Pyrimidine photohydrates

Two types of addition to pyrimidine bases appear to exist. The first, the formation of pyrimidine photohydrates, has been the subject of a detailed review.251 Results suggest that two reactive species may be involved in the photohydration of 1,3-dimethyluracil.252 A recent example of this type of addition is to be found in 6-azacytosine (308) which forms a photohydration product (309) analogous to that found in cytosine.253 The second type of addition proceeds via radical intermediates and is illustrated by the addition of propan-2-ol to the trimethylcytosine 310 to give the alcohol 311 and the dihydro derivative 312.254 The same adduct is formed by a di-tert-butyl peroxide-initiated free radical reaction. Numerous other photoreactions involving the formation by hydrogen abstraction of hydroxyalkyl radicals and their subsequent addition to heterocycles have been reported. Systems studied include 3-aminopyrido[4,3-c]us-triazine,255 02,2 -anhydrouri-dine,256 and sym-triazolo[4,3-fe]pyridazine.257 The photoaddition of alcohols to purines is also a well-documented transformation. The stereospecific addition of methanol to the purine 313, for example, is an important step in the synthesis of coformycin.258 These reactions are frequently more... 

Photochemical reactions of the purines and pyrimidines assume special significance because of the high molar extinction coefficients of the nucleic acids present in cells. Light is likely to be absorbed by nucleic acids and to induce photoreactions that lead to mutations.190 Both pyrimidines and purines undergo photochemical alterations, but purines are only about one-tenth as sensitive as pyrimidines. Photohydration of cytidine (Eq. 23-25) is observed readily. The reaction is the photochemical analog of the hydration of a,P-unsaturated carboxylic acids. Uracil derivatives also undergo photohydration. 

A theoretical study of pyrimidine photohydrates and a proposed mechanism for the mutagenic effect of ultraviolet lighP ... 

Only the most general remarks will be made here about the photohydration reaction because, as in the case of dimerization phenomena, most experimental observations have been made on only single compounds and cannot be safely generalized to other pyrimidines. These will therefore be discussed under the headings of individual compounds and any general aspects mentioned at that time. Existing information about the hydration of pyrimidines is summarized in Table I. 

The photohydration of uridylic acid53 is one of the earliest known photolytic reactions of a pyrimidine derivative and has been the subject... 

Little such information is at hand to support proposed mechanisms for the photohydration and photodimerization of the pyrimidine bases and their derivatives discussed in this chapter, and much of what is known is confusing or self-contradictory. 

There is as yet insufficient evidence for proper evaluation of electron transfer reaction as the possible key process in the photohydration reactions (and perhaps in some of the dimerization processes). It has also been suggested that there is a distinct charge separation in the (overall) neutral excited pyrimidine molecule, and that the charge is sufficiently localized that reaction of the excited molecule with OH or H+ (or both successively) can become a competitive reaction pathway.116 Such a dipolar reactant species has also been specifically proposed by Wacker et al.60 (Chart 8). This is an electrophilic attack on water similar to that proposed above for uracil photohydration. 

Becker et al.174 find that the photoproduct of cytidine-3 -phosphate is the photohydrate. The quantum yield is dependent on the pH, and is higher for the neutral form than for the acidic form. Albert176 has published a review article on nonphotolytic hydration of the C=N bond in many heteroaromatic substances, a reaction similar in many respects to photohydration of the pyrimidines. Shapiro and Klein175 report that cytidine and cytosine are deaminated at 95°C by a variety of aqueous buffers. The reaction is pH sensitive. 

All these bases absorb around 260 nm. Thymine and cytosine are most sensitive to irradiation. Two most important types of photochemical reactions that have been observed for these pyrimidine bases are photohydration and photodimerization. In vivo systems, interactions between protons and nucleic acids can also be initiated by radiations of wavelength dlorter than 300 nm. 

On irradiation uracil and related pyrimidines undergo photohydration across the 5,6-bond. 

Gajewski E, Fuciarelli AF, Dizdaroglu M (1988) Structure of hydroxyl radical-induced DNA-protein crosslinks in calf thymus nucleohistone in vitro. Int J Radiat Biol 54 445-459 Gajewski E, Rao G, Nackerdien Z, Dizdaroglu M (1990) Modification of DNA bases in mammalian chromatin by radiation-generated free radicals. Biochemistry 29 7876-7882 Garner A, Scholes G (1985) Mechanism of the photohydration of pyrimidines a flash photolysis study. Photochem Photobiol 41 259-265... 

Lability of the radiolabel during photolysis occurs in other types of molecules and the investigator should be aware of the peculiarities of those that he deals with. For example, 3H can be lost from photohydrates of pyrimidines (Wang, 1976). 

The pyrimidine nucleobases have the highest quantum yields for photoreactivity, with thymine uracil > cytosine. The purine nucleobases have much lower quantum yields for photochemistry, but can be quite reactive in the presence of oxygen. As can be seen from Figure 9-3, thymine forms primarily cyclobutyl photodimers (ToT) via a [2ir + 2tt cycloaddition, with the cis-syn photodimer most prevalent in DNA. This is the lesion which is found most often in DNA and has been directly-linked to the suntan response in humans [65]. A [2Tr + 2Tr] cycloaddition reaction between the double bond in thymine and the carbonyl or the imino of an adjacent pyrimidine nucleobase can eventually yield the pyrimidine pyrimidinone [6 1]-photoproduct via spontaneous rearrangement of the initially formed oxetane or azetidine. This photoproduct has a much lower quantum yield than the photodimer in both dinucleoside monophosphates and in DNA. Finally, thymine can also form the photohydrate via photocatalytic addition of water across the C5 = C6 bond. 

Uracil has a similar photoreactivity to thymine, but in RNA. Although the rate of photoreaction is similar, the photoproduct partitioning is different. While uracil forms the cyclobutyl photodimer and photohydrate, there is no evidence that it forms the pyrimidine-pyrimidinone [6 1] photoproduct. Also, the major photoproduct in... 

The photochemical stability of the nucleic acid bases under UV irradiation is no longer a major concern after the formation of the ozone layer, and the role of the state in the form of modern life is significantly reduced. However, Kohler s group suspects that the tt state might still be involved in the formation of photohydrates and pyrimidine/pyrimidine photoproducts, while the formation of... 

Many exeunples of photorearrangement arising by 4tt- and 6ir-electrocyclic pathways have been reported. Full experimental details for the photohydration of the pyrimidin-4-ones (10) to the enamides (11) via the Dewar pyrimidinones (12) have been published. 6fr-Electrocyclizatlon followed by elimination of benzoic acid is observed in the conversion of the 4-aryl-N-... 

The other photoproduct which is formed is one resulting from the light-induced addition of water across the 5,6-double bond of the pyrimidine, shown in II for uracil. This product will be denoted in this paper by the term photohydrate. Little has been reported about the mechanism of this reaction, or about the nature of the responsible intermediate (4). It is the purpose of this paper to report some observations about the variation with pH and salt concentration of the rate of the photohydration process in four uracil derivatives—(1) uracil (R = H), (2) 1-ethyluracil (EU, R = ethyl), (3) 1-cyclohexyluracil (CU, R = cyclohexyl), and (4) uridine (R = ribosyl). 

Figure 1. The fraction of photohydrate formed in mixtures of acetonitrile and H20 or D20 initial pyrimidine concentrations...

---