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PubMed2001

Synthesis and thermodynamic studies of oligonucleotides containing the two isomers of thymine glycol.

Thymine glycol is a major type of base damage, which is formed in DNA by reactive oxygen species. I describe the synthesis of oligonucleotides containing the 5S isomer of thymine glycol, which was not obtained by the oxidation of the oligonucleotides. Before the 5S isomer was synthesized, a building block without the protection of the tertiary hydroxy function at the C5 position of thymine glycol was tested by the use of the previously reported 5R isomer. In the presence of imidazole, migration of the silyl group between the C5 and C6 positions was observed, while the result of the oligonucleotide synthesis was identical to the case of the fully protected building block. Therefore, oligonucleotides containing the (5S)-thymine glycol were synthesized with the disilylated building block. In contrast to the 5R derivative, two products were detected in the HPLC analysis of the crude mixture after deprotection. Analysis by matrix-assisted laser-desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry revealed that the larger peak was the desired oligonucleotide, and it was found that the by-product was completely degraded by a short treatment with ammonium hydroxide at room temperature. I also report the application of oligonucleotides containing each isomer of thymine glycol to thermodynamic analyses of base-pair formation. The thermodynamic parameters obtained for the duplexes containing either the (5R)- or (5S)-thymine glycol indicated that the thymine glycol cannot form a base-pair with any nucleobase, regardless of the configuration at the C5 position.

Amides↗
PubMed1995

Molecular dynamics simulations of the effects of ring-saturated thymine lesions on DNA structure.

The effect of thymine lesions produced by radiation or oxidative damage on DNA structure was studied by molecular dynamics simulations of native and damaged DNA. Thymine in position 7 of native dodecamer d(CGCGAATTCGCG)2 was replaced by one of the four thymine lesions 5-hydroxy-5,6-dihydrothymine, 6-hydroxy-5,6-dihydrothymine (thymine photohydrate), 5,6-dihydroxy-5,6-dihydro-thymine (thymine glycol), and 5,6-dihydrothymine. Simulations were performed with Assisted Model Building with Energy Refinement force field. Solvent was represented by a rectangular box of water with periodic boundary conditions applied. A constant temperature and constant volume protocol was used. The observed level of distortions of DNA structure depends on the specific nature of the lesion. The 5,6-dihydrothymine does not cause distinguishable perturbations to DNA. Other lesions produce a dramatic increase in the rise parameter between the lesion and the 5' adjacent adenine. These changes are accompanied by weakening of Watson-Crick hydrogen bonds in the A6-T19 base pair on the 5' side of the lesion. The lesioned bases also show negative values of inclination relative to the helical axis. No changes in the pattern of backbone torsional angles are observed with any of the lesions incorporated into DNA. The structural distortions in DNA correlate well with known biological effects of 5,6-dihydrothymine and thymine glycol on such processes as polymerase action or recognition by repair enzymes.

Base Sequence↗
PubMed2001

Factors influencing the removal of thymine glycol from DNA in gamma-irradiated human cells.

The toxic and mutagenic effects of ionizing radiation are believed to be caused by damage to cellular DNA. We have made use of a novel immunoassay for thymine glycol to examine the removal of this lesion from the DNA of irradiated human cells. Because of the sensitivity of the assay, we have been able to keep the radiation doses at or below the standard clinical dose of 2 Gy. Our initial observations indicated that although removal of thymine glycol is > 80% complete by 4 h post-irradiation with 2 Gy, there is a lag of 30-60 min before repair commences. However, if cells are irradiated with 0.25 Gy 4 h prior to the 2-Gy dose, removal of the thymine glycols commences immediately after the second irradiation, suggesting that repair of thymine glycol is inducible. Our current studies are directed at two aspects of the repair process, (1) factors involved in the repair process leading up to and including glycosylase-mediated removal of thymine glycol and (2) the control of the inducible response. We have observed that mutation of the XPG gene drastically reduced the level and rate of global removal of thymine glycol (induced by 2-Gy irradiation), and there was no evidence for an inducible response. Similar results were seen with a Cockayne syndrome B (CSB) cell line. We have also examined repair in quiescent and phytohemagglutinin-stimulated human lymphocytes. Both show similar kinetics for the rate of removal of thymine glycol under induced and noninduced conditions.

Animals↗
PubMed1998

Requirement for DNA mismatch repair proteins in the transcription-coupled repair of thymine glycols in Saccharomyces cerevisiae.

Defects in DNA mismatch repair have been shown to lead to increased genomic instability and mutability. We recently found that human cells defective in the DNA mismatch repair gene, hMSH2, were deficient in the transcription-coupled repair (TCR) of both oxidative DNA damage, including thymine glycols, and UV-induced DNA damage. However, in a hMLH1 mutant, only a reduction in the TCR of UV damage was observed. In this study, we examined whether TCR of thymine glycols in Saccharomyces cerecisiae also requires the genes involved in DNA mismatch repair. We found that yeast cells containing mutations in MSH2 were deficient in the removal of thymine glycols from the transcribed strand of the RPB2 gene, while cells with mutations in either MLH1 or PMS1 alone showed near normal levels of TCR of thymine glycols. Interestingly, double mutants in the MLH1 and PMS1 genes were deficient in TCR of thymine glycols. Taken together, these results suggest that these two MutL homologues can act independently of each other, but that they have overlapping roles in TCR. Overall levels of thymine glycol removal were not reduced in the mismatch repair mutants. In contrast to the results with thymine glycols, no defects in TCR of pyrimidine dimers were found in cells with mutations in MSH2, MLH1, PMS1, and MLH1/PMS1.

DNA Damage↗
PubMed1992

Photoreactions of thymine and thymidine with N-acetyltyrosine.

We report here the photoinduced formation of a thymine-N-acetyltyrosine adduct. Irradiation of dilute solutions of thymine in the presence of N-acetyltyrosine (NAT) leads to the formation of N-acetyl-4-hydroxy-3-(6-hydrothymin-5-yl)phenylalanine (I), isolated as a mixture of the 5R and 5S diastereoisomers; the photoreaction occurs when irradiation is done either at lambda = 254 nm or at wavelengths of lambda > 290 nm. Irradiation of thymidine in the presence of NAT and of thymine in the presence of tyrosine leads to analogous photoadducts. The photoreaction of thymine with NAT is completely quenched by oxygen and cannot be sensitized by acetone. The likely mechanism involves initial photoionization of the amino acid and deprotonation to form the phenoxyl radical. Thymine then probably captures the released aqueous electron, leading to protonation at C6 of the resulting radical anion. Combination of the phenoxyl and 5,6-dihydrothymin-5-yl radicals would then lead to formation of the final products. The quantum yield for production of the thymine-NAT adduct at pH 7.8 was estimated to be about 5.5 x 10(-4), while a value of 2.3 x 10(-3) was estimated for production of corresponding thymidine adduct at pH 8.1. The dependence of the quantum yield for adduct formation on pH has been determined for both the thymine and thymidine reactions with NAT; the maxima in the quantum yield profiles occur at pH 8-8.5, while appreciable values were measured at pH 7.5. We have also demonstrated that a similar reaction occurs when tyrosine is located within a peptide.(ABSTRACT TRUNCATED AT 250 WORDS)

DNA-Binding Proteins↗
PubMed1987

Functional effects of cis-thymine glycol lesions on DNA synthesis in vitro.

Thymine glycol, a DNA lesion produced by ionizing radiation, has been introduced site specifically at high frequency into a synthetic oligonucleotide by chemical oxidation of the single thymine residue within the sequence. The lesion-containing template was then annealed to a complementary synthetic primer and used to study the effects of cis-thymine glycol lesions on DNA polymerase function in vitro. Synthesis by polymerase I (Klenow fragment), T4 DNA polymerase, and polymerase alpha 2 was arrested quantitatively at the site of the lesion. AMV reverse transcriptase was less inhibited and was able to synthesize past a significant fraction of the lesions. Changing the template base immediately 5' to thymine glycol from A to C did not significantly alter the pattern of synthesis arrest for any of the polymerases. The correct nucleotide, dAMP, was inserted opposite the lesion more than 90% of the time by all four polymerases, suggesting that thymine glycol forms a reasonably stable base pair with adenine. However, the 3'-5' exonuclease activity of polymerase I removed a 3'-terminal dAMP residue more rapidly from an A . thymine glycol base pair than from an A.T base pair. These results suggest that increased nucleotide turnover at the site of the lesion contributes to the inhibitory effects of thymine glycol lesions on DNA synthesis in vitro, at least for polymerases such as polymerase I that have intrinsic or associated editing exonuclease functions.

Base Sequence↗
PubMed1975

A kinetic study of thymine 7-hydroxylase from neurospora crassa.

The steady-state kinetics of thymine 7-hydroxylase (thymine, 2-oxoglutarate dioxygenase, EC 1.14.11.6) has been investigated. Initial velocity plots were all found to be linear and intersecting. Variation in concentration of two of the substrates, when the third substrate was at a constant high or low concentration, gave initial velocity plots that conform to an ordered sequential mechanism, where thymine is the second substrate to add. With 5-carboxyuracil, which is the end product in the sequential oxygenation of thymine, a competitive inhibition pattern was observed when 2-ketoglutarate was the variable substrate. When either thymine or oxygen was the variable substrate a noncompetitive inhibition pattern was obtained. When either 2-ketoglutarate or thymine was the variable substrate the inhibition patterns observed with bicarbonate were noncompetitive. With succinate noncompetitive inhibition patterns with hyperbolic intercept replots were obtained. These results are consistent with an ordered sequential kinetic mechanism, where 2-ketoglutarate is added first, followed by thymine and oxygen, and the products are released in the order: bicarbonate, succinate, and 5-hydroxymethyluracil. The order of the two last mentioned products, however, is changed in the presence of succinate.

Ketoglutaric Acids↗
PubMed2007

Investigation of proton transport tautomerism in clusters of protonated nucleic acid bases (cytosine, uracil, thymine, and adenine) and ammonia by high-pressure mass spectrometry and ab initio calculations.

The energetics of the ion-molecule interactions and structures of the clusters formed between protonated nucleic acid bases (cytosine, uracil, thymine, and adenine) and ammonia have been studied by pulsed ionization high-pressure mass spectrometry (HPMS) and ab initio calculations. For protonated cytosine, uracil, thymine, and adenine with ammonia, the measured enthalpies of association with ammonia are -21.7, -27.9, -22.1, and -17.5 kcal mol-1, respectively. Different isomers of the neutral and protonated nucleic acid bases as well as their clusters with ammonia have been investigated at the B3LYP/6-31+G(d,p) level of theory, and the corresponding binding energetics have also been obtained. The potential energy surfaces for proton transfer and interconversion of the clusters of protonated thymine and uracil with ammonia have been constructed. For cytosine, the experimental binding energy is in agreement with the computed binding energy for the most stable isomer, CN01-01, which is derived from the enol form of protonated cytosine, CH01, and ammonia. Although adenine has a proton affinity similar to that of cytosine, the binding energy of protonated adenine to ammonia is much lower than that for protonated cytosine. This is shown to be due to the differing types of hydrogen bonds being formed. Similarly, although uracil and thymine have similar structures and proton affinities, the binding energies between the protonated species and ammonia are different. Strikingly, the addition of a single methyl group, in going from uracil to thymine, results in a significant structural change for the most stable isomers, UN01-01 and TN03-01, respectively. This then leads to the difference in their measured binding energies with ammonia. Because thymine is found only in DNA while uracil is found in RNA, this provides some potential insight into the difference between uracil and thymine, especially their interactions with other molecules.

Adenine↗
PubMed2005

Interfacial molecular recognition of dissolved thymine by medium chain dialkyl melamine-type monolayers.

Systems consisting of an amphiphilic melamine-type monolayer and a pyrimidine derivative dissolved in the aqueous subphase are good candidates for the formation of interfacial supramolecular assemblies by molecular recognition of hydrogen-bond nonsurface-active species. In the present work, the change in the thermodynamic, phase, and structural properties as a result of molecular recognition of dissolved thymine by 2,4-di(n-undecylamino)-6-amino-1,3,5-triazine (2 C11H23-melamine) monolayers is studied. The combination of surface pressure studies with Brewster angle microscopy (BAM) imaging and grazing incidence X-ray diffraction (GIXD) measurements is optimal for the characterization of the change in structure and phase behavior at the interfacial recognition process. The molecular recognition of the nonsurface-active thymine dissolved in aqueous subphase changes drastically the characteristic features (surface pressure-area isotherms, morphology of the condensed phase domains) of the 2 C11H23-melamine monolayer. It is demonstrated that the kinetics of the recognition process affect largely the main characteristics (phase behavior, morphology of the condensed phase domains) of the interfacial system. The monolayers of 2 C11H23-melamine-thymine assemblies form dumbbell-shaped condensed phase domains not yet observed in other Langmuir monolayers so far. GIXD results show that the molecular recognition of thymine causes only quantitative changes in the two-dimensional lattice structure. Complementary hydrogen bonding of two thymine molecules by one 2 C11H23-melamine molecule is concluded from the chemical structure of both components. Additional information about the nature of the hydrogen bonding on the basis of supramolecular assemblies is obtained by using the quantum chemical PM3 approximation. Energy and lengths of the hydrogen bonds of the optimized thymine-2 C11H23-melamine-thymine structure are calculated.

Chemistry, Physical↗
PubMed2005

Fragmentation dynamics of condensed phase thymine by low-energy (10-200 eV) heavy-ion impact.

We report measurements of the formation and desorption of ionic fragments induced by very low-energy (10-200 eV) Ar(+) irradiation of thymine (T) films, deposited on a polycrystalline Pt substrate. A multitude of dissociation channels is observed, among which the major cation species are identified as HNCH(+), HNC(3)H(4) (+), C(3)H(3) (+), OCNH(2) (+), [T-OCN](+), [T-OCNH(2)](+), [T-O](+), and [T+H](+) and the major anions as H(-), O(-), CN(-),and OCN(-). Cation fragment desorption appears at much lower threshold energies (near 15 eV) than anion fragment desorption, where the latter depends strongly on the film thickness. It is proposed that anion fragment formation and desorption results from projectile impact-induced excitation of either (1) a neutral thymine molecule, followed by fragmentation and charge exchange between the energetic neutral fragment and the substrate (or film) and/or (2) a deprotonated monoanionic thymine molecule to a dissociative state, followed by a unimolecular fragmentation of the excited thymine anion. The H(-) and O(-) fragment formations may have a further contribution from dipolar dissociation, e.g., formation of electronically excited neutral thymine, followed by dissociation into O(-)+[T-O](+), due to their reduced sensitivity to the film thickness. Positive-ion fragment desorption exhibits no significant dependence on film thickness before the emergence of surface charging, and originates from a kinetically assisted charge-transfer excitation. The results suggest that the potential energy of the incident ion plays a significant role in lowering the threshold energy of kinetic fragmentation of thymine. Measurements of the time-dependent film degradation yields for 100-eV Ar(+) suggest a quantum efficiency for degradation of about six thymine molecules per incident ion.

Cations↗
PubMed2001

Effect of thymine glycol on transcription elongation by T7 RNA polymerase and mammalian RNA polymerase II.

Thymine glycols are formed in DNA by exposure to ionizing radiation or oxidative stress. Although these lesions are repaired by the base excision repair pathway, they have been shown also to be subject to transcription-coupled repair. A current model for transcription-coupled repair proposes that RNA polymerase II arrested at a DNA lesion provides a signal for recruitment of the repair enzymes to the lesion site. Here we report the effect of thymine glycol on transcription elongation by T7 RNA polymerase and RNA polymerase II from rat liver. DNA substrates containing a single thymine glycol located either in the transcribed or nontranscribed strand were used to carry out in vitro transcription. We found that thymine glycol in the transcribed strand blocked transcription elongation by T7 RNA polymerase approximately 50% of the time but did not block RNA polymerase II. Thymine glycol in the nontranscribed strand did not affect transcription by either polymerase. These results suggest that arrest of RNA polymerase elongation by thymine glycol is not necessary for transcription-coupled repair of this lesion. Additional factors that recognize and bind thymine glycol in DNA may be required to ensure RNA polymerase arrest and the initiation of transcription-coupled repair in vivo.

Animals↗
PubMed1978

Incorporation of labelled degradation products of radioactive thymine into non DNA material.

When thymine auxotrophs are grown in the presence of methyl labelled [3H] or [14C] thymine which has been stored at 4 degrees C, two classes of material are labelled which are not DNA. One class sediments on neutral sucrose gradients with spontaneously single stranded Okazaki pieces, is unstable in alkali, migrates on alkaline gels as very small material and is digested by ribonucleases and micrococcal nuclease, but not by DNAase I. This class is presumably RNA. The second class sediments more slowly on both neutral and alkaline sucrose gradients than Okazaki pieces, but co-migrates on alkaline gels with DNA whose size is between 700 and 4000 nucleotides. It is not digested by alkali, ribonucleases or deoxyribonucleases. Its identity is unknown. The proportion of the total incorporated counts in these two classes depends on the time of storage of the thymine and is already sufficient to interfere with certain types of experiments when the thymine is only a few weeks old. Thymine is easily purified by paper chromatography and this purified thymine does not label the two non DNA classes of material. It is recommended that radioactive thymine be purified in this way before use.

Carbon Radioisotopes↗
PubMed1990

Formation of cyclobutane thymine dimers photosensitized by pyridopsoralens: a triplet-triplet energy transfer mechanism.

The 365 nm irradiation of thymine thin films in the presence of pyridopsoralens is shown to induce the formation of cyclobutane thymine dimers, in contrast to other compounds such as 8- and 5-methoxypsoralen. In order to elucidate the mechanism of such a photosensitized reaction, we have determined the energy of the lowest triplet state (T1) of these compounds, using phosphorescence spectroscopy and CNDO/S quantum chemistry calculations. The T1 energy values were found to be significantly higher for pyridopsoralens--up to 0.3 eV--than for 8- and 5-methoxypsoralen (approximately 2.8 eV), which are not able to photoinduce cyclobutane thymine dimers. The determination of the relative efficiency of cyclobutane thymine dimer formation was performed using chromatographic analysis. A good correlation was found between the energy of the T1 state of the psoralen derivatives and the related cyclobutane thymine dimer formation. Moreover, the photosensitized cyclobutane thymine dimer formation appeared to be temperature-dependent. Our results are consistent with a mechanism involving a triplet energy transfer from the pyridopsoralen to thymine.

Energy Transfer↗
PubMed1990

Quinone sensitized electron transfer photooxidation of nucleic acids: chemistry of thymine and thymidine radical cations in aqueous solution.

The 2-methyl-1,4-naphthoquinone (MQ) sensitized photooxidation of nucleic acid derivatives has been studied by laser flash photolysis and steady state methods. Thymine and thymidine, as well as other DNA model compounds, quench triplet MQ by electron transfer to give MQ radical anions and pyrimidine or purine radical cations. Although the pyrimidine radical cations cannot be directly observed by flash photolysis, the addition of N,N,N',N'-tetramethyl-1,4-phenylenediamine (TMPD) results in the formation of the TMPD radical cation via scavenging of the pyrimidine radical cation. The photooxidation products for thymine and thymidine are shown to result from subsequent chemical reactions of the radical cations in oxygenated aqueous solution. The quantum yield for substrate loss at limiting substrate concentrations is 0.38 for thymine and 0.66 for thymidine. The chemistry of the radical cations involves hydration by water leading to C(6)-OH adduct radicals of the pyrimidine and deprotonation from the N(1) position in thymine and the C(5) methyl group for thymidine. Superoxide ions produced via quenching of the quinone radical anion with oxygen appear to be involved in the formation of thymine and thymidine hydroperoxides and in the reaction with N(1)-thyminyl radicals to regenerate thymine. The effects of pH were examined in the range pH 5-8 in both the presence and absence of superoxide dismutase. Initial C(6)-OH thymine adducts are suggested to dehydrate to give N(1)-thyminyl radicals.

Cations↗
PubMed1968

Control of the synthesis of macromolecules during amino acid and thymine starvation in Bacillus subtilis.

Studies of Maaløe, Lark, and others with amino acid- and thymine-starved cultures revealed successive steps in the biosynthesis of Escherichia coli chromosomes. In this study, the corresponding mechanisms in Bacillus subtilis 168 were examined. Using a strain requiring both thymine and tryptophan, we found that, 3 hr after the start of amino acid starvation, when the deoxyribonucleic acid (DNA) content of the culture had increased 40 to 50%, DNA synthesis ceased. After 4 to 5 hr, 100% of the cells were immune to thymineless death; their chromosomes had presumably been completed. Immune cultures slowly incorporated (3)H-thymine. Thymine incorporation increased 20-fold 30 min after readdition of amino acids, indicating reinitiation of chromosome synthesis. Simultaneous presence of amino acids and thymine was required for reinitiation. If 5-bromouracil (5-BU) was added instead of thymine, newly replicated DNA segments could be separated by centrifugation in CsCl. Analysis of the CsCl fractions by a transformation assay showed that the order in which the markers were synthesized was ade-16, thr-5, leu-8, metB5. Less than half the chromosomes started resynthesis synchronously in 5-BU. Nevertheless, chromosome alignment in the amino acid-starved culture is probably very good: marker frequency analysis of its DNA gives the same normalized frequencies as DNA from "perfectly" aligned spores. Full viability is maintained in the chromosome-arrested culture for 10 hr in thymine-free medium in the absence or presence of amino acids. In the latter condition, protein synthesis proceeds, and the cells filament and become more lysozyme-sensitive. Such cells must be incubated and plated on hypertonic or on slow-growth media; otherwise, they undergo "quasiosmotic" thymineless death. This death is thus apparently not directly attributable to any damage of chromosomal DNA. Further, weakening of the teichoic acid portion of the cell wall is not involved, since (32)P incorporation into teichoic acid is normal. Chloramphenicol prevents quasiosmotic thymineless death and also inhibits (32)P incorporation into teichoic acid. Chromosome-synthesizing cultures suffer thymineless death of two types: quasiosmotic death, and death insusceptible to osmotic rescue.

Amino Acids↗
PubMed1995

The yeast RAD2, but not RAD1, gene is involved in the transcription-coupled repair of thymine glycols.

Transcription-coupled repair of H2O2-induced thymine glycols and UV-induced pyrimidine dimers has been shown to occur in the yeast Saccharomyces cerevisiae. In order to determine whether the preferential repair of thymine glycols is carried out by the same nucleotide excision repair complex that removes pyrimidine dimers, we examined the repair of thymine glycols in two yeast mutants, rad1 delta and rad2 delta, in which the nucleotide excision repair pathway was disrupted. We find that in both wild-type and a rad1 delta mutant, repair of thymine glycols occurs faster on the transcribed strand of the GAL7 gene than on the nontranscribed strand. This indicates that transcription-coupled repair can occur in the absence of nucleotide excision repair and that repair of oxidative DNA damage initiated by an N-glycosylase can be coupled to transcription. In contrast, the initial rate of repair of thymine glycols on the transcribed strand of the GAL7 gene in a rad2 delta mutant is significantly slower than that for the wild-type cells, with kinetics similar to that of the nontranscribed strand. However, by 60 min post-treatment, the amount of repair on the transcribed strand in the rad2 delta eventually reaches that of the wild-type cells. We conclude that repair of oxidative DNA damage, such as thymine glycols, can be coupled to transcription and that RAD2 facilitates transcription-coupled repair of oxidative DNA damage in yeast.

DNA Probes↗
PubMed2004

Synthesis and evaluation of thymine-derived carboxamides against mitochondrial thymidine kinase (TK-2) and related enzymes.

Based on the structure of our previously identified mitochondrial thymidine kinase (TK-2) inhibitors, three series of thymine-derived carboxamides have been synthesized and tested against TK-2 and related enzymes. The methodology employed has been a solution-phase parallel synthesis based on the coupling of three thymine-derived acids [4-(thymin-1-yl)butyric acid (I), [4-(thymin-1-yl)-butyrylamino]acetic acid (II) and 6-(thymin-1-yl)hexanoic acid (III)] with different commercially available primary amines that carry cyano and/or phenyl groups. The couplings were performed in good yields (from 60% to 90%), with the exception of those that incorporate the highly crowded triphenylmethylamine (e). From the new synthesized compounds, the N-trityl-6-(thymin-1-yl)hexanamide (IIIe) was the most active TK-2 inhibitor (IC(50)=19+/-2microM).

Amino Acids↗
PubMed1968

-irradiation of thymine dimers in aqueous solution.

Thymine dimers were irradiated in aqueous solution with (60)Co gamma-rays in N(2) or O(2). Thymine and unidentified non-UV-absorbing products appeared. The thymine was identified by spectrophotometry, chromatography, and ability to support the growth of Escherichia coli 15 T(-). Residual dimer was determined by a UV-reversibility assay. The G-values for dimer breakage were approximately equal in N(2) and O(2). At low gamma-doses, about two thymines were produced per dimer broken in N(2), whereas only about one thymine appeared per dimer broken in O(2). For dimer irradiated in frozen solution, the yield of thymine was at least 100 times less than in liquid.

Biological Assay↗