Evidence for spontaneous, reversible paracyclophane formation. Aprotic solution structure of the boron neutron capture therapy drug, L-p-boronophenylalanine

Abstract

The simple, straightforward ¹H NMR spectrum of the neutron capture therapy drug, L-p-boronophenylalanine (L-p-BPA), in D₂O changes to a more complex one in DMSO-d₆ in which the ratio of new species observed is highly concentration dependent. The new species detected can only be explained by an additional stereocenter being formed at the boron center by intermolecular chelation of the amino acid of another molecule of L-p-BPA. This gives rise to the presence of an oligomeric species as well as another whose aromatic protons appear as a pair of sharp AB quartets centered further upfield at 6.66 and 6.84. Due to these shifts and couplings observed between the benzylic protons and the proton at the stereocenter of the amino acid, it is proposed that this species is a paracyclophane dimer of L-p-BPA in which one molecule of L-p-BPA chelates head-to-tail with another. This cyclophane dimer predominates in low concentrations (<50 mM) while the oligomer predominates at higher (>90 mM) concentrations. The formation of these two species is completely reversible, the addition of D₂O completely regenerating L-p-BPA. Variable temperature ¹H NMR found that the two pairs of aromatic protons of the cyclophane dimer coalesce at T_c = 141 °C, corresponding to a ΔG^‡ = 20.6 kcal mol⁻¹.