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2007, 111, 9186-9188 Published on Web 07/14/2007
An Extended Phase Transition in Crystalline L-Cysteine near 70 K Igor E. Paukov,*,† Yulia A. Kovalevskaya,† Valeri A. Drebushchak,†,‡ Tatiana N. Drebushchak,§,| and Elena V. Boldyreva*,§,| Institute of Inorganic Chemistry SB RAS, LaVrent’eVa 3, NoVosibirsk, 630090, Russia, Institute of Geology and Mineralogy SB RAS, Koptyuga 3, NoVosibirsk, 630090, Russia, Institute of Solid State Chemistry and Mechanochemistry SB RAS, Kutateladze 18, NoVosibirsk, 630128, Russia, and REC-008 NoVosibirsk State UniVersity, PirogoVa 2, NoVosibirsk, 630090, Russia ReceiVed: May 13, 2007; In Final Form: June 18, 2007
An anomaly of heat capacity near 70 K was registered in the crystalline L-cysteine (orthorhombic polymorph) by adiabatic calorimetry as a small, 3-5% height, diffuse “jump”, or a wide “hump” accompanied by the substantial increase in the thermal relaxation time. The Cp(T) dependence is characteristic for a transformation extended over a wide temperature range and sensitive to the thermal prehistory of the sample, that can be interpreted as nonsimultaneous changes in the dynamics and the orientation of the numerous thiol groups in the structure. The measurements of the cell parameters and cell volume on cooling suggest that a discontinuity in their values over the transition point exists.
Introduction Temperature-induced changes in crystalline amino acids are of interest for their ferroelectric, piezoelectrical, nonlinear optical properties1 and reveal the intrinsic motions of these structural fragments and their contribution to the dynamic properties of the proteins.2 The structure-forming units in the crystals of amino acids are similar to those in the biopolymers (head-totail chains of amino acids mimicking polypeptide chains, twodimensional layers mimicking β-sheets).3,4 Low-temperature “anomalies” (i.e., nonlinear changes in the temperature dependences of vibrational frequencies, volume and cell parameters, the mean square displacement of the atoms around their equilibrium position, or of the optical birefringence) were reported recently for several amino acids.1 However, only for a few systems the existence of a phase transition was confirmed by calorimetry measurements, which are very important for understanding the nature of an anomaly. Early calorimetric studies did not reveal phase transitions in the crystalline amino acids at temperatures below or close to ambient, with L-methionine (with a phase transition near 300 K) being considered as a rare exception.5 More recently, the occurrence of low-temperature phase transitions in taurine6 and in β-glycine7 was confirmed by calorimetry. L-Cysteine, +NH3-CH-(CH2SH)-COO-, plays an enormous role in the biological systems. Cysteine thiol or sulfhydryl (S-H) groups are the most chemically reactive sites in proteins under physiological conditions.8 The cysteine sulfhydryl may function as either a hydrogen bond donor (e.g., S-H‚‚‚O) or * To whom correspondence should be addressed. E-mail: paukov@ che.nsk.su (I.E.P.);
[email protected] (E.V.B.). † Institute of Inorganic Chemistry SB RAS. ‡ Institute of Geology and Mineralogy SB RAS. § Institute of Solid State Chemistry and Mechanochemistry SB RAS. | REC-008 Novosibirsk State University.
10.1021/jp073655w CCC: $37.00
Figure 1. Heat capacity, Cp, versus temperature in different temperature ranges: (a and b) Measured in pulse mode. (c) (black open circles) Measured in pulse mode; (red circles) a continuous mode, cooling with solid nitrogen (thermogram #1); (blue circles) a continuous mode, cooling with liquid helium (thermogram #2); (green triangles) a test measurement outside the anomaly region (thermogram #3).
© 2007 American Chemical Society
Letters
J. Phys. Chem. B, Vol. 111, No. 31, 2007 9187
Figure 2. Cell parameters and volume versus temperature; black symbols, data from ref 13.
acceptor (e.g., H‚‚‚S-H) group. Sulfhydryl hydrogen bonding in proteins is not well understood, primarily because such interactions are difficult to detect experimentally due to their weakness. The properties of the hydrogen bonded thiol groups can be mimicked using L-cysteine in the crystalline state, which can exist as the orthorhombic9,10 and the monoclinic polymorphs.11,12 At ambient temperature, as evidenced by X-ray9 and neutron10 diffraction, the sulfur atoms of the thiol groups are disordered over two positions. The distances between the oxygen and sulfur atoms of the neighboring molecules in the structure are consistent with the hypothesis on the formation of the two types of intermolecular hydrogen bonds, S-H‚‚‚S and S-H‚‚‚O.9,10 A recent structural study of L-cysteine at 30 K has shown the sulfur atoms to be completely ordered and located at those positions, which correspond to the formation of the S-H‚‚‚S hydrogen bonds.13 Therefore, a disorder-order transition should occur at some point on cooling and manifest itself in the temperature dependence of heat capacity. Previously, the heat capacity of L-cysteine was measured in the temperature range 85-298 K, but neither a hint on a phase transition nor a weak anomaly were registered.14 The aim of the present study was to measure the heat capacity of the orthorhombic L-cysteine at temperatures lower than 85 K in order to test the existence of a manifestation of a “disorder-order” phase transition in this system on cooling.
at 6 K, 0.3% at 15-40 K, and 0.1% at 40-300 K. A 3.9597 g portion of L-cysteine was put in a copper calorimeter (20.4 g, 5.7 cm3). The heat capacity of L-cysteine was measured in 90 experiments in the temperature range 5.7-304.8 K. After the sample was loaded, the calorimeter was filled with gaseous helium at the pressure 1 × 104 Pa, to improve the heat exchange. A correction for the excess heat capacity of helium in a filled calorimeter as compared to the empty one was introduced, and turned out to be significant (∼2%) only at very low temperatures, decreasing rapidly with increasing temperature (0.1% at 15 K). The measurements were carried out in pulse and in continuous heating modes.16,17 Pulse heating is basically used for precise heat capacity measurements in the temperature range not characterized by any anomalies. For the sake of this study, to avoid errors in the calculations of the averaged heat capacity values, we took special care to have the temperature after the heat input really equilibrated. The continuous heating measurements are commonly used for a detailed analysis of the temperature ranges near the anomalies in the Cp(T) functions. They allow one to achieve a much smaller temperature difference between the experimental points (0.01 K, as compared to 1-10 K in the pulse mode), thus increasing the sensitivity of the measurements to any anomalies in the Cp(T) function, which is important when detecting these anomalies, and testing the theoretical models describing phase transitions.
Experimental Section
Results
The sample of the orthorhombic crystalline L-cysteine (99.9%) was used as purchased from ICN Biomedicals. X-ray diffraction (Bruker GADDS) did not reveal the presence of any other crystalline phases in the sample. The low-temperature heat capacity was measured using an automatic vacuum adiabatic calorimeter as described elsewhere.15 The reliability of the measurements was confirmed by test measurements of the heat capacity of benzoic acid as a calibrant in the temperature range 6-300 K. The accuracy of the measurements was equal to 2%
The results of the heat capacity measurements in a pulse mode are shown in Figure 1. The anomaly in the heat capacity (that can be described as a small, ∼3-5% heigh diffuse jump, or as a broad “hump”) could be detected near 70 K. The temperature equilibration in the range 68-73 K took significantly longer than usual. The way, how temperature changed at the start of heating, also indicated at a slow thermal relaxation of the system. The heat capacity in the temperature range close to the anomaly (67-75 K) was followed in more detail in two series
9188 J. Phys. Chem. B, Vol. 111, No. 31, 2007 of the experiments with continuous heating (thermograms), using different cooling agents. In both cases, the occurrence of the anomaly was confirmed, and the size of the effect was higher during continuous heating than when measured on pulse heating. A very unusual observation is that, although the two thermograms were measured at the same heating rate, the anomaly near 70 K was much more pronounced, when cooling with helium. A reason can be sought in the different temperature reached prior to heating start (4 K for thermogram #2 and 60 K for thermogram #1), which has resulted in a different completeness of the transformation into the low-temperature phase. A test thermogram (#3) measured in the temperature range outside the anomaly region gave the same Cp values as those measured on pulse heating. Discussion The results of the heat capacity measurements can be interpreted as a manifestation of an extended dynamic transition, related to the ordering of the thiol groups on cooling. The ordering of the thiol groups due to the formation of the S-H‚ ‚‚S hydrogen bonds was observed earlier by single-crystal X-ray diffraction at 30 K.13 It can play an important role also in the pressure-induced phase transitions in L-cysteine at ambient temperature, although at 0.5 GPa the thiol groups still remain disordered.18 It is remarkable that the pressure-induced transitions show pronounced kinetic effects in the changes in the orientation of the thiol groups: an intermediate high-pressure phase was described as a structure with alternating thin domains with different orientations of thiol groups typical for another high-pressure phase and the ambient-pressure phase. The Cp(T) dependence measured for L-cysteine on cooling is characteristic not for a sharp disorder-order transition but for a transformation extended over a wide temperature range and depending on the thermal prehistory of the sample, that can be interpreted as nonsimultaneous changes in the dynamics and the orientation of the numerous thiol groups in the structure. The measurements of the cell parameters and cell volume (STOE STADI-4, Oxford Cryostreams) on cooling down to 100 K (the lowest temperature accessible) suggest that a discontinuity in their values over the transition point exists: the
Letters points reported for 30 K13 obviously deviate from the extrapolated curves (Figure 2). Further diffraction and NMR studies in the temperature range of the heat capacity anomaly will give a better insight into the nature of the transition. Acknowledgment. This work was supported by the Integration Projects #49 and #110 of the SB RAS, by a grant from RFBR (05-03-32468), and by a grant from BRHE (NO-008XI/BG6108). References and Notes (1) Boldyreva, E. V. Crystalline amino acids - a link between chemistry, materials sciences and biology. In Models, Mysteries, and Magic of Molecules; Boeyens, J. C. A., Ogilvie, J. F., Eds.; Springer-Verlag: BerlinHeidelberg-New York, 2007, pp 169-194. (2) Ringe, D.; Petsko, G. A. Biophys. Chem. 2003, 105, 667-680. (3) Vinogradov, S. N. Int. J. Pept. Protein Res. 1979, 14 (4), 281289. (4) Suresh, C. G.; Vijayan, M. Int. J. Pept. Protein Res. 1983, 22 (2), 129-143. (5) Mrevlishvili, G. M. Russ. Phys. ReV. 1979, 128 (2), 273-312. (6) Lima, J. A., Jr.; Freire, P. T. C.; Lima, R. J. C.; Moreno, A. J. D.; Mendes Filho, J.; Melo, F. E. A. J. Raman Spectrosc. 2005, 36 (11), 10761081. (7) Drebushchak, V. A.; Boldyreva, E. V.; Kovalevskaya, Yu. A.; Paukov, I. E.; Drebushchak, T. N. J. Therm. Anal. Calorim. 2005, 79, 6570. (8) Raso, S. W.; Clark, P. L.; Haase-Pettingell, C.; King, J.; Thomas, G. J. J. Mol. Biol. 2001, 307, 899-911. (9) Kerr, K. A.; Ashmore, J. P. Acta Crystallogr., Sect. B 1973, 29, 2124-2127. (10) Kerr, K. A.; Ashmore, J. P.; Koetzle, F. Acta Crystallogr., Sect. B 1975, 31, 2022-2026. (11) Harding, M. M.; Long, H. A. Acta Crystallogr., Sect. B 1968, 24, 1096-1102. (12) Go¨rbitz, C. H.; Dalhus, B. Acta Crystallogr., Sect. C 1996, 52, 1756-1759. (13) Moggach, S. A.; Clark, S. J.; Parsons, S. Acta Crystallogr., Sect. E 2005, 61, o2739-o2742. (14) Huffman, H. M.; Ellis, E. L. J. Am. Chem. Soc. 1935, 57, 46-48. (15) Paukov, I. E.; Kovalevskaya, Yu. A.; Rahmoun, N. S.; Geiger, C. A. Am. Mineral. 2006, 91, 35-38. (16) Paukov, I. E.; Kovalevskaya, Yu. A.; Rahmoun, N. S.; Geiger, C. A. Am. Mineral. 2007, 92, 388-396. (17) Bessergenev, V. G.; Kovalevskaya, Yu. A.; Paukov, I. E.; Shkredov, Yu. A. Thermochim. Acta 1989, 139, 245-256. (18) Moggach, S. A.; Allan, D. R.; Clark, S. J.; Gutmann, M. J.; Parsons, S.; Pulham, C. R.; Sawyer, L. Acta Crystallogr., Sect. B 2006, 62, 296309.
