T.J. Schaafsma

Excited-state properties of water-soluble porphyrin dimers

Abstract The excited-state properties of heterodimers oftetra(4-carboxyphenyl)porphyrin (TPPC) and tetra(N-methylpyridyl) porphyrin (TMPyP) are studied by absorption and emission spectroscopy, EPR and zero-field ODMR. The excited singlet and triplet states of dimers formed by pairing H 2 TPPC with H 2 TMPyP or ZnTMPyP are localized on H 2 TPPC. The dimers formed by pairing H 2 TPPC with CuTMPyP and H 2 TMPyP with ZnTPPC or CuTPPC are non-fluorescent due to intramolecular electron transfer.

The influence of the double reduction of QA on the fluorescence decay kinetics of Photosystem II

Abstract The acceptor QA of PS II was doubly reduced by treatment of PS II-enriched membranes (200–300 chlorophylls per PS II-reaction centre) with dithionite and benzyl viologen. After double reduction of QA, two major differences appeared in the fluorescence decay kinetics (at 4′C), as compared to the situation with all QA singly reduced: (1) a dominant fast phase (lifetime approx. 200 ps) was observed, similar to that in samples with QA oxidised: (2) a slow phase with a lifetime of approx. 7 ns was observed, which disappeared upon reoxidation of the sample. The fluorescence yield was approximately half of that of samples with singly reduced QA. The fast phase is interpreted as being indicative of a high efficiency of charge separation due to the absence of a negatively charged QA. This is explained by the double protonation of doubly reduced QA giving rise to the electrically neutral quinol. Similar observations were made in a core complex preparation (60–80 chlorophylls per reaction centre). This preparation involves a detergent solubilisation step and data from both EPR and fluorescence indicated that it was more susceptible to double reduction of QA by dithionite (as compared to PS II membranes). The possibility that this is a general phenomenon in detergent solubilised PS II preparations is discussed.

Sub-picosecond photoinduced electron transfer in water-soluble porphyrin dimers

Abstract Picosecond as well as femtosecond transient absorption measurements have been performed on eight water-soluble heterodimers consisting of combinations of oppositely charged free base and metal porphyrins which have been electrochemically characterized. The ultrafast non-exponential absorption increase, attributed to solvent-controlled, intradimer charge separation, is energy-gap independent. By contrast, the much slower charge recombination is energy-gap dependent.

Noninvasive measurement of plant water flow by nuclear magnetic resonance

Water flow through the stem of an intact cucumber plant has been measured by using pulsed NMR. This method yields the linear flow velocity of the sapstream, found to be proportional to the loss of weight due to evaporation. The presence of a large excess of stationary water (for cucumber 95% of the total water content) does not interfere with the detection of a small amount of flowing water, due to cancellation of the NMR signal of stationary water. This makes the method particulary suitable for application to biological systems with a high stationary water content.

Spin polarization in the lowest triplet state of chlorophyll

Abstract Chlorophyll-b in glassy solution has a spin-polarized lowest triplet state at and above 77 K. The magnitude of the effect is different for MTHF and ethanol as solvents, in contrast to what is found for the porphin free base. Chlorophyll-a does not exhibit spin-polarization under identical conditions as for chlorophyll-b. Zero-field parameters are found to be: chlorophyll-a (MTHF) D = (281 ± 6) × 10 −4 cm −1 ; E = (39 ± 3) × 10 −4 cm −1 ; chlorophyll-b (MTHF) D = (289 ± 4) × 10 −4 cm −1 ; E = (49 ± 3) × 10 −4 cm −1 , From ESR signal kinetics it follows that for chlorophyll-b, population and depopulation mainly involve the spin level y ⪢, describing a spin moving in a plane perpendicular to the molecular plane: P y ⪢ P x ⪢ P z ; k x = 240 ± 40 s −1 ; k y = 600 ± 120 s −1 ; k z ⩽ 75 s −1 , where P i and k i denote populating and decay rates. Thus, the kinetic scheme for the chlorophyll triplet is different from that of porphyrins with heavier metal ions, but very similar to that of the porphin free base. The spin-lattice relaxation time is found to be anisotropic and shorter than the decay rates of individual spin levels. Nevertheless, spin polarization can be observed, essentially because the ESR signal amplitude depends on population differences .

FLUORESCENCE SPECTRA AND ZERO-FIELD MAGNETIC RESONANCE OF CHLOROPHYLL a-WATER COMPLEXES

Abstract. –Fluorescence detected magnetic resonance (FDMR) and fluorimetry have been used to study chlorophyll‐water complexes at T= 4.2 K. By combining these methods, zero‐field splitting (ZFS)‐parameters can be assigned to the triplet state of the various species. It is found that these parameters decrease upon aggregation and/or complexation. These results can be rationalized by using a simple free‐electron model for chlorophyll including excitonic interaction. Evidence is presented favouring a structure recently proposed by Shipman and Katz for a chlorophyll‐dimer involving hydrogen‐bonded water, present in non‐polar solutions at low temperature with spectral properties similar to those of chlorophyll in vivo.

Quantitative measurement and imaging of transport processes in plants and porous media by 1H NMR

NMR and MRI have been applied to transport processes, that is, net flow and diffusion/perfusion, of water in whole plants, cells, and porous materials. By choosing proper time windows and pulse sequences, magnetic resonance imaging can be made selective for each of the two transport processes. For porous media and plant cells the evolution of the spatial distribution of excited spins has been determined by q-space imaging, using a 20 MHz pulsed 1H NMR imager. The results of these experiments are explained by including spin-relaxation and exchange at boundaries. A 10 MHz portable 1H NMR spectrometer is described, particularly suitable to study the response of net flow in plants and canopies to changing external conditions.

Fluorescence detected magnetic resonance (FDMR) of green sulfur photosynthetic bacteria Chlorobium sp.

Fluorescence Detected Magnetic Resonance (FDMR) spectra have been measured for whole cells and isolated chlorosomal fractions for the green photosyntheic bacteria Chlorobium phaeobacteroides (containing bacteriochlorophyll e, and isorenieratene as major carotenoid) and Chlorobium limicola (containing bacteriochlorophyll c, and chlorobactene as major carotenoid). The observed transition at 237 MHz (identical in both bacteria) and > 1100 MHz can be assigned, by analogy with published data on other carotenoids, to the 2E and D + E transitions, respectively, of Chlorobium carotenoids. Their zero field splitting (ZFS) parameters are estimated to be: |D|=0.0332 cm−1 and |E|=0.0039 cm−1 (chlorobactene), and |D|=0.0355 cm−1 and |E|=0.0039 cm−1 (isorenieratene). In the intermediate frequency range 300–1000 MHz the observed transitions can be assigned to chlorosomal bacteriochlorophylls c and e, and to bacteriochlorophyll a located in the chlorosome envelope and water-soluble protein. The bacteriochlorophyll e triplet state measured in 750 nm fluorescence (aggregated chlorosomal BChl e) is characterised by the ZFS parameters: |D|=0.0251 cm−1 and |E|=0.0050 cm−1.

Zero-field optically detected magnetic resonance of model compounds for pheophytins

Abstract ZFS parameters and kinetic constants of the lowest triplet state of chlorin and tetraphenylchlorin free base in n-octane have been determined by fluorescence-detected ODMR at 4.2 K. These compounds can be considered as model compounds for pheophytin, a compound of biological interest. For both compounds the middle spin-level is the most active one in the populating and depopulating pathway. In the lowest triplet state the NHNH axis in both chlorins is probably fixed to one orientation not involving the reduced ring, and no evidence was found for the occurrence of two tautomeric forms as in the corresponding porphyrins.

Optical detection and electronic simulation of magnetic resonance in zero magnetic field of dihydroporphin free base

An apparatus for the measurement of optically detected magnetic resonance(ODMR) in zero magnetic field at 4.2 K using commercially available parts is described. A simple electronic analogue device is used to obtain kinetic data from ODMR transients in a fast and reliable way. From these data decay rates of the three spin levels of the lowest molecular triplet state can be obtained, whereas populating rates can be determined within defined limits. Since the apparatus is suited for unguarded operation during long periods of time, signal averaging of weak signals, e.g., from biological molecules, is particularly easy. Kinetic data have been obtained for dihydroporphin, which can be considered as the structural basis for chlorophyll: k 1= 121±12 sec−1; k 2=277±28 sec−1; k 3=22±2 sec−1; 0.28<P 1<0.43; P 2=1; P 3<0.08, where k i and P i are the decay and populating rates of the ith spin level.