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Direct evidence for muonium radicals in water solutions
Volume 57, number 1
DIRECT EVIDENCE
CHEMICAL
PHYSICS
FOR MUONIUM WICALS
C. BUCCI, G. GUIDI, G.M. DE’MUNARI, Istiiuro di FE&a delfYfni~ecsir& Rmna, Italy and Gruppo NaSmude di Star ttura de& Hater&
LETMERS
IN WATER SOLUTIONS
M. MANFREDI,
1 July 1978
*
P. PODINL, R. TEDESCHI
C.. R., Parma, hly
and P.R. CRIPPA and A. VECLI Istitu?o di Fisk de#Tiniuersit& Parma, Italy and Gmpp
Nazionule di Cibemetica e Biofsica, C.N.R., Pama, ftafy
Received 28 November 1977 Revised manuscript received 31 Jauuaiy 1978
The precession frequencies of muoni,m in a moleculzr radical state have been observed by r.tSR technique in diluied solutions of 2,~~yd~~y-~-methyi-p~~idine
It-irather recent times the precession of the muon spin in a muonium atom in pure water was observed El] with the important consequence that the fast re-
actions of muonium in aqueous solutions can now be studied directly rather than by the indirect method which relies upon the residual polarisation and phase of the “free” muon precession signal [2]_ The muonium atom (Muo) in solutions is expected to be of unique importance in the investigation of the submicrosecond initial step of reactions in which normally atomic hydrogen is involved_ In the present experiments the reaction of Mua with DNA bases in water solutions is studied. In particular, the formation of an adduct radical on thymine is considered in connection with the problem of better understanding the nature of the fast stage radiation effects in DNA. The polarized muon beam available at CERN’s 600 MeV S.C. operating at an internal current of 3.5 + This work was supported by contract No. 75.00862.02 of the Consiglio Nazionaie delle Ricerche, Rome and by the Experimental Physics Division at C.E.R.N., Geneva. The analysisof data was performed at CERN. and C_I.N.E_C.A., Paxma computztion facilities.
ti is used to stop nearfy lo4 muonsfs, 80?5 polarized, in a 5 X 5 X 6 cm3 target. Positrons emitted in the muon’s decay are detected by two telescopes ar 590” relative to the incoming beam; in such conditions nearly 450 correlated events/second can be collected. A time-to-digital converter feeds each event in an online HP-2100 computer which also allows for a preliminary analysis of the accumulated time-histograms. The data from the two teiescopes can be added and total statistics of 8 X 106 events, or more, are obtained for final data analysis. Typical baclcground is of 5% and a small fraction of it is correlated at the cyclotron frequency, which manifests itself in the Fourier analysis as a line near I7 MHz whose equivalent asymmetry is rather small (OS to 1%). All liquid samples are outgassed according to the same procedure [l] _ The concentration of solute is determined spectrophotometrically both before and after t&e experiment. Samples of pure water, fused quartz, compact and powder graphite are systemaiitally used as reference for the calibration of the asymmetry and the magnetic field. The most in;eresting of these, of course, is pure water which is characterized, 41
Vohme
57, number 1
CHEMICAL
PHYSICS
1 JuIy 1978
LJZlTERS
in the range of external magnetic fields between 4 Oe and 30 Oe, by the following parameters: “free” muon = asymmetry, A, = 15.5%; muonium asymmetry, A, 2%; muonium damping, rrr, = 2 I.CS_ Since the full asymmetry in graphite for the present set-up is A, = X5%, both muon and MUOsignal in pure water agree with the recent determination [l] and, in particular, conf5r-m that nearlf 50% of the Mu0 are missing. In the thymine molecules studied here, the basic ring contains one double bond between carbons 5 and 6 [3] ; at this site ESR measurements on solid irradiated thyminc show that an extra hydrogen forms an adduct radical as a final long-lived radical state [4] _ In water (as well as in any other type of) solution the product of such a reaction is unknown probably because of the too short lifetime of the radicals to be detected by ESR Accordir&, the reaction of Mu0 is investigated, within the sohrbility range in water, starting from rather diluted solutions (10d5 M) up to the maximum solubiity which is ~3.5 X IOd2 M. A general feature of the data is that the Mu0 precession signal is increasingly damped at increasing solute’s concentration while the residud muon’s polarization maintains the same value as in pure water. A closer look at the data, however, indicates that one is dealing with a more complex system than the damped Mu0 precession_ Portions of the time-histograms are shown in fig. 1 for pure water and thymine solutions with an external field of 10 Oe. The muon’s lifetime decay and the background are not shown, Let us consider that such histograms contain exchisively Mu0 precession signals damped by a first order chemical reaction time, rch. The fate of Mu0 should then be described by the following expression: fit) = A0 exp(-t/rm){cos X [cos(oo
Fig l_ Direct “tie spectra of (a) pxe water, (b) thymine so_ lution 5 X 10e5 M_ (c) thymine dution 8 X lo4 M. The external field is near to 10 Oe. Free muon precession and decay are not shown. The time scale is 4 ns/ch.
42
wt
+ Cl) + cos S2LtJ ) exp(-r/r&)
,
0)
where w = 3(q2 f ~~9; CZ= 4(q2 - ~29; rm is the damping time determined for pure water. If the data for all concentrations and fields are actually analysed by considering only decaying Mu0 sigI& (i-e- according to expression (1) which obviously negiects the contribution from the polarization precessing in a radical state) one obtains the concentration dependence of T& shown in fig_ 2 at 10 Oe and, for a few concentrations, also at 5 Oe and 20 Oe_ Both
Volume 57. number 1
CHEMICAL PHYSKS LJSTERS
1 July 1978
splittings. It should be pointed out that, ultimately, all precession signalsdisappear(say above 1W2 M) thns rntig out *e possibility that stray sing& from the cell walls are present. The fact that one is dealing with a Mu0 reaction and simultaneous formation of radicalsis indicated by the persistence of precession signalsafter the apparent Muo decay. The characteristicsof the observed signals
Fig. 2. Initialdampingconstantof the signal(see text) versus solute’sconcentrationcorrectedfor the dampingfoend in pure w3ter. In the intermediateregionof concentrationsthe fittingwith a singlechemicalreactiontime is quite difficult for the reasonsexphined in the text,
ihe magnetic fieid dependence and the concentration
dependence are rather peculiar. As for the effect of the magnetic field on the damping of the precession signal, it might be argued that one is not dealing with chemical damping (but magnetic damping, for instance) or else that the observed signal does not belong excIusiveIy to ‘“unperturbed’ M@ precession_The possibility of magnetic damping can be ruled out at a qualitative stage because its dependence on the external field, when it can be observed, is opuosite to the present observations in which the dam&g rate increaseswith the field. A more plausible alternativeis the connection between the observed “damping” and the frequency splittings when muonium belongs to a radical complex: in this case expression (1) is not sufficient for the description of the time-evolution of the polarization. Similar to the field dependence, the concentration dependence also does not agree with the pictnre of an ‘hnperturbed” Muo pre$ession undergoing a monomolecular reaction (as it should in such diluted systems). The presence of the plateau for concentrations above 10-3 M seems to divide the range into two regions: a low concentration region in which the precession of “free” Mu0 can be obsenred for a iength of time without an appreciable formation of radicaisand a high concentration region in which Mumhas rapidly converted into radicalswith their typical frequency
pp--
:
/L5EC
:
c
n
Fii 3. (a) and (b) show tlte averageasymmetryof the w- signal in the 5 X 10” hf soiation respectivelyat 8 Oe and 16 Oe; the observedoAilations at times beyond 1~ are statistically significantsincethe nns noise is separatelydetermined for each Fourierhnsformed portion of the histogram (see
text). (c) showstie directtisr~e-spe~truxr~ dose FourierVlaE y.Gsin 200 ns portionsis reportedin @I. 43
Vohme 57. number 1
CHEMiCAL
PHYSICS LE’ITERS
for times up to 3 ps (for longer times there are serious signal-to-noise fbnitations~ are shown in fig_ 3, The points represent the sunrage asymmetry, Z, of the signal at the central frequency w_ ; they areseparated by 400 ns (fig_ 3a for 8 Oe) and by 200 ns (fig_ 35 for 16 Oe), time intervals over which 3 was derermined by Fcurier analysis. As a function of concentration the relative position of the observed “beatings” does net change but they are shifted to shorter times and, ultimately, disappear when one approaches concentrations of = 10-2 M. The overall behaviour can be then understood in terms of the welI known prediction for the time evolution of muon pokrization in the presence of muonium reacting to give rad.icaIs f5 j _ Basicelly the main assumption ;S the instantaneous change of the four characteristic frequencies when Mu0 forms a molecular radical. A crucial role is then pIayed by two factors. Fits, the rerm T&AU Z%1 (with Aw roughly speaking being the frequency difference between Mu9 and radical) determines whether one is likely to observe decaying Mu0(i.e. for large external fields or for small concentrations) or else radicals (i-e for very weak fields or large concentraticns). In intermediate conditions, T*ACJ = 1, the precession signals in both mueonium and radical can be Gbserved simuhaneor&y. Second, the radical’s reaction along presently undetermined paths must be taken into consideration especially at high concentrations of solute.
1 July 1978
A fitting procedure aimed to obtain a precise determination of *he Mu0 reaction constant, radical hyper-fine interaction and reaction constant is under way. Preliminary determinations of such parameters are: Kniu = (7 + 2) X 109 s-l M-l ; KR = (5 7 2) X 10’ s--l M-l; OR = (0.07 t 0.01)&+,. A more detailed account of these fmdings will be given in a fo~co~g extended version.
The authors thsnk Drs. P.W. Percival and H. Fischer for useful su=estions regarding the preparation of outgassed sampies.
References P-W. percival, H. Fischer, M. Camaui, F-N. Gygax. W, Riiegg,A. Schenck,I-LSc&iEingand H, Graf, Chem. Phys. Letters 39 (1976) 333. [2] J-H. Brewer, FLM. Crowe, F-N- Gygax, R-F. Johnson, 33-D.&Meson, D.G. Ffeming and A. Schenck, Phys Rev. [l]
Letters 31<1973) 143;
J.H. Brewer, KM. Crowe, F.N. Gygax, R.F. Johnson, D.G. Fleming and A. S&en&, Phys Rev, A9 (1974) 495. 131 H. Dertinger and N. Hart@ Intern. J. Radiat. BioL 21 (1972) 279, f4] A. Mueller, Progr. Biophys. MoL Biot 17 (1967) 101. [5] W.E. Fischer,Heiv. Phys. Acta 49 (1976) 629; P.W. Pexival and H. Fischer,Chem Phys. 16 (1976) 89.
DIRECT EVIDENCE
CHEMICAL
PHYSICS
FOR MUONIUM WICALS
C. BUCCI, G. GUIDI, G.M. DE’MUNARI, Istiiuro di FE&a delfYfni~ecsir& Rmna, Italy and Gruppo NaSmude di Star ttura de& Hater&
LETMERS
IN WATER SOLUTIONS
M. MANFREDI,
1 July 1978
*
P. PODINL, R. TEDESCHI
C.. R., Parma, hly
and P.R. CRIPPA and A. VECLI Istitu?o di Fisk de#Tiniuersit& Parma, Italy and Gmpp
Nazionule di Cibemetica e Biofsica, C.N.R., Pama, ftafy
Received 28 November 1977 Revised manuscript received 31 Jauuaiy 1978
The precession frequencies of muoni,m in a moleculzr radical state have been observed by r.tSR technique in diluied solutions of 2,~~yd~~y-~-methyi-p~~idine
It-irather recent times the precession of the muon spin in a muonium atom in pure water was observed El] with the important consequence that the fast re-
actions of muonium in aqueous solutions can now be studied directly rather than by the indirect method which relies upon the residual polarisation and phase of the “free” muon precession signal [2]_ The muonium atom (Muo) in solutions is expected to be of unique importance in the investigation of the submicrosecond initial step of reactions in which normally atomic hydrogen is involved_ In the present experiments the reaction of Mua with DNA bases in water solutions is studied. In particular, the formation of an adduct radical on thymine is considered in connection with the problem of better understanding the nature of the fast stage radiation effects in DNA. The polarized muon beam available at CERN’s 600 MeV S.C. operating at an internal current of 3.5 + This work was supported by contract No. 75.00862.02 of the Consiglio Nazionaie delle Ricerche, Rome and by the Experimental Physics Division at C.E.R.N., Geneva. The analysisof data was performed at CERN. and C_I.N.E_C.A., Paxma computztion facilities.
ti is used to stop nearfy lo4 muonsfs, 80?5 polarized, in a 5 X 5 X 6 cm3 target. Positrons emitted in the muon’s decay are detected by two telescopes ar 590” relative to the incoming beam; in such conditions nearly 450 correlated events/second can be collected. A time-to-digital converter feeds each event in an online HP-2100 computer which also allows for a preliminary analysis of the accumulated time-histograms. The data from the two teiescopes can be added and total statistics of 8 X 106 events, or more, are obtained for final data analysis. Typical baclcground is of 5% and a small fraction of it is correlated at the cyclotron frequency, which manifests itself in the Fourier analysis as a line near I7 MHz whose equivalent asymmetry is rather small (OS to 1%). All liquid samples are outgassed according to the same procedure [l] _ The concentration of solute is determined spectrophotometrically both before and after t&e experiment. Samples of pure water, fused quartz, compact and powder graphite are systemaiitally used as reference for the calibration of the asymmetry and the magnetic field. The most in;eresting of these, of course, is pure water which is characterized, 41
Vohme
57, number 1
CHEMICAL
PHYSICS
1 JuIy 1978
LJZlTERS
in the range of external magnetic fields between 4 Oe and 30 Oe, by the following parameters: “free” muon = asymmetry, A, = 15.5%; muonium asymmetry, A, 2%; muonium damping, rrr, = 2 I.CS_ Since the full asymmetry in graphite for the present set-up is A, = X5%, both muon and MUOsignal in pure water agree with the recent determination [l] and, in particular, conf5r-m that nearlf 50% of the Mu0 are missing. In the thymine molecules studied here, the basic ring contains one double bond between carbons 5 and 6 [3] ; at this site ESR measurements on solid irradiated thyminc show that an extra hydrogen forms an adduct radical as a final long-lived radical state [4] _ In water (as well as in any other type of) solution the product of such a reaction is unknown probably because of the too short lifetime of the radicals to be detected by ESR Accordir&, the reaction of Mu0 is investigated, within the sohrbility range in water, starting from rather diluted solutions (10d5 M) up to the maximum solubiity which is ~3.5 X IOd2 M. A general feature of the data is that the Mu0 precession signal is increasingly damped at increasing solute’s concentration while the residud muon’s polarization maintains the same value as in pure water. A closer look at the data, however, indicates that one is dealing with a more complex system than the damped Mu0 precession_ Portions of the time-histograms are shown in fig. 1 for pure water and thymine solutions with an external field of 10 Oe. The muon’s lifetime decay and the background are not shown, Let us consider that such histograms contain exchisively Mu0 precession signals damped by a first order chemical reaction time, rch. The fate of Mu0 should then be described by the following expression: fit) = A0 exp(-t/rm){cos X [cos(oo
Fig l_ Direct “tie spectra of (a) pxe water, (b) thymine so_ lution 5 X 10e5 M_ (c) thymine dution 8 X lo4 M. The external field is near to 10 Oe. Free muon precession and decay are not shown. The time scale is 4 ns/ch.
42
wt
+ Cl) + cos S2LtJ ) exp(-r/r&)
,
0)
where w = 3(q2 f ~~9; CZ= 4(q2 - ~29; rm is the damping time determined for pure water. If the data for all concentrations and fields are actually analysed by considering only decaying Mu0 sigI& (i-e- according to expression (1) which obviously negiects the contribution from the polarization precessing in a radical state) one obtains the concentration dependence of T& shown in fig_ 2 at 10 Oe and, for a few concentrations, also at 5 Oe and 20 Oe_ Both
Volume 57. number 1
CHEMICAL PHYSKS LJSTERS
1 July 1978
splittings. It should be pointed out that, ultimately, all precession signalsdisappear(say above 1W2 M) thns rntig out *e possibility that stray sing& from the cell walls are present. The fact that one is dealing with a Mu0 reaction and simultaneous formation of radicalsis indicated by the persistence of precession signalsafter the apparent Muo decay. The characteristicsof the observed signals
Fig. 2. Initialdampingconstantof the signal(see text) versus solute’sconcentrationcorrectedfor the dampingfoend in pure w3ter. In the intermediateregionof concentrationsthe fittingwith a singlechemicalreactiontime is quite difficult for the reasonsexphined in the text,
ihe magnetic fieid dependence and the concentration
dependence are rather peculiar. As for the effect of the magnetic field on the damping of the precession signal, it might be argued that one is not dealing with chemical damping (but magnetic damping, for instance) or else that the observed signal does not belong excIusiveIy to ‘“unperturbed’ M@ precession_The possibility of magnetic damping can be ruled out at a qualitative stage because its dependence on the external field, when it can be observed, is opuosite to the present observations in which the dam&g rate increaseswith the field. A more plausible alternativeis the connection between the observed “damping” and the frequency splittings when muonium belongs to a radical complex: in this case expression (1) is not sufficient for the description of the time-evolution of the polarization. Similar to the field dependence, the concentration dependence also does not agree with the pictnre of an ‘hnperturbed” Muo pre$ession undergoing a monomolecular reaction (as it should in such diluted systems). The presence of the plateau for concentrations above 10-3 M seems to divide the range into two regions: a low concentration region in which the precession of “free” Mu0 can be obsenred for a iength of time without an appreciable formation of radicaisand a high concentration region in which Mumhas rapidly converted into radicalswith their typical frequency
pp--
:
/L5EC
:
c
n
Fii 3. (a) and (b) show tlte averageasymmetryof the w- signal in the 5 X 10” hf soiation respectivelyat 8 Oe and 16 Oe; the observedoAilations at times beyond 1~ are statistically significantsincethe nns noise is separatelydetermined for each Fourierhnsformed portion of the histogram (see
text). (c) showstie directtisr~e-spe~truxr~ dose FourierVlaE y.Gsin 200 ns portionsis reportedin @I. 43
Vohme 57. number 1
CHEMiCAL
PHYSICS LE’ITERS
for times up to 3 ps (for longer times there are serious signal-to-noise fbnitations~ are shown in fig_ 3, The points represent the sunrage asymmetry, Z, of the signal at the central frequency w_ ; they areseparated by 400 ns (fig_ 3a for 8 Oe) and by 200 ns (fig_ 35 for 16 Oe), time intervals over which 3 was derermined by Fcurier analysis. As a function of concentration the relative position of the observed “beatings” does net change but they are shifted to shorter times and, ultimately, disappear when one approaches concentrations of = 10-2 M. The overall behaviour can be then understood in terms of the welI known prediction for the time evolution of muon pokrization in the presence of muonium reacting to give rad.icaIs f5 j _ Basicelly the main assumption ;S the instantaneous change of the four characteristic frequencies when Mu0 forms a molecular radical. A crucial role is then pIayed by two factors. Fits, the rerm T&AU Z%1 (with Aw roughly speaking being the frequency difference between Mu9 and radical) determines whether one is likely to observe decaying Mu0(i.e. for large external fields or for small concentrations) or else radicals (i-e for very weak fields or large concentraticns). In intermediate conditions, T*ACJ = 1, the precession signals in both mueonium and radical can be Gbserved simuhaneor&y. Second, the radical’s reaction along presently undetermined paths must be taken into consideration especially at high concentrations of solute.
1 July 1978
A fitting procedure aimed to obtain a precise determination of *he Mu0 reaction constant, radical hyper-fine interaction and reaction constant is under way. Preliminary determinations of such parameters are: Kniu = (7 + 2) X 109 s-l M-l ; KR = (5 7 2) X 10’ s--l M-l; OR = (0.07 t 0.01)&+,. A more detailed account of these fmdings will be given in a fo~co~g extended version.
The authors thsnk Drs. P.W. Percival and H. Fischer for useful su=estions regarding the preparation of outgassed sampies.
References P-W. percival, H. Fischer, M. Camaui, F-N. Gygax. W, Riiegg,A. Schenck,I-LSc&iEingand H, Graf, Chem. Phys. Letters 39 (1976) 333. [2] J-H. Brewer, FLM. Crowe, F-N- Gygax, R-F. Johnson, 33-D.&Meson, D.G. Ffeming and A. Schenck, Phys Rev. [l]
Letters 31<1973) 143;
J.H. Brewer, KM. Crowe, F.N. Gygax, R.F. Johnson, D.G. Fleming and A. S&en&, Phys Rev, A9 (1974) 495. 131 H. Dertinger and N. Hart@ Intern. J. Radiat. BioL 21 (1972) 279, f4] A. Mueller, Progr. Biophys. MoL Biot 17 (1967) 101. [5] W.E. Fischer,Heiv. Phys. Acta 49 (1976) 629; P.W. Pexival and H. Fischer,Chem Phys. 16 (1976) 89.