An account is presented of the mechanistic aspects of hydrothermal zeolite synthesis. The introduction provides a historical and experimental perspective and is followed by a summary of proposed ...mechanisms and associated modelling studies. The central section of the review contains a description of the most probable mechanistic pathways in zeolite formation. In this, the reaction stages of the induction period, nucleation and crystal growth are examined in chronological sequence. Finally, particular aspects of the synthesis process such as the constitution of growth species, template–framework interactions and the nature of zeolite solubility are treated in more detail.
Emphasis is placed upon the chemical basis of zeolite synthesis. Fundamental to this are the T
O
T bond-making and bond-breaking reactions which establish the equilibration between solid and solution components. The consequent generation of order, driven by energy differences and strongly moderated by kinetic limitations, is essentially one of continuous evolution. However, the discreet step of nucleation provides a discontinuity in which isolated regions of local order are superceded by the establishment of a periodic crystal lattice, capable of propagation. Crystal growth occurs through an in-situ, localised construction process from small, mobile species ordered by the participating cations.
The process of hydrothermal zeolite synthesis can be most adequately explained by a mechanism based upon the solution–mediation model, whether or not there is a visible liquid phase. The common presence of mobile species emphasises the overall similarity of zeolite synthesis reactions so that the need to distinguish any separate “gel rearrangement” or “solid-phase transformation” mechanism becomes unnecessary.
In this survey, an account is given of the main discoveries and advances in thinking in the field of zeolite synthesis from the 1940s up to late 2002. This present survey is principally concerned ...with the pattern of discovery and the consequent progression of ideas.
As root symbionts of cycad trees, cyanobacteria of the genus Nostoc produce β-methylamino-L-alanine (BMAA), a neurotoxic nonprotein amino acid. The biomagnification of BMAA through the Guam ecosystem ...fits a classic triangle of increasing concentrations of toxic compounds up the food chain. However, because BMAA is polar and nonlipophilic, a mechanism for its biomagnification through increasing trophic levels has been unclear. We report that BMAA occurs not only as a free amino acid in the Guam ecosystem but also can be released from a bound form by acid hydrolysis. After first removing free amino acids from tissue samples of various trophic levels (cyanobacteria, root symbioses, cycad seeds, cycad flour, flying foxes eaten by the Chamorro people, and brain tissues of Chamorros who died from amyotrophic lateral sclerosis/Parkinsonism dementia complex), we then hydrolyzed the remaining fraction and found BMAA concentrations increased 10- to 240-fold. This bound form of BMAA may function as an endogenous neurotoxic reservoir, accumulating and being transported between trophic levels and subsequently being released during digestion and protein metabolism. Within brain tissues, the endogenous neurotoxic reservoir can slowly release free BMAA, thereby causing incipient and recurrent neurological damage over years or even decades, which may explain the observed long latency period for neurological disease onset among the Chamorro people. The presence of BMAA in brain tissues from Canadian patients who died of Alzheimer's disease suggests that exposure to cyanobacterial neurotoxins occurs outside of Guam.
We here report biomagnification (the increasing accumulation of bioactive, often deleterious molecules through higher trophic levels of a food chain) of the neurotoxic nonprotein amino acid ...β-methylamino-L-alanine (BMAA) in the Guam ecosystem. Free-living cyanobacteria produce 0.3 µ/g BMAA, but produce 2-37 µg/g as symbionts in the coralloid roots of cycad trees. BMAA is concentrated in the developing reproductive tissues of the cycad Cycas micronesica, averaging 9 µg/g in the fleshy seed sarcotesta and a mean of 1,161 µg/g BMAA in the outermost seed layer. Flying foxes (Pteropus mariannus), which forage on the seeds, accumulate a mean of 3,556 µg/g BMAA. Flying foxes are a prized food item of the indigenous Chamorro people who boil them in coconut cream and eat them whole. Chamorros who die of amyotrophic lateral sclerosis/parkinsonism-dementia complex (ALS-PDC), a neurodegenerative disease with symptoms similar to amyotrophic lateral sclerosis, Parkinson's disease, and Alzheimer's disease, have an average of 6.6 µg/g BMAA in their brain tissues. The biomagnification of BMAA through the Guam ecosystem fits a classic triangle of increasing concentrations of toxic compounds up the food chain. This may explain why the incidence of ALS-PDC among the Chamorro was 50-100 times the incidence of amyotrophic lateral sclerosis elsewhere. Biomagnification of cyanobacterial BMAA may not be unique to Guam; our discovery of BMAA in the brain tissue from Alzheimer's patients from Canada suggests alternative ecological pathways for the bioaccumulation of BMAA in aquatic or terrestrial ecosystems.
Beta-methylamino-L-alanine (BMAA) occurs in higher levels in museum specimens of the Guamanian flying fox than in the cycad seeds the flying foxes feed on, confirming the hypothesis that cycad ...neurotoxins are biomagnified within the Guam ecosystem. Consumption of a single flying fox may have resulted in an equivalent BMAA dose obtained from eating 174 to 1,014 kg of processed cycad flour. Traditional feasting on flying foxes may be related to the prevalence of neuropathologic disease in Guam.
Cyanobacteria produce the neurotoxic amino acid β-N-methylamino-l-alanine (BMAA), which in contaminated marine waters has been found to accumulate in shellfish. Exposure to BMAA has been associated ...with an increased risk of neurodegenerative disease. Analysis of blinded samples found BMAA to be present in neuroproteins of individuals who died from ALS and ALS/PDC, but generally not in the brains of patients who died of causes unrelated to neurodegeneration or Huntington's disease, an autosomal dominant neurodegenerative disease. We here report support for a link between a patient with ALS and chronic exposure to the cyanobacterial neurotoxin BMAA via shellfish consumption. The patient had frequently eaten lobsters collected in Florida Bay for approximately 30 years. LC-MS/MS analysis of two lobsters which this ALS patient had placed in his freezer revealed BMAA at concentrations of 27 and 4 μg/g, respectively, as well as the presence of 2,4-diaminobutyric acid (DAB), a BMAA isomer. Two additional lobsters recently collected from Florida Bay also contained the neurotoxins BMAA and DAB. These data suggest that invertebrates collected in water where cyanobacterial blooms are present, if consumed, may result in direct human exposure to these neurotoxic amino acids. The data support the assertion that prolonged exposure to BMAA may have played a role in the etiology of ALS in this patient.
•β-N-methylamino-l-alanine (BMAA) is present in lobsters from Florida Bay.•A patient caught and ate lobsters regularly for 30 years before being diagnosed with ALS.•Direct human exposure to neurotoxins is possible through shellfish collected in areas where cyanobacterial blooms are present.•Further studies are needed on chronic low-dose exposure to BMAA as prolonged exposure may play a role in ALS etiology.
Cyanobacteria can generate molecules hazardous to human health, but production of the known cyanotoxins is taxonomically sporadic. For example, members of a few genera produce hepatotoxic ...microcystins, whereas production of hepatotoxic nodularins appears to be limited to a single genus. Production of known neurotoxins has also been considered phylogenetically unpredictable. We report here that a single neurotoxin, β-N-methylamino-L-alanine, may be produced by all known groups of cyanobacteria, including cyanobacterial symbionts and free-living cyanobacteria. The ubiquity of cyanobacteria in terrestrial, as well as freshwater, brackish, and marine environments, suggests a potential for wide-spread human exposure.
The Chamorro people of Guam have been afflicted with a complex of neurodegenerative diseases (now known as ALS-PDC) with similarities to ALS, AD, and PD at a far higher rate than other populations ...throughout the world. Chamorro consumption of flying foxes may have generated sufficiently high cumulative doses of plant neurotoxins to result in ALS-PDC neuropathologies, since the flying foxes forage on neurotoxic cycad seeds.
Fanihi – flying foxes (
Pteropus mariannus mariannus,
Pteropodidae) – are a highly salient component of the traditional Chamorro diet. A neurotoxic, non-protein amino acid, β-methylamino-
l-alanine ...(BMAA) accumulates in flying foxes, which forage on the seeds of
Cycas micronesica (Cycadaceae) in Guam's forests. BMAA occurs throughout flying fox tissues both as a free amino acid and in a protein-bound form. It is not destroyed by cooking. Protein-bound BMAA also remains in cycad flour which has been washed and prepared by the Chamorro people as tortillas, dumplings, and thickened soups. Other animals that forage on cycad seeds may also provide BMAA inputs into the traditional Chamorro diet.
We tested the brain tissues of the Chamorro people of Guam who died of amyotrophic lateral sclerosis/Parkinsonism dimentia complex (ALS/PDC) for the neurotoxin β‐methylamino‐l‐alanine (BMAA). We used ...validated high‐pressure liquid chromatography and liquid chromatography‐mass spectrometry analyses to test well‐characterized archival tissues of the superior frontal gyrus from eight Chamorros from Guam and a comparison group of 15 Canadians. BMAA was found as a free amino acid in 83% of Chamorro ALS/PDC patients (3–10 μg/g) as a protein‐associated amino acid in 100% of the Chamorro individuals (149–1190 μg/g). Both forms of BMAA were also found at comparable levels in two Canadians who died of progressive neurodegenerative disease. BMAA, which is produced by cyanobacteria, may be associated with some cases of neurodegenerative disease.
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