Biomedical subjects
Publications and source records attributed to Tetsuro Majima.
Rapid cleavage of the naphthylmethyl-oxygen bond of 1- and 2-[(4-benzoylphenoxy)methyl]naphthalenes in higher triplet excited states occurred within a laser flash of 5 ns to give 1- and 2-naphthylmethyl radicals with formation quantum yields of 0.042 +/- 0.004 and 0.020 +/- 0.002, respectively, during two-colour two-laser flash photolysis.
The kinetics of hole transfer in DNA by adenine-hopping mechanism was investigated by the combined pulse radiolysis-laser flash photolysis method. The hole transfer from Ptz*+* to oxG across the (A)n-bridge preceded by the A-hopping mechanism and the weak distance-dependent hole transfer with the rates faster than 108 s-1 over the distance range of 7-22 A was demonstrated. In contrast, hole transfer from oxG*+ to Ptz followed the single-step super exchange mechanism. Thus, two different processes for the hole transfer across the identical (A)n-bridge in DNA have been demonstrated. The results clearly show that the mechanism of hole transfer in DNA strongly depends on the redox nature of the oxidant, whether it produces only G*+ or both A*+ and G*+.
Doubly pyrene (Py)-conjugated oligodeoxynucleotides (ODNs) were synthesized and used for measurement of the formation rates of Py dimer radical cation (Py(2)(.+)) upon one-electron oxidation during the pulse radiolyses. Formation of Py radical cation (Py(.+)) in the time scale of less than 5 micros was monitored at 470 nm after an electron pulse during pulse radiolysis of D(2)O solution of doubly Py-conjugated ODN in the presence of K(2)S(2)O(8). Concomitant with the decay of Py(.+), formation of Py(2)(.+) with an absorption peak at 1500 nm (charge resonance band) was observed in the time range of approximately 100 micros. The formation rate of Py(2)(.+) in DNA reflected the dynamics of DNA which allows the interaction between Py(.+) and Py, since transiently formed DNA structure is trapped by the attractive charge resonance (CR) interaction to give Py(2)(.+). The formation rate of Py(2)(.+) with a characteristic CR absorption band in the near-infrared (near-IR) region was demonstrated to be useful to obtain the structural and dynamical information of transiently formed DNA in the time range of 1 micros to 1 ms.
A time-delayed, two-color pulse laser photolysis technique was used for a kinetic study of short-lived transient species through product analysis, the determination of the rate constant of the cycloaddition of o-quinodimethane (1) and maleic anhydride (2) in room-temperature solutions. o-Quinodimethane (1) was generated from 1,2-bis[(phenylseleno)methyl]benzene (3) by the irradiation of a pulse of a KrF excimer laser (248 nm) in the presence of excess 2, and a successive pulse of a XeCl excimer laser (308 nm) was irradiated to the reaction mixture after varied delay times from 0 to 0.1 s for the decomposition of the remaining 1 to quench the cycloaddition reaction. The rate constant of the cycloaddition of 1 and 2 was 2.1 x 10(5) M(-1) s(-1), which was obtained by the analysis of the delay-time dependence of the product yields.
Naphthalene in the higher triplet excited state Np(Tn) was generated from the two-step excitation method using two-colour two-laser flash photolysis technique and the lifetime of Np(Tn) was estimated to be 4.5 ps from the triplet energy quenching by quenchers such as p-dichlorobenzene, o-dicyanobenzene and carbon tetrachloride.
Transient phenomena of benzophenone (BP) in the higher triplet excited state (Tn) have been investigated by the two-colour two-laser excitation method. Triplet energy transfer from BP(Tn) to quenchers (Q) occurred within the duration of a laser pulse (5 ns) to give Q(T1) with higher triplet energy than that of BP(T1). The quantum yield of the triplet energy-transfer quenching of BP(Tn) by CCl4 was found to be 0.0023 +/- 0.0002 from the bleaching of the transient absorption of BP(T1) and the absorbed photon number. It appears that internal conversion from BP(Tn) to BP(T1) is the predominant process. The lifetimes (tauTn) of BP(Tn) and several substituted benzophenones (BPs) in the higher triplet excited state [BPs(Tn)] were estimated from the dependence of the Q concentration on the efficiency of the triplet energy-transfer quenching of BP(Tn) by Q, and found to be 110-450 ps, depending on the nature of the substituents on the BPs. The effect of the substituents on tauTn may be explained by the energy gap between the Tn and T1 states, because the main deactivation pathway for BPs(Tn) is the internal conversion process. In contrast, the substituent effect on the lifetimes of BPs(T1) cannot be explained by the energy gap law. The transient behaviour of Q(T1) depends on the properties of the quencher. Sequential triplet energy transfer from Q(T1) to BP occurred for p-dichlorobenzene and tert-butylbenzene as quenchers, while Q(T1) reacted partly with Q to form triplet excimers (3Q2*) for benzene, chlorobenzene, and o-dichlorobenzene as quenchers. When CCl4 was used as the quencher, the homolytic cleavage of a C-Cl bond of CCl4(T1) occurred to give Cl* and Cl3C* radicals.
Synthesis and fluorescence properties of oligodeoxynucleotides containing 4-methylamino-1,8-naphthalimide (NI) have been described. NI was successfully incorporated into DNA without significant destabilization of DNA whilst retaining its high fluorescence quantum yield. The attachment site of the NI greatly affected its property as an energy acceptor in FRET analysis.
Photodenitrogenation of the diazenes 4 affords exclusively the housanes 5 through intramolecular cyclization of the spectrally detected and characterized singlet diradicals 3. The lifetime of singlet diradical 3, determined by transient absorption measurements, depends on the Y and Z substituents at the para position of the phenyl ring and has the following order: Y, Z = OMe, OMe > OMe, CN > CN, CN > OMe, H > Cl, Cl approximately CN, H approximately Me, Me > H, H. This unprecedented substituent effect reveals stabilization of the singlet 2,2-dimethoxycyclopentane-1,3-diyl diradicals 3 through radical, zwitterionic, pi-bonding, and hyperconjugative structures.
Effects of base pairing on the one-electron oxidation rate of guanine derivatives, guanine, 8-bromoguanine, and 8-oxo-7,8-dihydroguanine have been studied. The one-electron oxidation rate of guanine derivatives was determined by triplet-quenching experiments, using N,N'-dibutylnaphthaldiimide (NDI) in the triplet excited state (3NDI*) and fullerene (C(60)) in the triplet excited state ((3)C(60*)) as oxidants. In all three guanine derivatives studied here, acceleration of the one-electron oxidation was observed upon hydrogen bonding with cytosine, which demonstrates lowering of the oxidation potential of guanine derivatives by base pairing with cytosine. When a methyl or bromo group was introduced to the C5 position of cytosine, acceleration or suppression of the one-electron oxidation relative to the guanine:cytosine base pair was observed, respectively. The results demonstrate that the one-electron oxidation rate of guanine in DNA can be regulated by introducing a substituent on base pairing cytosine.
The photoinduced electron transfer of tri-1-naphthyl phosphate and related compounds sensitized by 9,10-dicyanoanthracene (DCA) in acetonitrile produces 1,1'-binaphthyl and the corresponding biaryl. The quenching rate constant of the DCA fluorescence is calculated to be equal to the diffusion-controlled rate constant from the Stern-Volmer analysis and the fluorescence lifetime measurement. The free energy change calculated from the redox potentials and excitation energy of the singlet excited DCA indicates that the quenching process occurs exergonically to give the tri-1-naphthyl phosphate radical cation and DCA radical anion through electron transfer from tri-1-naphthyl phosphate to the singlet excited DCA at the diffusion-controlled rate. On the basis of spectroscopic and kinetic studies with laser flash photolysis, pulse radiolysis, and gamma-radiolysis, the radical cation of tri-1-naphthyl phosphate forms an intramolecular pi-dimer radical cation with face-to-face interaction between the two naphthyl groups within 8 ns of the electron pulse. The 1,1'-binaphthyl radical cation is eliminated at the rate constant of k(r) = 5.3 x 10(5) s(-)(1) from the intramolecular pi-dimer radical cation. Branching ratios of the reaction pathways are estimated for the reactive intermediates such as the tri-1-naphthyl phosphate radical cation and its intramolecular pi-dimer radical cation from the rate constants and quantum yields. The electron-withdrawing character of the P(O) group in the O-P(O)-O spacer is responsible for the elimination of the binaphthyl radical cation. The DCA-sensitized photoinduced electron-transfer reaction of the tri-1-naphthyl phosphate is compared with the direct photoreaction.
Isomerization from cis stilbene derivatives (c-S (S = RCH=CHC(6)H(5): 1, R = C(6)H(5); 2, R = 4-CH(3)C(6)H(4); 3, R = 4-CH(3)OC(6)H(4) (= An); 4, R = 2,4-(CH(3)O)(2)C(6)H(3); 5, R = 3,4-(CH(3)O)(2)C(6)H(3); 6, R = 3,5-(CH(3)O)(2)C(6)H(3); 7, AnCH=C(CH(3))C(6)H(5); 8, AnCH=CHAn)) to trans isomers (t-S) and oxidation of S with O(2) were studied in gamma-ray radiolyses of c-S in Ar-saturated 1,2-dichloroethane (DCE) and of S in O(2)-saturated DCE, respectively. On the basis of product analyses, it is suggested that a smaller barrier to c-t unimolecular isomerization for c-3(*+)-5(*+) and 8(*+) than for c-1(*+), 2(*+), and 6(*+) due to the single bond character of the central C=C double bond for c-3(*+)-5(*+) and 8(*+) with a p-methoxyl group but not for c-1(*+), 2(*+), and 6(*+) without a p-methoxyl group because of the contribution of a quinoid-type structure induced by charge-spin separation. The isomerization proceeds via chain reaction mechanisms involving c-t unimolecular isomerization and endergonic hole transfer or dimerization and decomposition. The isomerization of c-3(*+) to t-3(*+) is catalyzed by addition of 1,4-dimethoxybenzene but terminated by triethylamine. The regioselective formation of 3d in oxidation of 3(*+) with O(2) is explained by spin localization on the beta-olefinic carbon in 3(*+). The results of product analyses are compared with the rate constants of the unimolecular isomerization and the oxidation for S(*+) measured with pulse radiolyses.
A novel hydrophilic and negatively charged azobenzene-bearing amino acid, 4'-carboxyphenylazophenylalanine (azoAla 1), has been designed and synthesized for investigation of the photochemical regulation of the enzyme activity. The properties of photoisomerization and thermal stability of the cis-isomer were similar to those of a commonly used phenylazophenylalanine (azoAla 2). For photochemical control of the enzyme, these two azobenzene-bearing amino acids were incorporated into the specific position at the dimer interface of a restriction enzyme BamHI. These trans-azobenzene derivatives in the BamHI suppressed the enzymatic activity, and the following photoirradiation at 366 nm induced the recovery of its activity. Although the activities of both azoAla-BamHI mutants were same level after a long time irradiation, the recovery of the activity of azoAla 1-BamHI was faster than that of azoAla 2-BamHI with a short time irradiation. This result suggests that the negatively charged carboxylate group introduced into an azobenzene moiety affects the behavior of azoAla in the protein scaffold during the trans-cis photoisomerization.