Extension of slow-tumbling calculation to samples containing two spin probes to determine distances from 2D-ELDOR spectrum

This extension is being attempted as there is a considerable interest at the ACERT to measure distances between two NO spin probes in membranes from 2D-ELDOR spectra taking into account the electronic dipolar interaction between two spin labels. The advantage is that here one can measure these distances in membranes at ambient temperatures. At present, there is available no computational technique to rigorously take into account the interaction between two spin probes, that is between two electrons and two nuclei. In order to accomplish this, the technique developed by Schneider and Freed applicable to one spin label, that is one electron coupled to one nucleus, was studied in detail for possible extension using Lanczo’s algorithm. First, the extension was made for a system of one electron spin coupled to two nuclear spins by extending the “starting vector”, and taking into account the interaction of the additional nucleus with the electron. This extension was tested on a sample system, and was found to work satisfactorily. We are now in the process of including an additional electron, and its dipolar interaction with the other electron. The dipolar interaction will be ultimately averaged out to take into account the random orientations of the various dipoles in the system constituted by electron-electron pairs.