The primary photochemical electron donors of photosynthetic organisms eject a single electron when illuminated with light. In purple photosynthetic bacteria such as Rhodopseudomonas sphaeroides the oxidized primary donor P865+ exhibits a single Gaussian ESR signal possessing a line width that is narrowed by 1 / 4 2 relative to that of monomeric BChl a+ in vitro.' The ENDOR spectrum of P865' yields hyperfine splittings that are one-half those of monomeric BChl a+.2,3 The assignment of the ENDOR signals to specific protons in BChl a is accomplished by selective biosynthetic deuteration of the chromophore. This assignment is crucial because the observed line width can then be compared with that predicted by simulation using the observed hyperfine splittings. Once this is accomplished the total observed line width of P865+ can be accounted for only if the spin is shared between two BChl a molecules.A similar analysis of the green plant photosystem I donor P700 has not been possible due both to an inability to selectively deuterate the chlorophyll in the organism and to the poor quality of P700+ ENDOR d a h 2 On the basis of the fact that the lime width of the P700' ESR signal is about 1 / 4 2 that of Chl a+ and that the P700' ENDOR spectrum does not reveal any large hyperfine splittings, it was concluded that P700+ is a dimer of Chl a from which one electron is missing.We now present data that do not depend on the ambiguous nature of the P700' ENDOR results. These data show that P700' is a single oxidized Chl a type macrocycle. In addition, using the same methodology and criteria we present new data which confirms that P865+ is a dimer of a BChl a type macrocycle. The central difficulty in using the line width of the P700' or P865+ ESR signal alone to determine its possible multimeric nature stems from the possibility that the internal spin distribution of a particular chlorophyll in vitro may differ substantially from that in vivo. Moving A spin density away from carbon atoms adjacent to the Chl a methyl groups onto carbon atoms with only (Y protons or with no protons at all will significantly diminish the ESR line width due to a decrease in both the number and the magnitude of hyperfine splittings. It has been shown that the ESR line width of Chl a+ in vitro is a function of the strength of ligands bonding to its magnesium atom.4 Molecular orbital calculations predict the presence of a second low-lying doublet state of Chl a+ that possesses a rearranged spin distributi~n.~.~ Recently we have shown that tautomerization of the ring V p-keto ester of Chl a to the enol followed by oxidation yields a cation radical which possesses significantly reduced spin density on carbon atoms adjacent to the methyl groups.6Since the ESR signals of chlorophyll cations both in vivo and in vitro are single broad resonance consisting of many overlapping hyperfine split lines and since internal spin redistribution may change the line width of the single broad line without resulting (1) Norris, J. R.; Uphaus, R. A.; Crespi, H. L.; Katz, ...

Changes in rheology during the heating of bovine plasma protein suspensions were examined at temperatures between 50°C and 90°C and for concentrations ranging from 6-12% protein isolate over a 3-hr period. An annular pumping device was used to determine a viscosity index for the suspensions. A model was developed to explain the data. The viscosity index increased exponentially with time at a particular temperature while being linearly dependent upon the concentration. The Arrhenius Equation described the effect of temperature on the rate of change of the viscosity index during heating.