Transformations Of L-arginine Molecular Conformation In Phosphoric Acid Aqueous Solution And Preparation Of Novel Crystal

As an outstanding nonlinear optical (NLO) crystal, L-arginine phosphate monohydrate crystal (LAP) is initially created and developed by Shandong University. This crystal has been awarded "1st Award of National Technology Invention" in1988because of its excellent properties:A. Wonderful transparency property in ultraviolet (UV) band; B. High nonlinear optical efficiency, which is2-3.5times higher compared with KDP crystal; C. High conversion rate, about90%, especially for the deuterated LAP crystal (DLAP).In addition, researchers have found several abnormal phenomenons when LAP crystal is reacted with different forms of energy, such as laser, heat and magnetic field. Firstly, LAP crystal has extremely high laser damage threshold. When radiated with a beam of laser with wavelength1053nm and pulse width1ns, LAP crystal can reach as high as63GW/cm2without getting damaged. Secondly, scientists have discovered that LAP crystal has low SBS threshold and high SBS reflection, which is much higher compared with fused silica. Finally, molecules with special conformations enable LAP crystal a pyroelectrical property. These issues still remain enigmatic while the solution of which will has significant meanings. Via novel characterization methods and analysis of crystalline structure of LAP, we believe these special phenomenons of LAP crystal may have relations with its molecular conformations and inter-molecular forces.Recently we have found that Phosphate Arginine (PA), which has similar chemical composition with LAP crystal and serves as a key biological substance in energy storage and conversion, is widely existed in the body of invertebrate animals. Biological scientific research has shown that, after combining biological energy, L-arginine and phosphate acid can react and transfer to be PA. In this process, large amount of energy is stored in a special phosphate-guanidine group interaction (the so-called "high-energy phosphate bond"). In another aspect, acting as the "phosphate source" and "energy source", PA can provide phosphate group and energy for other biological functions. During the energy storage and conversion process of PA, transformation of molecular conformation of L-arginine as well as the interaction between phosphate and guanidine group has important functions. Noticing that PA and LAP crystal have similar chemical composition, we believe that the reaction between LAP crystal and high energy may has something in common with the energy conversion process of PA, such as that the molecular conformation of L-arginine may get changed when LAP crystal reacts with laser, heat etc. To test this hypothesis, we have conducted the following experiments:1. Circular Dichorism (CD) spectrum and specific rotation test on the aqueous solution of L-arginine and phosphate acid with temperature rises. Tests show that:A. L-arginine has a positive "Cotton" effect with the peak locating at207nm. B. When temperature rises, the intensity of the CD spectrum as well as the value of specific rotation of the aqueous solution decrease.2. Molecular simulation on the chirality property of L-arginine based on Molecular Mechanics and ab inito. In this part, we have calculated the CD spectrum and the specific rotations of L-arginine aqueous solution with software including Gaussian and Hyperchem. Simulation shows that:A. the CD spectrum of L-arginine aqueous solution has one peak locating at around212nm. B. With temperature rising, the intensity of CD spectrum and specific rotations decrease, which is the same with experimental results. Calculation shows that the molecular conformation of L-arginine changes beween "extended way" and "partly folded way".3. Investigation on the molecular interaction in LAP crystal employing two-dimensional infrared spectrum. Research focus is located at the guanidine and phosphate acid group. Correspondingly, the wavenumber should be1700cm-1～1000cm-1and1400cm-1～600cm-1, respectively. Research shows that with temperature rising, phosphate group changes intensively while no obvious evidence observed for the interaction between guanidine group and phosphate acid.4. Investigation of possible molecular interaction between L-arginine and phosphate acid in aqueous solution utilizing ultraviolet spectrum and molecular simulation. Research focus locates between220nm and300nm. Research shows that the waveband of220-300nm can be divided into three different parts while different phenomenon occur in each band. The assignments of the absorptions in these three bands are further researched.5. Discovery of a new NLO crystal——L-arginine4-nitrophenolate4-nitrophenol dehydrate crystal (LAPP). The optimal growth condition of the crystal is investigated. XRD is used to characterize the crystalline structure and shows that the crystal belongs to the monoclinic crystallographic system with space group P21, a=7.8669(5) A, b=10.3764(2) A, c=13.8301(2) A. The thermal stability, IR and UV/Vis/NIR spectrum are tested, which show that the cutting wavelength of LAPP crystal is around500nm. NLO characterization shows that LAPP crystal has a similar NLO efficiency with urea and is phase matchable.