Marcia Brody's research while affiliated with CUNY Graduate Center and other places

The ability of an exogenous long-chain unsaturated fatty acid (linolenic acid) to induce changes in the circular dichroism (C.D.) spectra of chlorophyllous systems of various levels of organization is demonstrated and attributed to its deaggregating influence. In the case of chlorophyll in solution (CCl(4) or CCl(4)-hexane), deaggregation is by direct action on the chromophore. Evidence is also given for an indirect mechanism when chlorophyll is attached to protein (e.g., in HP-700 complexes); in this case, deaggregation results from a conformational change in the protein. Interpretations are given for the differences in C.D. spectra of nonmembranous and membranous chlorophyll-containing systems. (The latter include "digitonin-isolated" system I particles, subchloroplast particles obtained by means of sonication, and specially prepared intact chloroplasts.)

The ability of long-chain unsaturated fatty acids to serve as models for the action of the protein fraction of Ricinus leaf extract on chloroplasts (and algae) has been determined experimentally for several parameters. These include steady-state fluorescence emission and excitation (measured at −196 °), fluorescence induction, light-induced absorption changes of chlorophylls aII and aI, Hill activity, and chloroplast ultrastructure. In addition, the sequential inhibition of system II- and system I-associated electron flow has been demonstrated as a function of increasing concentration of exogenous fatty acid.

In one of the photochemical reaction systems of photosynthesis (system I), the primary process is believed to involve an electron transfer between cytochrome and chlorophyll. As an in vitro model for this system, a study has been made of the photochemical reactions between chlorophyll a and hemin, in pyridine solution. A slow dark reduction, undergone by hemin in pyridine, is accelerated by the presence of chlorophyll, and both red and green light further increase the rate of the reaction. A mechanism proposed for this reaction is electron transfer from chlorophyll to hemin, with recovery of oxidized chlorophyll at the expense of solvent. Chlorophyll also sensitizes the photooxidation of reduced hemin in air. The kinetics of this reaction are similar to those of other sensitized photooxidations, indicating a similar mechanism. Quenching of chlorophyll fluorescence by low concentrations of hemin or reduced hemin suggest the existence of a complex between chlorophyll and hemin.

Absorption difference spectra between highly concentrated and dilute solutions of chlorophyll a have been measured in ethanol, maintaining the product of concentration and path length constant. A log-log plot of dimer versus monomer concentration, derived from the data, was linear with a slope of 2.0, which is consistent with the interpretation that dimers are present in this solvent. The absorption spectrum of the dimer in ethanol was calculated, and a value of 4.5 ± 0.8 M−1 was obtained for the equilibrium constant for dimerization.

Abstract— A detailed in vitro study was made of the flavin sensitized photoinactivation of indoleacetic acid, using primarily riboflavin as sensitizer. The dependence of the quantum yield on reactant concentrations, pH, presence of oxygen, viscosity, temperature, KI concentration, and solvent was determined. The involvement of a limiting dark reaction was demonstrated, using an intermittent light technique. The results are consistent with a mechanism involving a metastable state of riboflavin as the photochemically reactive species. The calculated rate constant for intersystem crossing to this state was found to be 2.5 times 108/sec. Riboflavin, in the metastable state, is believed to oxidize indoleacetic acid to indolealdehyde, with subsequent recovery of riboflavin by autoxidation. The maximum quantum yield of the photoinactivation of IAA is 0.71, indicating a highly efficient process, approaching 100% when energy loss due to riboflavin fluorescence is taken into account. Both carotenoids and pure chlorophyll-a were found to be inactive as sensitizers.

The state of chlorophyll in chloroplast fragments is affected by such factors as the ionic strength and pH of the suspending medium. With increasing ionic strength or at pH values other than neutrality, there is a decrease in the fluorescence yield of the form of chlorophyll with fluorescence maximum at 715 to 736 mmu (aggregate) and an increase in the yield of the form with fluorescence maximum at 685 mmu (monomer). (Positions of maxima cited are for 77 degrees K.) These changes in yield are accompanied by modifications in absorption and fluorescence excitation spectra. It is also noted that these effects are similar to the ones brought about by pancreatic lipase, wheat germ lipase, pancreatic trypsin or urea. An interpretation is given which is consistent with the experimental data, namely, that the effects originate in conformational changes in the proteins to which the pigments are attached. These conformational changes give rise to an increase in the size of the aggregate and a decrease in the probability of energy transfer between the monomer and aggregates.

It is shown that aggregates are present at high chlorophyll concentrations in pyridine and in Euglena at room temperature. The distribution between monomer and aggregate is not significantly altered by cooling to 77° K. The chlorophyll species are identified by their respective fluorescence emission and excitation spectra.

SYNOPSIS. In the present study, we show for various photosynthetic organisms in the mature state the diversity which exists in the relative fluorescence intensities of the aggregate and monomer of chlorophyll (at both room temperature and the temperature of liquid nitrogen). For a number of forms, but especially for Euglena gracilis, the spectral transformations which accompany greening are reported. The onset of photosynthetic activity is correlated with a steep (almost step-like) rise in the ratio of aggregate to monomer fluorescence. From this ratio we calculate the “effective” concentration of chlorophyll and estimate lamellar area as a function of greening.