Secondary Photoreactions Research Articles

Influence of blocking groups on photo-oxidation of tyrosine and derivatives

One-electron oxidation of tyrosine primarily results in the formation of di-tyrosine, which can induce crosslinks leading to protein damage. In this study, we investigated the 3-carboxybenzophenone-sensitized photo-oxidation of Tyr derivatives through time-resolved and steady-state photolysis under anaerobic conditions to analyze the effects of blocking groups.The mechanism for primary and secondary photoreactions in the sensitized photo-oxidation of Tyr derivatives in aqueous solution was presented based on time-resolved analysis and mass spectrometric characterization of photo-oxidation products. Identified di-Tyr products (in addition to those mentioned more often in the literature, such as 3,3′/3,O’) were in some samples presented together with Tyr-CBH adduct (resulting from radical recombination between the tyrosyl radical and CBH•). This publication discusses the possible coulombic effects of interacting ionic species (sensitizer and quencher) on quenching rate constants and the effect of amine groups and steric factors on the distribution of stable products. However, a crucial finding of this work is that the more blocked the Tyr is, the more di-Tyr isomers are formed, suggesting that Tyr residue in proteins may form several forms of di-Tyr cross-links.

Modeling the Dynamics of Secondary Photoreactions

Modeling the Dynamics of Secondary Photoreactions

Early Events of Photosensitized Oxidation of Sulfur-Containing Amino Acids Studied by Laser Flash Photolysis and Mass Spectrometry.

The mechanism of photooxidation of methionine (N-Ac-Met-NH-CH3, 1) and methyl-cysteine (N-Ac-MeCys-NH-CH3, 2) analogues by 3-carboxybenzophenone triplet (3CB*) in neutral aqueous solution was studied using techniques of nanosecond laser flash photolysis and steady-state photolysis. The short-lived transients derived from 3CB and sulfur-containing amino acids were identified, and their quantum yields and kinetics of formation and decay were determined. The stable photoproducts were analyzed using liquid chromatography coupled with high-resolution mass spectrometry. Substantial differences in the mechanisms were found for methionine and S-methyl-cysteine analogues for both primary and secondary photoreactions. A new secondary reaction channel (back hydrogen atom transfer from the ketyl radical to the carbon-centered α-thioalkyl radical yielding reactants in the ground states) was suggested. The detailed mechanisms of 3CB* sensitized photooxidation of 1 and 2 are proposed and discussed.

Photopatternable Epoxy-Based Thermosets.

The present work provides a comparative study on the photopatterning of epoxy-based thermosets as a function of network structure and network mobility. Local switching of solubility properties by light of a defined wavelength is achieved by exploiting versatile o-nitrobenzyl ester (o-NBE) chemistry. o-NBE derivatives with terminal epoxy groups are synthetized and thermally cured with different types of cycloaliphatic anhydrides via nucleophilic ring opening reaction. By varying the structure of the anhydride, glass transition temperature (Tg) and surface hardness are adjusted over a broad range. Once the network has been formed, the photolysis of the o-NBE groups enables a well-defined degradation of the 3D network. Fourier transform infrared (FT-IR) spectroscopy studies demonstrate that cleavage rate and cleavage yield increase with rising mobility of the network, which is either facilitated by inherent network properties (Tg below room temperature) or a simultaneous heating of the thermosets above their Tg. The formation of soluble species is evidenced by sol-gel analysis, revealing that low-Tg networks are prone to secondary photoreactions at higher exposure doses, which lead to a re-crosslinking of the cleaved polymer chains. The change in solubility properties is exploited to inscribe positive tone micropatterns within the thermosets by photolithographic techniques. Contrast curves show that the resist performance of rigid networks is superior to flexible ones, with a contrast of 1.17 and a resolution of 8 µm.

Semiconductor Photocatalysis for Chemoselective Radical Coupling Reactions.

Photocatalysis at semiconductor surfaces is a growing field of general photocatalysis because of its importance for the chemical utilization of solar energy. By analogy with photoelectrochemistry the basic mechanism of semiconductor photocatalysis can be broken down into three steps: photogenerated formation of surface redox centers (electron-hole pairs), interfacial electron transfer from and to substrates (often coupled with proton-transfer), and conversion of primary redox intermediates into the products. Sun driven water cleavage and carbon dioxide fixation are still in the state of basic research whereas aerial degradation reactions of pollutants have reached practical application for the cleaning of air. In addition, a great variety of organic transformations (not syntheses) have been reported. They include cis-trans isomerizations, valence isomerizations, cycloaddition reactions, intramolecular or intermolecular C-N and C-C couplings, partial oxidations, and reductions. In all cases, well-known products were formed but very rarely also isolated. As compared to conventional homogeneous organic synthesis, the photocatalytic reaction mode is of no advantage, although the opposite is quite often claimed in the literature. It is also noted that a high quantum yield does not implicate a high product yield, since it is measured at very low substrate conversion in order to minimize secondary photoreactions. That is especially important in semiconductor photocatalysis since photocorrosion of the photocatalyst often prevents long-time irradiation, as is the case for colloidal metal sulfide semiconductors, which in general are photochemically too unstable to be used in synthesis. In this Account, we first classify the numerous organic photoreactions catalyzed by semiconductor powders. The classification is based on easily obtainable experimental facts, namely the nature of the light absorbing reaction component and the reaction stoichiometry. Next we discuss the problem of quantitative comparisons of photocatalytic activities or apparent quantum yields and propose a basic three-step mechanistic model. Finally, we address the question whether or not the unique photoredox properties of simple inorganic semiconductor powders may lead to previously unknown visible light induced organic syntheses. For that, we summarize novel radical C-C- and C-N- couplings photocatalyzed by self-prepared cadmium sulfide powders. Electron acceptor and donor substrates like imines or 1,2-diazenes, and cyclic olefins or unsaturated ethers, respectively, undergo a linear addition reaction. The hitherto unknown products have all been isolated in good to moderate yields and may be of pharmaceutical interest. In the first reaction step photogenerated electron-hole pairs produce through proton-coupled electron transfer the corresponding radicals. Their subsequent chemoselective heterocoupling affords the products, correlating with an insertion of the imine or 1,2-diazene into an allylic C(sp3)-H bond of the donor substrate. In the absence of an imine or 1,2-diazene, cyclic allyl/enol ethers are dehydrodimerized under concomitant hydrogen evolution. Even a visible light photosulfoxidation of alkanes is catalyzed by titania. In these heterogeneous photoredox reactions the role of the semiconductor photocatalyst is multifunctional. It induces favorable substrate preorientations in the surface-solvent layer, it catalyzes proton-coupled interfacial electron transfer to and from substrates generating intermediate radicals, and it enables their subsequent chemoselective coupling in the surface-solvent interface. Different from molecular photosensitizers, which enable only one one-electron transfer with one single substrate, photoexcited semiconductors induce two concerted one-electron transfer reactions with two substrates. This is because the light generated electron-hole pairs are trapped at distinct surface sites and undergo proton-coupled interfacial electron transfers with unsaturated donor and acceptor substrates. The radicals diffuse in a solid-solute-surface layer to undergo chemo- and stereoselective C-C and C-N bond formation. Thus, the semiconductor photocatalyst functions like an artificial leaf. Since several minerals are known to have semiconductor properties, solar photocatalysis may be also relevant for prebiotic and environmental chemistry.

The survival of glycine in interstellar ices: A coupled investigation using NEXAFS experiments and theoretical calculations

AbstractComplex organic molecules have been observed in the interstellar medium in comets and meteorites. Among these compounds are the building blocks that may have led to prebiotic systems and ultimately to the origin of life. In the panspermia hypothesis, the survival and the transfer of the amino acids from space to planets is a necessary condition for the appearance of life, and, especially, their resistance to the solar UV radiation in ice is a key issue. The case of glycine, which is the smallest molecule in the amino acid family, is presented here. To improve our knowledge on the decomposition mechanisms of glycine, we have undertaken a coupled experimental and computational study. On the one hand, it has been performed a near‐edge X‐ray absorption spectroscopy (NEXAFS) study at the oxygen K‐edge of solid glycine and of glycine diluted into H2O ice, and irradiated at 30 K with soft X‐rays. However, extensive quantum chemical simulations using density functional theory have been realized at the B3LYP/cc‐pVQZ level. A systematic investigation of the most plausible fragmentations has been performed for neutral glycine itself, ionized glycine, doubly ionized glycine, protonated glycine, and zwitterionic glycine, the well known stable form of glycine trapped in ices. We show that glycine easily decomposes under irradiation. Water does not enhance or protect glycine from photodecomposition, but it increases the production of CO2 in the secondary photoreactions. The concentration of glycine tends to a limit of ∼30% of the initial load in the ice. The role of water ice appears to be that of a containment environment allowing the fragments to remain close, thus leading to reformation of glycine in situ. Under these conditions, it can be concluded that glycine is partially, but not entirely, protected by ice during its journey to the Earth. It is a strong point to be credited to the panspermia hypothesis. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011

ChemInform Abstract: A Novel Photorearrangement of 1,3-Diketonatoborinate: A Photoinduced Intramolecular Alkylation on a Conjugated Carbonyl System.

Irradiation of borinates derived from 9-borabicyclo(3,3,1)nonane and 1,3-diketones at their π-π* transition band caused a rearrangement from their triplet excited state to give boronate intermediates that could be hydrolyzed to the corresponding aldols. The primary photolysis involves the scission of one of the B—C bonds followed by a 1,3- or 1,5-migration and the formation of a more stable B—O bond by radical pathways. The triplet state reaction was established by quenching and heavy atom effect experiments. The overall reaction pattern is an irreversible 1,2- or 1,4- addition of an alkyl-boron group to an α,β-conjugated ketone system by radical processes. Evidence showed that the non-isolated boronates also underwent secondary photoreactions followed by hydrolysis to afford enones corresponding to dehydration products of aldols; mechanisms are suggested for these secondary steps. The photoreaction was more facile in nonpolar solvents than in polar solvents; different solvents also gave different product compositions. In the presence of dilute acetic acid, the photolysis gave a high yield of aldols, suggesting a rapid acetolysis of the B—C bond in boronates to acetylborates. Under oxygen, the photolysis was complicated by radical oxidation to give totally different products.

Photolysis of an Intrachain Peptide Disulfide Bond: Primary and Secondary Processes, Formation of H2S, and Hydrogen Transfer Reactions

The photodissociation of intrachain disulfide bonds in a model peptide and salmon calcitonin generates a series of cyclic peptide products following the generation of a CysS(*) thiyl radical pair. Key to the formation of these cyclic products are disproportionation and reversible hydrogen atom transfer reactions as well as secondary photoreactions, which lead to C-S bond breakage of primary photoproducts. Depending on the wavelength of the incident light, disulfides ultimately convert into cyclic thioethers. An important photolytic product is H(2)S, which is highly relevant for the production and storage of protein pharmaceuticals, where H(2)S can catalyze disulfide scrambling and protein degradation.

In situ Reduction in Photocycloadditions: A Method to Prevent Secondary Photoreactions

Secondary photoreactions are a common cause for the low yields often observed in photochemical reactions, preventing their widespread deployment in synthesis. In situ reductions which remove the chromophore from the product as it is formed provide a convenient method to prevent these secondary photoreactions.

Benzophenone‐(phenylthio)acetic acid phosphonium salts as initiators of free‐radical photopolymerization of vinyl monomers: Mechanistic studies

AbstractThe mechanism of the benzophenone‐sensitized photooxidation of phosphonium salts of (phenylthio)acetic acid was studied as a means for understanding how these salts function as coinitiators in the free‐radical photopolymerization of vinyl monomers. Both steady‐state and nanosecond flash photolytic methods were used to determine, in a quantitative manner, the mechanism of primary and secondary photoreactions for three quaternary phosphonium salts containing butyl and/or phenyl groups, i.e., P+(C4H9)4, P+(C4H9)(C6H5)3, and P+(C6H5)4. It was found that the initial polymerization rates were the same for all three phoshonium salts of (phenylthio)acetic acid and were equal to those found previously for tetralkyl ammonium salts. The polymerization rates were more than twice the rates found for direct initiation by benzophenone and by the benzophenone‐(phenylthio)acetic acid initiating system. These results correlate well with the large quantum yields of •CH2SC6H5 radicals (the main initiating radicals) found in the complementary photochemical investigation. It was found that a detailed knowledge of the photochemical reactions in the photoinitiating systems was critical to understand the kinetics of polymerization. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 8013–8022, 2008

Selective Formation of Triplet Alkyl Nitrenes from Photolysis of β-Azido-Propiophenone and Their Reactivity

Photolysis of beta-azido propiophenone derivatives, 1, with built-in sensitizer units, leads to selective formation of triplet alkyl nitrenes 2 that were detected directly with laser flash photolysis (lambdamax = 325 nm, tau = 27 ms) and ESR spectroscopy (|D/hc| = 1.64 cm-1, |E/hc| = 0.004 cm-1). Nitrenes 2 were further characterized with argon matrix isolation, isotope labeling, and molecular modeling. The triplet alkyl nitrenes are persistent intermediates that do not abstract H-atoms from the solvent but do decay by dimerizing with another triplet nitrene to form azo products, rather than reacting with an azide precursor. The azo dimer tautomerizes and rearranges to form heterocyclic compound 3. Nitrene 2a, with an n,pi* configuration as the lowest triplet excited state of the its ketone sensitizer moiety, undergoes intramolecular 1,4-H-atom abstraction to form biradical 6, which was identified by argon matrix isolation, isotope labeling, and molecular modeling. beta-Azido-p-methoxy-propiophenone, with a pi,pi* lowest excited state of its triplet sensitizer moiety, does not undergo any secondary photoreactions but selectively yields only triplet alkyl nitrene intermediates that dimerize to form 3b.

Acidity and photolability of ruthenium salen nitrosyl and aquo complexes in aqueous solutions

The photochemical behavior of nitrosyl complexes Ru(salen)(NO)(OH 2) + and Ru(salen)(NO)Cl (salen = N,N′-ethylenebis-(salicylideneiminato) dianion) in aqueous solution is described. Irradiation with light in the 350–450 nm range resulted in nitric oxide (NO) release from both. For Ru(salen)(NO)Cl secondary photoreactions also resulted in chloride aquation. Thus, in both cases the final photoproduct is the diaquo cation Ru III ( salen ) ( OH 2 ) 2 + , for which p K a’s of 5.9 and 9.1 were determined for the coordinated waters. The p K a of the Ru(salen)(NO)(OH 2) + cation was also determined as 4.5 ± 0.1, and the relative acidities of these ruthenium aquo units are discussed in the context of the bonding interactions between Ru(III) and NO.

Proximate,syn-Periplanar Diazene/Diazene(di)oxy, Diazeneoxy/Diazene(di)oxy, and Diazenedioxy/Diazenedioxy Skeletons: Syntheses, [2+2]Photocycloadditions, Metathesis

Of two very proximate syn-periplanar bisdiazenes (1,2) mono-, di-, tri- and tetra-N-oxides were prepared, representing six combinations of the individual NN/NNO/ONNO chromophores. According to DFT calculations (B3LYP/6-31G*), [2+2]photocycloaddition to the respective oxidized tetrazetidines is significantly to moderately endothermic. The metathesis isomerization of the oxidized tetrazetidines is generally highly exothermic and kinetically increasingly favorable with increasing oxidation state. In practice, four out of the six bichromophoric combinations undergo selectively, in competition with N2 elimination from a DBH unit (13) still partially, metathesis isomerization upon π → π* excitation (monochromatic 254 nm light). In the case of the syn-NNO/NNO combinations (5/6, 14), the photoaddition is thermally reversed. For a ONNO/NN combination (30), internal electron transfer is responsible for a complex reaction pattern. The preparative value of the metathesis reactions, though, is limited: The metathesis-derived bis[diazene mono(di)oxides] undergo relatively fast secondary photoreactions, while the tri(tetra)oxides undergo rapid thermal transformations. For the NN/NNO systems (12), of three potential pathways for its metathesis isomerization, the one that takes place via σ-symmetric intermediates (63, 64) is excluded by virtue of the retention of optical purity in the photometathesis of a highly enriched enantiomer [(−)-12]. Matrix irradiation experiments (12 K, IR control) with 12 result in the appearance of a kinetically highly labile transient. Supported by DFT calculations it is concluded that in the metathesis reactions, the respective tetrazetidine oxides (increasingly destabilized by interactions between oxygen lone pairs and NNσ* orbitals) function as vibrationally excited transients. That thermal reversion of these transients might be a general, nonproductive competition, is suggested by the experimental verification of a “reversed photometathesis” (51 → 15) and by the generally low rates in product formation upon irradiation. The question remains to be answered why in structurally analogous molecular skeletons, [2+2]photocycloaddition occurs in the CC/NN and variously oxidized NN/NN, and not, however, in the parent NN/NN combinations.

A novel photorearrangement of 1,3-diketonatoborinate: a photoinduced intramolecular alkylation on a conjugated carbonyl system

Irradiation of borinates derived from 9-borabicyclo[3,3,1]nonane and 1,3-diketones at their π-π* transition band caused a rearrangement from their triplet excited state to give boronate intermediates that could be hydrolyzed to the corresponding aldols. The primary photolysis involves the scission of one of the B—C bonds followed by a 1,3- or 1,5-migration and the formation of a more stable B—O bond by radical pathways. The triplet state reaction was established by quenching and heavy atom effect experiments. The overall reaction pattern is an irreversible 1,2- or 1,4-addition of an alkyl-boron group to an α,β-conjugated ketone system by radical processes. Evidence showed that the non-isolated boronates also underwent secondary photoreactions followed by hydrolysis to afford enones corresponding to dehydration products of aldols; mechanisms are suggested for these secondary steps. The photoreaction was more facile in nonpolar solvents than in polar solvents; different solvents also gave different product compositions. In the presence of dilute acetic acid, the photolysis gave a high yield of aldols, suggesting a rapid acetolysis of the B—C bond in boronates to acetylborates. Under oxygen, the photolysis was complicated by radical oxidation to give totally different products.Key words: borinate photochemistry, charge transfer absorption, alkylborane addition, boronate intermediates, B—C bond homolytic scission

A novel photorearrangement of 1,3-diketonatoborinate: a photoinduced intramolecular alkylation on a conjugated carbonyl system

Irradiation of borinates derived from 9-borabicyclo[3,3,1]nonane and 1,3-diketones at their π-π* transition band caused a rearrangement from their triplet excited state to give boronate intermediates that could be hydrolyzed to the corresponding aldols. The primary photolysis involves the scission of one of the B—C bonds followed by a 1,3- or 1,5-migration and the formation of a more stable B—O bond by radical pathways. The triplet state reaction was established by quenching and heavy atom effect experiments. The overall reaction pattern is an irreversible 1,2- or 1,4-addition of an alkyl-boron group to an α,β-conjugated ketone system by radical processes. Evidence showed that the non-isolated boronates also underwent secondary photoreactions followed by hydrolysis to afford enones corresponding to dehydration products of aldols; mechanisms are suggested for these secondary steps. The photoreaction was more facile in nonpolar solvents than in polar solvents; different solvents also gave different product compositions. In the presence of dilute acetic acid, the photolysis gave a high yield of aldols, suggesting a rapid acetolysis of the B—C bond in boronates to acetylborates. Under oxygen, the photolysis was complicated by radical oxidation to give totally different products.Key words: borinate photochemistry, charge transfer absorption, alkylborane addition, boronate intermediates, B—C bond homolytic scission

Sensitized photo-oxidation of sulfur-containing amino acids and peptides in aqueous solution

Qualitative and quantitative studies of the photo-oxidation of sulfur-containing amino acids and methionine-containing dipeptides and tripeptides in aqueous solution sensitized by 4-carboxybenzophenone (CB) are reviewed. The mechanism of the photo-oxidation reaction was investigated using the techniques of flash photolysis, steady state photolysis and pulse radiolysis. The rate constants for quenching of the CB triplet by twelve sulfur-containing amino acids and six methionine-containing peptides were determined to be in the range 10 8–10 9 M −1 s −1 for both neutral and alkaline solutions. The amino acids varied in structure, having different numbers of COCH and NH 2 terminal groups and their sulfur atom at different locations relative to the terminal groups. The methionine-containing peptides were MetGly, GlyMet, MetMet MetGlyGly, GlyMetGly and GlyGlyMet. Time-resolved transient spectra accompanying the quenching events were assigned to the triplet states of CB, ketyl radicals of CB, radical anions of CB and radical anions of CB and radical cations derived from the amino acids and peptides. The radical cations identified were intermolecularly (S.·.S) +-bonded cations, intramolecularly (S.·.N) +-bonded cations and an intramolecularly (S.·.S) +-bonded radical cation that was observed in experiments with MetMet. The quantum yields of the transients and their kinetics of formation and decay were measured by flash photolysis. The quantum yields of CO 2 formation were determined by steady state photolysis. Electron transfer from the sulfur atom to the triplet state of the ketone was found to be a primary photochemical step. A detailed mechanism of the CB-sensitized photo-oxidation of sulfur-containing amino acids and methionine-containing peptides, including primary and secondary photoreactions, is proposed and discussed. Within the mechanism, contrasting behavior between the peptides and amino acids as quenchers is emphasized.

Factors affecting the formation of an M-like intermediate in the photocycle of 13-cis-bacteriorhodopsin.

The light-induced proton pumping activity of bacteriorhodopsin (bR) is based on the photocycle of its light-adapted all-trans-retinal protein pigment. The photocycle of the 13-cis pigment lacks the M intermediate (which carries a deprotonated retinal Schiff base, characteristic of the all-trans photocycle) and is not associated with proton release and uptake. Aiming at establishing the reasons for the lack of light-induced Schiff base deprotonation and proton pumping in 13-cis-bR, we carried out pulsed-laser and continuous excitation experiments with artificial 13-cis-bR pigments derived from 13-demethylretinal, 13-demethyl-14-fluoro-bR, and 13-demethyl-12,14-difluoro-bR. Pulsed-laser photolysis shows that both M formation and proton pumping are restored in 13-cis-13-demethyl-bR by raising the pH to 8.5-9. M formation, but not proton pumping, is restored at neutral pH by 14-fluorine substitution. Continuous-illumination experiments lead, in all cases, to the generation of extremely long-lived (minutes to hours) M photoproducts. We show that such species are due to secondary photoreactions of late intermediates of the primary photolysis. Feasible mechanisms accounting for Schiff base deprotonation in the all-trans photocycle, but not in that of 13-cis-bR, are considered. Our findings favor a mechanism which attributes the lack of light-induced Schiff base deprotonation of 13-cis-bR to an insufficient change in the relative pKa of the donor (Schiff base) and acceptor (probably Asp-85) groups and/or to a high activation barrier for the proton transfer. The required change in relative pKas may be achieved either by deprotonation of a protein moiety (YH) with pKa approximately 8.5 or by fluorine substitution at position 14. Similarly, both YH titration and 14-fluorine substitution may reduce the barrier for proton transfer by affecting H-bonding interactions in the vicinity of the Schiff base linkage. The lack of proton release and uptake in the photocycle of 13-cis-13-demethyl-14-fluoro-bR, despite the presence of an M intermediate, is discussed. It appears that Schiff base deprotonation does not essentially imply proton release and uptake. Our conclusions bear on the molecular mechanism of the photocycle and of proton pumping in all-trans-bR.

ChemInform Abstract: 4‐Carboxybenzophenone‐Sensitized Photooxidation of Sulfur‐Containing Amino Acids in Alkaline Aqueous Solutions. Secondary Photoreactions Kinetics

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The 4-carboxybenzophenone-sensitized photooxidation of sulfur-containing amino acids in alkaline aqueous solutions. Secondary photoreactions kinetics

Sulfur-containing amino acids and alanine were oxidized via photosensitization by 4-carboxybenzophenone (CB) in alkaline aqueous solutions. The mechanism of this reaction was examined using steady-state and laser flash photolysis techniques. The rate constants were determined for the quenching of the CB triplet state by five sulfur-containing amino acids and alanine and were found to be ∼10 9 and 1.8×10 8 M 8 s -1 , respectively. The observation of the (S..S) + radical cations of some of the amino acids showed that the quenching process involves an electron transfer from the sulfur atom to the triplet state of CB

QUANTUM YIELDS AND SECONDARY PHOTOREACTIONS OF THE PHOTOPRODUCTS OF dTpdT, dTpdC AND dTpdU

Abstract— The photoproducts of the dinucleoside monophosphates, dTpdT, dTpdC and dTpdU, have been purified by high performance liquid chromatography and characterized by UV absorption spectroscopy, fast atom bombardment mass spectrometry and by secondary thermal and photoreactions. Four types of photoproducts were analyzed: (1) cyclobutane dimers including cis‐syn isomers and two diastereomers of the trans‐syn isomers; (2) 6‐4 photoadducts and the corresponding Dewar valence isomers; (3) photohydrates comprising two diastereomers and (4) a new photoproduct resembling nucleobase amine adducts, which occurs only for dTpdC. The quantum yields of formation of these photoproducts and for some secondary photoreactions were measured by kinetic analysis of the photoproduct yield as a function of photon fluence. These results indicate that cis‐syn cyclobutane dimers are the photoproducts formed with highest efficiency with dT[p]dC dimers being formed with 50–75% the efficiency of dT[p]dT dimers. The 6‐4 photoadducts are formed with 5–10% the efficiency of cis‐syn cyclobutane dimers and the 6‐4 photoadduct of dTpdC is formed two to three times more efficiently than that of dTpdT. Photohydrates are also formed efficiently due to an equilibrium between stacked and unstacked complexes of the dinucleoside monophosphates. It is shown that three of these photoproducts, namely the cyclobutane dimers of dTpdC, the 6‐4 photoadducts and the possible nucleobase amine adduct, undergo photolysis in the UV‐B region resulting in either photoreversion or secondary photoreaction.