recorded the triplet absorbance-difference spectrum (triplet-minus-singlet spectrum) of P-700 T minus P-700 of the primary donor of PS I at 1.2 K. This method is considerably more sensitive and selective than conventional flash absorption spectroscopy. The ADMR-monitored triplet-minus-singlet spectra of fresh and aged PS I particles are compared to published triplet-minus-singlet spectra of monomeric and dimeric chlorophyll (Chl) a in vitro. The triplet-minus-singlet spectrum of P-700 can be explained as a superposition of the triplet-minus-singlet spectrum of a dimeric Chl a complex and an additional red shift at about 689 nm, attributed to an adjacent Chl a molecule. We conclude that the primary donor of PS I is a dimeric Chl a complex with its long-wavelength absorption band peaking at 697 nm.

ESR in zero field of the photoinduced triplet state in isolated reaction centers of rhodopseudomonas sphaeroides R-26 detected by the singlet ground-state absorbance

Abstract We have measured zero-field resonance transitions of the triplet state of the primary donor monitoring the transmittance at 890 nm at 1.2 K in isolated reaction centers of Rhodopseudomonas sphaeroides R-26. The transitions correspond to a decrease in transmittance, confirming the energy transfer model for the transitions detected via the antenna fluorescence in whole cells.

Polarized triplet-minus-singlet absorbance difference spectra measured by absorbance-detected magnetic resonance. An application to photosynthetic reaction centres

Abstract A method is presented to measure linear dichroic triplet-minus-singlet absorbance difference spectra using absorbance-detected magnetic resonance of the triplet state in zero magnetic field. Orientational selection is achieved by using a linearly-polarised microwave field. The microwave-induced change in the absorbance of light with the electric vector parallel or perpendicular to the microwave field vector is monitored using a Morvue photoelastic modulator and an analyzing polarizer. The method is applied to reaction centres of the photosynthetic bacterium Rhodopseudomonas viridis.

The triplet state of the primary donor of the green photosynthetic bacterium Chloroflexus aurantiacus

The technique of absorbance‐detected electron spin resonance in zero magnetic field (ADMR) was applied to investigate the structure of the reaction center of the facultatively aerobic green bacterium Chloroflexus aurantiacus. The triplet‐minus‐singlet absorbance difference spectrum thus obtained at 1.2K shows a clear resemblance to those earlier reported for Rhodopseudomonas viridis and Rps. sphaeroides R‐26. The most prominent features are the bleaching of the Qy band of the primary electron donor at 887 nm and the appearance of a narrow band at 807 nm upon triplet formation. We conclude that the primary electron donor P‐865 of Chloroflexus aurantiacus is a BChl a dimer with Qy and Qx absorbance bands at 887 and 606 nm, respectively, at 1.2 K; apparently the triplet state is localized on an optical time scale on one of the constituent pigments of the dimer. The zero field splitting parameters |D| and, |E| of P‐865 are 197.7 (± 0.7) × 10−4 cm−1 and 47.3 (± 0.7) × 10−4 cm−1, respectively. Decay rates of 12 660 (± 750) s−1, 14 290 (± 800) s−1 and 1690 (± 50) s−1 were observed for the x, y and z triplet sublevels, respectively.

The triplet state of the primary donor of the photosynthetic bacterium Rhodopseudomonas viridis

The absorbance-detected magnetic-resonance technique has been applied to the study of the triplet state of the primary donor in chromatophores of the photosynthetic bacterium Rps. viridis. The results confirm the triplet-minus-singlet absorbance-difference spectrum and its interpretation as previously obtained for isolated reaction centers (Den Blanken, H.J. and Hoff, A.J. (1982) Biochim. Biophys. Acta 681, 365–374). Our present results affirm that the primary donor is a bacteriochlorophyll b dimer, and that there is no blue exciton band at 850 nm. We show that the reaction centers are not identical, but have a small heterogeneity in their properties. In chromatophores and sometimes in isolated reaction centers a shoulder is observed in the long-wavelength absorbance-difference band of the primary donor. This shoulder is possibly caused by charge transfer interaction of the donor with an adjacent chromophore (Vermeglio, A. and Paillotin. G. (1982) Biochim. Biophys. Acta 681, 32–40; Maslov, V.G., Klevanik, A.V., Ismailov, M.A. and Shuvalov, V.A. (1983) Doklady Akad. Nauk. SSSR 269, 1217–1221) or it reflects a slight heterogeneity in the reaction-center geometry, which cannot be removed with the selection offered by the magnetic resonance technique. The zero-field triplet-ESR spectrum and the sublevel decay rates of the triplet state of the primary donor are presented, as detected in whole cells at the antenna fluorescence, and in chromatophores and isolated reaction centers at the absorbance-difference band at 838 nm. We do not observe the expected reversal of the sign of the ESR transitions monitored with the two techniques. A tentative explanation is given in terms of energy transfer from unrelaxed excited states of the antenna pigments to the reaction center.

Sublevel decay kinetics of the triplet state of bacteriochlorophyll a and b in methyltetrahydrofuran at 1.2 K

Abstract The decay rates of the triplet sublevels of monomeric BChl a and b have been reexamined with pulsed microwave excitation. Our values of k x and k y are significantly higher than published values. The new results are discussed in relation to the exciton model of the special pair making up the primary electron donor in photosynthetic purple bacteria.

Resolution enhancement of the triplet—singlet absorbance-difference spectrum and the triplet-esr spectrum in zero field by the selection of sites. An

Abstract Selection of sites by narrow-banded detection and by electron—electron double resonance results in an enhancement of the resolution in the absorbance

Linear-dichroic triplet-minus-singlet absorbance difference spectra of reaction centers of the photosynthetic bacteria Chromatium vinosum, Rhodopseudomonas sphaeroides R-26 and Rhodospirillum rubrum S1

Abstract For the first time, linear-dichroic triplet-minus-singlet (LD-(T - S)) spectra of reaction centers of the photosynthetic bacteria Chromatium vinosum, Rhodopseudomonas sphaeroides R-26 and Rhodospirillum rubrum S1 have been measured using an extension of the technique of absorbance-detected magnetic resonance (ADMR) of the triplet state. For all bacteria studied the LD-(T - S) spectra exhibit a bleaching of the long-wavelength absorbance band that is either split or has a clear shoulder to longer wavelengths. The components are approximately parallel-polarized, indicating that they do not form an exciton pair. Around 800 nm a band appears with a width of about 7 nm, which does not form part of a band shift and that may be attributed to an appearing monomer band. Small features in the LD-(T - S) spectra at both sides of this band are well explained by band shifts of the two components of the 800 nm reaction center absorption band. The transition moment of the component at about 818 nm in reaction centers of Rps. sphaeroides R-26 is at an angle larger than 55° with both the x and the y triplet spin axes. In none of the bacteria do we find evidence for the bleaching of an exciton component of P-860 near 810 nm.