ROTATIONAL SPECTRA OF BIOMOLECULES - HISTAMINE

The rotational spectrum of histamine has been observed in a free-expansion jet spectrometer in the range 59-64 GHz. Four species of histamine have been identified as the most stable conformers, all of them gauche, in contrast to results of X-ray and NMR solution studies. Parallel ab initio molecular orbital calculations (3-21G and 6-31G bases) indicate that 20 tautomer/conformer possibilities exist and helped to identify the 4 most stable conformers. Identification was based on rotational constants, inertial defects, dipole moment components, quadrupole hyperfine multiplets, and studies of the pentadeuterio derivative of histamine. All four of the identified conformers are gauche, one of the N(1)-H tautomer and three of the N(3)-H tautomer (in order of stability the conformers are G-IVa of N(1)-H, G-Ib, G-Vc, and G-Ic of N(3)-H (see Figure 2), the expanding beam being estimated to contain comparable amounts of the first two species and 40% and 30%, respectively, of the last two). All four species seem to be stabilized by some form of hydrogen bonding involving the amino group of the side chain.