DFT calculations of isomer effects upon NMR spin-Hamiltonian parameters of prostate polyamines

1H Nuclear Magnetic Resonance (NMR) spin-Hamiltonian parameters: chemical shifts δ and spin-spin coupling constants J have been calculated using density functional theory, for the three polyamines: putrescine, spermidine and spermine present in prostate tissue. The Boltzmann weighted average of the chemical shifts and spin-spin coupling constants over a large number of stable conformers have been evaluated for each molecule. The comparison of such average chemical shifts with experimental values shows a significant improvement from values corresponding to the lowest-energy conformers, with rms errors of 0.15 ppm for putrescine and 0.05 ppm for both spermidine and spermine. From the comparison between spectra simulated from calculated δ and J parameters and experimental ones, the B3LYP/6-311++G** level of theory was seen to be a good compromise between accuracy and computational costs.