Abstract
The crystal structure of hexagonal L-cystine has been determined at room temperature at pressures between 0.4 and 3.7 GPa; unit-cell dimensions were measured up to 6.4 GPa. The structure of this phase consists of molecules in their zwitterionic form, and crystallizes in the hexagonal space group P6(1)22. The structure consists of hydrogen-bonded layers which are strongly reminiscent of those seen in alpha-glycine, and consist of R_4;4(16) hydrogen-bonded ring motifs. These layers are connected on one side by the disulfide bridges within the cystine molecules, and on the other by NH...O hydrogen bonds to other glycine-like layers. The most compressible unit-cell dimension, and the direction of greatest strain in the structure, is along the c-axis, and application of pressure pushes the layers closer together. The compression occurs approximately equally in the regions of the interlayer hydrogen bonds and the disulfide bridges; in the latter, changes in the C-S-S-C torsion angles allow the cystine molecules to act like springs. The effects of pressure can be interpreted in terms of closing-up of voids in the structure, and this leads to (i) a lessening of the N-C-C-O and C-S-S-C torsional angles, (ii) shortening of the N-H...O hydrogen bonds by 0.10-0.60 A and (iii) a further shortening of an already short S...S contact from 3.444 (4) A to 3.264 (4) A.
Highlights
The application of pressure is becoming an ever-more popular technique with which to analyse the nature of intermolecular interactions [for example, Oswald, Allan, Motherwell & Parsons (2005), Oswald, Allan, Day, Motherwell & Parsons (2005), Wunschel et al (2003), Boldyreva (2004a,b), Allan et al (1999, 2001)]
The structure of hexagonal L-cystine at ambient pressure l-Cystine crystallizes in two polymorphic forms: a tetragonal phase (P41) that was characterized by Chaney & Steinrauf (1974) and a hexagonal phase (P6122) first investigated by Oughton & Harrison (1959) but more recently in a charge density study by Dahaoui et al (1999)
The same comment applies to the C—S—S—C torsion angle, which is positive in both forms [69.3 (2) and 75.18 (5), respectively]
Summary
The application of pressure is becoming an ever-more popular technique with which to analyse the nature of intermolecular interactions [for example, Oswald, Allan, Motherwell & Parsons (2005), Oswald, Allan, Day, Motherwell & Parsons (2005), Wunschel et al (2003), Boldyreva (2004a,b), Allan et al (1999, 2001)]. In -amino acids the compression of numerous weak CHÁ Á ÁO interactions may be extremely important, as they have a supporting role to medium-strength hydrogen bonds under ambient pressure conditions, e.g. NHÁ Á ÁO (Desiraju & Steiner, 1999; Derewenda et al, 1995). The compression of such interactions has been discussed in other studies, such as the compression of l-serineI (Moggach et al, 2005). Structural data to 3.7 GPa, and unit-cell dimensions up to 6.4 GPa, are reported
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
