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Increased Selectivity of a Simple Photosensitive Probe in the Presence of Large Proportions of Cholesterol.

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more, this analysis reveals that in the Pt+-mediated oxidation,
methane is converted into methanol (10 YO),formaldehyde
(25 YO),and "CH,O," (65 YO).The turnover of the system may
be enhanced by increasing the oxygen excess to favor reactions IT and 111; however. this will result in a lowering of the
overall rate of conversion. Similar to the effect observed in the
reaction of Ir' with methane in the presence of oxygen,[51reaction I1 accelerates slightly during the course of the catalysis; this
may indicate the formation of excited [PtCH,]' ions o r of the
presence of a structural isomer in reaction 111a. This point will
be addressed in forthcoming. more detailed kinetic studies.
In conclusion, the reaction sequence reported here describes
for the first time a relatively efficient catalytic system for the
gas-phase oxidation of methane by a bimolecular ion -molecule
process. However, even in the gas phase the well-known problem of over-oxidation cannot yet be circumvented: the formation of "CH,O," is observed as main reaction channel.
E-xperimental Procedure
Pt ' ions were generated in the external ion source of a Spectrospin CMS 47X FTICR mass spectrometer [I71 by either laser desorption;laser ionization by focusing
a beam of a Nd.YAG laser ( i = 1064 nm) onto a platinum target [1X] o r glow-discharge ionization o f a platinum wire In an argon plasma [19]. The cations were
extracted from the source and transferred to the analyzer cell by a system ofelectric
potentials and lenses. The isolation of the lY5Pt+isotope and all subsequent isolation steps were performed by using FERETS [20],a computer-controlled ion-ejection protocol that combines single frequency ejection pulses with frequency sweeps
to optimize ion isolation. After isolation the ions of interest were thermalized by
collisions with either pulsed-in argon gas o r the reaction gases. and the thermalized
ions were subsequently reisolated. The degree of thermalimtion was assumed to be
complete if no further change in reactivity occurs upon increasing the amount of
pulsed-in ai-gon gas [7i]. Methane and oxygen were admitted to the FTICR cell
mbar,
through leak valves (typical preswres about 4 x lo-' mbar and 8 x
respectively). The pseudo-first order rate constants were derived from the decay of
the precursor ion signals and converted to absolute rate constants by calibrating the
ionization gauge measurements with rates of well-known ion-molecule processes
[14b, 21. 221; the error in the absolute rates is k 2 5 % [14b]. All functions of the
instrument were controlled by a Bruker Aspect 3000 minicomputer. Methane
(Linde AG. 99.999%). [DJmethane (Cambridge Isotope Laboratories, >98
a t o m % D), and oxygen (Linde ACT. 99.995%) were used w'ithout further purification.
Received: December 15. 1993 [Z 6562 I€]
Gerinan version: Angew. Cl7rm. 1994. 105, 1232
[l] a) H. Schwarz. Anpew. Chern. 1991, 103, 837; Angeir. Chen?. In?. Ed. Enyl.
1991. 30, 820; b) K. K . Irikura, J L. Beauchamp. J. PIijs. Chern. 1991, 95,
8344; c) J. M. Fox. Caial. Rfw. Sci. Eny. 1993. 35, 169.
[2] J. Sommer, J. Bukula, Ace. Chem. Re.>.1993. 26. 370.
[3] See for example a) M. A. Benvenuto, A. Sen. J. Chem. Soc. Chem. Commun.
1993, 970; b) M. Del Todesco Frisone. F. Pinna. G. Strukul, 0rganonietullic.s
1993, 12. 148, c) J. A. Labinger. A. M. Herring, D. K. Lyon, G. A. Luinstra,
J. E. Bercaw. I. T. Horvith, K. Eller, 2nd. 1993.12.895. and references therein
[4] Recent reviews on gas-phase transition metal chemistry: a ) K . Eller. H.
Schwarz, C h m . Res. 1991. 91. 1121: b) P. B. Armentrout. Annu. Rev. PhLs.
Chcm. 1990, 41, 313.
[5] K . K. Irikura. J. L. Beauchamp. J. Am. Chem. Soc. 1991, 113, 2769.
[6] Further examples of cationic Pt species in the gas phase: a) T. F. Magnera.
D. E. David. J. Michl. J A m . Chew. Sue. 1987.109. 936; b) D. J. Trevor. D. M.
Cox. A . Kaldor. ihid. 1990. 112. 3742; c) M. Hada. H . Nakatsuji. H. Nakai. S.
Gyobu. S. Miki. J. Mof.Sfrucr. (Theochem) 1993. 281. 207.
[7] Other catlilytic process under FT-ICR conditions: a) M. M. Kappes. R. H.
Staley. .
I
Am. Chmn?.So(,.1981. 103. 1286: b) S . W. Buckner, B. S . Freiser. ihid.
1988. 110, 6606; c) D. Schroder, H. Schwarz, Angeii. Chem. 1990, 102, 1468;
An,qrw Chem /nt. Ed. Eizgl. 1990. 29, 1433; d ) P. Schnabel, M. P. Irion, K. G.
Weil, J. Phm. Cherii. 1991, Y5. 9688; Chem. Ph.vs. Lett. 1992. 190. 255; e j M. P.
Irion. I n / . J. Mu.r.s Spectrum loti ProceJses 1992. 121, 1 ; f j M. P. Irion. P.
Schnabel. &r. Bims~w,qor.Plys. Chem. 1992. 96, 1101; g) P. Schnabel, K . G.
Weil. M. P. Irion. Angebt. Chem. 1992, 104. 633: Angiw. Chpm. in/.Ed. Engl.
1992. 31, 636; b) S. Karrafi. D. Schroder. H. Schwarz. Chrm. Ber. 1992, 125,
751 : i) D. Schroder. A. Fiedler. M. F, Ryan, H. Schwarz, J PhJs. Chmz., 1994,
98. 6X: j) R. Wesendrup. Diplomarbeit. Technische Universitlt Berlin, 1994.
[XI FT-ICR:MS: A n u l ~ ~ ~ iApplicorions
cul
of Fourier Trunsforin ion Cwlorron Resuiiunw mas^ Spw[wmeriw (Ed: B. Asamoto). VCH. Weinheim. 1991.
[9] T. Su. M. T. Bowers, In?. J Muss Specfrom Ion Phys. 1973. 12. 347.
1176
(
VCH Veilugr~erellschuftm h H , 0-69451 Weinherm 1994
[lo] a) If not mentioned otherwise, all thermochemical data were taken from: S. G .
Lias. J. E. Bartmess, J. F. Liebman. J. L. Holmes, R. D. Levin. W G. Mallard.
J Phys. Chem. Ref Datu 1988.17, Suppl. 1 . h) In two recent theoretical studies.
the BDE (Pt+-CHI) were calculated at 123 k 5 and 119 kcalmol-': K. K.
Irikura. W. A. Goddard 111. J Am. Chem. Soc.. submitted. C. Heinemann. R
Hartwig, R. Wesendrup. W. Koch. H. Schwarz. J. A m . ('hem. Soc., submiited.
(111 D Schroder. H. Schwarz. C h i u 1989, 43. 317.
1121 a) G . Hvistendahl. D. H. Williams. J Cliem. So(. Chem. Commun, 1975, 4.
b) H. Schwarz. W. Franke. J. Chandrasekhar. P von R Schleyer. Tetrahedron
1979. 35. 1969, and references therein.
(131 a) P. J. Derrick. K . F. Donchi in Cuinprehmsiw Cheiiiical Kinetics, Vol. 24
(Eds.. C. H. Bamford. C. F. H . Tipper). Elsevier. Amsterdam. 1983, p 53;
b) D . Schroder. D. Sulzle. 0 . Dutuit. T Baer. H. Schwarz. J Am. Chen?. Soc.,
111 press.
[14] a) Ref. [7a-c]: b) D . Schroder, Dissertation, Technische Universitat Berlin.
1993.
(151 Even i f H C 0 , H is formed in a highly vibrationally excited state, in view of the
significant barriers (> 60 kcal mol- ') associated with its dehydration or dehydrogenation, it is likely to survive: a) P. G. Blake. H. H. Davies, G. E. Jackson.
J Chem. Soc. B 1971. 1923; b) P. Ruelle. J Am. Chem. Sac 1987, 109. 1722.
[16] a) Ref. [ 7 c ] ; b) D. Schroder. A. Fiedler. J. Hrusak, H. Schwarz. J. Am. Chem.
Soc 1992, 114, 1215.
1171 a) K. Eller. H. Schwarz. Int. J Mass Specrruin. Ion Proce.s.ses 1989, Y3, 243.
b) K. Eller. W Zummack. H. Schwarz. J. Am. Chem. Soc. 1990, f / 2 . 621.
[I81 B. S. Freiser, Tuluntu 1985. 32, 697; A n d Chiin. Acru 1985. 178. 137.
1191 Recent examples. a ) W. S. Taylor. W R. Everett. L. M. Babcock. T. L. McNeal. Inr. J Musr Spwtrum. fon Processec 1993. 125. 45; h) D. M. Chambers.
S. A. McLuckey, G. L. Glisb, A n d Chern. 1993. 65, 778.
[20] R. A. Forbes. F. H. Laukien. J. Wronka. Int. J. Mass Spectrom. /on Prucessa
1988,83. 23
1211 J. E. Bartmess, R. M. Georgiadis. Vacuum 1983, 33, 149.
[22] For example: Y. Lin. D. P. Ridge, B. Munson. Org. Mass Spectrom. 1991. 26.
550.
Increased Selectivity of a Simple Photosensitive
Probe in the Presence of Large Proportions
of Cholesterol **
Siw Bodil Fredriksen, Valerie Dolle, Masakuni
Yamamoto, Yoichi Nakatani,* Maurice Goeldner and
Guy Ourisson
We have recently described a photoactivatable phospholipidic
transmembrane probe
which had a marked advantage
over the simpler membrane probes described earlier by Khorana
et al.[31and Breslow et aLr4IIt was much more site-selective and
attacked preferentially the terminal carbons of neighboring
phospholipids. This selectivity was excellent in the presence of
the near-physiological concentrations (33 mol YO)of cholesterol,
a normal constituent of eucaryotic membranes, which it reinforces by favoring a compact and regular arrangement of the
phospholipids. The probe 1, when used together with choles["I Prof. Y. Nakatani. S. B. Fredriksen. V. Dolle. M.Yamamoto. Prof. M. Goeldner
Prof. G. Ourisson
Laboratoire de Chimie Organique des Substances Naturelles associi au CNRS,
Universite Louis Pasteur
5 rue Blaise Pascal. F-67084 Strasbourg (France)
Telefax: Int. code (88)607620
[**I This work was supported by the CNRS (Action Imitative de I'lnterface
Chimie-Biologie), by the Supermolecules Research Project of the Research
Development Corporation of Japan and Universite Louis Pasteur.
+
0570-0833/9411111-1176$ 10.00+.2SiO
Anyen. Chem. in[. Ed. EnyI. 1994, 33, No. 11
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MeOwO-.,.--,C02Me
terol, is therefore a
useful new tool for
the investigation of
the topography of
membranes.
However, its synthesis is
not easy. Furthermore, we noted that
our predecessors had
not attempted to improve the very low selectivity of their simpler probes by adding
same cholesterol.
is true o f The
two
1) hv
2)MeONa
3)LH-20.
DMPC + 2 +- ‘ H K C
cholesterol
M e O A
b-CO2Me
HO ;
-{
1
@Ocholesterol
t
methyl myristate
+
X
X
Iz/AcOH
1
n
fi
+ M e O v C k ~ - -1)dRuCI$Na104
% M e x
-
C
O
2
M
e
studies recently pubMe02C
<
lished: one describmethyl n-oxomyristate
0
2) CHZNZ
n
( n = 4, 5, _ _ _ 12, 13, E 14)
ing a different type
Me02C
of photoactivatable
Scheme 2. Reaction sequence for the determination by the photoreaction of the functionalized positions in the rnyristoyl chains in DMPC
membrane - spanning
X = probe fragment linked by photoactivation to cholesterol or methyl myristate.
and the
other‘‘] dealing with
diation in vesicles of D M P C containing varying proportions of
simple benzophenone probes very similar to the one described
cholesterol. The products of the reaction were analyzed, as in
below. but not more selective than those of Khorana or Breslow
ref. [2], by the reaction sequence summarized in Scheme 2, and
for the same reason.
the results obtained with increasing cholesterol content are reWe have synthesized the phospholipidic photoactivatable
ported in Figure 1 ; in Figure 2 they are compared at 33 mol YO
“half-probe” 2 by the route shown in Scheme 1 (all products
cholesterol content[’] with those previously obtained with the
have been fully characterized by ‘H N M R and 13C N M R spectransmembrane probe 1.
trometry and, for the end product, by M S and microanalysis).
In analogy to its trdnSmembrane counterpart I, addition of
5 m o l % of the new probe 2 to 1,2-dimyristoyl-sn-glycero-3phosphocholine (DMPC) and organization by freeze- thaw
cycles in water hardly depresses the phase-transition temperature T , . which shows that 2 indeed does not perturb the double
layers to be investigated. We then tested its behavior upon irra-
* 0%
* 10%
0
Me0
HO
60 %
* 20%
NaH I I(CH2),&02Me
NaH I Me1
-0-
DMF
33%
20
95 %
A
0
EtOH I THF
4
t
KOH,
53 70
6 7 8
5
91011121314
n-
Fig. 1. Effect of cholesterol (mol% indicated on the right) on the distribution 01‘
DMPC positions functionalized by irradiation of the half-probe 2. /:Percentage
functionalization at carbon atom number n of the myristoyl chain.
0
c
-
LMPC I CdCI, I CsF
DMF
Me0
D
0
U
54 %
o4
2
€it.
Engl. 1994, 33. No. 11
6
7 8
91011121314
n-
Scheme 1 . Synthesisoi‘thehalf-probe2.A:m.p. 139-140’C;B: m.p.93-94-C;C:
m.p. 138-13Y C ; D : m . p . 127-128”C;Z: T,=46.3’C(pure);23.6’C(5rnol%in
DMPC). DMF = dirnethylformamide, LMPC = 2-lysomyr~stoylphosphocholine.
A n g m . . <’h[w.I n ! .
5
,I;;
Fig. 2. Comparison of the effect of 33 mol% cholesterol on the distribution of
DMPC positions functionalized by irradiation of the probes 1 and 2 I and n as for
Fig. 1 .
VCH Vrrlugsgeselfschaft mhH, 0.69451 Weinheim, 1994
0570-0833/94/1111-117?8 10.00+.2.5!0
1177
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Without cholesterol or with low proportions of this membrane reinforcer, the selectivity is very low: all the carbon atoms
of the chain up to C-4 are attacked, and there is no marked
preference for the end of the chain, as expected by analogy with
the results of Khorana. of Breslow, or of Lala.[3,4361
H owever,
near 33 mol YOcholesterol, the picture changes drastically, and
the selectivity becomes nearly as good as that observed with the
transmembrane probe.
The loss of selectivity observed in all cases by halving the
probe is in agreement with expectation : the transmembrane
probe nrust be more ordered than the half probe. However, the
difference of selectivity is so low at high cholesterol concentrations, and the synthesis of the half-probe is so much more
straightforward, that its use, or that of analogues, will probably
be warranted for the exploration of the topography of the inside
of membranes, in particular of membrane-inserted proteins.
The radius around a photoexcited benzophenone carbonyl
group within which hydrogen can be abstracted to oxygen
has been evaluated by Winnik['] to be about 2 A. Thus, our
results indicate in both cases highly ordererd bilayer systems;
the selectivity can probably not be increased with this chromophore.
The effect of added cholesterol should lead also unavoidably
to its attack. This is indeed the case: we have isolated small
amounts of products which, after transmethylation, were separated and analyzed by mass spectrometry and 'H N M R and I3C
N M R spectroscopy. Three of these products have the expected
mass for the dehydrated cholesterol + probe photoproduct.
Their N M R spectra suggest they are substituted in the side
chain, but they have not yet been more precisely characterized.
The structure of the half-probe leads necessarily, in its photoreaction with chiral substrates such as cholesterol or proteins,
to diastereomeric heterogeneity. This, and the relative inconvenience of the necessarily complex handling of the photoproducts. justifies improvements in the design of future analogues.
Received. Nokember 29. 1993 [Z 6524IEI
German version: A q y i v . Chem. 1994. 106. 1230
[ I ] Y. 1.DiyiLou. A. Genevois. T. Lazrdk, C. Wolff. Y Nakatani. G . Ourisson.
Pirahedron Lrrt. 1987, 28. 5743 5746.
[2] a ) M. Yamamoto, W. A. Wdrnok, A. Milon, Y. Nakatani. G. Ourisson, A / i p i ~ .
Chem. 1993. 105. 302-304; Angcw. Chnii. I n r . Ed. E ~ i g / .1993, 32, 3 9 ~ 2 6 1
h) M. Yamanloto, V. Dolle. W. A. Warnock. Y. Diyzou. M. Yamada. Y.
Nakatani. G. Ourisson. Bid/ Soc. Chin?. Fr., in press.
[3] A. H. Ross, R. Radhakrishnan, R. J. Robson. H. G. Khorana. J. Biol. Chm?.
1982. 257. 412-4161. and references therein.
[4] M F. Ciarniecki. R. Breslow. J1 A m Chon. So<. 1979, 101, 3675-3676, and
references therein.
[S] .I. M Delfino. S. 1.Schreiber. F M. Richards. J. Am. Chmi. Sm. 1993. l l 5 .
3458 3474.
[6] A. K Lala. E. R. Kumer. J. A m Chon. Suc. 1993. 115. 3982-3988.
(71 Up to 33 m o l % cholesterol does not perturh DMPC bilaycrs: R. A . Demcl. B.
de Kruyff. Biod~irf?.
Biop/ij.s. Aciu 1976. 457. 109- 132
[XI M. A. Winnik. Arc. Cheni. Rc,\. 1977. 10. 173-179
-
Synthesis and Characterization of Thiolato
Complexes with Two-Coordinate Iron(rr)**
Jeffrey J. Ellison, Karin Ruhlandt-Senge.
and Philip P. Power*
Stable complexes with two-coordinate, open shell, transition
metal centers are quite rare and, almost without exception, are
confined to derivatives of bulky alkyl or amido ligands.['] For
ligands with bonding atoms from the third period, for example.
sulfur or phosphorus, twofold coordination is currently unknown in the condensed phase and three-coordinate species are
very rare.['] Transition metal complexes with low coordination
numbers (2 o r 3) have recently assumed a new importance owing
mainly to the disclosure of the crystal structure of the nitrogenfixing FeMo cofactor of Azotobacter vinelnndii in which six of
the seven iron centers are, most probably, three-coordinate and
ligated by sulfur atoms (sulfido l i g a n d ~ ) ~(Fig.
~ " ] 1). Moreover.
Fig. 1. Schematic draw,ing (from ref. [3b]) for the binding of nitrogen to MoFe
cotictor of nitrogenasc.
it has been proposed that the dinitrogen molecule coordinates to
the six iron centers within the central Fe, cage which is in its
reduced form.[3b]The, as yet incompletely defined,[3'.dl site Y
(Fig. 1 ) may also involve an N or 0 species. The participation of
this ligand site in facilitating access of the dinitrogen molecule to
the core irons is also a possibility. In this case, it is conceivable
that an iron center coordinated by two sulfido ligands may be
involved as an intermediate during the reduction process. In this
paper the first two thiolato complexes 1 and 2 (Mes = 2,4,6Me,C,H,) which have essentially two-coordinate iron centers
are now described.
Compounds 1 and 2 were synthesized by the reaction of one
or two equivalents of the new thiol HS(2,6-Mes,C,H3) with
[Fe(S(2,6-MesZC,H,)) {N(SiMeJ2j.]
I
[Fe(S(2,6-Mes2C,H,)) ,]
2
[Fe(N(SiMe,),}J. The X-ray crystallographic dataC4]show that
1 (Fig. 2) and 2 (Fig. 3) are monomers with no close interactions
between neighboring molecules. Two crystallographically independent. but chemically identical, monomers and two toluene
molecules are observed in each asymmetric unit in the case of 1.
Both 1 and 2 possess a bent geometry at the iron center; the
S-Fe-N angles in 1 are 118.9(2) and 120.8(2)", while in 2 the
S-Fe-S angle i s very similar, 121.8(1)'. The Fe-S and Fe-N
bond lengths in 1 are 2.308(2) and 2.314(2), and 1.913(6) and
1.923(5)A, respectively; in 2 the Fe-S distances are 2.277(2) and
[*] Prof. P. P. Power. J J. Ellison. Dr. K. Ruhlandt-Seiige
Department of Chemisty. University of California
Davis. CA 95616 (USA)
Telefax: Int. code + (916)752-8995
[**I
This work was supported by the National Science Foundation and the
Petroleum Research Fund.
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