SynopsisWith the increased use of graphite-reinforced composites as replacements for metals has come concerns about durability under harsh environmental conditions. Degradation is expected to begin on the surface and progress toward the center of the resin a s a function of time, and reflectance infrared techniques are ideal for monitoring structural changes on surfaces. The present paper describes the application of Fourier transform infrared spectroscopy using internal reflectance elements to the determination of the degree of environmental aging of Hercules 3501-6 resin. The results indicate that degradation occurs via hydrolysis, oxidation, and dehydration reactions at specific locations in the polymer chain. Of special interest is the unique reaction of the tertiary m i n e of the epoxy portion of the molecule.

Neither of the two previously proposed secondary structures for eukaryotic 5.8S RNA is consistent with the present laser Raman results. A new, highly stable "cloverleaf" secondary structure not only fits the Raman data but also accounts for previously determined enzymatic partial cleavage patterns, base sequence and pairing homologies, and GC and A'U base pair numbers and ratios. The new cloverleaf model also conserves several structural features (constant loops, bulges, and stems) consistent with known 5.8S RNA functions. Finally, we propose a similar new cloverleaf secondary structure for Escherichia coli 5S RNA, consonant with many known properties of prokaryotic 5S RNA. 5S RNA and 5.8S RNA belong to a class of small RNA molecules (including tRNA) that function in protein synthesis. The structure and function of the smaller tRNA is now understood largely because (i) tRNAPhe has been successfully crystallized and its three-dimensional structure has been determined (1, 2), and (ii) tRNA function is present in part in the free cytoplasm. In contrast, 5S RNA and 5.8S RNA are intimately bound to the ribosome. Their three-dimensional crystal structures have not yet been reported, and conclusions based on low-field hydrogen-bonded proton nuclear magnetic resonance spectra have been severely limited by poor resolution (3, 4). Less-direct structural information from other techniques has been reviewed (5), and more recent results are now discussed.Pene et al. (6) demonstrated that 5.8S RNA is strongly hydrogen-bonded to the 28S RNA of the large ribosomal subunit of eukaryotes. This interaction is between the 3' end of the 5.8S RNA molecule and a complementary segment of the 288 RNA and is stabilized by the presence of a G-C-rich loop near the 3' end of the 5.8S RNA (7). Moreover, all 5.8S RNA nucleotide sequences determined to date contain a sequence of bases that is complementary to the T4/CG-loop of tRNA (8-10). These facts suggest that 5.8S RNA in eukaryotes binds tRNA at the ribosome during transcription.In addition, the following facts connect the structure and function of eukaryotic 5.8S RNA to prokaryotic 5S RNA. First, yeast 5.8S RNA can bind the same ribosomal proteins (EL-18 and EL-25) that Escherichia coli 5S RNA binds most strongly, and this 5.8S RNA-E. colh protein complex exhibits GTPase and ATPase activities similar to those for the homologous 5S RNA-protein complex (11). Second, all prokaryotic 5S RNA molecules contain the complement of the TXCG-loop of tRNA and can bind the tetranucleotide UUCG (5). Third, both the prokaryotic 5S RNA sequence and the eukaryotic 5.8S RNA sequence are contained in the large ribosomal RNA transcription units (12). Therefore, because eukaryotic 5.8S RNA and prokaryotic 5S RNA appear to have similar origin and function, their secondary structures should be similar.Raman spectroscopy has proved to be a sensitive probe ofThe publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in a...

The heat-induced melting of yeast 5S RNA and tRNAPhe has been monitored by UV, CD, and 360-MHz 1H NMR spectroscopy in order to determine the extent of base stacking and base pairing in the native and denatured structures. In the presence of Mg2+, the optical data indicate less than or equal to 40 base pairs in native yeast 5S RNA, a 60:40 ratio of GC to AU base pairs, with more single-stranded stacking and a slightly less stable structure (half-melted at 67 degrees C) than for tRNAPhe (half-melted at 71 degrees C). In the absence of Mg2+, the NMR results identify a minimum of approximately 32 base pairs at 25 degrees C (increasing to a minimum of approximately 35 base pairs in the presence of Mg2+), of which more than half are still intact at 48 degrees C. The native structure (25 degrees C) shows only minor dependence upon Mg2+ concentration, and no denatured forms could be detected. Finally, the present results support a previously proposed cloverleaf secondary structure for eukaryotic 5S RNA.

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