J . Phys. Chem. 1990, 94, 7363-7366
7363
An Analysis of the Electronic States of Aceanthrylene B. F. Plummer* and Scott F. Singleton Department of Chemistry, Trinity University, San Antonio, Texas 78212 (Received: July 31, 1989; In Final Form: May 16, 1990)
The electronic spectrum of aceanthrylene (AA) is examined experimentally and theoretically by use of simple perturbational molecular orbital theory and by use of a semiempirical PPP SCF CI calculation. The long-wavelength transition near 540 nm is assigned to a new state characteristic of a 4N annulene perimeter called a S transition that is not present in the precursor analogue anthracene. Stretched polyethylene sheet spectra are used to verify the calculated polarization assignments. The excited states contain extensive configuration interaction. The lowest triplet state is characterized by MNDO calculation and its electron density is used partially to explain the triplet-state photodimerization of AA. The emission behavior of 2-methylaceanthryleneis characterized by a weak fluorescence near 460 nm. This is an upper excited state emission or anomalous fluorescence.
Benzo-fused derivatives of acenaphthylene (A) are currently of interest because of their presence as trace contaminants in the environment and because of their potential car~inogenicity.'-~ Most studies have focused on their synthesis and ground-state reactivity.e6 We are interested in learning more about their photophysical and photochemical behavior because of their relationship to A whose excited-state reactivity has challenged numerous investigator^.'-'^ Aceanthrylene (AA) is of particular interest because of its significant mutagenicity' and because of its unique properties (vide infra). We have perfected a ready synthesisI3 of AA which allows us to study it in detail. In this study, the electronic states of AA are analyzed by qualitative M O theory and by M N D O and PPP SCF CI calculations.
TABLE I: Experimental and Calculated Absorption Bands (in nm) for AA calculated A,, experimental ,A, 560 450 423 415 399 378 360 344 290
(e)
(100) (1500) (sh) (3900) (3650)
(5480) (4900) (9000) (4900)
(9000)(sh)
polarizn
(oscill strength)
polarizn"
b b T T T
540 (0.08)
L
409 (0.27)
T
361 (0.21)
L
(0.10)
L L L L
L L L
Results and Discussion The absorptionS and emission spectra of AA have been reported.14 AA photodimeri~es'~ to produce four stereoisomeric products (eq I ) . The reaction is a triplet excited-state process
253 (58200) 236 (48600) 220 (21300) (sh)
'L
T L L
290 274 272 245 228 219
(0.18) (0.28j (1.70)
(0.09) (0.03)
T
L
= longitudinal or z polarized; T = transverse or y polarized; both
in plane. *Too weak to be determined in a single sheet. 2
1
9
3
L
syn HH
syn HT
(1)
8
4
5
6
1
A4
anti HH
anti HT
( I ) Kohan, M. J.; Sangaiah, R.; Ball, L. M.; Gold, A. Murat. Res. 1985, 155,95-98. (2) Sangaiah,
R. A.; Gold, A.; Toney, G. E.; Toney, S.H.; Claxton, R.; Easterling, R.; Nesnow, S. Muror. Res. 1983, 119, 259-266. (3) Fu, P. P.; Beland, F. A.; Yang, S.K. Carcinogenesis (London) 1980, I , 725-727. (4) Chung, Y.-S.; Kruk, H.; Barizo, 0. M.; Katz, M.; Lee-Ruff, E. J . Org. Chem. 1987.52, 1284-1288. (5) Becker, H.-D.; Hansen, L.;Andersson, K. J. Org. Chem. 1985, 50,
277-279. (6) Sangaiah, R.; Gold, A. OPPI Briefs 1985, 17, 53-55. (7) Cowan, D. 0.; Drisko, R. L. Tetrahedron Lett. 1967, 1255. (8) Koziar, J. C.; Cowan, D. 0. Acc. Chem. Res. 1978, 1 1 , 334. (9) Cowan, D. 0.; Drisko, R. L. Elements of Organic Photochemistry; Plenum: New York, 1976. (IO) Plummer, B. F.; Hall, R. A. J . Chem. SOC.,Chem. Commun. 1970, 44. (1 I ) Plummer, B. F.; Scott, L. J.; OBoyle, T. E. J . Org. Chem. 1979.44, 514. (12) Plummer, 8. F.; Hopkinson, M. J.; Zoeller, J. H. J. Am. Chem. Soc. 1979. 101, 6779. (13) Plummer, B. F.; AI-Saigh, Z. Y.;Arfan, M. J . Org. Chem. 1984,49, 2069. (14) Plummer, B. F.; AI-Saigh, 2.Y.; Arfan, M. Chem. Phys. Lett. 1984, 104, 389. (15) Plummer, B. F.; Singleton, S. F. J. Phys. Chem. 1989, 93, 5515.
0022-36S4/90/2094-7363%02.50/0
and the mechanism is postulated to proceed through a 1,Cdiradical intermediate. A particularly intriguing aspect of the reaction is the preference for H H dimers over HT dimers in a variety of solvent systems. In contrast to the triplet-state photodimerization of A, in which trans photodimer is generally favored,* the syn dimers of AA are formed competitively with anti dimers. The calculated and experimental results for the absorption spectrum of AA are shown (Table I). The absorption and emission spectra for 2-methylaceanthrylene (MeAA) are exhibited (Figure 1) to illustrate the presence of the upper excited state emission that we first reported for AA. AA and MeAA are orange-red, crystalline compounds. The first absorption band begins with a diffuse vibrational progression near 540 nm with a very weak oscillator strength and it rises on the edge of a transition that occurs near 400 nm. The striking color of AA is undoubtedly due to this first band which finds a parallel in the S state of acephenanthrylene,Is and cyclopenta[~,~pyrene.'~ Additional bands with significant vibronic structure occur at higher energies. A simple perturbational M O model, based on the analysis of nonalternant hydrocarbons by Michl and Thulstrup,I6 is used to simulate the spectrum of AA (Figure 2). The spectrum is also analyzed by use of the semiempirical Pariser-PopleParr PPP S C F CI calculation2"-22as well as by the use of MND023,24 (16) Thulstrup, E. W.; Michl, J. J . Am. Chem. SOC.1976, 98, 4533. ( I 7) Michl, J. J . Am. Chem. SOC.1976, 98, 4546. (18) Plummer, B. F. J . Phys. Chem. 1987, 91, 5035. (19) Plummer, B. F.; AI-Saigh, Z. Y. J . Phys. Chem. 1983, 87, 1579. (20) Parker, R.; Parr, R. G.J . Chem. Phys. 1953, 21, 466. (21) Pople, J. A. Trans. Faraday SOC.1953, 49, 1375. (22) Michl, J. J . Am. Chem. SOC.1978, 100, 6801-6819.
0 1990 American Chemical Society
1364 The Journal oJPhysical Chemistry, Vol. 94. No. 19, 1990
Plummer and Singleton
LUMO
HOMO
LOWEST TRCLET YO
LOWESI SINGLET YO
Figure 3. The lowest singlet- and triplet-statemolecular orbital diagram for aceanthrylene derived from MNDO calculations (see legend to Figure 2).
8.0 6.0
4.0
-
2.0
250
351
45@
550
AImI Figure 1. Absorption and emission spectrum of 2-methylaceanthrylene
showing upper excited state fluorescence (see Experimental Section for accurate molar absorptivities). .I"",.".
ethylene has the correct symmetry to interact in phase strongly with the L U M O of A N to form a new LUMO for AA labeled -I, whose energy is now lower than that of either contributor due to the stabilizing interaction brought about by the in-phase combination of the orbitals. The new H O M O in AA (labeled + I ) is the result of an out-of-phase combination of the H O M O of ethylene with an appropriate lower energy orbital of AN. This combination creates a new molecular orbital in AA whose potential energy is the highest of all the occupied M O s of AA while the remaining orbitals in A N are lowered in energy due to the per- I ) is similar to the turbation. The new transition ( I charge-transfer transition in A; therefore, we suggest calling it a S state also, since it is unique and not directly attributable to the perimeter states characterized by Platt for [4N + 21 annulenesZs Michl and co-workers have shown that these transitions can be related to a 4N-electron annulene perimeter by a perturbational treatment?6 The transitions 2 -I near 400 nm -1, near 360 nm are characteristic of the transitions and 3 associated with AN. In fact, the vibrational progression found in A N shows a peak distribution at 24938, 26316, 27624, and 28986 cm-'" which is nearly congruent with the peaks in AA in this region. The vibrational progression for the 'La transition in A N is (A"),, = 1430 cm-I (about 21 nm) and a similar progression is also found in the transitions of AA. Thus, the simple perturbational M O model for AA represents a fortuitously close parallel between the perturbed states of A N and those in AA. The results of the PPP S C F C I calculations are in reasonable agreement with the experimental results derived for AA. The is calculated oscillator strength for the first transition, I --I, small, suggesting a Franck-Condon forbidden transition. Significant configuration interaction with upper excited states can be identified with about 30% admixture coming from the 2 -1 configuration and 15% from the 3 - - I . The computed M O s for AA are shown in Figure 2. The MNDO M O s are illustrated in Figure 3 for comparison to those derived from PPP S C F CI. The A 0 coefficients derived from both approaches are similar. The second transition, 2 - - I , calculated to occur near 400 nm is assigned to IL. 'A because its vibrational progression and its transverse polarization are almost identical with those of the same transition in AN. Substantial CI occurs with a 30% admixture of 1 -1 configuration and an additional 30% coming from other electronic configurations. The width of this vibrational progression in AA is larger than that in AN, and it may be that the ILb IA transition is beginning to appear in AA, although it is submerged in the spectrum of AN. In fact, the peak at 360 nm shows an increased intensity that is anomalous when compared with A N and suggests that this transition which is calculated to be longitudinally polarized is probably the '4- 'A that is masked in AN. Therefore, the 3 -1 transition is assigned to the 'Lb 'A at 360 nm in AA.'*
.u.nlh,"l.n.
nlhr.o.,"
I
-
-
-
i"'
-
Figure 2. Perturbational molecular orbital diagram for the correlation of states of aceanthrylene. The circles are drawn with their diameters proportional to the size of the coefficients of the atomic orbitals at the atom indicated in the molecular orbital. The energy levels are wmputed from simple Huckel molecular orbital theory and plotted in units of 8. The HOMO and LUMO of aaanthrylene are those wmputed from PPP SCF CI calculation.
for the triplet-state electronic configuration The first transition in AA is characterized by a low-energy HOMO L U M O energy difference that is not found in anthracene (AN), the compound from which AA is formally derived. In the linear combination of molecular orbitals, the L U M O of
-
(23) Dewar. M. J. S.:Thicl. W. J . Am. Chcm. Sm.1977, 99. 4899. (24) Dewar. M. J. S.;Trinajstic. N. 3. Chcm. S a . , Chcm. Commun. 1970, 646.
-
-
-
-
(25) Platt, J. R. J . Chem. Phyr. 1949. 17, 484. (26) Howeler. U.; Chatterjce. P. K.; Klingensmith. K. A,: Waluk. J.; Michl, 1. Pure Appl. Chem. 1989.61. 2117-2128. (27) Jafle. H. H.:Orchin. M. Theory w d Applications of Ulrrouiolcr Spectroscopy; Wiley: New York. 1962. (28) Stciner. R. P.; Michl. J. J. Am. Chem. Soe. 1918. 100. 6861.
The Journal of Physical Chemistry, Vol. 94, No. 19, 1990 1365
Electronic States of Aceanthrylene
200
250
300
350
400
450
500
A
Figure 4. Polarized UV-visible absorption of aceanthrylenein stretched polyethylene at ambient temperature. Reduced absorption spectra A, = E, - 1 .2Ey and A, = E, - 0.4EZ. The absorbance scale is in arbitrary units.
-
The transition occurring a t 253 nm in AA is most likely the intense IBb ’A state. Other transitions in this congested region are currently not identifiable. Further verification of these assignments comes from the analysis of the polarized transition moments in stretched polyethylene.29” The reduced spectra (Figure 4) of AA after stepwise reduction of the polarized absorptions are illustrated. The best fit of the data for those bands that are approximately transversely polarized is found when the reduced spectrum A, = E, - 0.4E, is plotted. The reduced spectrum A, = E , - 1.2Ey reproduces reasonably well those bands that are longitudinally polarized. From these results, the orientation factors K, = 0.55 and K, = 0.38 are derived. Interestingly, this places the orientation of AA near the orientation of 1,4-dimethylanthracene in the orientation triangle.,O These measurements offer corroboration for the assignment of the polarizations of the computed transitions. The transition near 360 nm is seen to appear as a peak in the reduced spectrum A,, verifying its longitudinal polarization in agreement with the theoretical calculations. The transition at 540 nm is too weak for a simple polarized measurement to be made. The reduced spectra in the region 220-360 nm of AA also confirm some of the calculated polarizations, although experimental error due to light scattering makes this region more difficult to interpret. The calculated polarizations in this region with one exception are predicted to be longitudinally polarized. The transition at 290 nm seen as a shoulder in the reduced spectrum is z-polarized in agreement with the calculations. The only transverse band calculated to occur in this region is near 228 nm while the reduced spectrum A, suggests that a transversely polarized band occurs near 255 nm. The lowest triplet HOMO of AA is illustrated (Figure 3). The fact that the triplet state and the ground electronic state of AA both have a significantly large A 0 coefficient at carbon 2 of the bridge suggests that attack by triplet on ground-state AA in a multistep biradical process would favor initial bond formation at carbon 2 because of the enhanced resonance integral. Thus, the small preference for H H photodimers (about 60%) in competition with the less sterically encumbered HT photodimers (about 40%) can be rationalized on this basis. It is also plausible that the intermediate I ,Cbiradical that results from this attack is electronically favored by resonance delocalization of the intermediate through the meso position of the anthracene chromophore when compared to the alternative 1,4-biradical. Thus, the HH transition state probably is favored because of both of the foregoing reasons. More speculative in this regard is an interpretation that explains the competition between syn (about 50%) and anti dimers during the photodimerization. We have suggested that perhaps the (29) Michl, J.; Thulstrup, E. W .Spectroscopy with Polorized Light, VCH: New York, 1986. (30) Michl. J.; Thulstrup, E. W . Acc. Chem. Res. 1987, 20. 192-199.
formation of a nonemissive triplet exciplex in a syn geometry is competitive with the anti geometry transition state of the triplet attack which presumably minimizes steric effects. Only an extensive calculation of the presumed exciplex interaction can yield a more definitive answer. In Figure 1 the excited-state emission of MeAA is recorded. The normal Stokes-shifted fluorescence from the first excited state is too weak to be detected, which is characteristic of many of these benzo-annelated acenaphthylenes. However, we do observe a weak emission between 430 and 520 nm when the band near 420 nm is excited. The excitation spectrum monitored a t either peak of the emission spectrum (440 or 480 nm) simulates the absorption spectrum of MeAA, thereby corroborating that this emission is not impurity derived. This emission occurs when the sample is excited into the second excited state and is an example of S2 So fluorescence. This result corroborates our earlier reportI4 that AA undergoes anomalous fluorescence. Upper excited state fluorescence is dependent on the energy gap between the first and ~ ~ the S , and S2 second excited states, (Es, - B s , ) . ~ ’ - When excited-state energies are widely separated, the S2-SI internal conversion is inefficient, allowing fluorescence from upper excited states to c ~ m p e t e . l ~ *The ~ ~ -presence ~~ of the S state in the perturbed 4 N annulene perimeter creates a low-energy transition which is suitably separated in energy from the second excited state of AA such that the opportunity for anomalous fluorescence arises.
-
Experimental Section General. Molecular orbital calculations (PPP S C F CI) were carried out as previously describedI8 with the 65 lowest singly excited configurations used in the calculation. The MNDO results were calculated by using the MOPAC3’ QCPE 455 program using a Sun data station. Aceanthrylene was synthesized as previously described.I3 Proton nuclear magnetic resonance (NMR) spectra were obtained on a Varian VXR-300 spectrometer at 300 MHz, using 1% CDCI, solutions with internal tetramethylsilane (TMS), and I3C N M R were recorded on the VXR at 75 MHz in deuteriochloroform with CDC13 reference set at 77.0 ppm unless otherwise indicated. Deuterium locking was used to maintain field and frequency stability. Electronic absorption spectra were recorded on a Cary 23 15 UV-visible-near-IR spectrophotometer with a Cary DS-15 stand-alone data station running Varian Multiscan software. Stretched sheet spectra were run by using Glan polarizers and were corrected for background absorption. Spectra were digitized every 1 nm and data reduction was achieved by using Cricket Graph on a Mac 11. Excitation and emission spectra were obtained on a Perkin-Elmer MFP-44B fluorescence spectrophotometer using an excitation slit width of 5 nm and emission slit width of 10 nm and a photomultiplier detector type R928. Mass spectra were determined with a Hewlett-Packard 5995-C gas chromatographic mass spectrometer at 70-eV ionizing radiation. Melting point determinations were made with a Fisher-Johns hot stage melting point apparatus and are uncorrected. Elemental analyses were performed by Galbraith Laboratories. Synthesis of 2-Methyl-2-aceanthrenol. A solution of methylmagnesium iodide (9.6 mmol) (Aldrich) in ether (4.0 mL) was added to a continuously nitrogen purged suspension of 2(0.50g, 2.3 “01) dissolved in ether (150 mL) a~eanthrenone’~ and stirred magnetically. After 1 h, an additional 1 .O mL (2.4 mmol) of the Grignard reagent was added to the reaction mixture and the reaction was continued for 2 h. The solution was acidified with aqueous NH4CI, and the separated organic phase washed with two 50-mL portions of water. The ether layer was dried (Na2S04)and concentrated to 10 mL by rotary evaporation and (31) Engelman, R.; Jortner, J. Mol. Phys. 1970, 18, 145. (32) Beer, M.; Longuet-Higgins, H. C. J . Chem. Phys. 1955, 23, 1390. (33) Birks, J. B.;Easterly, C. E.; Christophorou, L. G. J . Chem. Phys. 1977, 66,423 1. (34) Viswanath, G.; Kasha, M. J . Chem. Phys. 1956, 24, 574. (35) Eber, G.; Schneider, S:;Dorr, R. Chem. Phys. Lett. 1977, 52, 59. (36) Plummer, B. F.; AI-Sacgh, Z . Y . Chem. Phys. Lerr. 1982, 91, 425. (37) Stewart, J. J. P.;Seiler, F. J. Quantum Chemistry Program Exchange: Indiana University: Bloomington, IN.
7366
J . Phys. Chem. 1990, 94, 7366-7371
the remaining solvent removed by a stream of nitrogen gas. The red-orange residue was recrystallized from petroleum ether (bp 35-60 "C), to produce 0.47 g (88%) of a bright yellow powdery solid: mp 132.1-133.4 "C; ' H N M R (CDC13) 6 7.3-8.3 (m, 8 H), 3.7 (s, 2 H), 2.4 (s, 1 H), 1.7 (s, 3 H) ppm; I3C NMR (CDC13) 6 28.8,48.0, 80.6, 116.6, 122.0, 124.6, 124.8, 125.1, 125.3, 127.3, 127.8, 129.3, 129.5, 133.9, 134.2, 137.0, 150.4ppm; IR 3350, 3060, 2970,2930, 1620, 1430, 1420, 1350, 1260, 1170, 1080,900,880, 850, 800, 770, 760, 740 cm-I; M S m / z (relative abundance) 95 (25), 94 (lo), 1 IO (17), 189 (40), 190 (22), 191 (72), 192 ( l l ) , 202 (16), 213 ( I l ) , 215 (42), 216 (30), 217 (14), 218 (15), 219 (76), 220 (1 5 ) , 234 (loo), 235 (20). Anal. Calcd for C I 7 H l 4 0 : C, 87.14; H, 6.04. Found: C , 86.95; H, 6.08. Synthesis of 2-Methylaceanthrylene. Aluminum oxide (0.5 g, 5 mmol) was added to a suspension of 2-methyl-2-aceanthrenol (0.10 g, 0.43 mmol) in 15.0 mL of toluene in a 25-mL roundbottom flask equipped with a Dean-Stark trap. The mixture was refluxed with magnetic stirring for 2 h. The alumina was removed by filtration and the filtrate rotary evaporated. The red residue was dissolved in heptane and chromatographed on aluminum oxide (neutral) with benzene as the eluent. Rotary evaporation followed
by recrystallization from petroleum ether (bp 35-60 "C) produced 40 mg (43%) of a dark red solid. Vacuum sublimation yielded: mp 88.0-90.0 "C; 'HNMR(CDC13) 6 8.34 (s, 1 H), 8.17 (d, 1 H), 8.05 (d, 1 H), 7.97 (d, 1 H), 7.75 (d, 1 H), 7.50-7.60 (m, 2 H), 7.22 (s, 1 H), 2.53 (s, 3 H);"C N M R (CDCI3) 6 141.6, 138.0, 135.5, 134.4, 130.2, 128.2, 127.9, 126.9, 124.5, 123.9, 123.0, 13.1 ppm; IR 3060, 1620, 1440, 1100,880,850,770,750, 735, 700 cm-I; M S m / z (relative abundance) 95 (lo), 107 (8), 107 (9), 213 (25), 214 (9), 215 (loo), 216 (63), 217 (12); UV-vis, A,, (cyclohexane) 585 nm (c 180), 470 (1400), 419 (4340), 411 (4150), 395 (6050), 378 (5400), 360 (8020), 345 (5400), 255 (47400), 239 (46000). Anal. Calcd for C I 7 H i 2 :C, 94.40; H, 5.60. Found: C, 94.29; H, 5.36.
Acknowledgment. The support of the donors of Petroleum Research Fund, administered by the American Chemical Society, and the support of the National Science Foundation with an RUI grant are appreciated. We thank Professor J. Michl for computational support (PPP S C F CI) and N S F and the Keck Foundation for partial support to purchase the VXR-300 and the Cary 23 15.
Pressure Tuning Infrared Spectroscopy of Two Phospholipids and Their Common Fatty Acid W. W. Ley and H.G.Drickamer* Materials Research Laboratory and School of Chemical Sciences, University of Illinois, Urbana, Illinois 61801 (Received: August 2, 1989; I n Final Form: March 30, 1990) Infrared spectra of solid palmitic acid, L-a-dipalmitoylphosphatidylcholine(DPPC), and L-a-dipalmitoylphosphatidicacid (DPPA), are studied at pressures up to 125 kbar to probe the fundamental nature of their molecular vibrations. In palmitic acid spectral changes are observed, which indicate that the stochastic conversion from the cis to the trans isomer occurs with increasing pressure. In palmitic acid and DPPA splitting is observed in the carbonyl region and appears to be related to hydrogen bonding. Correlation splitting in the CH2 bending region increases with pressure and eventually the correlation component becomes stronger than the fundamental and dominates this region of the spectrum. Also a tentative assignment is made for the (Y methylene bending mode, b(C,H,), in DPPA.
Introduction TABLE I: Behavior of Selected Peaks in the IR Spectrum of Palmitic Acid' Phospholipids are the major components of a wide variety of cell membranes. The study of the molecular dynamics of phosenergy a t pholipids by vibrational spectroscopy is a well-defined field and 5 kbar is summarized in recent reviews.'s2 Many assignments of vi(cm-I) assignment behavior brational peaks have been worked out, and dichroism has been 1408 b(C,H2) overall 12 cm-l blue shift used in studies on lipid bilayer systems to determine the orientation at 125 kbar of the oscillating dipoles with respect to the membrane ~ u r f a c e . ~ - ~ 1350 w(CH2) mixed with large increase in intensity with 39 There have also been high pressure infrared investigations involving cm-l blue shift at 125 kbar carboxyl modes 1312 w(CH2) mixed with small decrease in intensity with 44 aqueous dispersions of phospholipids,8-' * but the study of solid, cm-' blue shift at 125 kbar carboxyl modes less hydrated samples" has been more limited. 1280 to w(CH2) decrease in intensity; no longer In this work the vibrational infrared spectra of solid samples 1200 observable at 125 kbar of L-a-dipalmitoylphosphatidylcholine (DPPC), L-a-di1102 r(CH2)/p(CH2) 30 cm-' blue shift palmitoylphosphatidic acid (DPPA), and palmitic acid have been "Symbols for the vibrational modes are defined in the text. ( I ) Amey, R. L.; Chapman, D. In Biomembrane Structure and Function; Chapman, D., Ed.; Macmillan: London, 1983; pp 199-256. (2) Casal, H.L.; Mantsch, H. H. Biochim. Biophys. Acta 1984,779,381. (3) Goni, F. M.; Arrondo, J. L. R. Faraday Discuss. Chem. Soc. 1986.81, 117.
(4) Fookson, J . E.; Wallach, D. F. H. Archiu. Biochem. Biophys. 1978, 189, 195. ( 5 ) Brandenburg, K.; Seydel, U. 2.Naturforsch. 1986, I I C , 453. (6) Akutsu, H.; Kyogoku, Y. Chem. Phys. Lipids 1975, 14. 113.
(7) Akutsu, H.; Kyogoku, Y.; Nakahara, H.;Fukuda. K. Chem. Phys. Lipids 1975, 15, 222. ( 8 ) Siminovitch, D. J.; Wong, P. T. T.; Mantsch, H. H . Biophys. J . 1987, SI, 465. (9) Wong, P. T.T.; Mantsch, H. H. J . Chem. Phys. 1985.83 (7), 3268. (IO) Wong, P. T.T.; Siminovitch, D. J.; Mantsch, H. H. Biochim. Biophys. Acfa 1988, 947, 139. ( 1 1 ) Wong, P. T. T.;Huang, C. Biochemistry 1989, 28, 1259.
0022-3654/90/2094-7366$02.50/0
studied as a function of pressure. The aim of doing so is to gain more understanding of the fundamental nature of the molecular vibrations of phospholipids by examining them in an environment other than the lipid bilayer.
Experimental Procedure All spectra were recorded by a Nicolet 7199 FTIR fitted with a 4X Perkin-Elmer beam condenser to focus the IR radiation on the sample. The samples were held in a diamond anvil cell with type-I1 diamonds and confined by an Inconel gasket;I2 pressure (12) Sherman, W. F.; Stadtmuller, A. A. Experimental Techniques in High Pressure Research; Wiley: New York, 1987.
0 1990 American Chemical Society
