Results for “isomeric shift”

NASA NTRS1990

An ionic mechanism of carbon formation in flames

The formation of incipient carbon in flames can be described by a series of elementary reactions in which the precursor of soot is the ion C3H3+, and the major building block is acetylene, polyacetylenes, or other hydrocarbon fragments which are present in large concentrations. The precursor ion is produced by nonequilibrium chemi-ionization reactions, e.g., CH asterisk plus C2H2 yields C3H3+ plus e or CH asterisk plus O yields CHO+ plus e. The CHO+ rapidly becomes C3H3+ through a series of ion-molecule reactions. The chemi-ion then adds acetylene in a series of very rapid ion-molecule reactions producing larger and larger ions. Ions isomerize very rapidly to produce aromatic structures overcoming one of the major problems with neutral species mechanisms: how to form the first carbon ring. As the ions grow, their recombination rate coefficients with electrons increase so that the larger ions are removed by dissociative recombination. These neutral species now continue to grow by the addition of more acetylene producing even larger neutral species. With increasing size, the rate coefficient of electron attachment also increases causing the reaction to become important, depending upon the temperature. Appreciable concentrations of negative ions shift the ion removal process to dissociative ion-ion recombination, which is orders of magnitude slower than ion-electron recombination. As the neutral species continue to grow by the addition of acetylene, they gradually take on the aspect of particles, i.e., the bulk properties of the substance CxHy dominate over the chemical properties. This change is gradual and there is no distinct size at which a large molecule becomes a particle. Evidence will be presented supporting the above mechanism. All of the ions proposed in the mechanism have been observed by mass spectrometric analysis, their concentrations measured, and their rates of reaction in the flame environment determined to be adequate to account for the observed rate of formation of carbon.

Calcote, H. F.↗

NASA NTRS1992

The rhodopsins: structure and function. Introduction

Nature makes use of the propensity of retinal for light-dependent double-bond isomerization in a number of systems and in a variety of ways. The common theme for light receptors based on this kind of chemistry is that (1) the retinal is bound in most cases to a small membrane protein via a protonated lysine-retinal Schiff base, (2) the absorption maximum in the visible is tuned to a suitable wavelength largely by electrostatic interaction with polar protein residues, and (3) the light-induced bond rotations and strains in the retinal set off reaction chains during which at least part of the excess free energy acquired is transferred to the protein and causes pK shifts of acidic residues and/or backbone conformational changes. The physiological consequence of the process initiated by absorption of light is either the activation of an information transfer chain (sensory and visual rhodopsins) or energy transduction which drives the electrogenic movement of ions across the membrane (ion-motive rhodopsins). Rhodopsins with these functions occur in bacteria and in higher organisms; from an evolutionary standpoint they are not related to one another. Nevertheless, all of these proteins are remarkably similar and form a distinct family.

NASA Discipline Number 52-30↗