7864
J . Am. Chem. SOC.1991, 113, 7864-7869
Rotational Spectra of Biomolecules: Histamine Beat Vogelsanger, Peter D. Godfrey, and Ronald D. Brown* Contribution from the Centre for High Resolution Spectroscopy and Opto-Electronic Technology, Chemistry Department, Monash University, Clayton, Victoria, Australia. Received December 26, 1990
Abstracb 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(,,-H tautomer and three of the N(,).-H tautomer (in order of stability the conformers are G-IVa of N(,)-H,G-lb, G-Vc, and G-ICof NorH (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.
Introduction The biological significance of histamine, (2-aminoethy1)imidazole,' is attested by a voluminous literature on this molecule. Its primordial activity at the central nervous system level and in the regulation of insomnia gave it a significance similar to that of adrenaline and acetylcholine. It is also a powerful vasodilator and a stimulator of gastric secretion in higher animals. Its biological activity has stimulated a number of studies of its tautomerism, its conformation, and its overall geometry, ranging from N M R studies in solution,2 an X-ray crystallographic study,, and molecular orbital calculations of varying sophistication.* In the crystal, histamine is the trans conformer of histamine N(,)-H3while in solution it appears to be roughly equal proportions of trans and gauche conformers of the N,,,-H tautomere2The molecular orbital calculations are not mutually entirely agreed as to the lowest energy tautomer/conformer, but the most elaborate ab initio calculations (Hartree-Fock SCF using the STO-3G basis) considered only the trans conformer, similar to that found in the X-ray studies of histamine. Because the effect of the environment, crystal lattice or surrounding solvent, is unknown, we have set out to establish the most stable tautomer/conformer of the isolated molecule by detection and analysis of the rotational spectrum of histamine, the general style of the work being the same as that employed in our previous studies of biologically important molecules.s In parallel, we have also undertaken an extensive exploration of the energies of various tautomer/conformers using ab initio molecular orbital calculations and a more elaborate basis set (3-21G and 6-31G) than previously used. Molecular Orbital Study Calculations were performed on the histamine molecule by using the GAUSSIAN 88 package6 and the 3-21G basis set. The primary ( I ) Although the standard nomenclature for histamine is either 1Himidazole-4-ethanamine or 1 H-imidazole-5-ethanaminedepending upon the tautomer considered, all previous papers dealing with theoretical calculations on histamine and also the biochemical literature adopt the numbering system used here, where the skeleton numbering does not change with tautomerism. (2) Canellin, C. R.; Pepper, E. S.; Port, G. N. J.; Richards, W. G. J. Med. Chem. 1973, 16, 610-616. Ham, N. S.; Casy, A. F.; Ison, R. R. J. Med. Chem. 1973, 16, 470-475. Byrn, S. R.; Graber, C. W.; Midland, S. L. J. Org. Chem. 1976,41, 2283-2288. Reynolds, W. F.;Tzeng, C. W. Con. J. Biochem. 1977,55,576-578. Wasylishen, R. E.; Tomlinson, G. Can. J. Biochem. 1977, 55, 579-582. (3) Bonnet, J. J.; Ibers, J. A. J . Am. Chem. Soc. 1973, 95, 4829-4833. (4) Kang, S.; Chou, D. Chem. Phys. Lett. 1975,34, 537-541. Topiol, S.; Weinstein, H.; Osman, R. J. Med. Chem. 1984, 27, 1531-1534. Gresh, N.; Claverie, P.; Pullman, A. Theor. Chim. Acra 1984, 66, 1-20. Smeyers, Y. G.; Romero-Sanchez, F.J.; Hernandez-Laguna, A. J. Mol. Srrucr.: T H E 0 CHEM 1985. 123. 431-442; 1990, 207. 157-167. Topiol, S.J. Compur. Chem. 1987, 8, 142-148. ( 5 ) Brown, R. D.; Godfrey, P. D.; McNaughton, D.; Pierlot, A. J. Am. Chem. SOC.1988, 110.2329-2330; Chem. Phys. Lett. 1989, 156, 61-63; J. Am. Chem. SOC.1989, 1 1 1 , 2308-2310. Vogelsanger, B.; Brown, R. D.; Godfrey, P. D.; Pierlot, A. P. J. Mol.Specirosc. 1991, 145, 1-1 1.
structural features to be explored in order to determine the shape of histamine (Le., its most stable tautomeric/conformational form) are the tautomerism of the imidazole ring and the t h r p torsional , the conformation with respect parameters, 7'.72, and T ~ defining to the ethane skeleton, as illustrated in Figure 1. We anticipated that energy minima with respect to 7 2 would be found in the vicinity of the trans (T) (fully extended chair, 7 2 = 180') and gauche (G) (folded chair, 7 2 = 60' or 300'). This proved to be the case; all of our geometry optimizations, starting with arbitrary values of 72, ended with 7 2 values of 60°, 180°, or 300'. Variations in 7, indicated that stable conformers corresponded to values of this parameter representing staggered positions of the amino group. Variations of T~ indicated that the rotation of the imidazolyl group is much less restricted. In geometry optimizations, the imidazole ring was kept planar but all parameters were optimized. We noticed, for example, that the ring angle C(5)-C(4)-N(3)changed from about 110.2' for the N,,,-H tautomer to around 106.3' for the N,,,-H tautomer, scarcely affected by the side chain conformation (see Table I), but our attention was focused primarily on the overall shape, Le., side chain conformation. In all we have found 20 energy minima corresponding to the shapes illustrated in Figure 2. These include the sole minimum previously reported (T-c) although this conformer is predicted to be appreciably higher in energy than a number of others that we have now discovered. Table I lists the calculated total energies, dipole moments, optimized geometrical parameters and derived spectroscopic constants for each conformer. For the trans conformers, we started with the X-ray crystallographic structural parameters,2 the geometry optimizations being performed with the imidazole portion being constrained to planarity. The species of local minimum energy so derived (T-c) for both the N,,,-H and N,,,-H tautomers had values of T~ and T~ and of the other structural parameters that closely coincide with those obtained in STO-3G based calculations of Topiol et al.) However, with the larger basis set (3-21G) and also with 6-31G basis, the N,,,-H tautomer is predicted to be more stable than the N,,,-H tautomer, contrary to the results when the STO-3G basis is used. In addition, we were able to detect two further trans species (T-a and T-b) for both tautomers, with different conformations of the amino group. These species differed from T-c by less than 7O in the values of T~ and T* and by less than 2 kJ mol-' in energy. For the N,!,-H tautomer, T-b is 0.1 kJ mol-) more stable than T-c. We did not search for further trans species with T~ significantly different from 60-70' because single-point calculations for the trans conformer (Figure 3) indicate that this range of T~ ( 6 ) GAUSSIAN 88: Frisch, M. J.; Head-Gordon, M.; Schlegel, H.B.; Raghavachari, K.; Binkley, J. S.; Gonzalez, C.; Defrees, D. J.; Fox, D. J.; Whiteside, R. A.; Seeger, R.; Melius, C. F.; Baker, J.; Martin, R. L.; Kahn, L. R.; Stewart, J. J. P.; Fluder. E.M.; Topiol, S.;Pople, J. A. Gaussian, Inc., Pittsburgh, PA.
0002-7863/91/1513-7864%02.50/0 0 1991 American Chemical Society
Rotational Spectra of Biomolecules
J . Am. Chem. SOC.,Vol. 113, No. 21, 1991 1865 50
Tautomer N(11-H 45
(a)
I
8 40 E
I
Tautomer N(1yH Figure 1. Parameters T
2 5u 35
Tautomer N(3l-H
u
B
defining conformers of the histamine tautomers. Torsion angle 71 is viewed along the c&5 bond from cp to C5, 7* is viewed along c,
