J. Phys. Chem. 1989, 93, 5345-5347 ca. 0.7 eV; this value is roughly consistent with work-function changes upon water adsorption observed on Pt(lll).& As noted above, a similar effect is antici ated in the electrochemical system in that xs > 0, so that EM> E*=. Given that dvkO/dE is positive, evaluated at.E ! (Le., at an uncharged electrode) should also be smaller than at EM. In the analysis employed in Figure 3, then, this electrostatic effect of coadsorbed water upon vko for the electrochemical system is nullified by evaluating vko at EMrather than E:., thereby yielding higher vk0 values in accord with those for the anhydrous uhv system.51
R
(5 1) It should be noted that the CO-covered electrochemical interface extrapolated to E M is in a sense a hypothetical state since rapid CO electrooxidation occurs at these relatively positive (0.7-1.2 V vs SCE) EM values.
5345
Nevertheless, the lack of marked specific effects of the double-layer environment upon the vco frequencies (or bandshape) is perhaps surprising. The coverage-dependent reconstruction of and Pt(1 known in uhv might well be different in Pt( the electrochemical systems. Quite apart from these effects of solvation on the CO surface bonding, one also might expect coadsorbed solvent to influence significantly the extent of adsorbate dipole-dipole coupling. The characterization of such coupling via CO isotopic substitution (cf. ref 25) would be clearly of interest. A detailed infrared study of the electrooxidation as well as adsorption of CO on low-index platinum surfaces will be submitted in the near future.
Acknowledgment. This work is supported by the National Science Foundation.
Polarization Assignments in the 270-nm Band of the Adenine Chromophore Leigh B. Clark Department of Chemistry, University of California, San Diego, La Jolla, California 92093-0342 (Received: February 27, 1989)
Polarized electronic spectra taken from two crystal systems containing the adenine chromophore (9-methyladenine and 6-(methylamino)purine) indicate that the previous assignment of transition moment directions should be revised. The first UV absorption region (-270 nm) is dominated by a transition polarized 25' away from the short molecular axis while another weaker, overlapped transition at slightly lower energy is polarized close to the long molecular axis.
Introduction In 1963 Stewart and Davidson published the first electronic absorption spectra of single crystals of nucleic acid bases.' In that study (and its subsequent refinement2), 9-methyladenine (9-MA) was examined in both the neat crystal and the 1:l hydrogen-bonded complex with 1-methylthymine. After a complicated analysis it was concluded that the 270-nm region (267 nm in solution) was composed of two overlapping transitions. The great bulk of the intensity (97%, f = 0.28) arose from a transition polarized close to the short molecular axis (-3' according to the Tinoco-DeVoe convention as positive toward C6from the C4-C5 line). The existence of a weak, second band cf= 0.008, X = 255 nm) polarized along the long molecular axis was indirectly identified owing to a gradual change of the dichroic ratio found for this absorption region of the (100) face of 9-MA single crystals. Although there has been persistent discussion regarding the weak transition, the assignment of the strong band as short axis polarized has stood unchallenged for 25 years and has been "supported and confirmed" by numerous other s t ~ d i e s . Further, ~ these results constitute the basic optical parameters of the adenine chromophore that have been routinely used in calculations and interpretations of the optical properties of polynucleotides containing this chromophore. They have served as the experimental standard with which to judge the adequacy of theoretical calculations of the electronic structure of these complex molecules. In this Letter we present evidence that suggests that the original assignments ought be reconsidered.
Results We have reexamined the crystal spectra of 9-methyladenine and, in addition, obtained new data from 6-(methy1amino)purine ( 1 ) Stewart, R. F.; Davidson, J. J . Chem. Phys. 1963, 39, 255. (2) Stewart, R. F.; Jensen, L. H. J . Chem. Phys. 1964, 40, 2071. (3) Callis, P. R. Annu. Rev. Phys. Chem. 1983, 34, 329.
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(6-MAP) single crystals. The crystal absorption spectra reported here are obtained by Kramers-Kronig analysis of polarized reflection spectra. Although the data extend well into the vacuum-UV, we focus on the 270-nm (3.7 X lo4 cm-') region for the purpose of this Letter. Absorption spectra for radiation polarized along the b and c axes of the (100) face of 9-MA are given in Figure 1. This face is nearly parallel with the planes of the molecules (-9'). Stewart took the value for the dichroic ratio (b:c)of 5.1 f 0.5 from the low-energy portion of the band envelopes and found it could arise from the two possible in-plane transition moment directions of -3' or +45' for the dominant transition. The A:T dimer spectra were then shown to be more consistent with the -3' possibility than the +45' choice, so the -3' alternative was provisionally chosen. Since crystal field caused intensity borrowing among the vibronic components of a given electronic transition can lead to different contours for band components along different crystal axes, it is felt that the dichroic ratio taken from the region of the 0,O is not an appropriate characteristic of the transition as a whole. To minimize the effects of such intensity borrowing, we prefer to employ band areas or oscillator strengths. In this regard we find for 9-MA a value for fbfcof 4.8 and the corresponding in-plane polarizations of -3O and +46' which however are virtually coincident with Stewart's two possibilities. However, from the spectra taken along the a, b, and c axes of the (001) and (100) faces of 6-(methylamino)purine a different band pattern emerges (Figure 2), for there are what appear to be two significant bands in the 3.7 X 104 cm-' (270 nm3 absorption region. A lower energy, structured transition starting a t about 3.4 X lo4 cm-' (294 nm) appears dominantly along the c axis, while apparently another, more intense transition centered at 3.7 X lo4 cm-' (270 nm) appears strongly along the b and a axes. It should be noted here that the X-ray determination of the crystal structure of 6-MAP located all the protons in the structure and identified the 9-H tautomer as the form p r e ~ e n t . ~Given the
0 1989 American Chemical Society
5346 The Journal of Physical Chemistry, Vol. 93, No. 14, 1989 Wovelength (nm) 310 300 290 280
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Figure 1. Absorption spectra of the (100) face of 9-methyladenine single crystals derived from polarized reflection spectra by Kramers-Kronig analyses. The projection onto (100) of two of the four molecules of the unit cell is shown. The other two molecules of the unit cell can be obtained by reflection of the two shown across the ac plane. The crystal structure data were obtained from: Kistenmacher, T. J.; Rossi, M. Acfa
Crystallogr. 1977, 833, 253. Wovelength (nm) 310 300 290 280 I
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Figure 2. Absorption spectra of the (100) and (001) faces of 6-(methy1amino)purine obtained by Kramers-Kronig analysis of polarized reflection spectra. The projections of a representative molecule of the unit cell are shown. The other three can be obtained by first inverting the one shown and then reflecting the two so generated through the ac plane. The crystal structure data are from ref 4.
similarity of the electronic framework of these two molecules, the apparent spectral differences are puzzling.
Discussion We have considered various possible assignment schemes of these data within the context of the assumption that the spectra of 6-MAP and 9-MA are closely related (viz., similar band sequence, oscillator strengths, and polarization directions). For example, if the spectra of 6-MAP are to be considered as derived from a single, dominant transition, then the band shape differences (notably along c) must be attributable to different amounts of intensity borrowing among the various vibronic components along the different crystal axes. Assuming such is the case, then the measured component oscillator strengths = 0.24, f b = 0.46, f, = 0.23) lead to the dichroic ratios of fb/ fa/ f,of 2.1 / 1.1 / I .O. These ratios are consistent with but one nearly in-plane polarized transition moment at an angle of 0 = +33'. This result is not
ua
(4) Sternglanz, H.; Bugg, C. E. Biochim. Biophys. Acta 1973, 308, 1.
Letters far from Stewart's rejected alternative (+46') for 9-MA. Actually, the observed crystal oscillator strengths for 6-MAP when worked up in this manner lead to a small out-of-plane cant of 8 '. In any event the calculated solution (i.e., randomized) oscillator strength is 0.32 and compares well with the value of 0.34 found in trimethyl phosphate or water solvents. On the other hand, if the 6-MAP data are worked up supposing that there are the two substantially overlapped transitions mentioned above, then the spectra can plausibly be resolved to yield two transitions individually polarized close to the molecular plane (C2') with the following component oscillator strengths: I (3.5 X lo4 cm-' or 286 nm) f, = 0.03,fb = 0.1 1, f,= 0.15; I1 (3.7 X lo4 cm-' or 270 nm) fa = 0.21, fb = 0.35, f, = 0.08. These resolutions result in I at 0 = +83' with f = 0.10 and I1 at +25' and f = 0.22. This scheme is the more compelling one based on the appearance of the spectra. The question is, how are the 9-MA spectra to be rationalized with this two-band scenario? If we impose the 6-MAP results onto 9-MA, then the strong band, I1 (0 = +25'), will project almost entirely (>99%) onto the b axis while the weaker band, I (0 = +83'), projects dominantly along c V,/fb = 3.4). The band contour along b and that along c therefore reflect the characteristic appearances of transitions I1 and I, respectively. The fact that these contours are so similar is worrisome. If our premise about the similarity in spectral properties for the two molecules holds and the two-band scheme of 6-MAP is correct, then we must conclude that for 9-MA the two bands are almost perfectly degenerate and, furthermore, exhibit very similar vibronic intensity patterns. Both of these necessary conclusions are, of course, possible. The difference between the vibronic intensity patterns for transition I of 6-MAP and 9-MA could plausibly arise from crystal field effects if it is not owed to inherent molecular differences. The observed oscillator strengths in 9-MA are generally in conformance with such an assignment. Scaling the 6-MAP oscillator strengths by the ratio of the individual solution spectrum values (Le., 9-Ma/6-MAP = 0.29/0.34 = 0.85) yields calculated values for fb and f, of 0.63 and 0.20, respectively, while the corresponding 9-MA experimental values are 0.6 1 and 0.13. If the above analysis is correct, then the spectra taken from 9-MA cannot be used independently to separate and assign the two transitions owing to their degeneracy in energy and similarity in shape. The results from 6-MAP therefore constitute the best indicator of transition moment directions of the adenine chromophore. That a transition lies buried on the red edge of a more dominant transition has been suspected since earliest times and has been the subject of considerable speculation for many years. A shoulder does appear on the low-energy side of the absorption band in solution spectra, and it is this shoulder that has been variously assigned as vibronic structure or as a distinct transition. Circular dichroism spectra invariably show a single band throughout the entire region and for this reason are equivocal regarding the presence of the low-energy second band.s,6 The MCD spectrum of adenine, on the other hand, shows the same kind of doublepeaked spectrum (negative then positive) as does guanine for which the presence of two transitions is clear.' Linear dichroism of the adenine chromophore dissolved in stretched poly(viny1 alcohol) films shows an "irregularity" in the red edge of the main absorption. This variation appears as a drop in the reduced dichroism and is weak for adenine and adenosine but is more pronounced for 9-methyladenine and especially apparent for 6-(methylamin0)-9-methylpurine.~*~ In fact, Matsuoka and Nord6n9 report for this last molecule that the angle between the two bands is either 12' or 47'. Considering the uncertainties involved, the latter value is in reasonable agreement with the difference of 58' found here. Finally, the occurrence of the two-band pattern suggested by our ( 5 ) Ingwall, J. S. J. Am. Chem. SOC.1972, 94, 5487. (6) Burnner, W. C.; Maestre, M. F. Biopolymers 1975, 14, 5 5 5 . (7) Voelter, W.; Records, R.; Bunnenberg, E.; Djerassi, C. J. Am. Chem. SOC.1968, 90, 1663. (8) Fucaloro, A. F.; Forster, L. S. J. Am. Chem. SOC.1971, 93, 6443. (9) Matsuoka, T.; Norden, B. J . Phys. Chem. 1982, 86, 1378.
J . Phys. Chem. 1989, 93, 5347-5349
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6-MAP spectra agrees with the results obtained when two vibronic progressions were fitted to absorption, CD, LD, and MCD spectra for several adenine-containing molecules.i0 In summary then, if we assume that methylation at the amino group or at the 9-position does not distort the electronic states
of the adenine chromophore to any large extent, then the two transitions shown in Figure 3 appear to constitute the first absorption region (267 nm in solution). For the purposes of this Letter we have been arguing along the lines of the oriented gas model. This model ignores crystal field effects that may be significant for closely spaced transitions. The resolutions into band components indicated above have considerable latitude; however, the results are not particularly sensitive to details of the resolutions. Within the bounds of plausible band shapes the moment directions vary by f5O or so, while out-of-plane cants of 5-loo can develop. The oscillator strength of the lowest energy band could be as small as 0.05 or so while that of the stronger band will vary by the corresponding difference. Although the numerical values for the angles-that the transition moments make to the C4-C5 reference axis reported here are presented provisionally, we feel that this general pattern and in particular the transition moment direction of the dominant band ought not be subverted by crystal field effects or modifications in the band resolutions. Further details of this work including results for four additional, strong vacuum-UV transitions found in both 9-MA and 6-MAP will be presented at a later date.
(10) Fornasiero, D.; R m , I. A. G.; Rye, K.-A.; SOC.1981, 103, 1908.
Acknowledgment. This work was supported by a National Institutes of Health grant (GM38575).
f=.20
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I I !
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I H Figure 3. Proposed transition moment directions for the two transitions in the first absorption region of 6-(methy1amino)purine.
Kurecsev, T. J. Am. Chem.
Photoenol Biradlcals as Singlet Oxygen Sensitizers' R. W. Redmond and J. C. Scaiano* Division of Chemistry, National Research Council, Ottawa, Canada K I A OR6 (Received: March 8, 1989)
The photoenolization of o-methylacetophenone p r d s via a 1,Cbiradical intermediate formed through intramolecular hydrogen transfer. This biradical has a lifetime, 78,of 830 ns in acetonitrile and is quenched by oxygen with a rate constant of 4.2 X lo9 M-' ,the quenching process was demonstrated to involve energy transfer resulting in the formation of singlet oxygen, detected by its emission at 1.27 pm. Oxygen dependence studies confirm the biradical as the sensitizing species. In addition, the singlet oxygen produced was shown to react efficiently with the ground-state photoenol product.
The photoenolization of ortho-alkyl-substituted benzophenones and acetophenones bears many similarities to the Norrish type I1 reaction.2 The reaction mechanism is now well-established; for example, in the case of o-methylacetophenone (I) intramolecular hydrogen abstraction from one of two kinetically distinct triplets (syn and anti) leads to a single triplet biradical (11) which upon decay yields two isomeric photoenols, IIIE and IIIZ (Scheme I).3-'0 The E isomer is normally shorter lived, and both isomers have longer lifetimes in hydrogen-bonding solvents." The bi(1) Issued as NRCC-30203. (2) Scaiano, J. C. Acc. Chem. Res. 1982, 15, 252-258. (3) Porter, G.; Tchir, M. F. Chem. Commun. 1970, 1372-1373. (4) Porter, G.; Tchir, M. F. J . Chem. SOC.A 1971, 3772-3777. (5) Findlay, D. M.; Tchir, M. F. J . Chem. SOC.,Faraday Trans. 1 1976, 72, 1096-1 100. (6) Wagner, P. J.; Chen, C.-P. J . Am. Chem. Sot. 1976, 98, 239-241. (7) Haag, R.; Wirz, J.; Wagner, P. J. Helu. Chim. Acta 1977, 60, 2595-2607. (8) Small, R. D., Jr.; Scaiano, J. C. J . Am. Chem. SOC.1977, 99, 7713-7714. (9) Das, P. K.; Encinas, M. V.; Small, R. D., Jr.; Scaiano, J. C. J . Am. Chem. SOC.1979, 101, 6965-6970. (10) Scaiano, J. C. Chem. Phys. Lett. 1980, 73, 319-322. (11) Johnston, L. J.; Scaiano, J. C. Chem. Rev. 1989, 89, 521-547.
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radical is an excellent electron donor as demonstrated by its efficient reduction of methylvi~logen~ and also has a longer lifetime in hydrogen-bonding solvents, both properties which are common to type I1 biradicals.* Biradical I1 and the triplet state of the photoenols can be regarded as one and the same species. While most reactions of the biradical tend to illustrate the radical-like properties that are common in type I1 biradicals, Das and coworkersI2 have been able to establish a rather unique example of excited-state behavior; quenching of I1 by 8-carotene occurs by triplet energy transfer, with concomitant formation of the triplet state of @-carotene,which can be readily characterized in laser flash photolysis experiments. In view of this background, we reasoned that quenching of the biradical by oxygen could involve energy transfer, since the excitation energy for O2(IAg) is 22.5 kcal/mol," Le., only a few kcal/mol higher than the triplet energy of @-carotene.14 The rate constant for oxygen scavenging of I1 has been reported as 4.6 X lo9 M-I s-l in methanol, where its lifetime is 300 ns.8 In aceto(12) Kumar, C. V.; Chattopadhyay, S.K.; Das, P. K. J . Am. Chem. SOC. 1983, 105, 5143-5144. (13) Gilmore, F. R. J . Quant. Spectrosc. Radiat. Transfer 1965, 5, 369-3 a 9. (14) Herkstroeter, W. G. J . Am. Chem. SOC.1975, 97, 4161-4167.
Published 1989 by the American Chemical Society
