A DFT STUDY ON THE ELECTRONIC CHARACTER OF P700+

This density functional study is devoted to the long debated electronic nature of P700+. We found that P700+ is intrinsically a dimer of chlorophyll molecules. The unpaired electron spin distributes equally over two chlorophyll molecule halves in the bare P700+, while the dressed P700+ shows the spin density asymmetry mainly coming from the H-bond donated to 131-keto-O of one half. The experimental contradictions on the electronic nature of P700+ are also discussed.

Macronutrients—Nitrogen

Important considerations concerning nitrogen in plants include the amount of nitrogen required, the forms of nitrogen (inorganic and organic) present in plant tissue, the ways that nitrogen is used in plants and affected by fertilization, and symptoms plants show when nitrogen is deficient. After the nonmineral elements, N is found in the next largest amount. More N is needed by plants than all the other nonmineral elements combined, except for K. The range of N concentrations in plants is from 0.5 to 6.0%, with most plants having 1.5 to 3.0%. Inorganic forms of N in plants are NO3- (nitrate) and NH4+ (ammonium). These forms are usually present in relatively small amounts. Other inorganic forms of N do not accumulate without injury to the plants. Organic forms of N predominate in plants, mainly as amino acids and proteins. During and after absorption, N often follows this pathway: Proteins consist of a number of amino acids linked together into a large molecular structure. Once the proteins are formed in plants, N moves to other parts of the plant only if the proteins are split apart by hydrolysis into amino acids. The amino acids then flow freely to other parts of the plant where they can recombine into proteins again. Proteins consist of 12 to 19% nitrogen. Other complex proteins formed from amino acids are enzymes that act as catalysts in biochemical reactions. Proteins also act as reserve food in the seeds that is released during germination for early seedling growth. Another type of N-containing material is chlorophyll (the green coloring matter in leaves necessary for photosynthesis). In the center of a chlorophyll molecule is a Mg atom surrounded by four N atoms. Therefore, N is a part of the chlorophyll molecule and if N is deficient, then plants become yellow since there is insufficient chlorophyll produced. Other important N-complexes are purine and pyrimidine bases that can form adenosine triphosphate (ATP) during the respiration process as an energy carrier.

Photochemically Active Chlorophyll-Containing Proteins from Chloroplasts and their Localization in the Thylakoid Membrane

After solubilization of stroma-freed chloroplasts with deoxycholate, the lipids and the detergent used are separated from the proteins by gel filtration. In this way not denatured pigment-con-taining protein preparations were obtained. The particles in fraction 1 exhibited a molecular weight of 600 000 and contained an average of 25 chlorophyll molecules. The circular dichroism spectrum showed exciton splitting of the red band. The particles in fraction 2 contained 1 chlorophyll molecule and exhibited a molecular weight of 110 000. The particles in fraction 3 also contained only 1 chlorophyll molecule and had a molecular weight of between 80 000 and 100 000. Pure preparations of fraction 1 only carried out the methylviologen Mehler reaction with the dichlorophenol indophenol/ascorbate couple as electron donor. Fraction 3 only reduced ferricyanide with diphenylcarbazide as an electron donor in the light. Fraction 2 exhibited both the photosystem I reaction and the photosystem II reaction. An antiserum to extracted fraction 1 does not inhibit electron transport in the intact lamellar system. The photoreduction of methylviologen is only inhibited after disruption of the thylakoids. The antiserum to fraction 2 inhibits the photoreduction of methylviologen in the intact lamellar system. Consequently, one inhibition site for this photosystem I reaction must be located on the inner and another on the outer surface of the thylakoid membrane. In addition, antibodies to fraction 1 are specifically adsorbed onto the lamellar system without any effect on electron transport and without a concomitant agglutination. Antibodies to fraction 3 partially inhibit the photoreduction of ferricyanide with diphenylcarbazide as an electron donor in the intact lamellar system. Hence, the inhibition site of this system II reaction is located on the outer surface of the thylakoids. We have reason to believe that the inhibition sites not reacting are located in the partitions, which are not accessible to antibodies.

CHLOROPHYLL MONOLAYERS IN CHLOROPLASTS

1. The data on the fine structure of the chloroplasts and the chlorophyll analyses, in two algal flagellates, Euglena gracilis and Poteriochromonas stipitata, are consistent with the assumption that the chlorophyll molecules are arranged in monomolecular layers at the interfaces between the lipid (dense) and the aqueous protein complex layers. 2. The cross-sectional area available to each chlorophyll molecule in the chloroplasts of Euglena and Poteriochromonas was found to be 222 x 10–16 cm2. and 246 x 10–16 cm2. By comparison, the cross-sectional area of the hydrophilic head of the chlorophyll molecule is approximately 240 x 10–16 cm2. 3. The model of the molecular network depicted in Text-fig. 1 is purely speculative. It predicts a lower limit for the number ratio of chlorophyll to other pigment molecules of about 1:1 and a weight ratio of 2:1. A loose packed structure such as that shown in Text-fig. 1 predicts a weight ratio of 4:1 to 6:1. These values bracket the large majority of experimental values found for this ratio. The relative constancy of the number of chlorophyll molecules per chloroplast and the volume of the chloroplast indicated by the data in Table III suggest that chloroplasts probably possess a similar structural arrangement in a variety of photosynthetic microorganisms and plants.

SOME REMARKS ON THE PHOTOSYNTHESIS OF GREEN PLANTS

1. It is suggested that in the assimilation process of green plants the reduction of the CO2 takes place with the help of Fe++ ions (present in the chloroplast) under the influence of light, which is absorbed by a sensitizing chlorophyll-CO2-complex. 2. It seems that the chlorophyll has to fulfill two different functions depending on its situation in the chloroplast. The chlorophyll molecules on the surface of the lipoid phase (in contact with an aqueous phase containing Fe++) combine with CO2 to form a light absorbing chlorophyll-CO2-complex and in this way take part in the reduction of the CO2. The light energy is also absorbed by the greater portion of the chlorophyll, which is dissolved in the interior of the lipoid phase, and eventually handed over to the chlorophyll molecules on the surface. 3. The photosynthetic unit of Emerson and Arnold may be determined by the ratio: See PDF for Equation so that for every chlorophyll molecule on the surface there are about 500 molecules in the interior, which provide it with the necessary quanta.