Base triplet

An opening frame contains a series of codons (base triplets) coding for amino acids without any termination codons. There are six potential reading frames of an unidentified sequence. 

Fig. 3 Schematic representation of triplex form with chemical structures of base triplets of DNA and RNA triplexes. Reprinted from [80] with permission from Wiley InterScience...

In 1994, a conference with the title Aminoacyl-tRNA Synthetases and the Evolution of the Genetic Code was held in Berkeley, California its patron was the Institute of Advanced Studies in Biology. The conference dealt with the development of the synthetases and that of the genetic code (see Sect. 8.2), i.e., the assignment of the various amino acids to the corresponding base triplets of the nucleic acids. 

Fig. 9.2 Part of a quantitative- /NN HNN-COSY spectrum of a 1.5 mM uniformly 13C/15N-labeled intramolecular DNA triplex. This triplex consists of five Hoogsteen-Watson-Crick T A-T and three Hoogsteen C+ G-C base triplets. The spectral region corresponds to the 10 imino resonances of the Hoogsteen-Watson-Crick T A-T triplets. The data matrix consisted of 250 (q) X1024 (i2) data points (where n refers to complex points) with acquisition times of 45 ms (tn) and 85 ms...

The many (possibly more than 30) types of collagens found in human connective tissues have substantially the same chemical structure consisting mainly of glycine with smaller amounts of proline and some lysine and alanine. In addition, there are two unusual amino acids, hydroxyproline and hydroxylysine, neither of which has a corresponding base-triplet or codon within the genetic code. There is therefore, extensive post-translational modification of the protein by hydroxylation and also by glycosylation reactions. 

Each codon consists of three bases (triplet). There are 64 codons. They are all written in the 5 to 3 direction. 

As there are 20 proteinogenic amino acids (see p. 60), the nucleic acid language has to contain at least as many words (codons). However, there are only four letters in the nucleic acid alphabet (A, G, C, and U or T). To obtain 20 different words from these, each word has to be at least three letters long (with two letters, there would only be 4 = 16 possibilities). And in fact the codons do consist of three sequential bases (triplets). 

These results of course are inconsistent with Dowd s finding of a triplet ground state from the matrix ESR spectrum. At present, the discrepancy is rationalized with the hypothesis that the triplet is metastable in the matrix and that its conformational conversion to the more stable singlet is slow, thus allowing the observation of the ESR signal and the linear Curie plot. Other cases of what seem to be con-formationally based triplet metastabUity in matrix ESR had been observed previously in 19931° ° and will be discussed later, together with further examples (see Section 7.2.1). 

ITABLE V.1. Degenerative base triplet on ribonucleotides that codes for specific amino acids... 

Alberola V, Camps C, Provencio M et al. Cisplatin plus gemcitabine versus a cisplatin-based triplet versus nonplatinum sequential doublets in advanced non-small-cell lung cancer a Spanish Lung Cancer Group phase III randomized trial. J Clin Oncol 2003 21 3207-3213. 

Figure 5-7 Two base triplets that form in triple-stranded DNA and involve both Watson-Crick and Hoogsteen base pairing. (A) The triplet T A T, where the T (marked T ) of the third strand is hydrogen bonded as a Hoogsteen pair...

Figure 5-24 Proposed structure of H-DNA which can be formed when a homopyrimidine segment and its complementary homopurine segment separate as a result of protonation of the cytosine rings or of negative supercoiling stress.239,240 The triple-helical portion contains base triplets of the kind shown in Fig. 5-7.

The 2 -hydroxyl groups of the cytosine and of the guanosine in the anticodon are methylated. Moving far up the anticodon stem one can see two groups of base triplets which utilize both Watson-Crick and Hoogsteen types of base pairing. Drawing courtesy of Alexander Rich. 

Metal complexes with Schiff base ligands have useful applications in organic optoelectronics due to their outstanding photoluminescent (PL) and electroluminescent (EL) properties, and their ease of synthesis, which readily allows structural modification for optimization of material properties.28 Hamada and co-workers pioneered the use of zinc(II) Schiff base complexes as blue to greenish white emitters for EL devices. We have demonstrated Pt(II) Schiff base triplet emitters as yellow dopants for organic light-emitting devices... 

Figure 5. Nucleobase quartet generated from diplatinated G, A, U base triplet and C. The coreceptor ligand 1-MeU is also involved in hydrogen bonding with the 1-MeC guest. [Reprinted with permission from (117).]...

The possible effects of metal ions on the stability of pyrimidine purine pyrimidine base triplets such as HC+ GC as well as TAT still await theoretical treatment. 

In the example discussed above, the heterotriad consists of a photosensitizer and an electron donor. In the following example, a ruthenium polypyridyl sensitizer is combined with an electron acceptor, in this case a rhodium(lll) polypyridyl center [15]. The structure of this dyad is shown in Figure 6.21 above. The absorption characteristics of the dyad are such that only the ruthenium moiety absorbs in the visible part of the spectrum. Irradiation of a solution containing this ruthenium complex with visible light results in selective excitation of the Ru(ll) center and in an emission with a A.max of 620 nm. This emission occurs from the ruthenium-polypyridyl-based triplet MLCT level, the lifetime of which is about 30 ns. This lifetime is very short when compared with the value of 700 ns obtained for the model compound [Ru(dcbpy)2dmbpy)], which does not contain a rhodium center. Detailed solution studies have shown that this rather short lifetime can be explained by fast oxidative quenching by the Rh center as shown in the following equation ... 

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