l-histidinium phosphite, [C6N3O2H10]+·[HPO3H]− is monoclinic P21 with crystal data: a = 8.497(7), b = 11.999(10), c = 4.967(3) Å, β = 104.15(6)°, V = 491(1) Å3 and Z = 2. The crystal structure was solved with a final R value of 0.030 for 1780 independent reflections. The organic and phosphoric entities build layers parallel to the (100) plane. The [HPO3H]− groups, not held together by H-bonds, are linked by O–H⋯O and N–H⋯O bonds to the histidine groups. The latter are laced together by N–H⋯O bonds to form chains running parallel to b.
In addition to this study, the atomic arrangement of the monoclinic form of l-histidine was reinvestigated, this time with an untwinned crystal. The space group is P21 with a = 5.166(6), b = 7.385(5), c = 9.465(4) Å, β = 98.16(5)°, V = 357.4(9) Å3 and Z = 2. The new refinement, based on 1182 independent reflections, led to a final R value of 0.039. A description of the structure, including the H-bond scheme, is given. In addition to an improvement of the experimental accuracy, some H atoms, not found in the first study were located. The main structure is in general agreement with that from the previous investigation.
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