Of guanine

A novel modification of the STM supplements images with those due to the thermopower signal across the tip-sample temperature gradient [49]. Images of guanine on graphite illustrate the potential of this technique. 

The structure of guanine illustrates an important feature of substituted pyrimidines and purines Oxygen substitution on the ring favors the keto form rather than the enol Ammo substitution does not... 

The hue of the colorant varies gready with the amount and type of pigment found in the fish scales. Carotenoids produce reds and yellows, melanin results in blacks, and combinations of guanine and melanin produce greens and blues. Only when guanine is found alone is the product silvery or peady white. 

Base composition- Proportion of the total bases consisting of guanine plus cytosine or thymine plus adenine base pairs. Usually expressed as a guanine -I- cytosine (G -I- C) value, e.g. 60% G-l-C. 

Samples of DNA isolated from different tissues of the same species have the same proportions of heterocyclic bases, but samples from different species often have greatly different proportions of bases. Human DNA, for example, contains about 30% each of adenine and thymine and about 20% each of guanine and cytosine. The bacterium Clostridium perfringens, however, contains about 37% each of adenine and thymine and only 13% each of guanine and cytosine. Note that in both examples the bases occur in pairs. Adenine and thymine are present in equal amounts, as are cytosine and guanine. Why ... 

One of the steps in the metabolic degradation of guanine is hydrolysis lo give xanthine. Propose a mechanism. 

The different possible adducts formed between mitomycin C and DNA have been isolated by degradation of DNA after in vitro alkylation/crosslinking reactions and structurally characterized. Monoadduct 21 (Scheme 11.3), derived from alkylation at C-l only [53], and monoadducts 22 [54] and 23 [55, 56] (derived from C-10 alkylation by 16 at N-7 or N-2 of guanine, respectively) have been isolated, together with bisadducts 24 [57] and 25 [58], derived from interstrand and intrastrand crosslinks, respectively, and adduct 26 [59], formed by addition of a molecule of water to C-10 instead of the second guanine. All of these adducts have also been isolated from DNA after in vivo crosslinking [60, 61]. 

The ternary LRG complex, in the absence of guanine nucleotide, is stable, and the agonist is bound with high affinity to the receptor. 

---