比熱和直流磁化率證明N_H_O_氫鍵的電子_省略_D_和L_丙氨酸單晶中的不對(duì)稱

1、Articledoi: HYPERLINK :/ / www比熱和直流磁化率證明 N+HO氫鍵的電子自旋翻轉(zhuǎn)在 D-和 L-丙氨酸單晶中的不對(duì)稱相變王文清 1,*沈新春 1,2吳季蘭 1龔申國華 1趙洪凱 11(1 北京大學(xué)化學(xué)與分子工程學(xué)院應(yīng)用化學(xué)系, 北京分子科學(xué)國家實(shí)驗(yàn)室, 北京 100871;2 山東大學(xué)化學(xué)與化工學(xué)院, 濟(jì)南 250061)摘要: 為了解決 D-和 L-丙氨酸在約 270 K 相變的分岐和機(jī)理, 對(duì)其單晶、多晶粉末及原料利用微分掃描量熱儀測(cè)定比熱. 用三線法以藍(lán)寶石作校正, 并與手冊(cè)的 D-和 L-丙氨酸標(biāo)準(zhǔn)比熱值比較. 在單晶中, 實(shí)驗(yàn)觀察到吸 熱相變峰最高處時(shí)的溫

2、度及熱焓為: D-丙氨酸, Tc K, H Jmol-1; L-丙氨酸, Tc K, H= Jmol-1; 熱焓差為 Jmol-1. 參比晶體 D- 纈氨酸, Tc K, H Jmol-1; L- 纈氨酸, Tc= K, H Jmol-1; 熱焓差為 Jmol-1. 實(shí)驗(yàn)發(fā)現(xiàn)已測(cè)量過的單晶磨成多晶粉末后再測(cè), 相變峰消失. 說明相變與晶格有關(guān). 變溫中子衍射排除了 DL 的構(gòu)型相變, 但發(fā)現(xiàn) N+HO-氫鍵沿 D-和 L-丙氨酸單晶的 c 軸反向變化. 變溫偏振拉曼散射反映相變機(jī)制與 N+HO-中電子的軌道磁偶極矩相關(guān), 觀察到偏振光的不 對(duì)稱散射. 在外加磁場(chǎng)強(qiáng)度 H 為+1 T 和-1 T

3、 下, 變溫測(cè)定 D-和 L-丙氨酸晶體的直流磁化率, 證明在 270 K 有電子自旋翻轉(zhuǎn)的相變. 電子自旋的向上或向下, 取決于晶格中 NH+的扭曲振動(dòng)及 N+HO-氫鍵沿晶體 c 軸的方向.3由于自旋的定軸性, 可以解釋單晶和多晶粉末比熱結(jié)果的分岐.關(guān)鍵詞: 比熱; 直流磁化率; N+HO-氫鍵; 電子自旋翻轉(zhuǎn); 不對(duì)稱相變; D-和L-丙氨酸單晶中圖分類號(hào): O642; O646Heat Capacity and DC-Magnetic Susceptibility Evidence for theAsymmetry of Electron Spin-Flip Phase Transit

4、ion ofN+HO- Bond in Chiral Alanine CrystalWANG Wen-Qing1,*SHEN Xin-Chun1,2WU Ji-Lan1GONG Yan1SHEN Guo-Hua1ZHAO Hong-Kai1(1Beijing National Laboratory for Molecular Sciences, Department of Applied Chemistry, College of Chemistry and MolecularEngineering, Peking University, Beijing 100871, P. R. China

5、; 2School of Chemistry and Chemical Engineering, Shandong University, Jinan 250061, P. R. China)Abstract: With a view to understanding the argument of phase-transition mechanisms of D- andL-alanine at around 270 K, the temperature dependence of heat capacity measurements was investigated, for single

6、 crystals, ground powders, and polycrystalline products, using differential scanning calorimetry (DSC). The Cp (heat capacity under constant pressure) values of D- and L-alanine were calibrated with standard sapphire by the triple-curve method; these values coincide with the standard Cp values in th

7、eliterature. Endothermic transition peaks were observed at Tc K, H Jmol-1 and Tc K,H Jmol-1 for D- and L- alanine, respectively, and Tc K, H Jmol-1 and Tc K,H Jmol-1 for the reference crystals D- and L-valine, respectively. The energy differences of JReceived: October 31, 2011; Revised: January 19,

8、2012; Published on Web: February 13, 2012.Corresponding author. Email: ; Tel: +86-10-62752457.The project was supported by the National Natural Science Foundation of China (21002006, 20452002) and Special Program for Key Basic Research of the Ministry of Science and Technology, China (2004-973-36).國

9、家自然科學(xué)基金(21002006, 20452002)和國家科技部基礎(chǔ)研究重大項(xiàng)目(2004-973-36)資助 Editorial office of Acta Physico-Chimica Sinica774Acta Phys. -Chim. Sin. 2012Vol.28mol-1 for D-and L-alanine and Jmol-1 for D- and L-valine, which were observed from pre-alignedmolecules in the single crystals and vanished in the ground powder

10、s and polycrystalline products, show that the phase transition is related to the crystal lattice. Neutron diffraction results exclude the possibility of a DL configuration change, and show that the hydrogen bonds run antiparallel to the c-axis in the D- and L- crystals. Polarized Raman vibrational s

11、pectroscopy shows that the transition mechanism may be related tothe electronic orbital angular momentum and magnetic dipole moments of N+HO- in the crystals. Externalmagnetic fields, H=+1, -1 T, were applied parallel to the c(z)-axis of the D- and L-alanine crystalline lattices, respectively. The D

12、C-magnetic susceptibilities show electron spin-flip transitions at around 270 K in D- andL-alanine. The spin is“up”or“down”relative to the direction of N+ HO- bond along the c(z)-axis. Basedon spin rigidity and magnetic anisotropy, the results help to explain the discrepancies among heat capacityand

13、 magnetic susceptibility data for single crystals and polycrystalline powders of D- and L-alanine.Key Words: Heat capacity; DC-magnetic susceptibility; N+HO- bonding; Electron spin-flip;Asymmetry transition; D- and L-alanine crystalsN + HO- in the crystals of D- and L-alanine. This article is infocu

14、s on the temperature-dependence measurement of heat ca- pacity and dc-magnetic susceptibility to search for the transi- tion mechanism.1IntroductionWhy is life based on L-amino acids and D-saccharides rath- er than D-amino acids and L-sacharides? A theory on the ori- gin of life suggests that the pr

15、eferential emergence of the L-amino acids in proteins could be due to intrinsic differences in energy between enantiomers as a result of the weak forces. The weak-neutral current between electrons and nucleons intro- duces a small energy difference between enantiomers of a chi- ral molecule owing to

16、 parity violation. Calculation of the pari- ty violating energetic differences (PVED) between L- and D- amino acids suggested that L-enantiomers are more stable than D-enantiomers by 10-17kTR, where k is Boltzman constant and TR is room temperature for amino acids.1 The tunneling time from left-hand

17、ed state to the right-handed state, then back to the left, is extremely long of order millions of years for typical amino acids. In 2004, MacDermott and Hegstrom2 proposed a novel method to measure the PVED between enantiomers us- ing chiral ammonia-like molecules (NHDT) with tunneling time between

18、10-7 and 10-2 s. The direct measurement of ener- gy difference displays a small but possibly detectable elec- troweak optical rotation for D- and L-amino acids.The concept of chirality comes from -C atom with four dif- ferent substituents based on Cahn-Ingold-Prelog rules with re- spect to geometry

19、which is a pseudo-scalar without connection to the physical world. Amino acid molecules not only have a spatial arrangement of atoms but also possess a spatial arrange- ment of electric dipoles. D- and L-alanine crystallizes as a lin-ear chain of molecules with the carboxylate anion (COO-) and2Exper

20、imental Crystallization of D- and L-alanineThe polycrystalline powders of D- and L-alanine (Sigma Chemical Co.) were twice recrystallized from sterilization aqueous solution at 277 K by slow evaporation for 2 to 3 weeks, producing well formed crystal elongated along the c ax-is and with principal fa

21、ces 110 washed with dropping abso-3,4lute alcohol, evacuated and kept in a desiccator. Crowell et al.5pointed out that evaporation time of less than one week in-creased the inhomogeneous contribution to the phonon line shapes and concluded no further improvement in crystal quali- ty passed through t

22、hrice recrystallization. According to the Kosic et al.6 report, the most possible residual impurities in L-alanine are glycine and D-alanine. The crystal purity was identified by the following procedure. Firstly, the positive-ion electrospray mass spectra (EI-MS) were recorded using a Bruk- er APEX

23、IV Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer (Bruker Daltonics, Billerica, MA USA) equipped with an external ESI source (Analytica, Bran- ford Inc. USA), which produced detection limits of 2 fmol for organic impurities. L-alanine crystal was dissolved in a solvent system c

24、omposed of methanol/water/formic acid (49.4:49.4:0.2, volume ratio), to the concentration of gmL-1. Sam-ple solution was electrosprayed by 1 Lmin-1 flow rate froma capillary (ID: m) biased at -4 kV. The infusion rate was60 Lh-1 and the scan range was from m/z 50 to 1000. EI-MS could detect the impur

25、ity less than 2 10-13 gmL-1.7 Secondly, the purity of the single crystals of D-, L-, DL-alanine and D-, L-, DL-valine were identified by gas chromatography mass spectrometry (GC-MS) of their N-trifluoroacetyl (TFA) isopro- pyl esters on Chirasil-Val capillary columns.8 Atomic force mi- croscopy (AFM

26、) images clearly showed an ordered molecularthe nitryl cation (NH +) at opposite ends. All three protons of3the NH + group form N HO bonds. The intermolecular3N+ HO- bond links up along the c(z)-direction. There are twoinequivalent hydrogen centers in D- and L-alanine: one chiral center is H-C (hydr

27、ogen bonded to -carbon), however, C HO interaction is too weak and appear to be non-directional;the second chiral center is polar H + (hydrogen bonded to nitro-Ngen). Neutron diffraction and polarized Raman vibration spec-tra showed the up and down directions of the strongest bondWANG Wen-Qing et al

28、.: Asymmetry of Spin-Flip Transition775morphology of orthorhombic alanine and monoclinic valineunit cells, which are in fair agreement with the X-ray diffrac- tion data.9 Thirdly, FT-IR spectrometer (Nicolet iN 10 MX, USA) was with detection limits of 10 g. Heat capacity measurementsThe heat capacit

29、y measurements of D- and L-alanine were performed from 240 to 320 K with a differential scanning calo- rimeter (pyris diamond DSC, Perkin Elmer Corp, USA), which is of the heat-flux type. All of the crystals were dried for sever- al days at 10-4 mm, in a system trapped with liquid nitrogen. For the

30、pyris diamond system, dry N2 gas was purged throughthe DSC cells with a flow rate of 10 mLmin-1 for 60 min be-fore the measurement. The empty reference pan had a mass of mg for all runs. No mass losses which could be consid- ered significant occurred. The scan rate was 10 Kmin-1 from240 to 320 K. Fi

31、nal heat capacity calibrations were made with mg of sapphire, and the sample measurements were car- ried out with sample and reference pan. Triple curve method was used for precise temperature measurements of the specific heat of the sample.10The heat capacities of D- and L-alanine crystals and poly

32、- crystalline powders of Sigma products were measured. The rig- orous drawing of the baseline is based on Mraws model11 andthe transition enthalpy can be calculated from the area en-closed by the baseline and the trace recorded in the experi- ments.12,13 In order to get high precision of heat capaci

33、ty data by standard DSC, the following procedure had to be carried out before the heat capacity measurement. D- and L-alanine single crystals and Sigma polycrystalline powdersThe heat capacity of L-alanine single crystal mg), L-alanine Sigma powder (A-5824 Lot 84H02461, 99% , mg) and D-alanine singl

34、e crystal mg), D-alanine Sigma powder (A-7377 Lot 16H1584, 98% , mg) were mea-sured from 250 to 320 K at the scan rate of 10 Kmin-1 by tri-ple curve method. L- and D-valine single crystals and Sigma polycrystalline powderThe heat capacity of L-valine single crystal mg), L-valine Sigma powder (V-500

35、Lot 80H0457, 98% , mg) and D-valine single crystal mg), D-valine Sigma pow- der (V-0250 Lot 16H1206, 98% , mg) were measuredfrom 250 to 320 K at the scan rate of 10 Kmin-1 by triplecurve method. Ground powders from pre-measured D-, L-alanine and D-, L-valine crystalsIt is worthy to note that the tra

36、nsition peak around 270 K was not observed in the Sigma products of D- and L-alanine (-valine), the polycrystalline L-alanine g), L-valine g), and D-alanine by Hutchens14 and Huffman15 et al. For the aim to understand the transition mechanism, we ground the pre-measured single crystals of D- and L-a

37、lanine (-valine)then performed the heat capacity measurement. DC-magnetic susceptibilityDC-magnetic susceptibility measurement of D- and L-ala- nine single crystals was performed in the temperature range of2-320 K with SQUID-based magnetometers of Quantum De- sign model MPMS-XL-5. The crystals were

38、selected (mass of mg for D-alanine and mg for L-alanine) and rectangularly mounted in the long plastic tube. The crystals were placed that the c axis was exactly parallel to the direction of the magnetic field (H/z axis). The external magnetic field strengths (H=1 T) were applied. The magnetic momen

39、ts were measured under three scannings. Neutron diffractionThe neutron diffraction data of D- and L-alanine at 295 and60 K were performed on the ISIS Facility of CLRC Rutherford Appleton Laboratory,16 and the temperature-dependent data of D-alanine at 300, 260, 250, and 240 K were measured at the VI

40、VALDI neutron beam line.17,18 Raman vibration spectroscopyTemperature-dependent polarized Raman spectra of the NH +3torsion mode in D- and L-alanine were performed with b(cc)bscattering geometry from 80 to 300 K. The wavenumber accu- racy of the instrument is 1 cm-1. The repeatability is 1 pixel wit

41、h a standard CCD (1024 pixel 256 pixel of 26 m) under the condition of temperature stability ( 1 C). In the alaninemolecule, the NH+ group could undergo simultaneously a dy-3namic distortion and a lifting of the degeneracy of its groundand excited states, producing the splitting of the NH + tor-3sio

42、nal mode (493 cm-1, 80 K) into NHO (481 cm-1) andN + HO- (464 cm-1) modes at 290 K, which was discovered by Forss.19 The Stokes vibrational Raman line of N + HO- mode was measured with the spectrometer T64000.3Results and discussion Identification of samplesIn Fig.1, the MS spectrum of alanine (M=89

43、.09) showed the first peak at m/z and the second peak at m/z which were due to loss of H2O and plus NH3 from protonated molecular ion M+H+, respectively.Nicolet photoacoustic spectrometer with a detection limit of10 ng was used to preclude the possible H2O content in crystal as shown in Fig.2. Heat

44、capacity measurementsAn endothermic transition occurs at K for L-alanine MS spectra of L-alanine with electrospray ionization-fourier transform ion cyclotron resonance mass spectrometer776Acta Phys. -Chim. Sin. 2012Vol.28 Temperature-dependent heat capacity of D-alanine() Sullivan data20 of D-alanin

45、e H=(5.80.1) Jmol-1; () Huffman data15 ofD-alanine; () this work, Sigma polycrystalline powder of D-alanine(A-7377 Lot 16H1584, 98%, mg); () this work, D-alanine singlecrystal mg). Inset: T K, T K,T K, H Jmol-1. M gmol-1sstandard values of Hutchens and Fig.6). There are nopeaks present around 270 K

46、in the Sigma polycrystalline pow- ders of L-valine and D-valine (Fig.7).The transition peak was vanished in the ground powder of D-, L-alanine and D-, L-valine (Fig.8) which demonstrates that the transition is observed only in pre-aligned molecules mediat- ed by lattice modes. The transition paramet

47、ers are given in Ta- bles 1-3 and help to understand the difference in the Cp data between our study and earlier reports.14,15,20,21From the above results, endothermic transitions in single crys- tals of D-, L-alanine and D-, L-valine were verified. The heats of transition in D-, L-alanine and D-, L

48、-valine are 1.87, 1.46, and 1.75, Jmol-1, respectively. The transition peak was ab-sent in the polycrystalline powders of D-, L-alanine and D-,L-valine of Sigma products and the ground powders from the pre-measured crystals, which excludes the unidentified impuri- ties imparted observable difference

49、s at transition temperature. Transition mechanism of D- and L-alanineIR spectra of D- and L-alanine crystals compared with thestandard spectrum of L-alanine (a) and L-valine and L-alaninecrystals compared with the standard spectrum of H2O (b)and K for D-alanine crystal, respectively. There is nopeak

50、 present around 270 K in the Sigma powders of L-alanine and D-alanine. The heat capacity data of L- and D-alanine were compared with Sullivan et al.20 and the standard values of Hutchens14 and Huffman15 et al. and Fig.4, the inset fig- ures show the sharp phase transition peak in this work).An endot

51、hermic transition occurs at K for L-valine crystal and K for D-valine crystal, respectively. Theheat capacity data of L- and D-valine were compared with the Temperature-dependent heat capacity of L-alanine() Sullivan data20 of L-alanine (H=(20.30.1) Jmol-1); () Hutchens data14 ofL-alanine g); () thi

52、s work, Sigma polycrystalline powder ofL-alanine (A-5824, Lot 84H02461, 99%, mg); () this work, L-alaninesingle crystal mg). Inset: T K, T K, Temperature-dependent heat capacity of L-valine() Sullivan data20 of Sigma L-valine powder; () Hutchens data 21 of L-valine; () this work, L-valine single cry

53、stal mg). Inset: T K, T K, T K, H Jmol-1.T K, H Jmol-1. M gmol-1M (L-valine)=1 gmol-1ssWANG Wen-Qing et al.: Asymmetry of Spin-Flip Transition777 Temperature-dependent heat capacity of D-valine() Sullivan data20 of D-valine, H=(17.80.1) Jmol-1; () Hutchens data21 ofL-valine; () this work, D-valine s

54、ingle crystal mg). Inset: T(onset)= K, T K, T K, H Jmol-1. Temperature-dependent heat capacity of D- and L-valineSigma powders() Hutchens data of L-valine polycrystal g); () this work, Sigma L-valine powder (V-500 Lot 80H0457, 98%, mg);() this work, Sigma D-valine powder (V-0250 Lot16H1206, 98%,-1Ms

55、(D-valine)=1 gmol mg). Ms (valine)=1 gmol-1 DC-magnetic susceptibility evidence for electronAlanine molecules exist as parallel chains of hydrogen bond-ed zwitterions that form electric dipoles with magnetic mo- ments B (B=1 Bohr magneton), which run parallel and antipar- allel to the c axis in D- a

56、nd L-alanine crystals. The magnitude of such a magnetic dipole moment would be of the order of N=e/mp , in which the proton mass (mp) is 1837 times as that ofthe electron. The potential energy of the field is required to turn the magnetic dipole antiparallel to the field. The energy is -B when the d

57、ipole is parallel to the field, and it is + B when the dipole is antiparallel to the field. So the energy re- quired to turn the dipole is 2BB 10-4 eV1 T. Nuclearmagnetic dipole moments are three orders of magnitude small-spin-flip transition of H+ atomNThe spin-flip transition in D-alanine is clear

58、ly observed inthe plots of versus T and d/dT versus T under H=1 T (N S) and vice versa L-alanine experiences a peak around 270 K under H=-1 T (SN), respectively (Fig.9). Because the di- pole moment of L-alanine was originally aligned antiparallel tothe direction of magnetic field at H=+ 1 T, it cann

59、ot turn to align itself parallel to the field unless it can release the same amount of energy. Zeeman effect involves the splitting of ener- gy levels of an atom, due to the orientational potential energy of its magnetic dipole moment in an external magnetic field ap-plied to the H+ atom of N+HO- bonding.22N Temperature-dependent heat capacity of ground powders from the pre-measured crystals(a) D-alanine, (b) L-alanine, (c) D-valine, (d) L-valine778Acta Phys. -Chim. Sin. 2012Vol.28Table 1 Transitional results of D-, L-alanine and D-, L-valinesingle crystalsTable 4Hydrogen bond geometry from S