RNA composition and base pairing.

If RNA may contain a small proportion of adenine-guanine base pairs, these could interrupt the continuity of helical structure in a polynucleotide, in keeping with current theories of RNA structure, and could also account for the experimentally observed tendency for 6-amino bases to equal 6-keto bases and for purines to exceed pyrimidines.

Detection of molecular oxygen on Mars.

Molecular oxygen was detected in martian spectra near 7635 angstroms and its abundance measured both during and after the 1971 dust storm. Its column abundance in the clear martian atmosphere is about 10.4 +/- 1.0 centimeters amagat, giving a mixing ratio of molecular oxygen to carbon dioxide of 1.3 x 10-(3). The mixing ratio of molecular oxygen to carbon monoxide (1.4 +/- 0.3) is quite different from the value of 0.5 that would result from the photolysis of a pure carbon dioxide atmosphere, which indicates that there is or was a net source of oxygen relative to carbon (probably water) in the martian atmosphere.

Stratospheric hydroperoxyl measurements.

The hydroperoxyl radical (HO(2)) plays a key role in stratospheric chemistry through the HOx catalytic cycle of ozone destruction. Earlier measurements of stratospheric HO(2) have given mixed results; some measured mixing ratios greatly exceed theoretical predictions. Measurements of HO(2) have now been made with a balloon-borne farinfrared spectrometer. The measured daytime profile is in excellent agreement with theory up to 40 kilometers; above this level the measurements exceed theory by 30 percent, perhaps because of underprediction of ozone at these altitudes. The nighttime HO(2) profile is strongly depressed with respect to the daytime profile, in general agreement with theory.

Heterogeneous reaction probabilities, solubilities, and the physical state of cold volcanic aerosols.

On 19 January 1992, heterogeneous loss of HNO(3), ClNO(3), and HCl was observed in part of the Mount Pinatubo volcanic cloud that had cooled as a result of forced ascent. Portions of the volcanic cloud froze near 191 kelvin. The reaction probability of ClNO(3) and the solubility of HNO(3) were close to laboratory measurements on liquid sulfuric acid. The magnitude of the observed loss of HCl suggests that it underwent a heterogeneous reaction. Such reactions could lead to substantial loss of HCl on background sulfuric acid particles and so be important for polar ozone loss.

The limiting collagen microfibril. The minimum structure demonstrating native axial periodicity.

Collagen fibers were grown from solutions of acid-soluble or neutral salt-soluble collagen in 0.5 M acetic acid by rapid dialysis. The collagen was obtained under conditions where protease inhibitors were present at every stage of extraction and purification. Under the conditions used, length-wise but not lateral filament growth proceeded rapidly and gel-like networks were formed, Water readily exuded from the networks. The networks were stretched to fibrous form during drying. Small-angle X-ray diffraction showed the stretched fibrils to be highly ordered, showing up to 20 orders of the 670 A meridional periodicity. Intermediate- and wide-angle photographs show equatorial reflections at a spacing corresponding to approximately 12.5 A which is related to the intermolecular distance but none related to a microfibrillar packing at the 35-40 A level. Electron microscopy of the gel networks before stretching shows the presence of thin filaments with diameters predominantly in the 35-40 A range. No cross-striated fibrils are seen in electron micrographs of either stretched fibers or unstretched fibers. Thus, intermolecular packing in accord with the 670 A axial periodicity can take place within approximately 40 A diameter thin filaments. These correspond to the structures previously postulated to be collagen 'microfibrils'.

Ultrastructural studies of bones from patients with osteogenesis imperfecta.

Bone samples from patients suffering from osteogenesis imperfecta (OI) types I, II, III and IV, as well as normal controls, were studied by scanning (SEM) and transmission electron microscopy (TEM). SEM views of normal bone at low magnification show coherent structure, with regular striations due to a lamellar plywood-like arrangement of the mineralized collagen fibrils. Compact lamellar bone was also found in various OI specimens, but in limited disconnected regions separated by open spaces. Furthermore, some OI, but not normal, bones have regions of loose unconnected fibers and others of apparently abnormally dense mineral deposition. High resolution TEM studies of OI bone fragments have served to elucidate the structures of these different textures. There appears to be a substantial, though reduced, proportion of normal lamellar bone even in quite severe OI. However, the regions of loose fibers are largely unmineralized and probably contain abnormal collagen. Other regions are overmineralized, with generally small unorganized apatite crystals deposited onto fibril surfaces or in separate clusters. These structural abnormalities, together with the paucity of normal bone, may explain the fragility of OI bones.

Structural study of a mutant type I collagen from a patient with lethal osteogenesis imperfecta containing an intramolecular disulfide bond in the triple-helical domain.

We have built molecular models of collagen type I from a patient with lethal osteogenesis imperfecta incorporating one or two mutant alpha 1(I)-chains which contain a cysteine substituting a glycine near the C-terminal end. In either case, the cysteines can only be accommodated with considerable distortion of the native collagen structure, which disrupts inter-chain contacts. The disturbance of the triple helix is limited to a small local region. This suggests that the most important consequence of the mutation is delayed helix formation leading to overmodification and decreased collagen production, rather than the structural abnormality of the folded molecules, which are only marginally unstable.

Organization of hydroxyapatite crystals within collagen fibrils.

Transmission electron micrographs of individual mineralized collagen fibrils show that hydroxyapatite crystals are located mainly within the fibrils at the level of the gap regions. The plate-shaped crystals are observed to be more or less uniformly stacked across the fibril diameter. We therefore suggest that the crystals are primarily located in 'grooves' created by contiguous adjacent gaps. The proposal is consistent with the observed crystal distribution in the fibril and with their average widths, which are almost 10-times greater than an individual gap diameter.

Crystal organization in rat bone lamellae.

The plate-shaped crystals of rat bone are arranged in parallel layers that form coherent structures up to the level of individual lamellae. The crystal layers of the thin lamellae are parallel to the lamellar boundary, whereas those of the thicker lamellae are oblique to the boundary. The basic structure of rat bone can be described as 'rotated plywood'; a structure hitherto unrecognized in either biologic or synthetic materials.

Rotated plywood structure of primary lamellar bone in the rat: orientations of the collagen fibril arrays.

A basic structural motif of lamellar bone is the arrays of parallel collagen fibrils, with successive arrays having different orientations to form a plywood-like structure. Measurements of the angles between adjacent arrays from cryomicrotomed and vitrified thin sections of demineralized rat bone, cut approximately parallel to the lamellar boundary plane, show that most angles are around 30 degrees, although a subset are around 70 degrees. A structural model for collagen organization based on these measurements is proposed in which an individual lamellar unit (thick and thin lamellae together with transition zones) is composed of five arrays of parallel collagen fibrils, each offset by 30 degrees.

Rostrum of a toothed whale: ultrastructural study of a very dense bone.

The rostral bones of the toothed whale, Mesoplodon densirostris, consist mainly of hypermineralized secondary osteons and have yielded among the highest values for density (2.6 g/cm3) and mineral content (86.7%) yet reported for any bone. Scanning and transmission electron microscopy show parallel rods of mineral oriented along the length of the rostrum. These consist of platey crystals of carbonated hydroxyapatite, which, judging from electron diffraction, are extremely well and coherently aligned. The collagen component of the rostral bone consists largely of very thin fibrils aligned in longitudinal register to form tubular networks. The collagen fibrils are also aligned with the lengths of the mineral rods, which are apparently accommodated in the tubular spaces of the collagenous network. This peculiar ultrastructure clearly differs from the densely packed mineralized fibrils commonly observed in vertebrate lamellar osseous tissues, although histological examination has indicated some vestiges of "normal" primary bone surrounding the secondary osteons. Thus, the bone tissue in the rostrum is characterized by a remarkably sparse collagenous component. This ultrastructure can explain the high density, stiffness, and brittleness of the rostrum that have been observed. It also raises interesting questions about possible modes of crystal growth during ongoing mineralization in normal bone, and may have some relevance in the mechanical behavior of dense bones in pathological conditions.

Transitional structures in lamellar bone.

Scanning electron micrographs of fractured surfaces of mineralized bone show a lamellar structure with alternating smooth and rough regions. These have been interpreted as corresponding to two distinct collagen fibril and mineral crystal orientations in a rotated plywood structure. However, in various bones, there are clear indications of transition zones between lamellae in which the fibrils, as well as the plate-like crystals, have intermediate orientations. Strong evidence for intermediate collagen fibril orientations comes from vitrified cryo-sections of demineralized bone. These show zones of fibril segments graded in length between more homogenous regions of fibrils roughly parallel to the specimen section. Evidence for intermediate crystal orientations comes from transmission electron micrographs and electron diffraction patterns of crushed bone fragments. A tentative scheme is presented for an interlamellar transition zone, involving rotation about the collagen fibril axis as well as tilting of this axis parallel to the plane of the interlamellar boundary. Although it may be convenient to think of the structure of lamellar bone as being composed of alternating thick and thin lamellae, it is probably more correct and biologically more relevant to consider one pair of lamellae as the product of a single depositional cycle of varyingly oriented collagen fibrils that subsequently mineralize.

Protein conformation in rat tooth enamel.

Orientated 0.47 nm reflections in X-ray diffraction patterns of developing rat tooth enamel were confirmed using a wide range of specimen preparations and experimental conditions. This indicates that some of the organic matrix proteins adopt a beta-sheet conformation.