Mössbauer effect investigations of the hyperfine interactions and relaxation phenomena in salts of thulium

Two new phenomena have been observed in Mössbauer spectra: a temperature-dependent shift of the center of gravity of the spectrum, and an asymmetric broadening of the spectrum peaks. Both phenomena were observed in thulium salts. In the temperature range 1˚K ≤ T ≤ 5˚K the observed shift has an approximate inverse temperature dependence. We explain this on the basis of a Van Vleck type of interaction between the magnetic moment of two nearly degenerate electronic levels and the magnetic moment of the nucleus. From the size of the shift we are able to deduce an “effective magnetic field” H = (6.0 ± 0.1) x 10⁶ Gauss, which is proportional to ‹r^-3›_M‹G|J|E› where ‹r^-3›_M is an effective magnetic radial integral for the 4f electrons and |G› and |E› are the lowest 4f electronic states in Tm Cl₃·6H₂O. From the temperature dependence of the shift we have derived a preliminary value of 1 cm^-1 for the splitting of these two states. The observed asymmetric line broadening is independent of temperature in the range 1˚K ≤ T ≤ 5˚K, but is dependent on the concentration of thulium ions in the crystal. We explain this broadening on the basis of spin-spin interactions between thulium ions. From size and concentration dependence of the broadening we are able to deduce a spin-spin relaxation time for Tm Cl₃·6H₂O of the order of 10^-11 sec.

Spatial filtering in tone reproduction and vision

This thesis is concerned with spatial filtering. What is its utility in tone reproduction? Does it exist in vision, and if so, what constraints does it impose on the nervous system?

Tone reproduction is just the art and science of taking a picture and then displaying it. The sensors available to capture an image have a greater dynamic range than the media that may be used to display it. Conventionally, spatial filtering is used to boost contrast; it ameliorates the loss of contrast that results when the sensor signal range is scaled down to fit the display range. In this thesis, a type of nonlinear spatial filtering is discussed that results in direct range reduction without range scaling. This filtering process is instantiated in a real-time image processor built using analog CMOS VLSI.

Spatial filtering must be applied with care in both artificial and natural vision systems. It is argued that the nervous system does not simply filter linearly across an image. Rather, the way that we see things implies that the nervous system filters nonlinearly. Further, many models for color vision include a high-pass filtering step in which the DC information is lost. A real-time study of filtering in color space leads to the conclusion that the nervous system is not that simple, and that it maintains DC information by referencing to white.

How far from Jerusalem? Tropical customs and the question of race in the Book of John Mandeville

The Book of John Mandeville, while ostensibly a pilgrimage guide documenting an English knight’s journey into the East, is an ideal text in which to study the developing concept of race in the European Middle Ages. The Mandeville-author’s sense of place and morality are inextricably linked to each other: Jerusalem is the center of his world, which necessarily forces Africa and Asia to occupy the spiritual periphery. Most inhabitants of Mandeville’s landscapes are not monsters in the physical sense, but at once startlingly human and irreconcilably alien in their customs. Their religious heresies, disordered sexual appetites, and monstrous acts of cannibalism label them as fallen state of the European Christian self. Mandeville’s monstrosities lie not in the fantastical, but the disturbingly familiar, coupling recognizable humans with a miscarriage of natural law. In using real people to illustrate the moral degeneracy of the tropics, Mandeville’s ethnography helps shed light on the missing link between medieval monsters and modern race theory.

Enzyme induction in Neurospora crassa

I. Studies on Nicotinamide Adenine Dinucleotide Glycohydrase (NADase)

NADase, like tyrosinase and L-amino acid oxidase, is not present in two day old cultures of wild type Neurospora, but it is coinduced with those two enzymes during starvation in phosphate buffer. The induction of NADase, like tyrosinase, is inhibited by puromycin. The induction of all three enzymes is inhibited by actinomycin D. These results suggest that NADase is synthesized de novo during induction as has been shown directly for tyrosinase. NADase induction differs in being inhibited by certain amino acids.

The tyrosinaseless mutant ty-1 contains a non-dialyzable, heat labile inhibitor of NADase. A new mutant, P110A, synthesizes NADase and L-amino acid oxidase while growing. A second strain, pe, fl;cot, makes NADase while growing. Both strains can be induced to make the other enzymes. These two strains prove that the control of these three enzymes is divisible. The strain P110A makes NADase even when grown in the presence of Tween 80. The synthesis of both NADase and L-amino acid oxidase by P110A is suppressed by complete medium. The theory of control of the synthesis of the enzymes is discussed.

II. Studies with EDTA

Neurospora tyrosinase contains copper but, unlike other phenol oxidases, this copper has never been removed reversibly. It was thought that the apo-enzyme might be made in vivo in the absence of copper. Therefore cultures were treated with EDTA to remove copper before the enzyme was induced. Although no apo-tyrosinase was detected, new information on the induction process was obtained.

A treatment of Neurospora with 0.5% EDTA pH 7, inhibits the subsequent induction during starvation in phosphate buffer of tyrosinase, L-amino acid oxidase and NADase. The inhibition of tyrosinase and L-amino acid oxidase induction is completely reversed by adding 5 x 10^-5M CaCl₂, 5 x 10^-4M CuSO₄, and a mixture of L-amino acids (2 x 10^-3M each) to the buffer. Tyrosinase induction is also fully restored by 5 x 10^-4M CaCl₂ and amino acids. As yet NADase has been only partially restored.

The copper probably acts by sequestering EDTA left in the mycelium and may be replaced by nickel. The EDTA apparently removes some calcium from the mycelium, which the added calcium replaces. Magnesium cannot replace calcium. The amino acids probably replace endogenous amino acids lost to the buffer after the EDTA treatment.

The EDTA treatment also increases permeability, thereby increasing the sensitivity of induction to inhibition by actinomycin D and allowing cell contents to be lost to the induction buffer. EDTA treatment also inhibits the uptake of exogenous amino acids and their incorporation into proteins.

The lag period that precedes the first appearance of tyrosinase is demonstrated to be a separate dynamic phase of induction. It requires oxygen. It is inhibited by EDTA, but can be completed after EDTA treatment in the presence of 5 x 10^-5M CaCl₂ alone, although no tyrosinase is synthesized under these conditions.

The time course of induction has an early exponential phase suggesting an autocatalytic mechanism of induction.

The mode of action of EDTA, the process of induction and the kinetics of induction are discussed.