The crystal structure of the ~ crystal form of glycylglycine, -CO2CH2NHCOCHzNH +, has been investigated. The cell dimensions are a= 7.70, b= 9"57, c= 9.48 A,,/~= 124 ° 35'. The space group is P2x/a with four molecules per cell. Mo Kct rays were used. The trial structure was derived by use of part-cell Patterson methods and modified Banerjee equations. Final refinement was by block-diagonal anisotropic least-squares adjustment to an R of 12.3 and yielded standard deviations of about 0.007/~ in bond lengths. The intramolecular chemical bond lengths and interbond angles agree well with those found for the same molecule in the previously reported//crystal form but the configuration of the molecule is somewhat different. There is an angle of about 22.4 ° between the plane of the amide group and that of the carboxyl group. In the/~ crystal these groups were coplanar within experimental error. Since the ~t form is probably the stable form, this distortion presumably permits better molecular packing with stronger hydrogen bonds and van der Waals interactions.
Some time before the war the determination of the crystal structure of glycylglycine was undertaken as a continuation of the X-ray diffraction studies2 which are part of a program of research upon the constitution and configuration of proteins. This investigation, the first concerned with a linear peptide, was stopped in 1941 because of the war, and a brief description of the results as of that time was published.s The work has now been completed and the details are reported below.A preliminary investigation had been made by BernaL4 He grew crystals from mixtures of n-propyl alcohol and water and found three different crystal forms to which he applied the identifying prefixes a, p and y. All three of these were observed growing side by side in the same mother liquor. From the unit cell constants given it seemed likely that the 0-form, having a short needle axis period of only 4.62 Kx., might be the easiest structure to solve and it was accordingly chosen as the subject of the present research. This has proved to be the case so far as finding a trial structure was concerned but the crystals, being of an exceedingly thin needlelike habit, are unsuitable for obtaining good intensity data about any but the needle axis of the crystal. With the discovery of part cell projections6 it has seemed that the a-form is best suited for the most precise determination of interatomic distances. Accordingly no effort has been made in the present research to achieve the accuracy possible with complete threedimensional data. A careful, complete investigation of the a-form is now under way and the y-form will be examined well enough to determine the configuration of the molecule.In the present investigation crystals were grown from mixtures of n-propyl alcohol and water. The plate-like a-form was always obtained and it was only after repeated attempts that one crystallization yielded also a few needles of the y-form. Finally after numerous further crystallizations one crop containing some pcrystals appeared. It is not known which of the three forms is stable a t room temperature. It was shown by X-ray photographs that upon heating for twenty-four hours a t 105' the pand y-forms (1) Present address: T'Tans. Faraday Soc., 41, 434 (1945).are converted into the a-form. The exceedingly fine 0-crystals show the (100) and { O O l ) forms. The faces terminating the needles were not determined.All X-ray photographs were made with CuKa rays with X taken as 1.54 Kx. A few well-exposed oscillation pictures were made about co to confirm the length of that axis. Complete sets of oscillation pictures were made with multiple filmss about a. and bo for intensities. For the a 0 pictures a section of needle was broken off which was only a few tenths of a millimeter in length but even so its length was a t least twice the thickness of the specimen and the resulting variation in the sizes of the X-ray spots complicated the estimation of the intensities. One zero layer bo Weissenberg picture was also prepared with multiple films for intensity measurements...
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