1. Electron Donor 2223 2. Electron Transfer Path 2225 3. Electron Transfer Mechanism 2226 4. Physiological Relevance 2226 D. Radical Reactions in Photolyase 2226 E. “Dark Function” of Photolyase 2227 F. Regulation of Photolyase 2227 III. (6−4) Photolyase 2228 IV. Cryptochromes 2230 A. Structure 2231 B. Function 2231 1. Circadian Rythm 2231 2. Mammalian Circadian System 2231 3. Cryptochromes and the Circadian Clock 2232 V. Perspectives 2234 VI. Acknowledgment 2235 VII. References 2235

Structure and function of DNA photolyase and cryptochrome blue-light photoreceptors.

Reactions of oxyl radicals with DNA.

Hyaluronan fragments: an information-rich system.

Intermolecular static and dynamic fluorescence quenching constants of eight coumarin derivatives by nucleobase derivatives have been determined in aqueous media. One common sequence of the quenching efficiency has been found for the nucleobases. The feasibility of a photoinduced electron transfer reaction for the nucleobase-specific quenching of fluorescent dyes is investigated by the calculation of the standard free energy changes with the Rehm−Weller equation. A complete set of one-electron redox potential data for the nucleobases are determined electrochemically in aprotic solvents for the first time, which are compared with values obtained by various other methods. Depending on the redox properties of the fluorescent dyes, the sequences of the quenching efficiencies can be rationalized by the orders of electrochemical oxidation potentials (vs NHE) of nucleosides (dG (+1.47 V) < dA < dC ≈ dT < U (≥ +2.39 V)) and reduction potentials (dG (< −2.76 V) < dA < dC < dT < U (−2.07 V)). The correlation between...

Nucleobase-specific quenching of fluorescent dyes. 1. nucleobase one-electron redox potentials and their correlation with static and dynamic quenching efficiencies

The immune and the detoxication systems of animals are characterized by allelic polymorphisms, which underlie individual differences in ability to combat assaults from pathogens and toxic compounds. Previous studies have shown that females may improve offspring survival by selecting mates on the basis of sexual ornaments and signals that honestly reveal health. In many cases the expression of these ornaments appears to be particularly sensitive to oxidative stress. Activated immune and detoxication systems often generate oxidative stress by an extensive production of reactive metabolites and free radicals. Given that tolerance or resistance to toxic compounds and pathogens can be inherited, female choice should promote the evolution of male ornaments that reliably reveal the status of the bearers' level of oxidative stress. Hence, oxidative stress may be one important agent linking the expression of sexual ornaments to genetic variation in fitness-related traits, thus promoting the evolution of female mate choice and male sexual ornamentation, a controversial issue in evolutionary biology ever since Darwin.

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Good genes, oxidative stress and condition-dependent sexual signals

Pulse radiolysis of N2O-saturated solutions of poly(U) in the presence of tetranitromethane showed that 81 per cent of the radicals formed are reducing in nature. Using data from other sources it has been estimated that 70 per cent of the OH radicals add to the base at C(5) and 23 per cent at C(6) while only 7 per cent abstract an H-atom from the sugar moiety. To a large extent the C(5) OH adduct radicals attack the sugar moiety of poly(U) thereby inducing strand breakage and base release. G (base release) = 2.9 can be subdivided into three components: (a) immediate (20 per cent), (b) fast (50 per cent) and (c) slow (30 per cent). The immediate base release must occur either during the free-radical stage or as a result of the rapid (t1/2 less than 4 min at 0 degree C) decomposition of a diamagnetic product. The fast and the slow processes are only readily observable at elevated temperatures, e.g. at 50 degrees C the half lives are 83 min and 26 h, respectively (Ea (fast) = 68 kJ mol-1, Ea (slow) = 89 kJ mol-1, A (fast) = 1.5 X 10(7) s-1, A (slow) = 1.9 X 10(9) s-1. It is concluded that there are three different types of sugar lesions giving rise to base release, structures for which are tentatively proposed.

Reactions of OH radicals with poly(U) in deoxygenated solutions: sites of OH radical attack and the kinetics of base release

Solvated electrons from the radiolysis of water were reacted with a number of uracil derivatives. The radical anions I so formed undergo rapid protonation at O(4) (I + H/sup +/ ..-->.. II) independent of the pyrimidine used. With the equilibrium I/II established a slower protonation occurs at C(6) (I + H/sub 2/PO/sub 4//sup -/ ..-->.. III + HPO/sub 4//sup 2 -/). Whereas the absorption maxima of I and II are less than or equal to 300 nm the absorption maxima of III for the various pyrimidines center around 400 nm. The rate constants of this C(6) protonation very much depend on the site of methyl substitution. Using tetranitromethane as a probe showed that the reducing radicals I and II are quantitatively converted by the slow protonation into the oxidizing radical III. These results clearly show the high selectivity of this protonation and exclude the possibility of any significant protonation on C(5). The relevance of these results for the radiation chemistry of DNA are discussed. 23 references, 8 figures, 2 tables.

Uracil derivatives: sites and kinetics of protonation of the radical anions and the UV spectra of the C(5) and C(6) H-atom adducts

A number of N(1)-substituted thymine derivatives have been oxidized in aqueous solution using the one-electron oxidant SO{sub 4}{sup {sm bullet}}{sup {minus}}. The rate constants for this reaction are in the range 1 {times} 10{sup 9} mol{sup {minus}1} dm{sup 3} s{sup {minus}1} (5{prime}-thymidylic acid) to 5 {times} 10{sup 9} mol{sup {minus}1} dm{sup 3} s{sup {minus}1} (1-methylthymine). It is proposed that in all cases a radical cation is the initial transient observed. The 1,3-dimethylthymine radical cation decays by a first-order process (t{sub 1/2} {approximately} 2 {mu}s) with the production of a proton and can be followed by both absorbance and conductance measurements. This process is made up of water addition at C(6) and deprotonation from the C(5) methyl group. In alkaline solution the rate constant for the decay of the radical cation is increased, with OH{sup {minus}} addition at C(6) being preferred to deprotonation from the C(5) methyl group. When N(3) is unsubstituted, deprotonation of the radical cation can occur to give the N(3)-centered radical. The pK{sub a} values for thymidine and 1-methylthymine radical cations are 3.6 and 3.8, respectively, i.e., more than 5 orders of magnitude lower than those of their parent compounds.

Direct evidence for the formation of thymine radical cations from the reaction of sulfate (SO4.cntdot.-) with thymine derivatives: A pulse radiolysis study with optical and conductance detection

The reactivity of various free radicals with hyaluronic acid: steady-state and pulse radiolysis studies

The sulphate radical SO4(.-) reacts with 1,3-dimethyluracil (1,3-DMU) (k = 5 X 10(9) dm3 mol-1 s-1) thereby forming with greater than or equal to 90 per cent yield the 1,3-DMU C(5)-OH adduct radical 4 as evidenced by its absorption spectrum and its reactivity toward tetranitromethane. Pulse-conductometric experiments have shown that a 1,3-DMU-SO4(.-) aduct 3 as well as the 1,3-DMU radical cation 1, if formed, must be very short-lived (t1/2 less than or equal to 1 microsecond). The 1,3-DMU C(5)-OH adduct 4 reacts slowly with peroxodisulphate (k = 2.1 X 10(5) dm3 mol-1 s-1). It is suggested that the observed new species is the 1,3-DMU-5-OH-6-SO4(.-) radical 7. At low dose rates a chain reaction is observed. The product of this chain reaction is the cis-5,6-dihydro-5,6-dihydroxy-1,3-dimethyluracil 2. At a dose rate of 2.8 X 10(-3) Gys-1 a G value of approximately 200 was observed ([1,3-DMU] = 5 X 10(-3) mol dm-3; [S2O8(2-)] = 10(-2) mol dm-3; [t-butanol] = 10(-2) mol dm-3). The peculiarities of this chain reaction (strong effect of [1,3-DMU], smaller effect of [S2O(2-)8]) is explained by 7 being an important chain carrier. It is proposed that 7 reacts with 1,3-DMU by electron transfer, albeit more slowly (k approximately 1.2 X 10(4) dm3 mol-1 s-1) than does SO4(.-). The resulting sulphate 6 is considered to hydrolyse into 2 and sulphuric acid which is formed in amounts equivalent to those of 2. Computer simulations provide support for the proposed mechanism. The results of some SCF calculations on the electron distribution in the radical cations derived from uracil and 1-methyluracil are also presented.

D.J. Deeble

Papers

Reactions of OH radicals with poly(U) in deoxygenated solutions: sites of OH radical attack and the kinetics of base release

Uracil derivatives: sites and kinetics of protonation of the radical anions and the UV spectra of the C(5) and C(6) H-atom adducts

Direct evidence for the formation of thymine radical cations from the reaction of sulfate (SO4.cntdot.-) with thymine derivatives: A pulse radiolysis study with optical and conductance detection

The reactivity of various free radicals with hyaluronic acid: steady-state and pulse radiolysis studies

The SO4(.-)-induced chain reaction of 1,3-dimethyluracil with peroxodisulphate