“Unusual” lines observed in low-frequency cw ENDOR of photoexcited triplet state molecules: the primary donor triplet in photosynthetic reaction centers as an example

The origin of frequently observed “negative” (opposite phase) ENDOR lines in the low-frequency region of triplet state ENDOR spectra is explained in terms of microwave hole burning and RF modulation phenomena. From this, a new method of detecting burnt side holes in EPR spectra is derived which is based on cw ENDOR instrumentation. The method uses the modulation satellites that are induced by a longitudinal RF field component and appear around any EPR line, including burnt holes (“negative” lines). The longitudinal RF field was generated by a coil oriented parallel to the external field, but a longitudinal component of the RF field also exists in most conventional ENDOR spectrometers because of slight misalignments of the ENDOR coil generating the transversal RF field. The lines it induces in the low-frequency part of ENDOR spectra are generally considered as artifacts. It is shown, however, that RF induced modulation satellites provide valuable information concerning the lines distant from the spectral position in the EPR spectrum chosen for ENDOR observation. This allows one to record the pattern of side holes burnt by microwave saturation through forbidden transitions that carries information about ENDOR frequencies comparable to what can be extracted from ESEEM experiments. Such comparability is demonstrated for examples of nitrogen ENDOR of photoexcited triplet states of the primary donor in photosynthetic reaction centers and related compounds.