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Intraproteic ligand diffusion in laser-photodissociated carboxyhemoglobin and subunits
Volume 79, number 3
PHOTODISSOCIATION
CHEhIlCAL PHYSICS LETTERS
1 hlay 1981
OF DMETHYLNITXOSAMINE
G GEIGER, H STAFAST, U. BRUHLMANN and J Robert HUBER Insmu t fur Phyaka:sche Chemie der Utubersmit Zurich, 805 7 Zurcch, Swrtzerland Received 23 December 1980. m final form 31 January 1981
Photodtssoclatlon of (CH9)2N-N0 followng Sl(na*) -So ewltatlon yields (CHs)zN- and NO with a quantum yield of 1 03 f 0 10. These fragments recombine leavmg no stable photoproducts A frachon of NO produced by photolysls LSviiranonally excited The rate of the NO&J = 1) relaxation m colllslon WA (CHJ)2N-N0, measured by IR fluorescence, IS (147 f 0 03) x 104 r’ ToiY’
hand, emlsslon kinetic bratronal fluorescence
I_ introduction
Drmethylmtrosamme, (CH,),N-NO (DMN), IS one of the srmplest chemically stable nitrosamme compounds. These molecules, notorious carcinogens, are characterized by the existence of an electron-donatmg group (R2N-) drrectly adJacent to an electron acccptmg group (-NO) The structure of DMN has been extensively mvestlgated by electron drffractlon [I] , microwave [2], IR and Raman spectroscopy [3] _ In Its ground state the molecule IS planar (C, structural symmetry) and possesses an internal rotation barrier of ~95 kJ/mole [4] around the N-N bond. Wl-ule the N-N bond length of 1 344 A [ 1,2] hes between that of a smgle and double N-N bond, the N-O bond length 1s 1.23 a as compared to 1.15 A u-t the NO molecule. The bond dlssoclatlon energy of the weakest bond, the R,N-NO bond, IS esumated to be =I70 kJ/mole [S] In this commumcatlon we report results from a photochemical mvestigation of DMN in the gas phase at room temperature When excited into the first electromc absorption band the apparently photostable compound IS, in reality, photodrssoclated mto (CH3)2N’and NO, where part of the NO molecules are vtbrationally excited. A subsequent effic!ent recombmatron of these two fragments leaves no photoproducts This process has been observed usmg, on one hand, mass spectrometric anai_Jsis and lSNO as a tracer and, on the other
(? 009-2614/81/0000-0000/S
02.50
0 North-Holland
spectroscopy morutonng of NO(u = 1 + u = 0)
the vr-
2. Experimental Nltrosamme was excited with an XeF excLmer laser (Lumomcs TE 860) at a repetrtron frequency of 50 Hz. The 1R emlsslon - observed at right angle to the laser beam - was co!lected with a NaCl lens u/l), passed through 2 7 cm gas cell as well as a Ge plate (UV filter), and focused onto 2 hquld-nitrogen cooled InSb detector (Hughes SBRC .3 mm diameter, rrse tune 1.4 ps) The signals were preamphfled (Hughes SBRC mode1 A 220) and recorded wrth a Blomatlon 8 100 transient recorder m conJunction with a Nlcolet 1070 multichannel srgnal averager and a xy recorder_ The st.ztlonary photolysls was performed wtth a 500 W mercury lamp (Osram HBO) and a Spex muumate monochromator . The analysts of the photoproducts was carried out with a gas chromatograph (Carlo Erba 4160) equrpped wth a 50 m long, open tubular glass column (UCON LB 550X, Jaeggl, Trogen, Switzerland) and a quadrupole mass spectrometer (Balkers QMG 5 11). For measurements of the photodecompositron quantum yield a radiometer (Optrorucs Laboratories, model 730A: was utlllzed. Dlmethylnitrosamine (Merck) was punfied by dlstdlatton at 77 K under hrgh vacuum. lsNO (isotopic punty > 99.5%; Prochern. London) was used as received. Publishing
Company
521
Volume 79, number 3
CHEMICAL
PHYSICS
3. Results
The gas-phase absorptron spectrum of drrrethylmtrosamme (DMN) shown m fig. 1 IS characterized by two drstmct electrontc transitions, a weak. slightly structured Sl(nn*) + S, transrtron with a maamum at 363.5 nm (e = 68 M-l cm-l) and a strong broad transrtron at 227 nm (e ~4600 M-1 cm-l) [6] mto the At a DMN pressure of =l Torr uradratron first absorption band at 363 5 nm produced nerther detectable photoproducts nor any observable decrease in the initial DMN concentration. Since the excrtatron energy was 330 kJ/emstem and the R2N-NO bond 1s expected to have an energy of =Z170 kJ/mole [S] , we assumed that the molecu!ar drssocratron
1s
followed
NO+RZN--
R,N’+
NO*,NO
by a very efficient kR RZN-NO
(1)
radical
I May 1981
number of absorbed emstcm s-* Q-1 leadmg to drssocratron drvrded by the molar concentrdtron of DMN, the formatron of the DMN isotope under stationary condrtions IS
3.1. Di~~~~tatron detected by rsotoprc exchange
F$N-NO*
LETTERS
recombmatron
[R2NJSNO]
= [R,NNOlo
f 1) = I,t
(4)
and thus In([R,N’SNO]/[R2NNO]
The ratro [R,Ng5NO] /[R?NNO] obtamed from the quantitative mass spectrometrrc analysrs excellently frt eq. (4) as demonstrated in ftg. 2 Using the same method, the number of produced R,NISNO molecules per number of absorbed photons yielded the quantum yreld of photodrssoctatron +p,,,(363.5 nm) = 1.03 _+0.10 Addrtron of 100 Torr N, as buffer gas had no influence on the photodecomposrtron. Moreover, It 1s noted that no tctramethylhydraztne was detected even after 24 h of contuwous photolysn, I (absorbed, 363 5 nm) = lOI photons/s cl 313
I
0 25
c;
/
0 2c
015
010
005
I
I
Oh
I
60
spectrum of dunethybutrosamme @= 0 75 Torr) m the gas phase at room temperature. For the Fist absorptton band the scale has been expanded
:
6b
L-IL L
-JL
/f
522
(3)
(2)
To examine thrs possrbrhty, photolysrs was carrred out on a mrxture of I 5 Torr DMN and 4 5 Torr JSNO The mass spectra of small samples taken at different time intervals durrng photolysis (cf. insert of frg 2) revealed an exchange of NO by t5NO n-r DMN consistent wrth the above mechanism Consequently as long as [NO] * [ISNO] , practtcahy every amme fragment unli recombine wrth lsNO to form R,NISNO. Smce [R,NNO] = [R,NNO] uexp(--Iat), where 1, IS the
Fig. 1. Absorption
[ 1 - e\p(-fat)]
I
1
180
I
300
th-ml1
Frg. 2. Isotoptc exchange by photolysc; accordmg to eq. (4). The pressures of RzNNO md “NO were 1 5 and 4 5 Torr. respectively. (The slope s -=_&a, where a IS the ratio of the urddrated to the totalcell volume ) The msert shows the mass spectra of R,NNO (m/e = 74) and R2N”NO (m/e = 75) of the reaction mzxture at zero tune and after 3 and 6 h of r~radlatlon.
Volume
CHEhIICAL
79, number 3
3 2 Detection
of the NO fragment
PHYSICS
by IR emuuon
It was found that dlssoclation of DMN produces vlbratlonally excited NO molecules. In contrast to a recent study on CF,NO where mterference by photoproduct emlsslon necessitated a flow system [7], the kmetlc measurements of NO*(u = 1) fluorescence (1876 cm-l) was easily detected following XeF (350 run) laser excitation of DMN. This emission 1s suppressed to <5% when a 7 cm long cell ftied with 30 Torr NO was posltloned between the photolysls ceU and the detector The decay of the IR fluorescence. as displayed m fig. 3, follows pseudo-first-order kmetits accordmg to kw NO* + %NNO -NO + %NNO, (5) WI& the measured rate comtant km = k, [R,NNO] . From the slope of a least-squares fit over the range OS-2 0 Torr DMN, the vibrational relaxation rate constant was found to be k, = (1.47 +- 0.03) X 10” s-l Torr-1, lying m the range obtamed by Stephenson [8] for C,H, (1.19 X lOa), CH, (0 61 X 104) and C,H,, (0 85 X IO4 s-l Torr-1) with NO*(v = !) Varlatlon of the laser pulse energy by a factor of five (a = (0 5-2.5)X lOI6 p h o t ons/cm2 pulse) did not alter the decay, I e. km.
1 May 1981
LEI-fERS
4. Discussion Excitation mto the St(mr*) + SO absorption of DMN results m the formatlon of R,N’ ralcals and NO molecules These fragments efficiently recombine leavmg no stable photoproducts Using isotopes such as IsNO or Nl80 as tracers a sunple and convenient photochemlcal way becomes available to produce DMN isotopes (e g. for medlcal apphcatlons). A further interestmg feature of the DMN photolysis 1s the production of vlbratlonally excited NO molecules At 2 Torr, a laser pulse of 2 5 X 1Ol6 photons cmm2 excites and dlssoclates ==G 5% of the molecules withm the u-radiated volume correspondmg to a partial pressure &NO) = p(R,N -) a 10 mTorr. In the light of these relatively high fragment concentrations It IS not mconcelvable that the reactions _k& NO* + R,N SNNO (6) and/or NO*@=
l)+NO*(u=
I)-
kt?Y
NO**@
= 2) + NO
(7)
contribute to the IR emlsslon decay [eq. (S)] . For the reactlon NO + ‘NH2 + product, which IS sumiar to that of eq. (6), Hancock et al [9] reported a rate constant 6 8 X lo5 s-l Torr-1, while Stephenson [IO] measured = 1 2 X 105 s-t Torr1 for the vlbratlonal energy a ke, exchange, eq (7). The lack of any measurable devlatlon from the exponential decay of the IR emission led us to conclude that both processes are not important under the present excltatlon comhtlons (I.e. k: < 3 X 105 s-l Torr-I) but should become significant at higher laser intensity .
Acknowledgement Support of this work by the Schwelzerischer National fonds IS greatly appreciated We thank Dr. Mary MahaneJ for crltlcdlly readmg the manuscript
References
i
i
p(torr)
Fig. 3. Stern-Volmer plot of the mverse decay tune (k,) of NO* (U = 1) versus the RzNNO pressure. The msert shows the fluorescence sgnal obtain& at 05 Torr RzNNO by signal averagmg for 2000 laser pulses.
[ 11 P. Rademacher (1969) [2]
and R StB:eti,
Acta Chem.
Sand.
23
660,672.
F Scappmi. A Guamten, H Dreizler and P Rademacher, 2. Naturforsch 27a (1972) 1329. A Guarnien, F Rohwer and F. Scappuu, Z Naturforsch. 30 (1975) 904 523
Volume
i9, number
3
CHELIICAL
PHYSICS LETTERS
(31 1-W. Levm, G-W A. hfllne and T. Axenrod. J. Chem. Phys 53 (1970) 2.505. P. Rademacher and W. Luttke, Spectrochim. Acta 27 (1971) 715. Ber. Bunsenges Physlk Chem. 78 (1974) 1353. 141 R.K. Harnsand R A. Sprag,Chem Commun. 1 (1967) 362: SM. Glidewell, Spectrochun Acta 33A (1977) 361, and references therem. [S] B G. Cowerdock, P Pritchard Jones and J.R hIaJer. Trans Faraday Sot. 57 (1961) 23.
524
1 hl.iy 1981
I P Fisher and E Henderson. Trans Faraday Sot 63 (1967) 1342. [6] K. Weiss and f.R Huber, unpubhshcd results, D R Batbste. L9 Davis and R V Naumann. J Am. Chem. sot 97 (1975) 5071. [ 71 M P Roelhg and A L Houston, Chem. Phys Letters 57 (1978) 75. 181 J.C. Stephenson, J. Chem Phys 60 (1974) 4289. [9]
G Hancock, W Lange. hf Lcnzl Phys Letters 33 (1975) 168. [ 101 J C. Stephenson. J. Chem. Phys
and K H Welgc, 59 (1973)
1523
Chem
PHOTODISSOCIATION
CHEhIlCAL PHYSICS LETTERS
1 hlay 1981
OF DMETHYLNITXOSAMINE
G GEIGER, H STAFAST, U. BRUHLMANN and J Robert HUBER Insmu t fur Phyaka:sche Chemie der Utubersmit Zurich, 805 7 Zurcch, Swrtzerland Received 23 December 1980. m final form 31 January 1981
Photodtssoclatlon of (CH9)2N-N0 followng Sl(na*) -So ewltatlon yields (CHs)zN- and NO with a quantum yield of 1 03 f 0 10. These fragments recombine leavmg no stable photoproducts A frachon of NO produced by photolysls LSviiranonally excited The rate of the NO&J = 1) relaxation m colllslon WA (CHJ)2N-N0, measured by IR fluorescence, IS (147 f 0 03) x 104 r’ ToiY’
hand, emlsslon kinetic bratronal fluorescence
I_ introduction
Drmethylmtrosamme, (CH,),N-NO (DMN), IS one of the srmplest chemically stable nitrosamme compounds. These molecules, notorious carcinogens, are characterized by the existence of an electron-donatmg group (R2N-) drrectly adJacent to an electron acccptmg group (-NO) The structure of DMN has been extensively mvestlgated by electron drffractlon [I] , microwave [2], IR and Raman spectroscopy [3] _ In Its ground state the molecule IS planar (C, structural symmetry) and possesses an internal rotation barrier of ~95 kJ/mole [4] around the N-N bond. Wl-ule the N-N bond length of 1 344 A [ 1,2] hes between that of a smgle and double N-N bond, the N-O bond length 1s 1.23 a as compared to 1.15 A u-t the NO molecule. The bond dlssoclatlon energy of the weakest bond, the R,N-NO bond, IS esumated to be =I70 kJ/mole [S] In this commumcatlon we report results from a photochemical mvestigation of DMN in the gas phase at room temperature When excited into the first electromc absorption band the apparently photostable compound IS, in reality, photodrssoclated mto (CH3)2N’and NO, where part of the NO molecules are vtbrationally excited. A subsequent effic!ent recombmatron of these two fragments leaves no photoproducts This process has been observed usmg, on one hand, mass spectrometric anai_Jsis and lSNO as a tracer and, on the other
(? 009-2614/81/0000-0000/S
02.50
0 North-Holland
spectroscopy morutonng of NO(u = 1 + u = 0)
the vr-
2. Experimental Nltrosamme was excited with an XeF excLmer laser (Lumomcs TE 860) at a repetrtron frequency of 50 Hz. The 1R emlsslon - observed at right angle to the laser beam - was co!lected with a NaCl lens u/l), passed through 2 7 cm gas cell as well as a Ge plate (UV filter), and focused onto 2 hquld-nitrogen cooled InSb detector (Hughes SBRC .3 mm diameter, rrse tune 1.4 ps) The signals were preamphfled (Hughes SBRC mode1 A 220) and recorded wrth a Blomatlon 8 100 transient recorder m conJunction with a Nlcolet 1070 multichannel srgnal averager and a xy recorder_ The st.ztlonary photolysls was performed wtth a 500 W mercury lamp (Osram HBO) and a Spex muumate monochromator . The analysts of the photoproducts was carried out with a gas chromatograph (Carlo Erba 4160) equrpped wth a 50 m long, open tubular glass column (UCON LB 550X, Jaeggl, Trogen, Switzerland) and a quadrupole mass spectrometer (Balkers QMG 5 11). For measurements of the photodecompositron quantum yield a radiometer (Optrorucs Laboratories, model 730A: was utlllzed. Dlmethylnitrosamine (Merck) was punfied by dlstdlatton at 77 K under hrgh vacuum. lsNO (isotopic punty > 99.5%; Prochern. London) was used as received. Publishing
Company
521
Volume 79, number 3
CHEMICAL
PHYSICS
3. Results
The gas-phase absorptron spectrum of drrrethylmtrosamme (DMN) shown m fig. 1 IS characterized by two drstmct electrontc transitions, a weak. slightly structured Sl(nn*) + S, transrtron with a maamum at 363.5 nm (e = 68 M-l cm-l) and a strong broad transrtron at 227 nm (e ~4600 M-1 cm-l) [6] mto the At a DMN pressure of =l Torr uradratron first absorption band at 363 5 nm produced nerther detectable photoproducts nor any observable decrease in the initial DMN concentration. Since the excrtatron energy was 330 kJ/emstem and the R2N-NO bond 1s expected to have an energy of =Z170 kJ/mole [S] , we assumed that the molecu!ar drssocratron
1s
followed
NO+RZN--
R,N’+
NO*,NO
by a very efficient kR RZN-NO
(1)
radical
I May 1981
number of absorbed emstcm s-* Q-1 leadmg to drssocratron drvrded by the molar concentrdtron of DMN, the formatron of the DMN isotope under stationary condrtions IS
3.1. Di~~~~tatron detected by rsotoprc exchange
F$N-NO*
LETTERS
recombmatron
[R2NJSNO]
= [R,NNOlo
f 1) = I,t
(4)
and thus In([R,N’SNO]/[R2NNO]
The ratro [R,Ng5NO] /[R?NNO] obtamed from the quantitative mass spectrometrrc analysrs excellently frt eq. (4) as demonstrated in ftg. 2 Using the same method, the number of produced R,NISNO molecules per number of absorbed photons yielded the quantum yreld of photodrssoctatron +p,,,(363.5 nm) = 1.03 _+0.10 Addrtron of 100 Torr N, as buffer gas had no influence on the photodecomposrtron. Moreover, It 1s noted that no tctramethylhydraztne was detected even after 24 h of contuwous photolysn, I (absorbed, 363 5 nm) = lOI photons/s cl 313
I
0 25
c;
/
0 2c
015
010
005
I
I
Oh
I
60
spectrum of dunethybutrosamme @= 0 75 Torr) m the gas phase at room temperature. For the Fist absorptton band the scale has been expanded
:
6b
L-IL L
-JL
/f
522
(3)
(2)
To examine thrs possrbrhty, photolysrs was carrred out on a mrxture of I 5 Torr DMN and 4 5 Torr JSNO The mass spectra of small samples taken at different time intervals durrng photolysis (cf. insert of frg 2) revealed an exchange of NO by t5NO n-r DMN consistent wrth the above mechanism Consequently as long as [NO] * [ISNO] , practtcahy every amme fragment unli recombine wrth lsNO to form R,NISNO. Smce [R,NNO] = [R,NNO] uexp(--Iat), where 1, IS the
Fig. 1. Absorption
[ 1 - e\p(-fat)]
I
1
180
I
300
th-ml1
Frg. 2. Isotoptc exchange by photolysc; accordmg to eq. (4). The pressures of RzNNO md “NO were 1 5 and 4 5 Torr. respectively. (The slope s -=_&a, where a IS the ratio of the urddrated to the totalcell volume ) The msert shows the mass spectra of R,NNO (m/e = 74) and R2N”NO (m/e = 75) of the reaction mzxture at zero tune and after 3 and 6 h of r~radlatlon.
Volume
CHEhIICAL
79, number 3
3 2 Detection
of the NO fragment
PHYSICS
by IR emuuon
It was found that dlssoclation of DMN produces vlbratlonally excited NO molecules. In contrast to a recent study on CF,NO where mterference by photoproduct emlsslon necessitated a flow system [7], the kmetlc measurements of NO*(u = 1) fluorescence (1876 cm-l) was easily detected following XeF (350 run) laser excitation of DMN. This emission 1s suppressed to <5% when a 7 cm long cell ftied with 30 Torr NO was posltloned between the photolysls ceU and the detector The decay of the IR fluorescence. as displayed m fig. 3, follows pseudo-first-order kmetits accordmg to kw NO* + %NNO -NO + %NNO, (5) WI& the measured rate comtant km = k, [R,NNO] . From the slope of a least-squares fit over the range OS-2 0 Torr DMN, the vibrational relaxation rate constant was found to be k, = (1.47 +- 0.03) X 10” s-l Torr-1, lying m the range obtamed by Stephenson [8] for C,H, (1.19 X lOa), CH, (0 61 X 104) and C,H,, (0 85 X IO4 s-l Torr-1) with NO*(v = !) Varlatlon of the laser pulse energy by a factor of five (a = (0 5-2.5)X lOI6 p h o t ons/cm2 pulse) did not alter the decay, I e. km.
1 May 1981
LEI-fERS
4. Discussion Excitation mto the St(mr*) + SO absorption of DMN results m the formatlon of R,N’ ralcals and NO molecules These fragments efficiently recombine leavmg no stable photoproducts Using isotopes such as IsNO or Nl80 as tracers a sunple and convenient photochemlcal way becomes available to produce DMN isotopes (e g. for medlcal apphcatlons). A further interestmg feature of the DMN photolysis 1s the production of vlbratlonally excited NO molecules At 2 Torr, a laser pulse of 2 5 X 1Ol6 photons cmm2 excites and dlssoclates ==G 5% of the molecules withm the u-radiated volume correspondmg to a partial pressure &NO) = p(R,N -) a 10 mTorr. In the light of these relatively high fragment concentrations It IS not mconcelvable that the reactions _k& NO* + R,N SNNO (6) and/or NO*@=
l)+NO*(u=
I)-
kt?Y
NO**@
= 2) + NO
(7)
contribute to the IR emlsslon decay [eq. (S)] . For the reactlon NO + ‘NH2 + product, which IS sumiar to that of eq. (6), Hancock et al [9] reported a rate constant 6 8 X lo5 s-l Torr-1, while Stephenson [IO] measured = 1 2 X 105 s-t Torr1 for the vlbratlonal energy a ke, exchange, eq (7). The lack of any measurable devlatlon from the exponential decay of the IR emission led us to conclude that both processes are not important under the present excltatlon comhtlons (I.e. k: < 3 X 105 s-l Torr-I) but should become significant at higher laser intensity .
Acknowledgement Support of this work by the Schwelzerischer National fonds IS greatly appreciated We thank Dr. Mary MahaneJ for crltlcdlly readmg the manuscript
References
i
i
p(torr)
Fig. 3. Stern-Volmer plot of the mverse decay tune (k,) of NO* (U = 1) versus the RzNNO pressure. The msert shows the fluorescence sgnal obtain& at 05 Torr RzNNO by signal averagmg for 2000 laser pulses.
[ 11 P. Rademacher (1969) [2]
and R StB:eti,
Acta Chem.
Sand.
23
660,672.
F Scappmi. A Guamten, H Dreizler and P Rademacher, 2. Naturforsch 27a (1972) 1329. A Guarnien, F Rohwer and F. Scappuu, Z Naturforsch. 30 (1975) 904 523
Volume
i9, number
3
CHELIICAL
PHYSICS LETTERS
(31 1-W. Levm, G-W A. hfllne and T. Axenrod. J. Chem. Phys 53 (1970) 2.505. P. Rademacher and W. Luttke, Spectrochim. Acta 27 (1971) 715. Ber. Bunsenges Physlk Chem. 78 (1974) 1353. 141 R.K. Harnsand R A. Sprag,Chem Commun. 1 (1967) 362: SM. Glidewell, Spectrochun Acta 33A (1977) 361, and references therem. [S] B G. Cowerdock, P Pritchard Jones and J.R hIaJer. Trans Faraday Sot. 57 (1961) 23.
524
1 hl.iy 1981
I P Fisher and E Henderson. Trans Faraday Sot 63 (1967) 1342. [6] K. Weiss and f.R Huber, unpubhshcd results, D R Batbste. L9 Davis and R V Naumann. J Am. Chem. sot 97 (1975) 5071. [ 71 M P Roelhg and A L Houston, Chem. Phys Letters 57 (1978) 75. 181 J.C. Stephenson, J. Chem Phys 60 (1974) 4289. [9]
G Hancock, W Lange. hf Lcnzl Phys Letters 33 (1975) 168. [ 101 J C. Stephenson. J. Chem. Phys
and K H Welgc, 59 (1973)
1523
Chem