Crystal structure end electron spin echo modulation (ESEEM) study of [Cu(II)(TEPA)NO2]PF6Crystal structure end electron spin echo modulation (ESEEM) study of [Cu(II)(TEPA)NO2]PF6 (TEPA)=Tris[2-(2-pyridyl)ethyl]amine): A model for the purported structure of the nitrite derivative of hemocyanin (TEPA)=Tris[2-(2-pyridyl)ethyl]amine): A model for the purported structure of the nitrite derivative of hemocyanin

The coordination properties of nitrite ion (NO2-) with copper are relevant to investigations of a variety of copper proteins, including those involved in nitrogen oxide processing. Here, we use X-ray diffraction and electron spin echo envelope modulation (ESEEM) spectroscopy to investigate the coordination and electronic structure of a nitrite-copper(II) model compound, [Cu(II)(TEPA)NO2]PF6 (TEPA = tris[2-(2-pyridyl)ethyl]amine). In the crystal (C21H24N5CuO2PF6 crystallizes in the monoclinic space group P2(1)/c with Z = 4, a = 11.817(8) angstrom, b = 13.124(6) angstrom, c = 15.692(7) angstrom, and beta = 92.39(4)-degrees), nitrite is shown to bind as an equatorial ligand to Cu(II) through oxygen, rather than nitrogen. In frozen solution, the detection of ESEEM from the nitrite N-14 indicates O-ligation as well. The quadrupole and electron-nuclear coupling tensors for the nitrite N-14 are obtained by field-dependent ESEEM spectral simulation. The quadrupole parameters, e2qQ = 5.66 MHz and eta = 0.31, are similar to those for diamagnetic metal nitrites. The net charge on the nitrite nitrogen calculated from the quadrupole parameters, based on Townes-Dailey theory, suggests that the ONO angle of bound nitrite is between 111-degrees and 116-degrees in frozen solution, as compared to 115-degrees in the crystal. The principal directions of the quadrupole tensor and the electron-nuclear coupling tensor relative to that of the g tensor suggest that the ONO orientation relative to the Cu(II)(TEPA) structure in frozen solution is essentially the same as that in the crystal. The ability to observe modulations arising from Cu(II)-coordinated (NO2-)-N-14 allows us to probe for an equatorial nitrito ligand to Cu(II) in solution samples. An attempt was made to substantiate the claim that nitrite binds equatorially through oxygen to Cu(II) in hemocyanin after nitrite treatment (Solomon, E. I. In Copper Proteins; Spiro, T. G., Ed.; Wiley-Interscience: New York, 1981; pp 43-108) by searching for (NO2-)-N-14 modulations in the ESEEM spectrum. None were found, suggesting that nitrite does not coordinate to the Cu(II) of hemocyanin as claimed.