EPOC supplementary material
Geometrical Parameters in Calculated Structures
The rotational angles 1 (corresponding to the dihedral NringCringCchainCchain)
and 2 (corresponding to the dihedral CringCchainCchainNchain) obtained after
geometry optimization at the HF/6-31G* are given in Table E1 (near here). Comparison
shows, that generally, the absolute values of the 2 angle are close to 180 and 60 in the trans
and gauche conformations, respectively. The absolute value of the 1 angle varies from about
40 to 150.
The ring CN1, CN2 and C=C bond lengths are almost the same in the trans and
gauche conformations for each tautomeric form of HA. The differences are not larger than
0.01 Å (1 Å=0.1 nm). Protonation of the chain N3-amino has a slightly stronger effect on the
CN bond lengths for the gauche than for the trans conformation. For the T1 tautomer in the
gauche conformation, the CN1 bond shortens (by 0.009 Å) and the C=N2 bond lengthens
(by 0.006 Å). A reverse effect takes place for the T2 tautomer in the gauche conformation.
The CN1 bond lengthens (by 0.004 Å) and the C=N2 bond shortens (by 0.002 Å). This
behaviour results from two opposite effects, which operate in the N3-amino protonated
tautomers: an intramolecular interaction between the basic neutral N2-imino and the acidic
protonated N3-amino called 'internal solvation' in the gauche-AmH+-T1, and a repulsion
effect between acidic hydrogens at the N1 and N3-amino in the gauche-AmH+-T2 (Fig. 1).
For the trans conformation, protonation of the chain N3-amino causes similar changes of the
CN bond lengths in both tautomers (T1 and T2). The CN1 bond slightly shortens (by 0.001
Å) and the C=N2 bond slightly lengthens (by 0.002 Å). Protonation of the ring N2-imino
leads to an equalization of the CN1 and CN2 bond lengths in the ImH+, that is typical for
the amidine group.33
In the solid state, the HA-T2 for the neutral (1 = 66.3 and 2 = -170.8) and the
AmH+-T1 for the monocation (1 = 89.7 and 2 = 177.3), both in the trans conformation,
were identified.22 The calculated 1 angle (69.3 for the trans-HA-T2 and -49.9 for the
trans-AmH+-T1) differs from that found in the crystal histamine species, due to
intermolecular interactions between histamine molecules in the solid state. The structure of
the crystal HA-T2 is stabilized by an H-bond between the ring N-H and the chain N-amino of
a neighbour molecule. In the crystal, the protonated AmH+-T1 is stabilized by a stronger Hbond between the chain NH3+ and the ring N-aza of another ion.
Table E1. Selected geometrical parameters for the isolated neutral and ionic forms of
histamine calculated at the HF/6-31G* level
2N
1N
H
CH2 CH2X

X=NH2 (HA), NH3+ (AmH+)

angle (in degree) a
bond length (in Å)
tautomer
conformation
1
2
CN1
C=N2
C=C
HA-T1
trans
-66.1
177.5
1.346
1.289
1.352
gauche
63.8
-68.0
1.345
1.290
1.352
trans b
-49.9
169.6
1.345
1.291
1.351
gauche
40.8
-63.1
1.336
1.296
1.349
trans c
69.3
-179.9
1.352
1.287
1.352
gauche
46.9
-67.6
1.348
1.290
1.354
trans
-86.6
-177.9
1.351
1.289
1.353
gauche
-151.2
-56.8
1.352
1.288
1.359
AmH+-T1
HA-T2
AmH+-T2
H
2N
+
1N
CH2NH2
 
H
ImH+
trans
-68.5
-180.0
1.309
1.315
1.344
gauche
40.6
-62.2
1.309
1.314
1.345
a  corresponds to the dihedral N
1
ringCringCchainCchain and 2 corresponds to the
dihedral CringCchainCchainNchain.
b For crystal structure: 1 = 89.7 , 2 = 177.3 , CN1 1.349 Å, CN2 1.328 Å and C=C
1.352 Å.22b
c For crystal structure: 1 = 66.3 , 2 = -170.8 , CN1 1.349 Å, CN2 1.303 Å and C=C
1.356 Å.22a
Implication of the ring nitrogen atoms in intermolecular interactions changes also the
CN1 and CN2 bond lengths. The difference between the C-N1 and C-N2 bond lengths is
smaller for the crystal (0.046 Å in HA-T2 and 0.021 Å in AmH+-T1) than for the isolated
HF/6-31G* structures (0.065 and 0.054 Å, respectively). Only the C=C bond lengths
calculated at the HF/6-31G* level (1.352 in HA-T2 and 1.351 Å in AmH+-T1) are close to
that found in the solid histamine (1.356 and 1.352 Å, respectively).
Effect of Solvent on Tautomeric Preferences
The PCM model applied to the neutral histamine species indicates that whatever is the
solvent, the gauche-T2 tautomer has the lowest energy, as in the gas phase. Total energies
calculated for the trans and gauche conformations of both neutral tautomers increase in the
following order in aqueous solution: gauche-HA-T2, trans-HA-T1, trans-HA-T2, gaucheHA-T1 (0.0, 1.3, 1.3, 1.6 kcal mol-1, respectively, 1 cal = 4.184 J). In the gas phase, this order
is slightly different: gauche-HA-T2, gauche-HA-T1, trans-HA-T1, trans-HA-T2 (0.0, 2.3,
3.1, 3.8 kcal mol-1). The small differences in total energies suggest that both tautomers in the
trans and gauche conformations may be present in tautomeric mixture of HA. This fact
together with high basicity of histamine, which easily takes a proton from water, especially if
CO2 is not excluded, complicates spectroscopic measurements for free histamine, and
partially explain, why interpretation of various spectra (IR and NMR) recorded in solution
have not led to converging conclusions.
For the monocationic mixture, tautomeric preference found in the gas phase is not the
same as in aqueous solution. In the later case, the gauche-AmH+-T1 has the lowest energy,
whereas the gauche-ImH+ is found in the gas phase. The PCM model indicates that total
energies calculated for the monocationic forms increase in the following order in aqueous
solution: gauche-AmH+-T1, gauche-ImH+, trans-AmH+-T1, trans-AmH+-T2, trans-ImH+,
and gauche-AmH+-T2 (0.0, 1.6, 3.0, 4.3, 5.1, 5.2 kcal mol-1, respectively). This order is
completely different from that found in the gas phase: gauche-ImH+, gauche-AmH+-T1,
trans-ImH+, trans-AmH+-T1, gauche-AmH+-T2, and trans-AmH+-T2 (0.0, 3.2, 12.1, 15.7,
22.9, 27.2 kcal mol-1, respectively), in which only two forms, gauche-ImH+ and gaucheAmH+-T1 should be taken into account. The other protonated forms are too high in energy to
be significant under room temperature conditions. In aqueous solution, the relative total
energies of monocationic forms are within 5 kcal mol-1, and thus three tautomers in both the
trans and gauche conformation may be present in significant amount. The free energy
perturbation (FEP) method32d and the Monte Carlo (MC) simulations9d were also applied to
the histamine monocation mixture. However, only two tautomers of the AmH+ form in
different conformations were considered. The ImH+ form was not taken into account. Both
methods predicted the lowest energy for the gauche-AmH+-T1 form in aqueous solution.
This discrepancy, between the conclusion on the order of the total energies of
tautomers-conformers in aqueous solution and that on the order of the protonation energies
(Table 8) may result from the fact that theoretical models are mainly based on the physical
solute/solvent interactions, and predict reasonable results for these processes or phenomena in
which the chemical solute/solvent interactions (e.g. hydrogen bonds) are similar in the series
considered. This is the case for the order of the protonation energies calculated for the species
of the same conformation. For the trans conformation, only intermolecular hydrogen bonds
are possible. For the gauche conformation, two competitive types of interactions, intra- and
intermolecular hydrogen bonds operate, and thus their influence on the total energy is different
from that in the trans conformation. Therefore, general conclusion on the order of the total
energies of different conformations even for the same tautomer in aqueous solution should be
derived very carefully.
Generally, in the highly polar aqueous solvent, intramolecular hydrogen bonds are
unlikely, due to interactions of polar groups of histamine with water molecules. In the case of
acyclic flexible diamines, it has been shown that a few water molecules are sufficient to open
the intramolecular hydrogen bond and to change the conformation of diamines from the
gauche (for the isolated molecule) to the trans conformation (for the solvated molecule).6b
The experimental (macroscopic) pKa value for the monoprotonation of histamine at the chain
N3 atom is close to those measured for monoamines.9a,c,10 Therefore, the basicity
enhancement observed for proton sponges (pKa > 6)54 is not operating in the case of
flexible histamine.9a At a physiological pH (7.4) the monocation trans-AmH+-T1 can be the
main form of histamine. This trans conformation of the monocationic form has been found as
the only conformation present in aqueous solution by ab initio analysis of IR and Raman
spectra.9e
Quite a different conclusion has been derived on the basis of 1H NMR experiments,11
Both conformations (trans and gauche) have been considered for the neutral, monoprotonated
and diprotonated forms of histamine. Noszál and co-workers11d have even estimated the
basicity of both nitrogen atoms (N2 and N3) for the trans and gauche conformers (e.g. for N3
pKa = 10.12 and 10.18, respectively). This estimation, however, needs some re-examination
because (i) the Authors did not take into account the tautomerization process in the neutral
(HA) and N-amino protonated forms (AmH+), and surprisingly (ii) the pKa estimated for the
gauche conformer (10.18) is almost the same as that of the trans one (10.12). The PCM model
indicates that generally, basicities of the gauche conformers are considerably higher than
those of the trans ones. The calculated absolute energies of protonation are larger by 2-4 kcal
mol-1. One exception is the protonation at the chain N3 atom occurring from the gauche-HAT2 to the gauche-AmH+-T2, for which the absolute Eprot is lower by 2 kcal mol-1 due to a
repulsion of two acidic groups, NH in the ring and NH3+ in the chain.
Solid State Conformations
Free base histamine: The side chain in HA-T2 is in the trans conformation (1 = 64.2 and
2 = 170.8).22a
Monoprotonated form: The side chain is stabilized in the trans conformation in the case of
the histamine hydrobromide (1 = 87.2 and 2 = 177.2)22b and diaqua-dichlorobis(histamine)-calcium diaqua-tetrachloro-calcium (1 = 178.7 and 2 = 175.3),56a
whereas the gauche conformation is obtained for bis[5-(2-ammonioethyl)-imidazole-N3]tetrakis(iso-thiocyanato)-nickel(II) (1 = 84.5 and 2 = 69.4).56b
Metal complexes: The absolute values of the 1 angle in metal/histamine complexes usually
vary from 28.8 [for Co(III) complex]59 to 65.5 [for Cu(II) complex]58i, while the absolute
values of the 2 angle vary from 57.8 [for Cu(II) complex]58h to 74.8 [for the other Cu(II)
complex]58f with mean values of 45.2 and 69.9, respectively. The substantial deviations can
be, however, observed in the case of tri(histamine)-nickel(II) perchlorate,61c where the angles
are significantly smaller (1 = 24.7 and 2 = 46.7), and especially in the case of (2oxalato-O,O',O",O'")-bis[aqua-(nitrato-O)-(histamine-N,N')]-copper(II),58d where the six
membered ring is almost planar (1 = 3.6 and 2 = 0.6). The large planarization in the
latter case can be attributed to strong intermolecular interactions in the crystal lattice. The
histamine molecules are held together by hydrogen bonding interactions to form ladder-like
chains, which in turn interact each other by - stacking interactions involving the imidazole
fragments (with dihedral angle equal to 0). The shortest interatomic distance and the
interplanar separation between the imidazole groups is 3.488 Å and 3.11 Å, respectively.58d
A very similar type of arrangement may be also found in the case of catena-{(2-oxalatoO,O',O",O'")-bis[(2-perchlorato-O,O')-(histamine-N,N')]-copper(II)]},58d where a ladder-
type structure is also formed and - stacking interactions involving the imidazole rings are
observed. However, the six membered ring is not planar (1 = 39.3 and 2 = 69.1), in
spite that the dihedral angle between the imidazole groups is 0, and the shortest interatomic
distance and the interplanar separation between the imidazole rings are 3.325 Å and 2.97-3.00
Å, respectively. Formation of an intramolecular - stacking interactions between the
imidazole fragment and the phenyl ring has also been observed in a few other complexes of
Cu(II),58g,h emphasising that indeed the interactions of this kind may also be important in
solutions, and play an important role in biological systems. The crystal packing forces
influence the dihedral angles 1 and 2 substantially as it can be observed by comparison of
the angles within one complex molecule. Typical examples are (carbonato-O,O')-bis(histamine-N,N')-cobalt(III) chloride monohydrate59 and aqua-(histamine)-(3,5,-diiodotyrosinato)copper(II) monohydrate,58h for which 1 = 39.3 or 39.4 and 2 = 60.4 or 69.5 in the
former case, and 1 = 36.3 or 49.6 and 2 = 57.8 or 67.5 in the latter. In copper(I)histamine complex [(2-histamine)-bis(carbonyl-histamine-copper) bis(tetraphenylborate)],57
the 1 and 2 are equal to 155.0 and 173.0, respectively for trans-HA-T2. In the case of
two gauche-HA-T1 fragments, the 1 and 2 are equal to 34.6 (33.6) and 66.2 (70.2),
respectively.
Computational details
Ab initio calculations for two selected conformations: 'essential' and 'scorpio' (trans and
gauche) of the neutral and monoprotonated histamine were carried out at the HF/6-31G*
level40 using the GAMESS program.64 The geometries of all species were fully optimized
without symmetry constraint and the stationary point on the potential energy surface found.
The calculated harmonic vibrational frequencies indicated that the optimized trans and gauche
structures corresponded to energy minima. Selected geometrical parameters are listed in Table
E1 (near here).
The HOMA values were based on the molecular geometries optimized at the HF/631G* (Table E1), while the NICS(1) was calculated 1 Å above the imidazole ring for these
geometries with use of the GIAO/HF/6-31+G* method (GIAO - Gauge-Independent Atomic
Orbital method66). The values of both parameters of aromaticity are given in Table 1 (full
text).
Single point energy calculations were performed using the MP2 and DFT methods and
6-31G* or 6-311++G** basis sets on geometries optimized at the HF/6-31G* level, using the
GAUSSIAN'94 program.65 The relative energies between the two neutral tautomers {E =
E(HA-T1)  E(HA-T2)} and between the different monoprotonated forms of histamine
{E(1-2) = E(AmH+-T1)  E(AmH+-T2) and E(1-3) = E(AmH+-T1)  E(ImH+)} are
summarized in Table 2 (full text). For comparison, the literature data29a,b are also included in
this table. The symbols E, E(1-2) and E(1-3) are the same as in previous papers.29
The thermal corrections of enthalpy and entropy were also calculated and included in
the Gibbs free energies at the HF/6-31G* level. These calculations assume an ideal gas
equation-of-state, a temperature of 298.15 K, and a pressure of 1 atm. The relative
thermodynamic parameters for the tautomerization process (generally abbreviated here as P
in kcal mol-1, 1 cal = 4.184 J) calculated according to eq. (1): E - relative total energies,
ZPVE - relative zero point vibrational energies, H - relative enthalpies, G - relative
Gibbs free energies, and pKT derived from the G according to eqs. (2)-(4) are listed in
Table 3 (full text). The 2.303 RT factor is equal to 1.3643 under the chosen standard state
(T = 298.15 K).
P = P(Ti)  P(Tj)
(1)
pKT(HA) = [G(HA-T1)  G(HA-T2)]/2.303 RT
(2)
pKT(AmH+) = [G(AmH+-T1)  G(AmH+-T2)]/2.303 RT
(3)
pK'T = [G(AmH+-T1)  G(ImH+)]/2.303 RT
(4)
The effects of solute/solvent interactions on the tautomeric preferences in the neutral
and monocationic forms of histamine for two selected stable conformations (trans and
gauche) were studied using the PCM method24 and geometries optimized at the HF/631G*level. The relative energies calculated for 12 solvents are listed in Table 4 and 5 (full
text). Variations of the dipole moment for each tautomer-rotamer calculated for the gas phase
and for 12 solvents are given in Table E2 (near here).
The thermodynamic gas-phase basicity parameters were calculated for each partial
monoprotonation reaction given in Scheme 1 according to general reaction (5), where B and
BH+ are the neutral (HA-T1 or HA-T2) and monoprotonated forms of histamine (AmH+-T1,
AmH+-T2 or ImH+). Protonation energies (Eprot) were calculated according to equation (6).
The energy of the isolated proton was taken equal to zero at the ab initio level. The Eprot
calculated in this way refers to 0 K. The proton affinities {PA, the negative of the enthalpy of
the protonation reaction, PA =  [H(BH+)  H(B)  H(H+)]}7 were calculated according
to equation (7) from the protonation energies corrected to 298.15 K (which take into account
the changes in the zero-point vibrational corrections and in the thermal contributions to the
total energy of each species from rotations, vibrations and translations) including the work
term [(pV) = RT = 0.592 kcal mol-1 at 298.15 K for the ideal gas]. For the proton, only
the translational energy term is not equal to zero (3/2 RT = 0.887 kcal mol-1 at 298.15 K for
the ideal gas). The gas-phase basicities {GB, the negative of the Gibbs free energy, GB =
 [G(BH+)  G(B)  G(H+)]},7 calculated according to equation (8), differ from PA by the
corresponding entropy term {TS =  [TS(BH+)  TS(B)  TS(H+)], where S is the sum
of the rotational, vibrational and translational entropies}.7 For the proton, only the
translational entropy is non zero [Stransl(H+) = 26.040 cal mol-1 K-1].7 All calculated
microscopic thermodynamic basicity parameters in the gas phase are summarized in Table 6
(ful text).
B + H+
BH+
(5)
Eprot = E(BH+)  E(B)  E(H+)
(6)
PA =  [Eprot(298.15) + (pV)]
(7)
GB = PA  TS
(8)
Protonation energies in aqueous solution [Eprot(aq)] corresponding to reaction (9),
were calculated according to equation (10) using the energies of neutral and protonated forms
of histamine obtained in the PCM model. The energy of the hydrated proton in this model was
calculated for the HF/6-31G* geometries as the difference between the energies of hydrated
H3O+ [E(H3O+-aq) = 76.423241 hartree, 1 hartree = 627.5095 kcal mol-1] and hydrated
water molecule [E(H2O-aq) = 76.020911 hartree].4 Calculated Eprot(aq) are summarized in
Table 8 (full text).
Baq + H+aq
BH+aq
(9)
Eprot(aq) = E(BH+aq)  E(Baq)  E(H+aq)
(10)