Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Optical Rotation”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,297 records · Page 72Linked to original sources
PubMed1980

Optical activity and evolution.

It is noted that the chemical reactions occurring in rarefied cosmic clouds (molecular concentration less than or approximately to 10(2) cm-3) differ from similar laboratory reactions by the much greater effect on the outcome of external force fields. In this light it is hypothesized that the synthesis of optically active substances may occur in the outer space under the conjoint stereospecific effect of a magnetic and other molecule-orienting field. It is further conjectured that the optically active substances of the Solar System had been produced in the course of its formation out of the primal rarefield cloud.

Biological Evolution↗
PubMed1980

Experiments on the amplification of optical activity.

By way of investigating possible mechanisms for the abiotic amplification of small enantiomeric excesses (e.e.'s) in almost racemic mixtures of amino acid enantiomers, we have undertaken a quantitative study of the base-initiated partial polymerization of leucine and valine N-carboxy-anhydride (NCA) enantiomer mixtures containing known excesses of both the R- and S-forms. Polymerization to the extent of ca. 50% of leucine NCA having an 8-70% e.e. of either the R- or S-enantiomer led to an e.e. enhancement in the polymer, which contained a 12-84% e.e. of that enantiomer which predominated in the original monomer NCA. A corresponding decrease in the e.e. of the initially predominant enantiomer was noted in the unpolymerized residue from each reaction. Polymerization to the extent of 25-50% of mixtures of valine NCA enantiomers containing a 12-13% e.e. of either R- or S-isomer led to polymers showing a 7-8% decrease in the e.e. of the initially predominant enantiomer, and to an increase of its e.e. in the unpolymerized residue. these divergent results, the latter of which is quite novel, are compared with earlier qualitative results in the literature and are discussed briefly from the viewpoint of both mechanism and the amplification of optical activity.

Amino Acids↗
PubMed1983

Possible mechanism for origin of chiral specificity during origins of life.

We have earlier (Origins of Life 10 (1980), 15-30) proposed a conformational theory for the origin of nucleic acid-directed adaptor-mediated ordered and proliferative synthesis of proteins and hence origin of life. Conjunction of L-amino acids and beta-D-ribonucleotides emerges as a natural consequence of a template fitting interaction in this theory of the origin of the genetic decoding apparatus. Here we propose an interesting new concept for the origin of chiral specificity, by showing that two autonomously developing systems of protein-synthesizing machinery, one manufacturing L-peptides (L-system) and the other, D-peptides (D-system) could have arisen and during early stages of evolution L-system could have developed a killer enzyme to destroy the D-system, causing the presently existing chiral specificity in all the evolved organisms on Earth. It would be interesting to look for such 'killer enzymes' in the present-day organisms. Of course, the existence of D-amino acid-containing antibiotics gives some credence to this theory.

Genetic Code↗
PubMed1975

Life and chirality beyond the earth.

An attempt is made to show that the phenomenon of chirality- of which optical activity is but one consequence- is by no means restricted to life on Earth, but is common throughout the universe. Several independent sources have been investigated including: statistical fluctuations; stereoselective physical factors; and energetic differences between enantiomeric molecules. It is emphasised that a search for chirality as an indicator for life elsewhere in space provides an excellent tool for the fascinating question of exobiology. Still one must be aware of the limitations of the experimental methods and their interpretations.

Extraterrestrial Environment↗
PubMed1984

Experimental evidence for beta-decay as a source of chirality by enantiomer analysis.

Earlier experiments testing the Vester-Ulbricht beta-decay hypothesis for the origin of molecular chirality are reviewed, followed by descriptions of our own experiments involving attempted asymmetric radiolysis of DL-amino acids using quantitative gas chromotography as a probe for optical activity. Our radiation sources included 90Sr-90Y, 14C and 32P Bremsstrahlen, longitudinally polarized electrons from a linear accelerator and longitudinally polarized protons from a cyclotron. With the possible exception of the linear accelerator irradiations, these experiments failed to produce g.c.-detectable enantiomeric excesses, even at 50-70% gross radiolysis. We thus find no unambiguous support for the Vester-Ulbricht hypothesis in any of the attempted asymmetric radiolyses performed to date. Radioracemization, a possible reason for these failures, is discussed.

Amino Acids↗
PubMed1994

A histochemical approach to the knowledge about the neuron nucleus: the "pre-alarm chromatin".

Chromatin as a functional whole. Since the nineteen-fifties (1,2), studies on the histochemistry of the nucleus have been based on its concept as a whole: measurement of the DNA content, and the ratio between nucleus size and cell size appeared to be (and were in effect) an indication of the functional status of the single cell and of the cell population. Two decades later, the already well-known morphological distinction between the chromatins as euchromatin and heterochromatin was reinterpreted on the basis of the degree of spiralization of the nucleosomal fiber and its complexity (3). Subsequently, considerable information about the non-random interphasic position of the chromosomal domains in the nucleus was obtained by in situ hybridization, and the successive reconstruction of their location in the nucleus by image processing with Normarski optics and rotating stage or by confocal microscopy (4-8). Moreover, immunological studies using monoclonal antibodies raised against the splicing factors acting on nuclear pre-mRNAs in discrete nuclear regions (spliceosomes) (9,10), lent support to the notion that the chromatin machinery operates as a whole.

Animals↗
PubMed1989

Synthesis of mono- and disaccharide amino-acid derivatives for use in solid phase peptide synthesis.

N-Fluorenylmethyloxycarbonyl-protected serine and threonine derivatives, carrying O-glycosidically alpha- or beta-linked peracetylated beta-D-Galp-(1-3)-D-GalNAcp carbohydrate chains, were prepared. These derivatives are intended for use in solid phase glycopeptide synthesis. Suitably protected mono- and disaccharide thioglycosides were used as carbohydrate intermediates. These were activated by treatment with bromine to give the glycosyl bromides, which were then used in silver triflate-promoted glycosidations of N-fluorenylmethyloxycarbonyl amino-acid phenacyl esters. Removal of the phenacyl esters with zinc gave the target free acids.

Amino Acids↗
PubMedDate not supplied

A technique for the determination of enantiomeric amino acids in biological samples.

Racemic amino acids can be separated into their enantiomers by means of gas-liquid chromatography. The most applied technique, today, is the conversion of chiral compunds into diastereoisomers with optically active reagents and subsequent chromatography on conventional optically inactive stationary phases. In previous studies it has been realized that this technique is associated with various problems. We studied the use of optically active stationary phases for separating enantiomers directly via a diastereoisomeric association complex. The optically acitve stationary phases employed are N- and C-terminal substituted dipeptides of the type N-trifluoroacetyl-dipeptide-cyclohexyl esters and have been synthesised by the I-hydroxibenztriazole dicyclohexylcarbodiimide method. The quality of these phases with respect to separation factors, resolution factors, and thermodynamical properties have been evaluated. All synthetic phases show excellent properties; however, when attempting separation of mixtures of naturally occurring amino acids extensive overlap in the elution diagram was detected. Only one phase - N-TFA-L-chi-amino-n-butyryl-L-chi-amino butyric acid cyclohexyl ester - gave complete resolution of the naturally occurring amino acids alanine, valine, glycine, threonine, leucine, isoleucine, serine and proline on a 400 ft x 0.02 in capillary column. Less volatile amino acids such as aspartic acid, phenylalanine, methionine, glutamic acid, tyrosine, arginine, and tryptophan can be resolved at a 100 ft x 0.02 in column.

Amino Acids↗
PubMedDate not supplied

Evolution in bioids: hypercompetitivity as a source of bistability and a possible role of metal complexes as prenucleoprotic mediators of molecular asymmetry.

Spontaneous production of optically active compounds can occur through kinetic instability of an asymmetric steady state in open systems, in which two enantiomeric autocatalysts compete for a common prochiral substrate in a stereoselective reaction of order n greater than 2. For the case of n=3, a proof of instability of a symmetric reacting state in the general case, and functions of reaction parameters ('Chemical Reynolds Numbers') governing the existence and stability of 7 different steady states are derived. The 'extinct state' (without autocatalyst) is stable; a finite amount of products is required to shift it into one of the reacting steady states. A mutation from one state into another in such system ('bioids') involves an amplification of different 'kinds of information', as 'stochastic' (noise into dissipative structures), 'molecular' (autocatalysts), and 'stoichimetric' information. Stereospecific third order kinetics are believed to be realizable on octahedral metal complexes with two-dentated ligands and to have played a role in the prebiological evolution of optically active compounds.

Biological Evolution↗
PubMed1995

Can biological homochirality result from a phase transition?

The problem of chiral purity in living organisms is still one of the prominent difficulties in the study of the origins of life. In particular the parity non-conservation known to occur in weak interactions could not be related to this lack of symmetry: these physical forces, though universal, are very weak and up to now no amplification process had been proposed. In 1991, A. Salam remarked that, due to the attractive character of the parity violating force in electro-weak interactions, a phase transition at low temperature should exist, leading eventually to enantiomeric purity. We undertook then a series of experimental tests, looking for a sizeable change in the optical activity of cystine molecules. We found no evidence for the phase transition down to 0.01 K. The interpretation of these negative results will be discussed, and future experiments proposed.

Amino Acids↗
PubMed1995

Enantioselective autocatalysis. II. Racemization studies on a 1,4-benzodiazepinooxazole derivative.

Since the bromofluoro-1,4-benzodiazepinooxazole derivative I may undergo spontaneous resolution on crystallization from methanol solution, it provides an ideal substrate on which to study the phenomenon of enantioselective autocatalysis involving spontaneous resolution under racemizing conditions (SRURC). In order to augment the understanding of factors which might be important in such a process we have now undertaken to study the racemization of I in more detail by observing its rate under a wide variety of experimental conditions.

Acetic Acid↗
PubMed1975

Crystallization and solid-state reaction as a route to asymmetric synthesis from achiral starting materials.

Many molecules which are achiral can crystallize in chiral (enantiomorphic) crystals and, under suitable conditions, crystals of only one chirality may be obtained. The formation of right- or left-handed crystals in excess is equally probable. Lattice-controlled (topochemical) photochemical or thermal solid-state reactions may then afford stable, optically active products. In the presence of the chiral products, achiral reactants may preferentially produce crystals of one chirality, leading to a feedback mechanism for the generation and amplification of optical activity. Amplification of optical activity can also be achieved by solid-state reactions. The optical synthesis of biologically relevant compounds by such routes may be envisaged.

Aminobutyrates↗
PubMed1975

Five simultaneous steady states in a flipping two-compartment-system with optical antipodes.

The previously given systems-theoretic model for the synthesis of optical antipodes (AD and AL) in strongly asymmetric yield, which shows mono-bistable behaviour depending on the degree of "openess" of the chemical reaction system is reconsidered for two equal compartments (subscripts 1 and 2 on A) with coupling by diffusion. In this configuration three threshold values, j1, is less than j2 is less than j3, for the influx j of the common precursor substance appear. For j is less than j1 only one steady state (s.s.) with no optical activity (ADi = ALi, i-1;2) and equal distribution of the antipodes in both compartments (AD1 = AD2, AL1 = AL2) exists. For j is greater than j 1, this totally symmetric s.s. becomes unstable and a pair of s.s. with optical activity (AD1 is less than AL1, AD2 is less than AL2 or AD1 is greater than AL1, AD2 is greater than AL2) but no spatial asymmetry emerges (parallel flipping), i.e. both compartments be8have as a whole, showing a preponderance of either the D- or the L-form. For j is greater than j2 in addition two new s.s. are possible with antiparallel flipping (AD1 is less than AL1, AD2 is greater than AL2 or AD1 is greater than AL1, AD2 is less than AL2), i.e. in one compartment the D-form has the majority and in the other one the L-form, but these are stable only beyond a third threshold value j3. A third thinkable pair with no optical activity, but different sum concentrations in both cells, does not exist in this special circuitry, but can be obtained in a slightly changed arrangement. So for j is greater than j2, 5 different (4 stable, 1 unstable) s.s., exist for the same set of parameters, one of which is chosen by the system.

Catalysis↗