J. I. Zink, G. E. Hardy, and J. E. Sutton
240
Triboluminescence of Sugars
Jeffrey 1. Zink,*la Gordon E. Hardy, and James E. Sutton Department of Chemistry,’ University of California, Los Angeles, California 90024 (Received July 14, 1975)
The triboluminescence of a variety of mono- and disaccharides including sucrose is luminescence from mo-. lecular nitrogen. The absence of triboluminescence from some saccharides is not a function of crystal size. The diminished intensity of triboluminescence excited in crystals under denitrogenated liquids in which they are insoluble is discussed.
The earliest record of triboluminescences (TL), the luminescence caused by the application of mechanical stress to crystals, is contained in the writings of Bacon2 who observed that lumps of sugar emitted light when scraped. The triboluminescence of sugar was known to many other early writers including Boyle who observed that “hard sugar being nimbly scraped with a knife would afford a sparkling light”.2 He also observed that the luminescence still occurred when the sugar was scraped under vacuum. In spite of the long history of the T L of sugar, the origin of the luminescence has never been determined. For organic crystals and some inorganic salts, the most common origins are crystal fluorescence3 or phosphore~cence.~-~ The previously postulated* gas discharge origin is rarer. In some cases, room temperature TL can occur from crystals which are not photoluminescent a t that temperature.j We have spectroscopically investigated the T L of mono- and oligosaccharides. We report here spectroscopic proof that the T L of saccharides originates from gas discharge and note general observations regarding the phenomenon. Samples of D-glucose; lactose, maltose, L-rhamnose, and sucrose are triboluminescent while samples of cellobiose, fructose, fucose, galactose, and mannose are not. The T L spectrum of sucrose, a typical member of the triboluminescent sugars, is shown in Figure 1. All T L was excited by grinding the sample in a Pyrex vial with a stainless steel rod. The T L could also be excited by grinding the sample between any hard objects including glass, wood, plastic, and other metals. The spectra were obtained using the method previously reported.6 The spectrum shown in Figure 1 is that of the second positive group of molecular n i t r ~ g e n As . ~ we have previously remarked,s an excitation energy of over 11 eV is necessary to excite Nz from the ground state to the emitting 37ru state. The vibrational progression shown arises from the vibrational levels of the 37rg state of Nz. In order to gain insight into the location of the emitting molecular nitrogen, we have measured the relative intensities of the TL of sucrose crystals in a variety of argon stripped solvents in which the sugar is insoluble. In all cases, the intensity of the TL excited under the liquid decreased compared to that excited in air. Typical decreases ranged from a minimum of 38% in benzene through 68% in chloroform to 100% in nujol. No correlation between the intensity and any bulk property of the liquid was found. The absence of a correlation may occur because the intensity is a sensitive function of the mechanical energy applied to the crystal. The lubrication properties of the solvent and/or the softening of the crystals due to solvent penetration could The Journal of Physical Chemistry, Voi. 80, No. 3, 1976
account for the decreased intensity. A sample of sucrose was vacuum degassed a t 80’ to remove nitrogen adsorbed in the crystals. No TL was observed when this sample was excited under argon stripped benzene. However, TL was observed when the degassed sample was excited in the atmosphere. From these observations, it is reasonable to conclude that ambient nitrogen gas is not necessary for the phenomenon to occur but that adsorbed or absorbed nitrogen is sufficient. The above experiments, combined with the insensitivity of the T L to the nature of the grinding implement, argue against the T L being caused by an electrical discharge between the crystal and the grinding implement. However, the detailed mechanism by which the high energy 3 ~ state , of Ne is populated is not yet known. The T L spectra of all the triboluminescent saccharides studied are similar and are assigned to Nz emission. The relative intensities of the T L from the various saccharides cannot be meaningfully compared because of the differences in the crystal sizes and the possible differences in their mechanical properties. In order to determine whether or not the apparent absence of T L from a given saccharide is caused by crystal size, we have examined both crystals and finely ground powders of representative samples of triboluminescent and nontriboluminescent saccharides. Weak T L could be visually observed in a dark room even from finely ground powders of the triboluminescent sugars,
3i
Sucrose
Wavelength Figure 1. The triboluminescence spectrum of sucrose. The emission is from the second positive group of molecular nitrogen. The error bars represent the standard deviation of at least five separate measurements.
249
Sugar Effects on Amino Acid and Model Peptides
while no T L was observed from fructose, mannose, galactose, fucose, or cellobiose. Thus, the presence or absence of T L is probably a function of the crystal structure or the packing of molecules in a crystal and not solely a function of the size of a crystal. We have observed similar behavior of the T L of N-acetylanthranilic acid and its derivatives. Minor changes of the substituents which barely affect the luminescence properties of the molecule prohibit the TL.1° Work is in progress to determine the structural factors necessary for a crystal to be triboluminescent.
Acknowledgment. The Army Research Office, Durham, is gratefully acknowledged for support of this work.
References and Notes (1) (a) Camille and Henry Dreyfus Teacher-Scholar, 1974-1979. (b) Contribution No. 3494. (2) (a) F. Bacon, “Of the Advancement of Learning”, 1605, G. W. Kitchin, Ed., J. M. Dent and Sons, London, 1915. (b) E. N. Harvey, “A History of Luminescence”, American Philosophical Society, Philadelphia, Pa., 1957, Chapter 10. (3) J. I. Zink and W. Klimt, J. Am. Chem. Soc.,96, 4690 (1974). (4) J. I. Zink and W. C. Kaska, J. Am. Chem. SOC.,95, 7510 (1973). (5) J. I. Zink, J. Am. Chem. SOC.,96, 6775 (1974). (6) J. I. Zink, Inorg. Chem., 14, 555 (1975). (7) C. R. Hurt, N. Mcavoy, S. Bjorklund, and N. Fillipescu, Nature (London), 212, 179 (1966). (8) J. I. Zink, Chem. Phys., Lett., 32, 236 (1975). (9) G. Herzberg, “Molecular Spectra and Molecular Structure. I. Spectra of Diatomic Molecules”, 2nd ed., Van Nostrand, New York, N.Y., 1950, p 32. (IO) W. C. Kaska, G. Hardy, and J. I. Zink, unpublished observatians.
Effects of Sugar Solutions on the Activity Coefficients of Aromatic Amino Acids and Their N-Acetyl Ethyl Esters T. S. Lakshml and P. K. NandP Protein Technology Discipline, Central Food Technological Research Institute, Mysore 5700 13,India (Received May 14, 1975)
Activity coefficients of N-acetyl ethyl esters of phenylalanine, tyrosine, and tryptophan have been determined in aqueous sucrose and glucose solutions. Similar measurements have been carried out with phenylalanine, tyrosine, and tryptophan in sucrose solution. The compounds show increased activity coefficients (decrease in solubility or “sugaring o u t ” ) in the solutions studied. The molar free energy of transfer, U t r , of these compounds from water to sugar solutions has positive values and indicates increased hydrophobicity in sugar solutions. The AFtr value is found to remain unaltered over a considerable range of temperature. These observations have been utilized to explain the thermal stability of protein in aqueous sugar solutions. Prevention of heat coagulation of ovalbumin in sucrose solution was recorded a long time back.l Similar observations of the stability of ovalbumin and serum proteins in sugar solutions have been r e p ~ r t e d . ~Simpson -~ and Kauzmann observed that the extent of ovalbumin denaturation in urea was reduced in presence of s u ~ r o s eGerlsma .~ and Stuur observed that the melting temperature of both ribonuclease and lysozyme increases in sorbitol solution indicating their increased thermal stabilities in the solution.6 Recently we have observed that the heat coagulation of aglobulin, the major protein component of sesame seed ( S e s a m u m indicum L.) proteins, is prevented in sucrose and glucose solutions (unpublished work). The manner in which sugar induces the increased heat stability of proteins or reduces the extent of denaturation by other reagents is not known. Studies during past few years have indicated how the groups which are mainly involved in the denaturation process of a protein molecule would behave in different solvent systems, e.g., simple electrolytes, urea, guanidine hydrochloride, and tetralkylammonium salt solution^.^-^^ This information was obtained from a study of the activity coefficients of model compounds which are representative of different groups in a protein molecule. In our previous studies we chose blocked
amino acid ethyl esters, e.g., CH3CQNHCW(R)CQQC2H5, R being a nonpolar side chain. Here we report measurements of the activity coefficient of the above compounds where R is phenylalanyl, tyrosyl, and tryptophanyl side groups in sucrose and glucose and also aromatic amino acids in sucrose solutions. The phenylalanyl derivative has also been studied in aqueous glycerol, ethylene glycol, and 1-butanol solutions. Temperature effect on the activity coefficient of the compounds has also been determined to obtain a better insight into the mechanism of sugar effects on model compounds and hence protein.
Experimental Section Materials and Methods. N -Acetyl-L-phenylalanine ethyl ester (Sigma Chemical Co.), N-acetyl-L-tyrosine ethyl ester (Grade I, Cyclo Chemical Corp.), N-acetyl-L-tryptophan ethyl ester (Grade I, Cyclo Chemical Corp.), L-phenylalanine (BDH), L-tyrosine (E. Merck), and L-tryptophan (E. Merck) were used. Solubility phase curves for these compounds showed the absence of impurities. Sucrose and glucose were reagent grade chemicals. Fractions of ethylene glycol (E. Merck) and I-butanol (E. Merck) distilling respectively at 115-117 and 194-196O were used; BDH glycerol was used. Glass distilled water was used. The Journal of Physical Chemlstfy, Voi. 80,No. 3, 7976
