Spectrum action

The most widely employed optical method for the study of chemical reaction dynamics has been laser-induced fluorescence. This detection scheme is schematically illustrated in the left-hand side of figure B2.3.8. A tunable laser is scanned tlnough an electronic band system of the molecule, while the fluorescence emission is detected. This maps out an action spectrum that can be used to detemiine the relative concentrations of the various vibration-rotation levels of the molecule. 

The existence of two separate but interacting photosystems in photosynthetic eukaryotes was demonstrated through analysis of the photochemical action spectrum of photosynthesis, in which the oxygen-evolving capacity as a function of light wavelength was determined (Figure 22.10). 

FIGURE 22.10 The photochemical action spectrum of photosynthesis. The qnantnm yield of photosynthesis as a fnnction of wavelength of incident light shows an abrnpt decrease above 680 nm, the so-called red drop. 

Ward, W. W., and Seliger, H. H. (1976). Action spectrum and quantum yield for the photoinactvation of mnemiopsin, a bioluminescent photoprotein from the ctenophore Mnemiopsis sp. Photochem. Photobiol. 23 351-363. 

The action spectrum plotted in Fig. 7c was recorded in the Br2 B X, 14—0 region. Features associated with transitions of Hes Br2 complexes are expected to become prominent in this spectrum since the Av = —3 product is being probed. Again following the band-shift rule the features associated with transitions of the (3,0)He Br2 and (2,l)He Br2 conformers are observed at about +4 cm from the (2,0)He Bt2 and (l,l)He Br2 features. Since the (0,l)He- Br2 feature is shifted by about +10cm from the monomer band origin, and the (l,l)He Br2 feature is found at about +14cm, the weak feature observed at about +24 cm is tentatively attributed to transitions of the... 

Figure 13. Action spectrum of the linear He I Cl complex near the He + I Cl(By = 2) dissociation limit obtained by scanning the excitation laser through the ICl B—X, 2-0 region and monitoring the l Cl E—>X fluorescence induced by the temporally delayed probe laser, which was fixed on the l Cl E—B, 11-2 band head, (a). The transition energy is plotted relative to the I Cl B—X, 2-0 band origin, 17,664.08 cm . Panels (b), (c), and (d) are the rotational product state spectra obtained when fixing the excitation laser on the lines denoted with the corresponding panel letter. The probe laser was scanned through the ICl B—X, 11-2 region. Modified with permission from Ref. [51].

Photoirradiation of the modified electrode with nanoclusters of Cj qN alone or the mixture of C oN and MePH afforded anodic photocurrents. The photocurrent action spectrum was in fair agreement with the absorption spectrum of the THF-H2O (2 1) mixed solution containing nanoclusters of the mixture of CfioN and MePH or C oN alone. These results strongly indicate that the photocurrents can be ascribed to photoexcitation of the nanoclusters of C qN. ... 

Since the function of most of the chlorophyll and the accessory pigments is thought to be the harvesting of light, one would expect the action spectrum... 

The defenders of the carotenoid-photoreceptor-hypothesis have always understood the shape of these action spectra in the blue to mean that the bluelight receptor is a carotenoid. Indeed, in Fig. 6 3 it can be observed, that the three-peak absorption spectrum of trans-0-carotenoid (in hexene) agrees well with the observed action spectrum of the avena coleoptile (Fig. 3 5). However, there remains one loose end which has been the crucial point of controversy in this field, ever since Galston and Baker66 suggested in 1949 that the photoreceptor for phototropism might be a flavin Flavin absorbs in the near UV, /3-carotenoid does not. 

Nevertheless, the avena coleoptile exhibits a curvature to unilateral UV-illumina-tion with a satisfactory log-linear response/time relationship38) (the bending mode is similar to that observed for the second positive curvature which develops from the coleoptile base cf. 2.2). Fig. 5 338) shows that the double-peaked action spectrum does not match neither flavin (Fig. 5 5,16S)) nor carotenoid absorption (Fig. 5 4,183)), most likely excluding both as photoreceptors. The growth hormone auxin (cf. 2.4 and Scheme 1) has been discussed to be a possible photoreceptor. However, in this case, this is not supported by the action spectrum either. 

Phycocymes shows a positive UV-induced light-growth-response (Fig. 3 3) as well as a negative phototropic curvature, as depicted in Fig. 5 1. The exceptionally good fit of this action spectrum and auxin absorption (Fig. 5 2) might indicate that auxin is the UV-photoreceptor. 

Fig. 6. Comparison of different action-and absorption spectra of (1) lactat oxidase1 1U, (2) 0-cis carotene183), (3) ft-trans carotene183), (4) lutein in ethanol/ water76). (5) same action spectrum as in Fig. 3 5...

Composite action spectra characteristics of carotenoid (Fig. 8 2,169)) and flavin (Fig. 8 1,49)), imitated by the low temperature absorption spectra, are compared with the avena action spectrum (Fig. 8 3). Song and Moore pointed out on this basis, that the carotenoid is a rather unlikely photoreceptor, whereas the flavin is169). 

Fig. 8. Action spectra as mimicked by low temperature absorption spectra (liquid nitrogen) of (1) flavin49) and (2) carotene169). These spectra are compared with the phototropic action spectrum (curve (S), same as Fig. 3 5)...

Fig. 9. Absorption spectra of several flavopro-teins compared with an action spectrum of pho-totropism (dotted line C ),39))- The spectra are arranged with respect to the position of their UV-peaks (1) succinate dehydrogenase78), (2)lipo-amide dehydrogenase179), (4) lactate oxidase111), (5) D-amino acid oxidase111), (6) flavodoxin110), (7) old yellow enzyme2), (8) ferredoxin NADP+ reductase16), (9) oxynitrilase111), (10) L-amino acid oxidase111)...

Therefore, the action spectrum for phototropism does not simply reflect the absorption spectrum of the active photoreceptor pigment itself, but instead, its absorption spectrum somehow modified by shading pigments. However, on this basis Thimann and Curry failed to calculate a curve fitting the experimental action spec-... 

Using the same model, Shropshire and Withrow concluded that in the limit of zero bending the influence of shading pigments on the action spectrum could be completely ignored, i.e., the action spectrum is exactly equal to the absorption spectrum 162 ... 

And only for that (extrapolated) case, the near UV-peak of the action spectrum vanishes, from which the authors conclude that the active photoreceptor is probably carotenoid in nature (cf., Fig. 6 3). 

Fig. 14. Action spectra for light-induced metabolic responses. (1) oxygen uptake in Chlorella200, and carotenogenesis of (S) Neurospora44), (4) Mycobacterium91), (5) Fusarium14l Curve (2) shows the absorption spectrum of an extract of Mycobacterium possibly containing the bluelight photoreceptor112). Compare curve (2) to the action spectrum (4)...

Another role of bluelight is exhibited by the moth Pectinophora gossypiella. A circadian rhythm of egg hatching can be initiated with a brief light pulse. The action spectrum (similar to Fig. 16 2,27)) again suggests a flavin photoreceptor. 

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