Effect of High Pressure on Crystalline Glycine: A New High-Pressure Polymorph

Abstract

Study of the effect of pressure on organic molecularcrystals is of considerable interest for predicting theresponse of drugs, materials, and devices based onmolecular crystals to high pressures and to othermechanical actions, as well as for upgrading theoreticalmodels used for predicting crystal structures andmolecular conformations as a function of conditions.Information obtained for crystals of small organic mol-ecules can be used for studying biological macromole-cules, in particular, for predicting the secondary struc-ture of polypeptide chains [1].Prominent among organic molecular crystals arecrystals of amino acids. They can serve as biomimet-ics, as well as medicines and materials for molecularelectronics. Studies of the effect of pressure on aminoacids are also of importance for geo- and cosmochem-istry [2]. The search for new high-pressure polymor-phs is believed to be one of the most central problems;however, reliable data on pressure-induced polymor-phous transformations in amino acid crystals have nothitherto been reported. The conclusion based onRaman spectra that pressure induces polymorphoustransformations in L -asparagine [3] and alanine [4]was not supported by later diffraction studies ofthese structures (J.A. Beukes and S. Parsons, privatecommunications). The existence of a high-pressurephase was suggested for glycine [Dawson, A.,Parsons, S., Allan, D., Loveday, J., and Guthrie, M.,http://www.isis.rl.ac.uk/BCA2001/Abstract%20files/bca1…];however, neither the data supporting this suggestion northe data describing the crystal structure of such a phaseare currently available. α -Glycine was not found toundergo pressure-induced phase transitions, at least upto pressures of about 4 GPa (X-ray diffraction) [5] or23 GPa (Raman spectroscopy) [6]. The X-ray diffrac-tion study of α -glycine containing an impurity of γ -glycine at pressures up to 4 GPa [5] showed that, at apressure of about 3.5 GPa, all reflections of the γ phaseabruptly disappeared and no new lines were observed.It was suggested that the γ phase thereby undergoes aphase transition at this pressure and the lines of a newphase are masked by the lines of the α modification [5].To verify this assumption, we prepared a pure phase of γ -glycine by the ad hoc procedure in [7]. This samplewas studied by X-ray diffraction at hydrostatic pres-sures of up to 8 GPa.The high-pressure experiments were carried out insitu in diamond anvil cells of the Merrill–Bassett type[8]. A methanol–ethanol mixture was used as the pres-sure-transmitting medium; this mixture was speciallydried in order to rule out the possible influence of watertraces on the polymorphous transformations of glycine.The X-ray powder diffraction experiment was carriedout with synchrotron radiation ( λ = 0.7195 A) at theEuropean Synchrotron Radiation Facility, Grenoble.X-ray diffraction patterns were recorded with anMAR345 image plate detector. Primary diffraction datawere processed and integrated using the Fit2D software[Hammersley, A., v. V11.012, hammersley@esrf.fr].Diffraction spectra were indexed with the TREOR pro-gram [10]. The subtraction of the background andstructure solution and refinement by the Rietveld tech-nique were performed with the MRIA program pack-age [11]. The PowderCell program package [Kraus, W.and Nolze, G., PowderCell for Windows, v. 2.3,http://www.bam.de/a_v/v_1/powder/e_cell.html] wasused for graphic representation and crystal-chemicalanalysis.The use of a pure sample allowed us, for the firsttime, to reliably detect a pressure-induced phase transi-tion resulting in the formation of a new polymorph andto solve and refine its structure. The initial γ modifica-tion and the new high-pressure phase coexisted in awide range of pressures. The first indications of a new