<scp>SCF</scp><scp>CI</scp> and <scp>SCF</scp> open‐shell studies of the base components of the nucleic acids

Kuprievich, V. A.

doi:10.1002/qua.560010504

Cited by 46 publications

(10 citation statements)

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“…The excited states of these bases have been investigated by SCF open-shell calculations [23] and the unrestricted Hartree-Fock metho4241 (see also lmamura et a1. [25] and Kuprievitch [26], and for comparative discussion on the properties of the triplet state, including the problem of the zero-field splitting parameters, A. Pullman [27]).…”

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confidence: 99%

Electronic Factors in the Photodimerization of Thymine

Pullman

1968

Photochem & Photobiology

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The role that quantum chemistry has played in the understanding of the photochemistry of pyrimidines is outlined. A variety of increasingly sophisticated calculations have shown that electronic excitation is localized largely at the double bond between carbons 5 and 6. which is the position of photodimerization ih thymine. So far. the theoretical procedures have not been sufficient to permit a distinction between different multiplicities for the excited levels, but refer only to a "first excited state". A discussion is given of why more extended calculations are necessary before any statement can be made as to whether it is the singlet or the triplet that is involved in thymine photodimerization. THE PHOTODIMERIZATION of thymine, first discovered upon U.V. irradiation of frozen thymine[ 1-41, and afterwards upon in uitro [5,6] and in vivo [7,8] irradiation of DNA, is one of the most important reactions in photobiology. The formation of such dimers in irradiated DNA probably constitutes one of the essential factors responsible for the sensitivity of nucleic acids and cells to the effects of u.v. light. It takes place through the C-5-C-6 bond of thymine and involves the formation of a cyclobutane ring. The reaction occurs also with more or less facility with different thymine analogues [9].1 wish to recall the contribution of the molecular orbital study of the problem[ 10,111 to the elucidation of the mechanism of the reaction, and to report some more recent results in this field.The first molecular orbital calculations on the problem[ 10,l I] were carried out within the general frame of the Huckel approximation. The purpose of these calculations was to determine the electronic characteristics of the first excited state of the compounds studied and to correlate these characteristics with the rate of photodimerization of a series of pyrimidine bases.Among the characteristics of the excited states, two quickly appeared to be particularly important to the problem under investigation:I . The first characteristic concerns the distribution on the molecular periphery of the "uncoupled" electrons of the first excited state, which in this approximation may be confounded with the "spin densities". Examination of this distribution has shown that: ( ( 1 ) In the majority of the pyrimidines the highest concentrations of the lone electrons in theirfirst excited state occur ut C-5 and C-6. The only cases in which this dues not happen are those of the bases that do not dimerize: in 5-nitrouraci1. and 2-thiothymine the maximum concentration of the lone electrons is at the extracyclic substituent, NO, or SH. respectively. ( b ) A striking parallel exists between the yield of photodimerizntion crnd the total concentrrrtion of uncoupled electrons at the C-5-C-6 bond of the pyrimidines in their first excited state. This parallel is illustrated in Table 1. The only exception to this rule is 6-azathymine. The inability of this molecule to undergo photodimerization may be due to its general insensitivity to U.V. irradiation or to the dif...

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confidence: 99%

Electronic Factors in the Photodimerization of Thymine

Pullman

1968

Photochem & Photobiology

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“…3), and with the same parametrization system as in Ref. 4; for the thymine molecule we neglected the weak effect of the methyl group upon the r-electronic shells and described this base with the same Hamiltonian as uracil.…”

Section: Methods Of Calculationmentioning

confidence: 99%

Role of electron correlation in the quantum‐mechanical calculations of the coulomb interaction energy in the DNA base pairs

Kuprievich¹,

Kudritskaya²,

Klymenko³

1979

Int J of Quantum Chemistry

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The intermolecular electronic correlation contributions to the Coulomb component of the nucleic acid base interaction energy are estimated. The Coulomb energy is evaluated with the use of atomic monopoles, which are determined from the rr-electronic densities calculated by the SCF method and by employing partially or completely optimized APSG wave functions. When the correlation is thus taken into account, a systematic decrease in atomic charges occurs; this effect is considerable only if an optimized orbital set is used. As a result, the Coulomb interaction energy due to the rr-electronic atoms decreases from -1.13 to -0.85 kcal/mol for the AT pair and from -7.15 to -4.61 kcal/mol for the GC pair.

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“…The matrix H has been previously defined in (8) and (9). In the Pople-Pariser-Parr approximation with zero differential overlap the elements of the matrices The results obtained for the energy of the first triplets are shown in Table 11, charges and spin-densities are shown in Tables IV and V. This method always results in eigenvalues of S2, as all the electrons in the closed-shell of nS doubly occupied orbitals are paired.…”

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confidence: 99%

“…Hence their electronic structure has been investigated by several authors [ 1-81 using the Huckel approximation, and the semiempirical SCF-LCAO-MO closed-shell treatment, with a n d without configuration interaction (P-P-P method). Most recently the open-shell method of Roothaan i n its semiempirical form has been applied for the first triplet excited states of the nucleotide bases [9]. Also the semiempirical form of the different orbitals for different spins (UHF) method applying spin projection has been used for the calculation of the triplet excited states of these systems [lo].…”

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confidence: 99%

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The first excited triplet states of the nucleotide bases calculated with different semiempirical SCF schemes

Packer

Avery

Ladik

et al. 1969

Int J of Quantum Chemistry

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AbstractsThe n-electron distributions, spin densities, and energies of the first triplets of the nucleotide bases, uracil, thymine, cytosine, adenine, and guanine, were investigated in various semiempirical approximations. Results are presented for calculations using the semiempirical form of the closed-shell SCF configuration interaction method, of the different orbitals for different spins (unrestricted Hartree-Fock) treatment, with and without spin projection, and of the Roothaan's open-shell procedure.On a ttudit les distributions des Clectrons T, les densites de spin, et les energies des premiers triplets des bases nuclkotides (uracile, thymine, cytosine, adCnine, et guanine), dans le cadre des approximations semi-empiriques difftrentes : la forme semi-empirique de la mtthode SCF-CI pour des couches completes, la mtthode "different orbitals for different spins" (mtthode Hartree-Fock sans restrictions) avec et sans projection de spin et la mtthode de Roothaan pour des couches incompletes.Es wurden n-Elektronenverteilungen, Spindichten, und Energien der ersten Tripletten der Nukleotidbasen (Uracil, Thymin, Cytosin, Adenin und Guanin) mit verschiedenen semi-empirischen Approximationen berechnet : die semi-empirische Form der SCF-CIMethode fur abgeschlossene Schalen, die Methode "different orbitals for different spins" mit und ohne Spin-projektion, und die Roothaansche Methode fur offene Schalen.

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SCFCI and SCF open‐shell studies of the base components of the nucleic acids

Cited by 46 publications

References 18 publications

Electronic Factors in the Photodimerization of Thymine

Electronic Factors in the Photodimerization of Thymine

Role of electron correlation in the quantum‐mechanical calculations of the coulomb interaction energy in the DNA base pairs

The first excited triplet states of the nucleotide bases calculated with different semiempirical SCF schemes

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