Rapid single- and multiple-scattering EXAFS Debye-Waller factor calculations on active sites of metalloproteins

This paper describes recent results using our approach to calculating self- consistently single (SS) and multiple-scattering (MS) Debye-Waller factors (DWF) on active sites of metalloproteins. The calculation of MS DWF, togeth er with the Feff7 program allows us to simulate ab-initio EXAFS spectra for a given temperature systems with no adjustable parameters. In our latest r eport (Dimakis N., and Bunker G., 1998) we calculate, using density functio nal and semiempirical approaches, the SS and MS DWF for small molecules and compared them to Raman, infrared and EXAFS spectra. In this report calcula tion of DWFs is done for tetrahedral Zn imidazole, a complex containing thi rty two atoms that is similar in certain respects to active sites of many m etalloproteins. Ab-initio calculation, although it is a more accurate and r eliable scheme, it is not at present practical on desktop computers; comput ation times are weeks. Therefore as an alternative we have tried the semiem pirical MNDO Hamiltonian, which is at least three orders of magnitude faste r than ab-initio, and can be expected to be of reasonable accuracy because it is parameterized for organic compounds. Our approaches take advantage of commercially available molecular orbital programs. We have written additio nal programs which, using normal mode calculations, calculate the MS paths, and transparently interface with Feff7 to produce the EXAFS spectra. Resul ts are in very good agreement with experimental data tested.