Identification of a compound in Chamaecyparis taiwanensis inhibiting the ice-nucleating activity of Pseudomonas fluorescens KUIN-1

Anti-Ice Nucleation Activities of Adenine and Poly-A Nucleotides

　Most of the ice nucleation activity inhibitor reported so far are compounds processing the hydroxyl group such as the polyphenolic derivative. After examining the anti-ice nucleation activity of the purine base, the highest compound is theophylline, and the activity showed 3.80±0.32℃ at a final concentration of 0.1 mg/ml. We found that the activity of the adenine which was essential to genome information DNA was higher than that of guanine. After examining effect of adenine concentration, high activity showed 9.1±1.2℃ and became approximately constant above 0.1 mg/ml. This active rise is a result of effect of concentration under alkaline condition. Therefore after examining effect of pH on the activity of adenine, this activity rose under an alkaline condition. The active rise predicts that an electric charge of adenine is a factor. Among four kinds of nucleotide of 6 bases, poly-A nucleotide was higher and showed 1.33±0.42℃ at a final concentration of 0.1 mg/ml. This activity of poly-A were proportional to the number of the base. From these results, it was suggested that the poly-A and adenine could be able to be applied to the field to preserve the blood and tissue which differentiated in the generative medicine.

Supercooling-Promoting (Anti-ice Nucleation) Substances

Studies on supercooling-promoting substances (SCPSs) are reviewed introducing name of chemicals, experimental conditions and the supercooling capability (SCC) in all, so far recognized, reported SCPSs and results of our original study are presented in order to totally show the functional properties of SCPSs which are known in the present state. Many kinds of substances have been identified as SCPSs that promote supercooling of aqueous solutions in a non-colligative manner by reducing the ice nucleation capability (INC) of ice nucleators (INs). The SCC as revealed by reduction of freezing temperature (°C) by SCPSs differs greatly depending on the INs. While no single SCPS that affects homogeneous ice nucleation to reduce ice nucleation point has been found, many SCPSs have been found to reduce freezing temperatures by heterogeneous ice nucleation with a large fluctuation of SCC depending on the kind of heterogeneous IN. Not only SCPSs increase the degree of SCC (°C), but also some SCPSs have additional SCC to stabilize a supercooling state for a long term to stabilize supercooling against strong mechanical disturbance and to reduce sublimation of ice crystals. The mechanisms underlying the diverse functions of SCPSs remain to be determined in future studies.

Characterization of Anti-Ice Nucleation Activity of the Extract from Coffee Refuse

　The supercooling-facilitating (SCF) activities, that is, the anti-ice nucleation activity of the hot water extracts from five types of processed food refuse was examined. The extract with the highest activity among five hot water extracts was coffee refuse, showing 1.50℃ of SCF activity at a final concentration of 0.1 mg/ml. From the hot water extract of coffee refuse, the coffee refuse extract containing various polyphenols was prepared by the ultrafiltration (less than MWCO 10,000), a solvent fractionation of ethyl acetate. The yield of coffee refuse extract was 0.9% (w/w) from dried coffee refuse. The SCF activity of the coffee refuse extract at a final concentration of 1.0 mg/ml was 4.2℃. HPLC analysis of the coffee refuse extract showed that caffeine and chlorogenic acid, which are major components of coffee, could be found at 173 and 62.3 µg/ml, respectively. However, the SCF activities of both compounds (0.70 and 1.06℃) at a final concentration of 0.1 mg/ml were lower than those of ferulic acid and coumaric acid, respectively at 3.40 and 2.35℃. This is the first report to our knowledge on the SCF activity of caffeine. The SCF activity of caffeine at a final concentration of 1.0 mg/ml was 2.3℃. The specificity of caffeine against various ice nuclei containing calcium oxalate, 9-fluorenon, and ice nucleating bacteria was examined. Caffeine at a final concentration of 1.0 mg/ml could inhibit the ice nucleation activity of calcium oxalate, and Pseudomonas fluorescens KUIN-1 at the same level that of as silver iodide. From these results, it was suggested that the extract could be able to be applied to the field to control the frost damage of the vegetables and that the harvested vegetables might be stored unfrozen even at 0℃ or less.

Screening of plant resources with anti-ice nucleation activity for frost damage prevention

Previous studies have shown that some polyphenols have anti-ice nucleation activity (anti-INA) against ice-nucleating bacteria that contribute to frost damage. In the present study, leaf disk freezing assay, a test of in vitro application to plant leaves, was performed for the screening of anti-INA, which inhibits the ice nucleation activity of an ice-nucleating bacterium Erwinia ananas in water droplets on the leaf surfaces. The application of polyphenols with anti-INA, kaempferol 7-O-β-glucoside and (-)-epigallocatechin gallate, to the leaf disk freezing assay by cooling at -4--6 °C for 3 h, revealed that both the compounds showed anti-INAs against E. ananas in water droplets on the leaf surfaces. Further, this assay also revealed that the extracts of five plant leaves showed high anti-INA against E. ananas in water droplets on leaf surfaces, indicating that they are the candidate resources to protect crops from frost damage.

Analysis of supercooling activities of surfactants

Supercooling-promoting activities (SCAs) of 25 kinds of surfactants including non-ionic, anionic, cationic and amphoteric types were examined in solutions (buffered Milli-Q water, BMQW) containing the ice nucleation bacterium (INB) Erwinia ananas, silver iodide (AgI) or BMQW alone, which unintentionally contained unidentified ice nucleators, by a droplet freezing assay. Most of the surfactants exhibited SCA in solutions containing AgI but not in solutions containing the INB E. ananas or BMQW alone. SCAs of many surfactants in solutions containing AgI were very high compared with those of previously reported supercooling-promoting substances. Cationic surfactants, hexadecyltrimethylammonium bromide (C16TAB) and hexadecyltrimethylammonium chloride (C16TAC), at concentrations of 0.01% (w/v) exhibited SCA of 11.8 °C, which is the highest SCA so far reported. These surfactants also showed high SCAs at very low concentrations in solutions containing AgI. C16TAB exhibited SCA of 5.7 °C at a concentration of 0.0005% (w/v).

Analysis of supercooling activity of tannin-related polyphenols

Based on the discovery of novel supercooling-promoting hydrolyzable gallotannins from deep supercooling xylem parenchyma cells (XPCs) in Katsura tree (see Wang et al. (2012) [38]), supercooling capability of a wide variety of tannin-related polyphenols (TRPs) was examined in order to find more effective supercooling-promoting substances for their applications. The TRPs examined were single compounds including six kinds of hydrolyzable tannins, 11 kinds of catechin derivatives, two kinds of structural analogs of catechin and six kinds of phenolcarboxylic acid derivatives, 11 kinds of polyphenol mixtures and five kinds of crude plant tannin extracts. The effects of these TRPs on freezing were examined by droplet freezing assays using various solutions containing different kinds of identified ice nucleators such as the ice nucleation bacterium (INB) Erwinia ananas, the INB Xanthomonas campestris, silver iodide and phloroglucinol as well as a solution containing only unintentionally included unidentified airborne ice nucleators. Among the 41 kinds of TRPs examined, all of the hydrolyzable tannins, catechin derivatives, polyphenol mixtures and crude plant tannin extracts as well as a few structural analogs of catechin and phenolcarboxylic acid derivatives exhibited supercooling-promoting activity (SCA) with significant differences (p>0.05) from at least one of the solutions containing different kinds of ice nucleators. It should be noted that there were no TRPs exhibiting ice nucleation-enhancing activity (INA) in all solutions containing identified ice nucleators, whereas there were many TRPs exhibiting INA with significant differences in solutions containing unidentified ice nucleators alone. An emulsion freezing assay confirmed that these TRPs did not essentially affect homogeneous ice nucleation temperatures. It is thought that not only SCA but also INA in the TRPs are produced by interactions with heterogeneous ice nucleators, not by direct interaction with water molecules. In the present study, several TRPs that might be useful for applications due to their high SCA in many solutions were identified.

Freezing activities of flavonoids in solutions containing different ice nucleators

In this study, we examined the effects on freezing of 26 kinds of flavonoid compounds, which were randomly selected as compounds with structures similar to those of flavonoid compounds existing in deep supercooling xylem parenchyma cells (XPCs) in trees, in solutions containing different kinds of ice nucleators, including the ice nucleation bacterium (INB) Erwinia ananas, INB Xanthomonas campestris, silver iodide, phloroglucinol and unidentified airborne impurities in buffered Milli-Q water (BMQW). Cumulative freezing spectra were obtained in each solution by cooling 2 μL droplets at 0.2 °C/min by a droplet freezing assay. Freezing temperature of 50% droplets (FT(50)) was obtained from each spectra in a separate analysis with more than 20 droplets and mean FT(50) were obtained from more than five separate analyses using more than 100 droplets in total in each flavonoid. Supercooling-promoting activities (SCA) or ice nucleation-enhancing activities (INA) of these flavonoids were determined by the difference in FT(50) between control solutions without flavonoids and experimental solutions with flavonoids. In mean values, most of the compounds examined exhibited SCA in solutions containing the INB E. ananas, INB X. campestris, silver iodide, and phloroglucinol although the magnitudes of their activities were different depending on the ice nucleator. In solutions containing the INB E. ananas, 10 compounds exhibited SCAs with significant differences (p<0.05) in the range of 1.4-4.2 °C. In solutions containing silver iodide, 23 compounds exhibited SCAs with significant differences in the range of 2.0-7.1 °C. In solutions containing phloroglucinol, six compounds exhibited SCAs with significant differences in the range of 2.4-3.5 °C. In solutions containing the INB X. campestris, only three compounds exhibited SCAs with significant differences in the range of 0.9-2.3 °C. In solutions containing unidentified airborne impurities (BMQW alone), on the other hand, many compounds exhibited INA rather than SCA. In mean values, only four compounds exhibited SCAs in the range of 2.4-3.2 °C (no compounds with significant difference at p<0.05), whereas 21 compounds exhibited INAs in the range of 0.1-12.3 °C (eight compounds with significant difference). It was also shown by an emulsion freezing assay that most flavonoid glycosides examined did not affect homogeneous ice nucleation temperatures, except for a few compounds that become ice nucleators in BMQW alone. These results suggest that most flavonoid compounds affect freezing temperatures by interaction with unidentified ice nucleators in BMQW as examined by a droplet freezing assay. The results of our previous and present studies indicate that flavonoid compounds have very complex effects to regulate freezing of water.

Ice nucleation in emulsified aqueous solutions of antifreeze protein type III and poly(vinyl alcohol)

Antifreeze protein (AFP) III and poly(vinyl alcohol) (PVA) are known as anti-ice nucleating agents (anti-INAs), which inhibit heterogeneous ice nucleation. However, the effectiveness of these anti-INAs in inhibiting ice nucleation in water-in-oil (W/O) emulsions, in which homogeneous ice nucleation can be experimentally simulated, is unclear. In this study, the ice nucleation temperature in emulsified solutions of AFP III, PVA, and other nonanti-INA polymers was measured, and then the nucleation rate was analyzed based on classical nucleation theory. Results showed that ice nucleation was surface-initiated and, except for PVA solutions, probably caused heterogeneously by the emulsifier, SPAN 65, at the droplet surfaces. In this nucleation mode, AFP III had no significant effect on the ice nucleation rate. In contrast, PVA exhibited ice-nucleating activity only at the droplet surfaces, suggesting that the nucleation is due to the interaction between PVA and SPAN 65.

Deep supercooling xylem parenchyma cells of katsura tree (Cercidiphyllum japonicum) contain flavonol glycosides exhibiting high anti-ice nucleation activity

Xylem parenchyma cells (XPCs) of boreal hardwood species adapt to sub-freezing temperatures by deep supercooling to maintain a liquid state of intracellular water near -40 degrees C. Our previous study found that crude xylem extracts from such tree species exhibited anti-ice nucleation activity to promote supercooling of water. In the present study, thus, we attempted to identify the causative substances of supercooling. Crude xylem extracts from katsura tree (Cercidiphyllum japonicum), of which XPCs exhibited deep supercooling to -40 degrees C, were prepared by methanol extraction. The crude extracts were purified by liquid-liquid extraction and then by silica gel column chromatography. Although all the fractions obtained after each purification step exhibited some levels of anti-ice nucleation activity, only the most active fraction was retained to proceed to the subsequent level of purification. High-performance liquid chromatography (HPLC) analysis of a fraction with the highest level of activity revealed four peaks with high levels of anti-ice nucleation activity in the range of 2.8-9.0 degrees C. Ultraviolet (UV), mass and nuclear magnetic resonance (NMR) spectra revealed that these four peaks corresponded to quercetin-3-O-beta-glucoside (Q3G), kaempferol-7-O-beta-glucoside (K7G), 8-methoxykaempferol-3-O-beta-glucoside (8MK3G) and kaempferol-3-O-beta-glucoside (K3G). Microscopic observations confirmed the presence of flavonoids in cytoplasms of XPCs. These results suggest that diverse kinds of anti-ice nucleation substances, including flavonol glycosides, may have important roles in deep supercooling of XPCs.

The structures and functions of ice crystal-controlling proteins from bacteria

Many organisms have evolved into unique mechanisms which minimize freezing injury due to extracellular ice formation. Specifically, certain bacteria have produced a few proteins each with different functions. For example, the ice nucleation protein acts as a template for ice formation, which is responsible for imparting ice nucleating activity. The anti-nucleating protein inhibits the fluctuation of ice nucleus formation by a foreign particle in the water drop. Also, the antifreeze proteins depress the freezing temperature, modify or suppress ice crystal growth, inhibit ice recrystallization, and protect the cell membrane from cold-induced damage. In this article, a review on the current knowledge of the structure and the function of these three types of proteins, which are capable of interacting with ice itself or its nuclei from bacteria.

Inhibition of bacterial ice nucleation by polyglycerol polymers

The simple linear polymer polyglycerol (PGL) was found to apparently bind and inhibit the ice nucleating activity of proteins from the ice nucleating bacterium Pseudomonas syringae. PGL of molecular mass 750 Da was added to a solution consisting of 1 ppm freeze-dried P. syringae 31A in water. Differential ice nucleator spectra were determined by measuring the distribution of freezing temperatures in a population of 98 drops of 1 microL volume. The mean freezing temperature was lowered from -6.8 degrees C (control) to -8.0,-9.4,-12.5, and -13.4 degrees C for 0.001, 0.01, 0.1, and 1% w/w PGL concentrations, respectively (SE < 0.2 degrees C). PGL was found to be an ineffective inhibitor of seven defined organic ice nucleating agents, whereas the general ice nucleation inhibitor polyvinyl alcohol (PVA) was found to be effective against five of the seven. The activity of PGL therefore seems to be specific against bacterial ice nucleating protein. PGL alone was an ineffective inhibitor of ice nucleation in small volumes of environmental or laboratory water samples, suggesting that the numerical majority of ice nucleating contaminants in nature may be of nonbacterial origin. However, PGL was more effective than PVA at suppressing initial ice nucleation events in large volumes, suggesting a ubiquitous sparse background of bacterial ice nucleating proteins with high nucleation efficiency. The combination of PGL and PVA was particularly effective for reducing ice formation in solutions used for cryopreservation by vitrification.