Special TRIPLE

A refinement of the ENDOR experiment is electron-nnclear-nnclear triple resonance, now commonly denoted TRIPLE. In TRIPLE experiments one monitors the effect of a simnltaneons excitation of two nnclear spm transitions on the level of the EPR absorption. Two versions, known as special TRIPLE (ST) and general TRIPLE (GT), are rontinely perfonned on connnercially available spectrometers. 

Another type of DOUBLE ENDOR, called special TRIPLE , has been introduced by Dinse et al.90 to study proton hf interactions of free radicals in solution. In a special TRIPLE experiment two rf fields with frequencies vp + Av and vp — Av are swept simultaneously. For systems with Tln < T,i this leads to a considerable signal-to-noise improvement and to TRIPLE line intensities which are directly proportional to the number of nuclei with the same hf coupling constant. It should be remembered, however, that in transition metal complexes in the solid state the resonance frequencies are not, in general, symmetrically placed about the free proton frequency vp and that the condition Tln < Tj,i is not always fulfilled. 

Figure 3 Special TRIPLE resonance spectra of the primary donor radical-cation P in the bRC of R. sphaeroides wild type and mutant HE(M202) (His - Glu) and of monomeric BChl a " in organic solvents, all spectra in isotropic solution. The isotropic hfcs are directly obtained from the special TRIPLE frequency nSJ = Ais,J2.H The oxidation potential of the primary donor is also given (vs. NHE). Adapted from reference 68.

The special TRIPLE technique [25, 26] is used for hyperfme couplings a /(2/j) < cOj. In atypical experiment,... 

A special triple issue of Journal of Molecular Catalysis contains the papers presented at the 7th International Symposium on Olefin Metathesis at Kingston-upon-Hull (U.K.), 24-28 August, 1987. ... 

Special TRIPLE [2] can give the number of equivalent / = Vi nuclei. The two RF-frequencies are swept to simultaneously correspond to the two ENDOR transitions for the same nucleus, Fig. 2.4(c). The method is employed particularly to radicals in liquids showing hyperfine structure due to H nuclei. The method is more sensitive than the usual ENDOR experiment. The theoretical background is described in original [2] and review literature [15, 16]. 

A nucleus with spin / gives rise to 21 + 1 hyperfine lines in ESR while only two lines occur in the ENDOR specttum corresponding to the electronic quantum numbers ms = /2 for an 5 = V2 species. The relative signs of several isotropic hyperfine coupling constants in a radical can be determined by the general TRIPLE method as described in Chapter 2. The number of equivalent nuclei in the radical systems discussed below can be obtained from the line intensities in ESR, Special TRIPLE, and pulsed ENDOR, but usually not from CW-ENDOR for reasons discussed in the previous chapter. 

The use of multi resonance techniques such as ENDOR together with general / special TRIPLE resonance techniques has now become much more common as the techniques move more into more general use, certainly towards the end of the review period. The use of such techniques has enabled very small hyperfine coupling constants (smaller than the linewidth) to be measirred which would normally be inaceessible to... 

Figure 1 First-derivative spectra obtained by continuous-wave techniques for the phenalenyl radical (A) ESR, (B) ENDOR, (C) special TRIPLE resonance, (D) general TRIPLE resonance, and (E) ENDOR-induced ESR. The arrows indicate the positions of the RF pumping frequencies there are several possibilities in (D). The RF pumping frequency in (E) is set in the same position as shown in (B). (Reprinted from Kurreck H, Kirste B, and Lubitz W (1988) Electron Nuclear Double Resonance Spectroscopy of Radicals In Solution. Weinheim VCH Publishers.)...

Figure 33. Typical ESR. ENDOR, general triple and special triple spectra of bacteriochlorophyll a (5) cation radical (iodine...

In the ENDOR/TRIPLE technique (also called Special TRIPLE) both transitions are induced simultaneously [14,15] by modulating a carrier of frequency with a frequency, v iple, resulting in two side bands... 

Electron nuclear-nuclear Special TRIPLE resonance [21 ] was used as an extension of ENDOR. In this technique the respective high- and low-frequency NMR transitions v+ and v are pumped simultaneously by sweeping two RF frequency sidebands symmetrically about the proton Zeeman frequency vjj [21 ]. The obtained signals (changes of EPR intensity) are much larger and - even more important - less dependent on the ratio of electron and nuclear relaxation rates than in a conventional ENDOR experiment. Additionally, the relative line intensities approximately... 

Fig. 2 shows Special TRIPLE spectra of Pg55 for two selected orientations of the crystal in the x-y plane and for isotropic liquid solution. The four RC sites in the unit cell are all equivalent for the shown crystal orientations, for other rotation angles in this plane the four sites are pairwise equivalent. The observed spectral resolution for the crystal is comparable to that obtained for isotropic solution. 

Special TRIPLE spectra of P355 in an RC single crystal of Rb. sphaeroides R-26 for rotation about the unit cell z-axis, and in liquid (buffer/LDAO) RC solutions. (Microwave power 20 mW, RF field ca. 10 gauss (rot. frame) for each sideband, 10 kHz fm, 70 kHz deviation). For the shown crystal orientations X II p and y Bp all four sites in the unit cell are equivalent [5,18,19]. The x-axis is close to the long and y is close to the short diagonal of the crystal morphological rhombic cross section as indicated. (See also refs. [18,19].) For identification of the X- and y-axis, see text. 

Special TRIPLE rotation patterns for lines 1-10 of P055 single crystal x-y plane. The frequencies correspond to... 

Special TRIPLE resonance experiments on in well-ordered RC single... 

To determine how the electronic structure of D is influenced by the mutation we studied the spin density distribution in the cation radical by ENDOR and Special TRIPLE [14-16] spectroscopy. These techniques were used earlier to measure the spin density distribution of D in RCs of Rb. sphaeroides R-26 [7-9]. It was established that the unpaired electron in is delocalized over both BChl a molecules. The spectral characteristics of this delocalization have been explained by a model in which the unpaired electron is equally shared between the two halves of the dimer [17] this results in a halving of the isotropic hyperfine coupling constants (hfc s) of the dimer with respect to the monomer. According to this model the width of the EPR spectrum of the dimer should be reduced by V2. More refined models for the delocalization of the unpaired electron have now been proposed to account, for example, for the fact that the reduction factors of the isotropic hfc s deviate from 2, (see, for example, [9]). Irrespective of the details of the model, a delocalization over both halves of the dimer should manifest itself in a reduction of the hfc s and a reduction of the EPR linewidth compared with the monomer, whereas when the unpaired electron (hole) is localized on one half of the dimer, the hfc s and linewidth should be that of the monomer. 

In the Special TRIPLE experiments, D was created by in situ illumination at 292K using a 500 watt projector with a 3 cm H2O filter. For the RC s of R-26 an 850 nm interference filter (Corion C850) and a 21% neutral density screen were used, whereas for the mutant RC s only a 50% neutral density screen was used. For the ENDOR experiments at 90K the RC s were illuminated with the 500 W projector at 293K for 5 sec and rapidly frozen in liquid nitrogen under continuous illumination. 

To determine the spin density distribution of D, ENDOR and Special TRIPLE [14-16] methods were used. The line positions Ve dor ENDOR spectra are given by the resonance condition... 

In the Special TRIPLE experiment, which is described in an accompanying paper [20] in more detail, both NMR transitions of a particular hfc are irradiated simultaneously. This often results in enhanced sensitivity and spectra that resemble those of the ENDOR spectra folded around V. ... 

Figure 3. Special TRIPLE spectra of D in reaction centers of (a) Rb. sphaeorides R-26...

In Fig. 3 the Special TRIPLE spectra at 292K of in both mutants and Rh. sphaeroides R-26 are shown. The isotropic hfc s are summarized in Table 1. It is seen that the isotropic hfc s of in the heterodimer mutants are larger than in Rb, sphaeroides R-26. For a comparison of the spin... 

Figure 2. ENDOR (a) and Special TRIPLE spectra (b) of D in single crystals of %-labeled RC s. For the shown orientations, the field is parallel to one of the three symmetry axes of the crystal, and all four RC s in the unit cell are magnetically equivalent. Line positions in (a) correspond to the high-frequency ENDOR transition (see eq. (2b)) = 1.46 MHz the indicated assignment to and...

---