Synthesis of conjugated polyene carbonyl derivatives of nitroxyl spin-labels and determination of their molecular structure and conformation by electron nuclear double resonance

Article abstract of DOI:10.1021/ja00062a037

Formic, propenoic, and pentadienoic acid derivatives of the nitroxyl spin-label 2,2,5,5-tetramethyl-1-oxypyrroline and their corresponding ethyl ester and aldehyde analogues have been synthesized and characterized by chemical methods. Because the π-bonding Orbitals of the olefinic side chains are conjugated to the vinyl group of the oxypyrrolinyl ring, these spin-labeled derivatives are excellent spectrophotometric probes, exhibiting high molecular absorption extinction in the near ultraviolet. The molecular structures and conformations of these spin-labeled derivatives have been determined by electron nuclear double resonance (ENDOR) spectroscopy and molecular modeling. ENDOR was performed on frozen, glassy solutions of the spin-labels, and ENDOR absorptions of protons on the side chains were assigned by selective deuteration. From the maximum and minimum ENDOR shifts that correspond to the principal hyperfine coupling components, the dipolar hyperfine contributions were calculated to estimate electron-proton separations. The electron-proton distances evaluated over a 4.5-10.5-A? range were associated with less than 3% uncertainty on the basis of ENDOR line widths. Conformational analysis based on torsion angle search calculations, constrained by ENDOR-determined electron-proton separations, showed that the polyene chain has an all planar, trans conformation. In the aldehydes the conformation of the C=O group with respect to the alkene chain is found to be s-trans, while in the acids and esters the -OH and -OC₂H₅ groups were found to be s-trans, placing the C=O group in an s-cis conformation. The different conformations of the carbonyl group in these compounds are explained on the basis of resonance energy and dipolar interactions of the polyene side chain and the carbonyl or carboxyl group.