Precision neutron diffraction structure determination of protein and nucleic acid components. X. A comparison between the crystal and molecular structures of L-tyrosine and L-tyrosine hydrochloride

The amino acid L‐tyrosine (C₉H₁₁NO₃) and its salt L‐tyrosine hydrochloride $({\mathrm{C}}_9{\mathrm{H}}_{11}{\mathrm{NO}}_3·\mathrm{HCl})$ crystallize respectively in the space groups P2₁2₁2₁, a=6.913(3) Å, b=21.118(10) Å, c=5.832(3) Å, and P2₁, a=11.083(5) Å, b=9.041(4) Å, c=5.099(3) Å, β=91.82(3) Å. Both structures have been refined by neutron diffraction techniques, and all the hydrogen atoms have been located precisely. The tyrosine molecule occurs in the zwitterion form in pure L‐tyrosine while both the amino group and the carboxyl group are protonated in the hydrochloride. In both crystalline compounds there is a three‐dimensional network of hydrogen bonds. Statistical tests show no significant differences between the bond lengths and valence angles in the compounds except for those atoms involved in hydrogen bonds. The conformational angles of the main chain (C, C^α, N, O¹, O²) and those of the side chain differ only slightly in the two compounds while the mutual orientations of the main chain and of the side chain differ completely; in L‐tyrosine N is gauche with respect to C^γ of the phenyl group while in the hydrochloride N is nearly trans to C^γ. The barriers to rotation of the ammonium groups are estimated to be 8.3 kcal/mole in L‐tyrosine and 5.0 kcal/mole in the more weakly hydrogen bonded hydrochloride. Detailed statistical tests for L‐tyrosine show very good agreement between the heavy atom structural parameters obtained from our work and those found in previous x‐ray diffraction studies [A. Mostad, H. M. Nissen, and C. Ro/mming, Tetrahedron Lett. 1971, 2131, and Acta Chem. Scand. (to be published); J. Donohue and R. Bogg (private communication)]. Such tests also show that the standard deviations are quite well estimated in both the neutron and one of the x‐ray studies (Mostad et al.).