Simulation of NOESY spectra of DNA segments: a new scaling procedure for iterative comparison of calculated and experimental NOE intensities.

A new algorithm for simulation of two-dimensional NOESY spectra of DNA segments has been developed. For any given structure, NOE intensities are calculated using the relaxation matrix approach and a new realistic procedure is suggested for 1:1 comparison of calculated and experimental intensities. The procedure involves a novel method for scaling of calculated NOE intensities to represent volumes of digitised cross peaks in NOESY spectra. A data base of fine structures of all the relevant cross peaks with Lorentzian line shapes and in-phase components, is generated in a digitised manner by two-dimensional Fourier transformation of simulated time domain data, assuming a total intensity of 1.0 for each of the cross peaks. With this procedure, it is shown that the integrated volumes of these digitised cross peaks above any given threshold scale exactly as the total intensity of the respective peaks. This procedure eliminates the repetitive generation of digitised cross peaks by two-dimensional Fourier transformation during the iterative process of structure alteration and NOE intensity calculation and thus enhances the speed of DNA structure optimization. Illustrative fits of experimental and calculated spectra obtained using the new procedure are shown.