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Biomedical subjects

A J Hoff

Publications and source records attributed to A J Hoff.

53 records · Page 3Linked to original sources
PubMed1979

Magnetic field-induced increase of the yield of (bacterio)chlorophyll emission of some photosynthetic bacteria and of Chlorella vulgaris.

In photosynthetic bacteria, in which the iron-ubiquinone complex X is prereduced, a magnetic field induces an increase of the emmission yield, which is correlated with the decrease in reaction center triplet yield reported previously (Hoff, A.J., Rademaker, H., van Grondelle, R. and Duysens, L.N.M. (1977) Biochim. Biophys. Acta 460, 547--554). Our results support the hypothesis that under these conditions charge recombination of the oxidized primary donor and the reduced primary acceptor predominantly generates the excited singlet state of the reaction center bacteriochlorophyll. In Chlorella vulgaris and spinach chloroplasts, at 120 K, the magnetic field has an effect similar to that found in bacteria, which suggests that an intermediary electron acceptor between P-680 and Q is present in Photosystem II also.

Bacteriochlorophylls↗
PubMed1978

Electron spin resonance in zero magnetic field of the reaction center triplet of photosynthetic bacteria.

The decay rates kx, ky, kz of the individual spin levels of the light-induced triplet state have been accurately measured by the zero-field resonance technique under conditions of very low light intensity and a microwave sweep rate of 2.5 MHz/microseconds, which is in excess of that commonly used in optical detection magnetic resonance experiments. The rates ku found correspond well with those previously determined under somewhat different conditions (Hoff, A.J. (1976) Biochim. Biophys. Acta 440, 765--771) and with those inferred from the decay at 4.2 degrees K of the triplet-triplet absorption after picosecond excitation (Parson, W.W. and Monger, T.G. (1977) Brookhaven Symp. Biology 28, 195--212). Thus there seems no reason to doubt that PR corresponds to the triplet state detected by ESR. In a recent publication Clarke and Connors (Clarke, R.H. and Conners, R.E. (1976) Chem. Phys. Lett. 42, 69--72) published values of the rates ku which differ substantially from ours and which lead to a mean lifetime in excess of that of PR. We show that erroneous rates are obtained when the microwave sweep rate is not made fast relative to the decay of the individual spin levels. Zero-field splitting parameters for a member of photosynthetic bacteria have been measured with an accuracy of better than 0.4% for D and 1% for E. The enhanced precision as compared to conventional ESR allows one to discriminate between species of one family. Deuteration reduces the ku values by a factor of about 2, with little spin selectivity. This effect is much larger than previously observed for chlorophyll a. The present results explain the decrease in fluorescence intensity observed on microwave saturation in zero-field optical detection magnetic resonance experiments, and they also show that the simple exciton model is inadequate to derive the geometry of the reaction center dimer from the observed zerofield splitting and decay rates.

Electron Spin Resonance Spectroscopy↗
PubMed1978

Nitrogen electron nuclear double resonance and proton triple resonance experiments on the bacteriochlorophyll cation in solution.

Electron nuclear double resonance signals of the pyrrole nitrogens of bacteriochlorophyll alpha cation in a solution of CH2Cl2/CH3OH (6:1) have been observed. The nitrogens are inequivalent: the hyperfine coupling constants were determined to be 2.36 and 3.18 MHz (0.84 and 1.14 G, respectively). Triple electron nuclear double resonance experiments have yielded the signs and values of nine proton hyperfine coupling constants. The assignment based on these data agrees well with previous work (Borg, D.C., Forman, A. & Fajer, J. (1976) J. Amer. Chem. Soc. 98, 6889-6893); in addition, two methine protons have been identified. Spin densities calculated from the hyperfine couplings are compared to reported molecular orbital calculations.

Bacteriochlorophylls↗
PubMed1977

On the magnetic field dependence of the yield of the triplet state in reaction centers of photosynthetic bacteria.

The yield of the triplet state in reaction centers of Rhodopseudomonas sphaeroides is dependent on the strength of an applied magnetic field. Reaction centers of the wild type that lack a functional iron complexed to the primary acceptor ubiquinone show a dependence similar to that of reaction centers of the mutant R-26 in which the iron-ubiquinone complex is intact. Apparently, the iron of the iron-ubiquinone complex is not essential to the effect, but it does exert an influence on its extent. Inchromatophores, the effect is about 2-fold decreased; the value of the magnetic field at which half the effect is found is about 500 G, in contrast to this value for reaction centers, which is 50--100 G. The magnetodependence of the triplet yield is discussed in terms of the Chemically Induced Dynamic Electron Polarization mechanism (CIDEP).

Bacteriochlorophylls↗
PubMed1976

Kinetics of populating and depopulating of the components of the photoinduced triplet state of the photosynthetic bacteria Rhodospirillum rubrum, Rhodopseudomonas spheroides (wild type), and its mutant R-26 as measured by ESR in zero-field.

Optically detected ESR spectra in zero magnetic field of the triplet state of three photosynthetic bacteria are presented. The zero field splitting parameters [D] and [E] and the widths of the resonances show small but significant differences for the three bacteria. The resonance lines are inhomogeneously broadened as demonstrated by hole-burning experiments. The populating probabilities and depopulating rates for the triplet sublevels have been measured. The populating kinetics are very similar for the three bacteria. The depopulating rates are more than one order faster than those of chlorophyll a and chlorophyll b and of porphyrin model systems. The populating probability of the lowest level is about 6 times less, and the depopulating rate about 6 times slower, than for the upper levels, identifying this level as the level connected to the molecular z-axis perpendicular to the plane of the molecule. The relative populations of the triplet sublevels are almost equal in zero magnetic field.

Electron Spin Resonance Spectroscopy↗
PubMed1976

Zero field resonance and spin alignment of the triplet state of chloroplasts at 2 degrees K.

Microwave induced transitions in zero magnetic field have been observed in the photoinduced triplet of chloroplasts treated with dithionite by monitoring changes in the intensity of the 735 nm fluorescence band at 2 degrees K. Similar results were obtained with chloroplasts treated with hydroxylamine plus 3-(3,4-dichlorophenyl)-1,1-dimethylurea and preillumination. The zero field parameters are D = 0.02794 +/- 0.00007 cm-1, E = 0.00382 +/- 0.00007 cm-1, i.e. equal to those of monomeric chlorophyll a to within the experimental error. The photoinduced triplet appears to be linked to Photosystem II. This indicates that the low temperatures 735 nm fluorescence band of chloroplasts is at least partly due to Photosystem II.

Chloroplasts↗