EVIDENCE FOR HEME-HEME EXCITONIC COUPLING IN THE SORET CIRCULAR-DICHROISM OF HEMOGLOBIN

In order to study interdimer heme-heme electronic interactions in huma n hemoglobin, the Soret circular dichroism spectrum of the carboxy add uct is measured as a function of protein concentration, the spectrum a t the highest concentration representing primarily that of alpha(2) be ta(2) tetramers (93%) and the lowest concentration representing primar ily alpha beta dimers (68%), The tetramer-dimer difference spectrum, o btained using singular value decomposition and linear least squares fi tting from a matrix of CD spectra measured at ten concentrations, is r oughly conservative, with a larger negative lobe at shorter wavelength s and a peak-to-trough magnitude that is 18% of the tetramer's maximum Soret CD magnitude, It is tentatively assigned to heme-heme excitonic interactions on the basis of theoretical predictions by R. W. Woody [ (1985) in Optical Properties and Structure of Tetrapyrroles (Blauer, G ., and Sund, H., Eds.), pp. 239-256, Waiter de Gruyter,New York]. (C) 1997 Academic Press.