Terahertz Time-domain Spectroscopy Of Amino Acid Interaction With Water

As the basic structural unit of protein,amino acids play particularly important roles in the fields of biology,materials science and chemistry.At the same time,amino acids are also considered as one of the cornerstones for studying the physical and chemical properties of proteins.We can gain a deeper understanding of the molding mechanism and related properties of various proteins by studying the interactions between amino acids and water.In addition,amino acid drugs and foods are highly susceptible to interact with water molecules during their procedures of production and storage,which tends to result in the changes of their own structures and properties.Therefore,the study on changes of the structure and properties of amino acids after it react with water molecules means a lot for achieving the performance detection of amino acid products.Terahertz Time Domain Spectroscopy(THz-TDS)is a kind of THz spectrum analysis technology based on the technique of ultrafast femtosecond lasers,which has been gradually applied in the fields of chemical analysis,environmental monitoring,physics,astronomy,safety inspection and biological materials in recent years.Compared with the traditional technique of infrared spectrum,THz spectroscopy has stronger sensitivity to the molecular skeleton vibration and weak intermolecular forces such as hydrogen bonding and van der Waals force,especially for most polar molecules(such as ammonia molecules,Water molecules,etc.),which will exhibit more intense absorption characteristics.Based on this,four kinds of amino acids(glycine,alanine,serine,tyrosine),which exist most in silk fibroin,were selected for the detection of THz spectroscopy,and the interaction mechanism between four kinds of amino acids mentioned above and different forms of water molecules(crystal water,film water,hydration water)were analyzed by the technique of THz spectroscopy.The main research work of the thesis:(1)Four anhydrous amino acids and their corresponding amino acid hydrates containing crystal water were detected by THz-TDS system.The vibration mode of anhydrous amino acids was classified utilizing the software of Gaussian,and based on the differences in absorption peaks between anhydrous amino acids and corresponding hydrated amino acids,the THz absorption mode of the corresponding amino acid hydrate has been identified.The experimental results show that there are significant differences of the THz absorption characteristics between four anhydrous amino acids and corresponding amino acid hydrates.It is proved that the identification of the same anhydrous amino acid and corresponding amino acid hydrates can be achieved by THz spectroscopy.The vibration absorption of anhydrous amino acids in the THz band mainly comes from the collective vibration and intermolecular vibration of amino acid molecules,and the vibration absorption of hydrated amino acids mainly comes from the hydrogen bond vibration between crystal water and amino acids.The order of the change of the absorption peaks after four amino acids combined with the crystal water is serine>glycine>alanine>tyrosine.This result above is just contrary to the order of the hydrophobicity index of the four amino acids,which indicates that the amino acid with stronger hydrophobicity contain less the crystallization water content in its hydrate.(2)The interaction between four amino acids and film water was studied based on the THz-TDS system.The process of hydrating and dehydrating film water of four kinds of amino acids was monitored and measured in real time,and according to the condition of change of absorption peaks in the process above,the vibration modes of different contents of membrane water have already been assigned.The experimental results show that the THz spectrum is extremely sensitive to the hydration and dehydration process of the film water on the surface of amino acid,which can be used for the quantitative analysis of the content of film water on the surface of amino acid.The order of the hydration and dehydration rate of the four amino acids for the membrane water is glycine > serine > alanine > tyrosine respectively,and this sorting was generally related to the hydrophobicity index and relative specific surface area of amino acids.The amino acid with weaker hydrophobicity and larger specific surface area has stronger affinity for film water.(3)The rules of interaction between four amino acids,which is in solution state,and hydration water was studied by THz-TDS.The effects of different amino acid types and different pH solutions on the interaction between amino acids and hydration water were analyzed.The THz spectral absorption characteristics of the four amino acid solutions under two systems(pure water,pH 7.2 phosphate buffer)were tested.The results show that the THz absorption spectra of the four amino acid solutions in the two solution systems both don't have obvious characteristic absorption peaks,and the absorption coefficients increase linearly with the increase of frequency.In the buffer system,the absorbance of THz of the four amino acid solutions decreased linearly with the increase of concentration,the order of reduction rates of four amino acid solutions mentioned above is tyrosine > alanine > glycine > serine,which is consistent with the order of hydrophobicity index of four amino acids.In the pure water system,the THz absorbance of the tyrosine solution still decreases linearly with the increase of concentration.However,the THz absorbance of glycine,serine and alanine changes nonlinearly with the increase of concentration,which is mainly result from the changes of the action between amino acid and hydration water caused by the variation of acidbase environment of pure aqueous solution with the addition of three amino acids mentioned above.In general,both the type of amino acid and the pH environment of the solution affect the interaction pattern between amino acid and hydration water,and the changes of this interaction pattern can be detected by the THz spectrum.