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The synthetic racemic monoester had the same absorption spectrum. Optically active sodium monoethyl a-acetamidomalonate (experiment L24) showed, in D20, absorption had the same absorption spectrum. bands at 2.95-3.05 7.25 (m), 7.34 (m), 7.55 (w).(w), 5.82 (m), 6.18 (s), 6.78-6.90 (w),The synthetic racemic salt [CONTRIBUTION FROM THEA study of the reaction between pyridoxal-5-phosphate and aminothiols was prompted by the observation that pyridoxal-5-phosphate is released in the form of a complex with cysteine when muscle phsphorylase a is incubatcd in a cysteine buffer at pH 6.8. In aqueous media over a wide range of pH values the 4-formyl group of pyridoxal-5-phosphate reacts with the sulfhydryl group of an addend that contains no amino group to form a highly dissociable thiohemiacetal; it also reacts with the amino group of an addend that contains no sulfhydryl group to form a highly dissociable Schiff base. When both functional groups are present in suitable proximity on the same addend, the product is a relatively stable complex containing a thiazolidine ring. The second order constants describing the rates of complex formation between pyridoxal-5-phosphate and both cysteine and its ethyl ester have been determined as a function of pH and compared with those found for the same reactions with 5-deoxypyridoxal. The results are believed t o bear upon the bonding of pyridoxal-5-phosphate in muscle phosphorylase.

Digitonin -fractionated photosystem -I subehloroplasts were titrated potentiometrically between -450 and -610 mV at pH 10. Examination of the titrated subchloroplasts by low-temperature (13'K) electron paramagnetic resonance spectroscopy revealed resonances centered at values of 2.05, 1.94, 1.92, 1.89, and 1.86 on the g-factor scale. The peak heights depended on the potentials at which the chloroplasts were poised. The resonances of at least three iron-sulfur centers can be recognized: one with lines at g = 2.05 and 1.94; one with lines at g = 2.05, 1.92, and 1.89; and one for which only a line at g = 1.86 has been resolved. The midpoint potentials of the iron-sulfur species fall into two distinctly separate regions: the titration profile of the g = 1.94 signal, the first segment of the g = 2.05 plot, and the rise phase of the g = 1.86 signal had a value of -530 i 5 mV; the upper segment of the g = 2.05 plot, the decrease phase of the g = 1.86 signal, and the g = 1.89 profile had a midpoint potential estimated to be <-580 mV. The oxidation-reduction reaction of each of the bound iron-sulfur species, as represented by the changes of the electron paramagnetic resonance spectra, was reversible and apparently involved a two-electron change.Titration at pH 9 could only be carried to -560 mV, and essentially only the first half of the titration behavior as found at pH 10 was seen. At any given potential more positive than -560 mV, the part of the iron-sulfur protein that was not reduced electrochemically could be reduced photochemically, but only to the maximum extent reduced electrochemically at -560 mV. Whereas, chloroplasts illuminated at room temperature and then frozen while still being illuminated developed a signal similar to that produced by electrochemical reduction at -610 mV, illumination at 770K did not bring about photoreduction beyond that accomplished electrochemically at about -560 mV.Dithionite alone in the dark and under anaerobic conditions brought about a partial reduction to the extent of the first electrochemical reduction step. Dithionite plus illumination at room temperature or dithionite plus methyl viologen in the dark produced the maximum signal. Electron paramagnetic resonance spectra due to either light or electrochemically reduced iron-sulfur proteins showed no detectable decay for at least 3 days when samples were stored in the dark at 77OK. These findings were subsequently confirmed by others (3-5).Leigh and Dutton (3) also found that chloroplasts poised at redox potentials from +350 to -250 mV were still capable of undergoing photoreactions, giving rise to the ferredoxin-type EPR spectrum. Mdore recently, Bearden and Malkin (6) reported that the stoichiometric relationship between the bound ferredoxin and P700 is unity, which lends further support to the idea that membrane-bound ferredoxin is the primary electron acceptor.Simultaneous with these developments a light-absorbing species was characterized by Hiyama and Ke (7) that exhibited kinetic behavior expected of the primary ...

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