北京大学：《近代物理实验 Modern Physics Laboratory》课程学生成果_开放性研究型实验示例（张淼、白凡）

Temperature dependence of optical rotation on parity-violating phase transition of D-, L, and DL alanine crystals 张淼白凡 北京大学物理系九九级 指导老师:杜为民王文清

Temperature dependence of optical rotation on Temperature dependence of optical rotation on parity - violating phase transition of D violating phase transition of D -, L -, and DL , and DL - alanine alanine crystals crystals 张淼 白凡 北京大学物理系九九级 北京大学物理系九九级 指导老师：杜为民 指导老师：杜为民 王文清

Part i abstract Chirality in Biological System Salam Theory: Theory and Supposed Experiment Previous Work Our experiment

Part I Abstract Part I Abstract ¾ Chirality Chirality in Biological System in Biological System ¾ Salam Theory: Theory and Supposed Salam Theory: Theory and Supposed Experiment Experiment ¾ Previous Work Previous Work ¾ Our Experiment Our Experiment

Introduction: Chirality all biomolecules have to be of one hand. so homochirality is a hallmark of life. a Life is based on L-amino acids and D-sugars rather than the enantiomeric d-amino acids and L-sugars. This broken symmetry is now believed to be a feature of fundamental physics a result of symmetry-breaking by the weak force which makes one enantiomer very slightly more stable than the other

Introduction Introduction ：Chirality Chirality „ All biomolecules biomolecules have to be of one hand, so have to be of one hand, so homochirality homochirality is a hallmark of life. is a hallmark of life. „ Life is based on L Life is based on L -amino acids and D amino acids and D -sugars rather than the rather than the enantiomeric enantiomeric D -amino acids amino acids and L -sugars. This broken symmetry is now sugars. This broken symmetry is now believed to be a feature of fundamental physics believed to be a feature of fundamental physics ⎯ a result of a result of symmetry symmetry-breaking breaking by the weak force weak force, which makes one , which makes one enantiomer enantiomer very slightly more stable than the other. very slightly more stable than the other

Introduction: Salam Phase Transition Theory a Salam postulated that chirality among the twenty amino acids, which make up the proteins, may be a consequence of a phase transition which is analogous to that due to bCs superconductivity Salam suggested a unique mechanism whereby the parity violating energy difference(PVED between enantiomers might lead directly to a homochiral product. He surmised that there should exist an abrupt phase transition at a critical temperature T, causing a change in the chirality from d-amino acid to that of more stable natural enantiomer. the L-amino acid

Introduction Introduction ：Salam Phase Transition Salam Phase Transition Theory „ Salam postulated that postulated that chirality chirality among the twenty among the twenty amino acids, which make up the proteins, may be amino acids, which make up the proteins, may be a consequence of a a consequence of a phase transition phase transition which is which is analogous to that due to analogous to that due to BCS superconductivity. superconductivity. Salam suggested a unique mechanism whereby the Salam suggested a unique mechanism whereby the parity violating energy difference ( parity violating energy difference (PVED) between between enantiomers enantiomers might lead directly to a might lead directly to a homochiral homochiral product. He surmised that there should product. He surmised that there should exist an abrupt phase transition at a critical exist an abrupt phase transition at a critical temperature temperature T c, causing a change in the , causing a change in the chirality chirality from D -amino acid to that of more stable amino acid to that of more stable “natural natural ” enantiomer enantiomer, the L , the L -amino acid. amino acid

Introduction: Salam Phase Transition Theory 1=1036o(1-4sinO exp ≈2.5×10K

Introduction Introduction ：Salam Phase Transition Salam Phase Transition Theory K g T eff C 2 3 2 ] 2 . 5 10 ( 1 4sin ) 2 exp[ 10 ≈ × − − = σ θ ϕ

Introduction: Salam Phase Transition Supposed Experiment Measuring differences of specific heats 2. Lower the temperature while measuring the optical activity while polarized light is shone upon a particular amino acid crystal. If the polarization vector gets rotated, one may be sure that the appropriate phase transition has taken lace pl Salan,A. Phys lett B.1992,288,153-160 Salam. A.J. MoL Evol. 1991.33.105-113

Introduction Introduction ：Salam Phase Transition Salam Phase Transition Supposed Experiment Supposed Experiment 1. Measuring differences of Measuring differences of specific heats specific heats. 2. Lower the temperature while measuring the Lower the temperature while measuring the optical activity optical activity while polarized light is shone while polarized light is shone upon a particular amino acid crystal. If the upon a particular amino acid crystal. If the polarization vector gets rotated, one may be sure polarization vector gets rotated, one may be sure that the appropriate phase transition has taken that the appropriate phase transition has taken place. Salam, A. Salam, A. Phys Lett B. 1992,288,153 288,153 -160 Salam, A. Salam, A. J.Mol.Evol J.Mol.Evol. 1991,33,105 ,33,105 -113

Introduction: Previous work on this subject Figureau et al conducted a series of experiments to test the validity of Salam prediction. They observed no changes of optical rotation in solution after exposing both racemic DL cystine and L-cystine to temperatures ranging from 77K to 0. 6K for three and four days, thus reported failing to validate PVED-induced phase transitions predicted by salam Figureau, A, Duval, E; Boukenter, A. Orig. Life EvoL. Biosphere. 1995, 25, 211-217

Introduction: Introduction: Previous work on this Previous work on this subject subject Figureau Figureau et al. conducted a series of experiments conducted a series of experiments to test the validity of Salam prediction. They to test the validity of Salam prediction. They observed observed no changes no changes of optical rotation of optical rotation in solution solution after exposing both after exposing both racemic racemic DL - cystine cystine and L -cystine cystine to temperatures ranging to temperatures ranging from 77K to 0.6K 77K to 0.6K for three and four days, thus for three and four days, thus reported reported failing to validate failing to validate PVED -induced induced phase transitions predicted by Salam phase transitions predicted by Salam. Figureau Figureau, A.; Duval, , A.; Duval, E.; Boukenter Boukenter, A. Orig. Life. Orig. Life. Evol. Biosphere . Biosphere.1995, 25,211 -217

Introduction: Inaccuracy of previous experiment a We argue that Figureau samples were taken out of the cryostat after the cooling period, then heated and dissolved in HCl solution at room temperature for measurement. Therefore, their observing no optical rotation in solution only proves that Salam phase transition is not an irreversible process But it is not a piece of negative evidence while the transition is reversible Moreover, their work only denied the configuration change from D-cystine to L-cystine

Introduction: Introduction: Inaccuracy of Inaccuracy of previous experiment previous experiment „ We argue that We argue that Figureau Figureau samples were taken out of samples were taken out of the cryostat after the cooling period, then heated the cryostat after the cooling period, then heated and dissolved in and dissolved in HCl solution at room temperature solution at room temperature for measurement. Therefore, their observing no for measurement. Therefore, their observing no optical rotation in solution only proves that Salam optical rotation in solution only proves that Salam phase transition is phase transition is not an irreversible not an irreversible process process, But it is not a piece of negative evidence ut it is not a piece of negative evidence while the transition is reversible while the transition is reversible. Moreover, their work only denied the Moreover, their work only denied the configuration change from D configuration change from D -cystine cystine to L -cystine cystine

Experiment: Experiment Design In order to study on Salam Phase Transition T was predicted as about 250K), we have to measure the optical rotation in crystalline amino acid while the temperature is very low. The variation of optical rotation angle of crystal with temperature is complicated causing by subtle changes in molecular structure e. g. changes of ability of certain groups(such as carboxylate methyl and amino group)to rotate. The angle depends on chemical bonding and molecular conformation as yet poorly understood way

Experiment: Experiment: Experiment Design Experiment Design „ In order to study on Salam Phase Transition ( In order to study on Salam Phase Transition ( T c was predicted as about 250K), we have to measure was predicted as about 250K), we have to measure the optical rotation in crystalline amino acid optical rotation in crystalline amino acid while the temperature is very low the temperature is very low. The variation . The variation of optical rotation angle of crystal with of optical rotation angle of crystal with temperature is complicated causing by subtle temperature is complicated causing by subtle changes in molecular structure e.g. changes of changes in molecular structure e.g. changes of ability of certain groups (such as ability of certain groups (such as carboxylate carboxylate, methyl and amino group) to rotate. The angle methyl and amino group) to rotate. The angle depends on chemical bonding and molecular depends on chemical bonding and molecular conformation as yet poorly understood way. conformation as yet poorly understood way

Experiment: Characterization of samples The powder of D-alanine. L-alanine and DL-alanine were obtained from Sigma Chemical Co. The amino acids single crystals were well-formed crystals elongated along the c axis, which were grown by slow evaporation of saturated aqueous solutions at 4 C, then washed with absolute alcohol, evacuated and kept in a desiccator

Experiment: Experiment:Characterization of samples Characterization of samples „ The powder of D The powder of D -alanine alanine, L -alanine alanine and DL -alanine alanine were obtained from Sigma were obtained from Sigma Chemical Co. The amino acids single Chemical Co. The amino acids single crystals were well crystals were well -formed crystals formed crystals elongated along elongated along the c axis, which were , which were grown by grown by slow evaporation of saturated slow evaporation of saturated aqueous solutions at 4 aqueous solutions at 4 ° C, then washed , then washed with absolute alcohol absolute alcohol, evacuated and kept , evacuated and kept in a desiccator desiccator