Wavelength Dependence Of Quantum Yield Research Articles

Photoaquation reactions of trans-chloro(dimethylformamide)tetraammine-chromium(III)

Abstract Upon ligand-field irradiation in acidic solution, the trans -Cr(NH 3 ) 4 (DMF)Cl 2+ cation undergoes simultaneous loss of DMF, Cl − and NH 3 . Photosolvation of DMF predominates at all excitation energies. From the wavelength dependence of quantum yields, the lower-lying 4 E state and the upper 4 B 2 state, are inferred to be the main precursors to aquation of the axial ligands and of NH 3 , respectively. The results are examined in the light of the current photolytic models. Differential spectrophotometry indicates that DMF and Cl − replacement takes place with complete trans → cis isomerization. Comparison with the photochemical behavior of trans -Cr(NH 3 ) 4 (H 2 O)Cl 2+ suggests that the photoisomerization of the latter occurs entirely through water exchange. Charge-transfer irradiation destabilizes the coordinated groups less selectively than ligand-field photolysis.

Wavelength-dependent quantum yields of chloroplast phosphorylation catalyzed by phenazine methosulphate

Studies of phenazine methosulphate (PMS)-catalyzed O 2 exchange and phosphorylation in spinach chloroplasts reveal that at short wavelengths (<680 m μ) PMS acts at a reduced quantum efficiency as an oxidant for O 2 evolution with concomitant phosphorylation. The quantum yield profile of phosphorylation obtained with PMS differs markedly from the yield profile of phosphorylation for normal chloroplasts with NADP + or ferricyanide as oxidant. Between 525 and 680 mμ the quantum yield of phosphorylation (∅ATP) catalyzed by PMS is less than half the constant maximum ∅ATP of the normal system. The maximum ∅ATP value for the normal system is approx. 0.16 ATP/ hv. With the PMS system a peak in the yield at 690 mμ is obtained approaching the ∅ATP value of the normal system. This yield falls again at longer wavelengths (>700 m μ). The addition of ascorbate to the PMS phosphorylating system decreases the short-wavelength (<680 m μ) phosophorylation activity but increases the long-wavelength (>690 m μ) phosphorylation activity. The quantum yield profile of this system, showing a long-wavelength rise in phosphorylation efficiency is obtained with or without the addition of 3(3,4-dichlorophenyl)-1, 1-dimethylurea. These experiments have been interpreted as indicating two separate electrontransfer processes catalyzed by PMS, one in which PMS acts at a reduced efficiency as a Hill oxidant, and the other in which PMS acts as an electron donor and acceptor in a cyclic fashion in sensitizing and essentially long-wavelength phosphorylation process.

Wavelength-dependent quantum yield of ATP synthesis and NADP reduction in normal and dichlorodimethylphenylurea-poisoned chloroplast

At short wavelengths (525–690 mμ) the direct measurement of the quantum yield of the photoreduction of NADP + in normal O 2-evolving spinach chloroplasts is constant (∅ approx. 0.3 equiv/ hv). At short wavelengths (<690 m μ) the quantum yield for NADP + reduction in 3(3,4-dichlorophenyl)-1,1-dimethylurea-poisoned chloroplasts supplied with the ascorbate-2,6-dichlorophenolindophenol couple (donor system) is approx. half as efficient as the normal system. At long wavelengths the quantum yield of NADP + reduction in the donor system increases by a factor of 2 (∅ approx. 0.3 equiv/ hv) when compared with the corresponding yield for the donor system at short wavelengths (∅ approx. 0.15 equiv/ hv). Between 525 and 690 mμ, the phosphorylation yield for the normal system is constant (∅ = 0.15 ATP/ hv), maintaining a constant P/2e ratio of unity. The P/2e ratios indicate a tight coupling between phosphorylation and electron transport encompassing a single phosphorylation site for the transfer of two electrons. Between 525 and 680 mμ, the phosphorylation yield for the donor system is constant (∅ approx. 0.04 ATP/ hv), maintaining a P/2e ratio of approx. 0.5. At longer wavelengths (>690 m μ) the phosphorylation yield of the donor system rises (∅ approx. 0.07–0.08 ATP/ hv) concomitant with the rise in the yield of electron flow. These experiments suggest the possibility that two types of phosphorylation processes operate in chloroplasts, (1) a short-wavelength process coupled to the normal O 2-evolving activity, and (2) a long-wavelength process coupled to the electron-donor activity of reagents such as DCIP.

Ultraviolet inactivation of transfer ribonucleic acid functions

Escherichia coli and yeast sRNA‡ have been irradiated with ultraviolet light at a number of wavelengths and under various conditions. Phenylalanine and lysine acceptance and transfer, as well as the binding of the corresponding aminoacyl tRNA's to ribosomes, were found to be inactivated with simple exponential kinetics. It is concluded that the target for inactivation of these tRNA functions is small, and that there is a high overlap between the three targets of the phenylalanine tRNA functions and a low overlap between those of the lysine tRNA functions. Two lysine tRNA's were shown to differ in the ultraviolet sensitivity of their amino-acid acceptance. It was found that the ATP-acceptance of partially degraded sRNA is sensitive to ultraviolet radiation. Various attempts to restore activity in ultraviolet-irradiated tRNA were unsuccessful. The irradiated sRNA was characterized by its absorption spectrum and its thermal denaturation profile. Irradiated phenylalanyl and lysyl tRNA's were co-chromatographed with the unirradiated tRNA's on columns of Sephadex and methylated albumin. Various results, particularly the wavelength dependence of quantum yield, led us to consider a hypothesis which implicates a change in secondary structure resulting predominantly from the formation of uracil dimers as cause of ultraviolet inactivation of E. coli phenylalanine and lysine tRNA's.