No abstract

Observed coronal loops have a surprisingly uniform thickness that cannot be easily understood in terms of standard coronal magnetic Ðeld models. We investigate the possibility that the uniform thickness can be explained by locally enhanced twist in the Ðeld, so that observed loops correspond to twisted coronal Ñux tubes. Our approach is to construct numerical models of fully three-dimensional force-free magnetic Ðelds. To resolve the internal structure of an individual loop embedded within a much larger dipole conÐguration, we use a nonuniform numerical grid of size 609 ] 513 ] 593, the largest ever applied to a solar problem to our knowledge. Our models show that twist promotes circular cross sections in loops. Such cross sections are typically assumed, and have recently been veriÐed from observations, but their physical cause has been heretofore unexplained.

The conformation of an uncomplexed form of the antibiotic valinomycin (C(54)N(6)O(18)H(90)) has been determined by direct methods including a novel technique for strong enantiomorph discrimination via the calculation and systematic analysis of cosine invariants of a special type. The intramolecular hydrogen bonding scheme and the isopropyl group stereochemistry of uncomplexed valinomycin are compatible with interpretations of spectral measurements for the complexed and uncomplexed molecule in solution but are different from any previously proposed structure. The simple conformational change of a hydrogen bond shift, which could be induced by the process of potassium ion complexing, transforms the uncomplexed into the complexed structure.

Some recently secured results make possible the implementation of a program for direct phase determination first proposed over ten years ago. Application is made to the evaluation of the phases for the estriol structure. The values of 1805 structure invariants, cos (tp~ + ~2+~p3), lead, by means of a novel least-squares technique, to the determination of 103 phases. Employing the tangent formula, these 103 phases are used to obtain the values of the 1023 phases whose corresponding normalized structure factors are not less than unity.

The crystal structure of bis-(L-isoleucinato)copper(II) monohydrate has been determined in order to discover typical conformations of the biologically important amino acid L-isoleucine. This is a cis copper complex, and the copper atom has fivefold coordination with its nearest neighbors disposed approximately in a square pyramid. The two isoleucine residues within a single molecule do not have the same conformation, and only one of the chelate rings is puckered. All amino and water hydrogen atoms are involved in hydrogen bonds which form layers of molecules parallel to (010). The crystals are orthorhombic (space group ° P212121) and have four molecules in a unit cell whose dimensions are a=9.451, b = 21.674, c= 7.629 A.