RHF-INDO

The 327-670 GHz EPR spectra of canthaxanthin radical cation were resolved into two principal components of the g-tensor (Konovalova et al. 1999). Spectral simulations indicated this to be the result of g-anisotropy where gn=2.0032 and gi=2.0023. This type of g-tensor is consistent with the theory for polyacene rc-radical cations (Stone 1964), which states that the difference gxx gyy decreases with increasing chain length. When gxx-gyy approaches zero, the g-tensor becomes cylindrically symmetrical with gxx=gyy=g and gzz=gn. The cylindrical symmetry for the all-trans carotenoids is not surprising because these molecules are long straight chain polyenes. This also demonstrates that the symmetrical unresolved EPR line at 9 GHz is due to a carotenoid Jt-radical cation with electron density distributed throughout the whole chain of double bonds as predicted by RHF-INDO/SP molecular orbital calculations. The lack of temperature... 

Piekara-Sady, L., M. M. Khaled et al. (1991). Comparison of the INDO to the RHF-INDO/SP derived EPR proton hyperfine couplings for the carotenoid cation radical Experimental evidence. Chem. Phys. Lett. 186 143-148. 

ENDOR and NMR studies in conjunction with theoretical AMI and/or INDO studies (in particular RHF-INDO/SP) have contributed greatly to the understanding of the carotenoid radical cation and the description of the charge delocalization along the polyene chain (Piekara-Sady et al., 1991 Hand et al., 1993 Piekara-Sady et al., 1993,1995). An improved crystal structure of j3-carotene reported by Senge et al. (1992) has been used and provided the basis for the success of some of the theoretical descriptions. ENDOR studies have also been successfully performed on /3-carotene and canthaxanthin radicals produced photochemically on Nafion films and silica gel (Piekara-Sady et al., 1991 Wu et al., 1991), and... 

The H-bond distance r U] between the GLU and the keto oxygen can be estimated from the point dipole relation (12). Assuming that the dipolar hfc Ax = 0.9 MHz and taking the oxygen spin density pj O.O6 from RHF-INDO/SP calculations, one obtains = 1.7 A. ... 

Dipolar hyperfine tensors of P365 were calculated from the spin densities obtained from an RHF-INDO/SP calculation performed for the X-ray geometry of [16]. Thereby spin densities of all valence... 

According to our MO (RHF-INDO/SP) calculations, the N nuclei carry significant densitiesjind, therefore, the largest dipolar contribution to the hf... 

Calculated off-diagonal elements of the hf tensors obtained from RHF-INDO/SP spin densities based on the X-ray coordinates.- 8,23 xhese values have been scaled by a constant factor of 0.8 to obtain agreement between the calculated N-ENDOR frequencies in eq. (2b) and the experimental ones. This scaling also gives a fair agreement between experimental and calculated sums of all isotropic N hfc s (16.7 MHz and 15.5 MHz, respectively), f) Isotropic N hfc s (1/3 (A -f A b + A, , )) the diagonal elements of the hf tensors are obtained from eq. (2b) using the calculated elements Ay (i j). 

Application of this model to D " of the wild type yields Aa/I3j 0.35 (Figure 5b). Independently, RHF-INDO/SP calculations on the supermolecule of the wild type... 

Spin densities of Pg6 5 have been calculated by the RHF-INDO/SP method described in ref. 7. The same method had been applied earlier to Pf o Whereas, in the latter case, only proton s-spin densities were evaluated to obtain theoretical isotropic proton hfc s, the complete set of (440) s- and p -, p -, Pz spin densities were used to calculate dipolar hyperfine tensors in 65 present work. 

Fig. 1. Comparison of experimental (dotted) and calculated (RHF-INDO/SP) s-spin densities (solid lines) of in Rb. sphaeroides R-26. Experimental values from isotropic proton hfc s using = Qjj Ph(1s) with Qji = 1420 MHz Geometry from X-ray structure analysis (refinement of Dec. 1990). For details of the calculation see text. S-spin densities are proportional to the areas of the respective squares (p < 0) and circles (p > 0).

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