Two-dimensional ENDOR-ESEEM correlation spectroscopy

A two-dimensional (2D) experiment that correlates electron-nuclear double r esonance (ENDOR) and electron spin-echo envelope modulation (ESEEM) frequen cies, useful for unraveling and assigning ENDOR and ESEEM spectra from diff erent paramagnetic centers with overlapping EPR spectra, is presented. The pulse sequence employed is similar to the Davies ENDOR experiment with the exception that the two-pulse echo detection is replaced by a stimulated ech o detection in order to enhance the resolution in the ESEEM dimension. The two-dimensional data set is acquired by measuring the ENDOR spectrum as a f unction of the time interval T between the last two microwave pulses of the stimulated echo detection scheme. This produces a series of ENDOR spectra with amplitudes that are modulated with T. Fourier transformation (FT) with respect to T then generates a 2D spectrum with cross peaks connecting spec tral lines of the ESEEM and ENDOR spectra that belong to the same paramagne tic center. Projections along the vertical and horizontal axes give the thr ee-pulse FT-ESEEM and ENDOR spectra, respectively. The feasibility of the e xperiment was tested by simulating 2D ENDOR-ESEEM correlation spectra of a system consisting of an electron spin (S = 1/2) coupled to two nuclei(I-1 = I-2 = 1/2) taking into account experimental conditions such as pulse durat ions and off-resonance irradiation frequencies. The experiment is demonstra ted on a single crystal of Cu2+ doped L-histidine (Cu-His), containing two symmetrically related Cu2+ sites that at an arbitrary orientation exhibit o verlapping ESEEM and ENDOR spectra, While the ESEEM spectrum is relatively simple and arises primarily from one weakly coupled N-14, the ENDOR spectru m is very crowded due to contributions from two nonequivalent nitrogens, tw o chlorides, and a relatively large number of protons. The simple ESEEM pro jection of the 2D ENDOR-ESEEM correlation spectrum is then used to disentan gle the ENDOR spectrum and resolve two sets: of lines corresponding to the different sites. (C) 2000 Academic Press.