EXSYCALC is a free program intended for the study by NMR of molecular systems undergoing chemical exchange. It does a quantitative analysis of the experimental intensities of the NMR peaks obtained in EXSY experiments to calculate the magnetization exchange rates k’ of the exchange equilibrium (related with the reaction rate constants k ). The program allows the calculation of systems with an arbitrary number of exchange sites, spins, populations and arbitrary longitudinal relaxation rates. The calculations are done according to a full relaxation matrix analysis of the intensities. The range of applicability of the approach used requires that the signal of each different species in the exchange process is conveniently separated from the others in the NMR spectrum (i.e. slow chemical exchange in the chemical shift time scale). For the calculation of rate constants, the program requires that the user supplies the experimental amplitudes of certain NMR peaks obtained in two different EXSY experiments, one is an EXSY experiment acquired at a certain mixing time ™, and the other is an EXSY experiment acquired at 0 or very short mixing time (reference experiment). In the former experiment the mixing time ™ need to be large enough for the magnetization exchange process to take place. In this experiment the amplitudes (intensities) of those signals in exchange, A(tm), have to be quantified for both diagonal and cross peaks. In the other EXSY experiment, the EXSY reference experiment, no cross peaks due to magnetization exchange should be observed (thermal equilibrium) and the amplitudes of just the diagonal peaks of those signals in exchange, A(0), have to be measured. It is important to mention that both experiments must be acquired and processed under identical conditions, temperature, number of scans etc.
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