3152
J . Phys. Chem. 1992, 96, 3152-3157
Solvent Diffusion in the Presence of Semipenetrable, Rodlike Polymers Roberto Olayo Departmento de Fisica, Universidad Autonoma Metropolitana- Iztapalapa, Mexico D.F. 09340, Mexico
and Wilmer G . Miller* Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455-0431 (Received: September 3, 1991)
The translational self-diffusion coefficient of a solvent (D)in two rodlike polymer/solvent systems, poly(benzy1 glutamate) in dimethylformamide (PBLG/DMF) and poly(n-butyl isocyanate) in benzene (PBIC/benzene), was measured by pulsed gradient spin echo nuclear magnetic resonance. D decreases monotonically with polymer concentration at all temperatures and polymer rod lengths studied. DMF diffusion was independent of PBLG molecular weight and dropped more than a factor of 5 as 50 vol 7% polymer was approached. Using known properties of PBLG, including the mixing of its sidechains with solvent, the obstruction of DMF diffusion by the presence of PBLG could be accounted for using the solvent penetrable prolate ellipsoid model of Jonsson et al. (Colloid Polym. Sci. 1986, 264, 77). Benzene diffusion in the presence of PBIC can also be understood using the same model. The obstruction to solvent diffusion by ‘fuzzy rods” was compared to that observed for random coil polymers. We show that in the dilute and semidilute regions this approach can also model experimental results with only a modest reduction of solvent diffusion coefficient within the domain of the polymer molecule.
Introduction The translational diffusion of small molecules in simple liquids has been studied for many years. Self-diffusion measurements, those not involving a concentration gradient, are typically made by isotopic tracer1 or pulsed gradient spin echo nuclear magnetic r e s o n a n ~ emeasurements. ~,~ When a second component becomes of colloidal dimensions and is impenetrable, it is effectively stationary with respect to the motion of the solvent and presents simply an obstruction about which the solvent must diffuse. If the solvent diffusion is followed for distances that are long compared to the dimensions of the obstruction, the observed diffusion coefficient will be reduced, dependent on the volume fraction and geometry of the obstruction. In addition to obstruction, dispersed particles may have specific interactions with the solvent, e.g., hydration of ionic groups or hydrogen bonding, or the solvent may have a limited though finite solubility in the particle. Each of these will have an effect on the solvent diffusion and frequently is the reason for solvent self-diffusion ~tudies.l.~-~ A number of models have been presented which may be used to correlate the concentration and temperature dependence of the diffusion coefficient of the s o l ~ e n t . ~ - One ’ ~ of the most clear-cut experimental systems is that of charge-stabilized spheroidal latex particles, which may be produced with diameters ranging from hundreds of angstroms to tens of microns. In aqueous dispersions of poly(methy1 methacrylate) latex, agreement with predictions is achieved up to 20 vol % latex, with solvent diffusion falling off more rapidly than predicted at higher concentrations.1° Deviations are considered to be the result of solvent interaction with the latex. With polystyrene latex, where solvent uptake should be minimal, both latexI3 and solvent14J5diffusion have been measured. As one would expect, the solvent diffusion falls off less rapidly with latex concentration than latex diffusion; moreover, the solvent diffusion agrees closely with the predicted values for obstruction out to a high volume fraction of latex. Obstruction factors for prolate and oblate ellipsoids have been formulated.’JO A prolate ellipsoid polymer latex of high aspect ratio has not been made. The closest equivalent is a DNA double helix or the rodlike tobacco mosaic virus. Both are polyelectrolytes requiring consideration of specific interactions with the solvent. Colloidal latex spheres may be sterically stabilized by diluent soluble nonionic polymer chains or “hairs” attached to their surfaces, instead of being charge stabilized. The rod polymer equivalent is a rodlike core with hairs attached to the rod. In this case part of the rod cross section is impenetrable to the solvent, Author to whom correspondence should be sent.
0022-3654/92/2096-3 152$03.00/0
while an outer shell, the hairs or side chains, is solvent penetrable. Upon dissolution the side chains mix with solvent and are extended with the rod length remaining unchanged. In this paper solvent diffusion in two such “fuzzy“ cross section, molecularly dispersed, rodlike polymersolvent systems is reported-poly(y-benzyl aL-glutamate) in dimethylformamide, PBLG in DMF, and poly(n-butyl isocyanate), PBIC, in benzene. Materials and Methods PBLG samples were obtained from Sigma Chemical Co., Inc. They will be referred to as PBLG 10 [ 10 5001, PBLG45 [45 1501, PBLGlO5 [105600], and PBLG250 [249700], where the numbers in brackets are the weight-average molecular weights as determined by the manufacturer based on low-angle light scattering data extrapolated to zero concentration (LALLS). PBIC, M , of 145 000, was synthesized and characterized as described previously.16 Solvents were reagent grade and dried before use. PBLG/DMF samples were prepared by weight in N M R sample tubes, sealed, and homogenized by mild warming plus back and forth centrifugation until uniform between crossed polars. PBIC/benzene samples were prepared similarly but homogenized at 40-50 OC by slow tumbling for up to 2 months. Solvent proton self-diffusion coefficients were determined by pulsed field gradient spin echo nuclear magnetic resonance (PGSE NMR) using either a 90-180 two-pulse or a stimulated 90-90-90 ( 1 ) Wang, J. H. J. Am. Chem. SOC.1954, 76, 4755. (2) Stejskal, E. 0.;Tanner, J. E. J. Chem. Phys. 1964, 42, 288. (3) Stilbs, P.; Moseley, M. E. Chem. Scr. 1980, 15, 176. (4) Salvinien, J.; Brun, B.; Kamenka, N. Eer. Bunsen-Ges Phys. Chem. 1971, 75, 199. ( 5 ) Sandeaux, J.; Kanenka, N.; Brun, B. J . Chim. Phys. Phys.-Chim. Biol. 1978, 75, 895. (6) Tirrell, M. Rubber Chem. Technol. 1984, 57, 523. (7) Duda, J. L. Pure Appl. Chem. 1985, 57, 1681. (8) Muhr, A. H.; Blanshard, J. M. V. Polymer 1982, 23, 1012. (9) Maxwell, J. C. A Trearise on Elecrricify and Magnetism; 2nd ed.; Claredon Press: Oxford, U.K., 1881; Vol. 1, p 435. (10) Jonsson, B.; Wennerstrom, H.; Nilsson, P. G.; Linse, P. Colloid Polym. Sci. 1986, 264, 7 7 . (1 1) Mackie, J. S.;Mears, P. Proc. R. SOC.London, A 1955, 232, 498. (12) Vrentas, J. S.; Duda, J. L. J. Polym. Sci., Polym. Phys. Ed. 1977, IS, 403. (13) Wang, L.; Miller, W. G. Theor. Chim. Acra, in press. (14) Chewer, E.; Blum, F. D.; Foster, K. R.; Mackay, R. A. J. Colloid Interface Sci. 1985, 104, 121. (15) Wang, L.; Miller, W. G. Manuscript in preparation. (16) Olayo, R. Tesis de Maestria, Universidad Autonoma Metropolitana, Mexico D. F., 1983.
0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 7, 1992 3153
Solvent Diffusion in the Presence of Rodlike Polymers
0.8
so
o.6 0.4
1
0
m
PBLGlO5
m
1 \ o
4
- 0
\
0.6
20 30 40 50 Volume Yo PBLG Figure 1. Diffusion coefficients of DMF in PBLG/DMF isotropic solutions scaled by the diffusion coefficient of pure DMF, D,. Also shown is the obstruction factor computed from eq 3 for a long prolate ellipsoid that is impenetrable and noninteractive with the dispersing solvent. 0
10
three-pulse sequence. All benzene and some DMF measurements were performed on a Nicolet 1180 N M R spectrometer operating at 300 MHz with a maximum field gradient of 0.25 T/m, temperature controlled to f0.2 "C. Most of the DMF results were determined using a JEOL 60-MHz instrument at 27-29 "C with the capability to give a field gradient as large as 17.7 T/m.17 The self-diffusion coefficient, D, is determined2 from the decay of its echo amplitude, A , given by A(2T) = A(0) exp(-h/T2) exp(-y2G2D62[A- 6/31) (1) in a two-pulse sequence or by A ( T ~ T ~ =) A(0) exp(-[~, T ~ ] / T , )e x p ( - 2 ~ ~ / Texp(-y2G2D62[A ~) - 8/31) (2)
+
from a stimulated echo, where T or T~ is the time between the first and second pulse, T~ - T~ is the time between the second and third pulse, y is the proton gyromagnetic ratio, and A is the interval between field gradient pulses of magnitude G and variable duration 6. TI and T2are the spin-lattice and spinspin relaxation times, respectively. Solvent diffusion coeffcients measured by a twepulse sequence were found to be indistinguishable from those measured from a stimulated echo and will not be differentiated in the results section. The time over which diffusion was followed, A - 6/3 A, could be varied from 1 to 1000 ms, corresponding typically to diffusion over distances of 0.1-10 p, depending on the magnitude of D. The decay in the echo from the solvent was at least 1 order of magnitude faster than that from the polymer, so there was no problem in separating solvent from polymer diffusion.
-
DMF diffusion coefficients in PBLG/DMF samples, scaled by the measured diffusion coefficient in pure DMF, 1.73 f 0.03 X lW5cm2/s, are shown in Figure 1. PBLG samples were prepared so that only the isotropic phase was present, as PBLG diffusion in the isotropic phase was also of interest; consequently only the lowest molecular weight could be extended to high polymer concentrations. Observation by light microscopy through crossed polars of the dissolution of PBLGlO during sample preparation showed clearly that this rod exceeded the minimum length needed to form a liquid crystalline phase at concentrations beyond those reported here; i.e., as the solvent contacted the dry polymer liquid crystalline spherulites were formed which eventually dissipated into the isotropic phase. Thus the liquid crystalline phase boundary is greater than the largest concentration studied, 48.5 ~ 0 1 % .It is evident from Figure 1 that the DMF diffusion is not dependent on the length of the PBLG rod, so further discussion of these data will not differentiate among rod lengths. However, the viscosities of the solutions are markedly rod length dependent, ranging from solutions with essentially solvent viscosity to solutions that do not flow easily when the sample tube is inverted, as the viscosity is dominated by the polymer rather than the solvent motion. (17) Callaghan, P. T.; Trotter, C. M.; Jolley, K. W. J . Ma@. Reson. 1980,
37, 241.
"
0
"
4
"
"
"
"
8 12 Volume % PBIC
"*
16
Figure 2. Diffusion coefficients of benzene in PBIC/benzene solutions at the temperatures indicated ("C) scaled by the diffusion coefficient of pure benzene. The obstruction factor computed from eq 3 is shown also.
The diffusion coefficients for benzene in PBIC/benzene solutions are shown in Figure 2 at four temperatures. The concentration dependence shows a variation with temperature which lies outside experimental error. It must be borne in mind that the PBIC/benzene solutions have problems not present with the PBLG/DMF solutions. Whereas DMF is a thermodynamically good solvent for PBLG,'* benzene is not so good a solvent for PBIC. Even 1% solutions are biphasic at 25 OC. However, solutions homogenized at 40-50 O C may exist as isotropic, single-phase metastable solutions at lower temperatures for long periods of time.19 The effect of phase separation will be discussed later.
Discussion PBLG/DMF. The effect of the geometry of impenetrable obstructions on solvent diffusion has received attention.l**J0 Rods of high aspect ratio may be approximated by long prolate ellipsoids. For long prolate ellipsoids impenetrable and noninteractive with the solvent, based on eq 49 of Jonsson et a1.I0 [C, = 0 and a >>
[I
D/Do = 1 - (2/3)[0/(1
+ 0)]
(3)
where 0 is the volume fraction of polymer. As seen in Figure 1, the obstruction factor as given by eq 3 does not match the experimental data. If solvent penetrates and mixes with the side chains, the obstruction factor, again based on eq 49 of Jonsson et al., becomes D/Do= (1/3)[1 - ( 1 - ~ , , / ~ , ) 0 , ] - ~ [ 3 ~ - 6 ~ ~ - 0 ~3 +~ 3 , + @;I/[x(1 - 2x - @,I (4) where csp= mean concentration of solvent in the swollen polymer, c, = concentration of the solvent in regions of pure solvent, 0, = volume fraction of polymer plus solvent within polymer domain, x = E1 - (Ds~/Ds)(csp/cs)]~~ (eq 5), Os, = mean diffusion coefficient of solvent within the swollen polymer, and 0,= diffusion coefficient of solvent in regions of pure solvent = Do. When the dry rodlike polymer, of mean cylindrical cross section dD,contacts solvent, the side chains mix with solvent and are extended to give a swollen rod of cross section d,, and the solvent penetrates until an impenetrable core, drip, is reached. These are shown schematically in Figure 3 and depend on the molecular and thermodynamic details of the particular side chain/solvent pair. Under the condition of no volume change on mixing, csp,cs, 0,and 0, are related by volume conservation to be csp/cs
= [ l - (dD/dM)21
if solvent is distributed through the rod. If a solvent impenetrable core of diameter dnpexists, then volume conservation requires csp/cs = [1 - (dD/dh4)21/[1 - (dr~p/dM)~]
(6)
(18) Goebel, K. D.; Miller, W . G. Macromolecules 1970, 3, 64. (19) Olayo, R.; Miller, W. G. J. Polym. Sci., Part B Polym. Phys. 1991,
29. 1473.
3154 The Journal of Physical Chemistry, Vol. 96, No. 7, 1992
7
-
Olayo and Miller
K
1
0.8
e0
0.6 0.4
0.2 0
R=-CHzCH,COOCH,UJ PBLC
n H
I/.\"/[
i?
R
I
n
.CH zCH,CH zCH, PBIC
R
Figure 3. Definition of rod cross sections and molecular formulas. T h e unswollen polymeric rod of cross section dDis swollen by solvent to cross section d M leaving a core dnPimpenetrable to solvent.
In either case the dry (0)and solvent swollen (0,) volume fractions are given by (7) *p = (dM/dd2@ = (Vsw/Vunsw)@ where V,, is the volume encompassed by the solvent swollen polymer molecule and Vu,,, is the dry volume of the polymer molecule. Taking the density of PBLG to be 1.274 g/cm3 2o and an axial translation of 1.5 A per monomer,2' dDis 15.5 A. The side chains in the dry polymer are undoubtedly mixed, as evidenced by their motion at 25 0C22-24and by their equal partial specific volume whether in dry films or in solution.22 It makes no difference that the side chains from neighboring rods in the solid polymer are mixed. With volume conservation on polymersolvent mixing, c,,/c, is not arbitrary but unambiguously related to the space-filling dry rod cross section dD,d,,, and dM. Solvent cannot penetrate the helical backbone nor probably the first side chain carbon, so d,, will be taken as 6.5 A. If the side chains are fully extended, the rod cross section is about 25 A, which would be a maximum value for dM. A rod diameter of 21.5 A can be Calculated from the high-temperature osmotic virial coefficient'* and 16 8, from the light-scattering virial coeffi~ient,2~ where the specific volume used in making this calculation does not include any contribution from solvent within the domain of the side chain of the rod. Inclusion of such a contribution26would increase these values. Taking a value of 20 A or larger for dMmakes the value ~ )small ~ that it makes only a few percent difference for ( d , , / ~ i so in DIDo if dnpis taken to be 6.5 or 0 A. Although eq 4 was derived on the assumption of complete solvent penetration, the small value makes alteration of eq 4 to include an impenetrable of (~d,,/d,)~ core, as has been done for partially penetrable spheres,1° a marginal correction. The maximum reduction factor will occur when D,, 0.50, a free volume approach to solvent diffusion seems to work ell.^*^^ In the dilute and semidilute regions, where solvent diffusion is significantly less than that predicted by impenetrable obstruction models, it has been suggested that a "surface drag" effect of polymer on solvent within the polymer domain is responsible for the discrepancy.M It is of interest to apply the model we have used for solvent diffusion in the presence of fuzzy rods to the random coil data. At low polymer concentration there is some analogy to the rod polymer case in that individual polymer molecules are solvent swollen and have nonoverlapping domains. There are also differences: (1) typically the domain of the polymer molecule is highly solvent swollen compared to rod side chains as a result of less molecular constraints and contains as much as 90% or more solvent; (2) there is no core that is not penetrable by solvent; and (3) the shape of the polymer domain is constantly and rapidly changing due to segmental motion and will be approximated as a sphere here though the shape should have only a small effect.1° With the polymer domain containing -90% solvent, D,, may be comparable to D,, unlike the situation with the fuzzy rod polymers which have limited solvent uptake. The impenetrable sphere equivalentlo to eq 3 is D/Do = 1/(1 @/2) (10)
+
and the solvent swollen equivalentlo to eq 4 is D/Do = 11 - (1 - ~ , , / ~ ~ ) @ p l-- 'P@,I/[l [~ + P@,/21 (11) where for spherical expansion, rather than cylindrical expansion as in eqs 3 and 4 csp/cs @p
=
= [ l - (dO/dM)31 = (dM/dD)3@ (vsw/vu"sw)@
where
P = 11 - (Dsp/Ds)(~s,/~,)l/[~+ (DS,/D~)(~S,/~,)/21 and where dM and dD are the diameter of the swollen and dry polymer molecules using spherical geometry. If one prefers to model the random coil as a cylinder rather than as a sphere, solvent swelling is still three dimensional and not two dimensional as in PBLG or PBIC as there are no constraints holding one dimension fixed. Calculations based on eqs 10 and 11 are shown in Figure (37) Olayo, R.;Miller, W. G. Latin American Symposium on Polymers, Guadalajara, Mexico, October, 1990; Sociedad Polimerica de Mexico A.C.: p 455. (38) von Meerwall, E. D. Adu. Polym. Sci. 1983, 54, 1. (39) Maklakov, A. I. Myskomol. Soedin. 2.1983, A25, 1631. (40) Pickup, S . ; Blum, F. D.Macromolecules 1989, 22, 3961.
3156 The Journal of Physical Chemistry, Vol. 96, No. 7, 1992 1
0.8
'
0.6 0.4
0.2 0
5 10 Volume % Polymer
0
15
1
0.8 0.6
? 0
0.4
0.2
0
0
10
20
30
40
50
Volume % Polymer
Figure 6. Treatment of solvent diffusion in dilute solutions of random coils as solvent swollen spheres, eq 11, with @/ap= 0.10 and D,,/D, = 0 (2), 0.50 (3), 0.75 (4), 0.82 (5), or 0.85 (6). Curve 1 is obstruction by nonswollen spheres, eq 10, while curves D and E are for penetrable rods, taken from Figure 4. Data points are representative random
coil-diluent solvent diffusion results.40 6a, where plot 4 is seen to fit solvent diffusion in random coil/ diluent systems rather well. Since and /3 appear together in eq 11, various additional combinations of @/@, and D,,/D, will give identical obstruction factors. These include @/a,= 0.08 and D,,/D, = 0.80, as well as @/ap = 0.15 and D,,/D, = 0.63. Using typical swelling factors for random coil polymers in moderate to good solvents, eq 1 1 will fit a wide variety of experimental data with a reduction of solvent diffusion coefficient that is only 15-3076 less than that in pure solvent. The reduction in solvent diffusion can be calculated using values of 9,> 1, which occurs when Q, > and still predict the observed solvent diffusion, Figure 6b. These modest reductions in solvent diffusion inside versus outside the polymer domains are quite different than the "surface drag" explanation.@ This approach also can explain, at least a t low concentrations, why random coil polymers seem to give similar obstruction factors independent of polymer or solvent. Although the radii of gyration vary, the domain of a dilute, swollen polymer molecule contains mostly solvent. The temperature dependence may also be explained without resort to more elaborate approaches. The approach we have used here is typically not used by those studying solvent diffusion in the presence of random coils. It appears to have merit at least up to the point of polymer domain overlap, and perhaps beyond.
Summary The diffusion of solvent in the presence of rodlike macromolecules with flexible side chains has been measured over a considerable range of polymer concentrations. The reduction in solvent diffusion coefficient by the presence of the polymer as an obstruction has been compared to a model whereby the solvent diffuses in two distinct domains-regions devoid of polymer and regions where the solvent is in the domain of the solvent swollen polymer side chains. Several assumptions were made: ( 1 ) the concentration and diffusion coefficient of solvent in the polymer domain is independent of polymer concentration up to the point where the swollen polymer is space filling; ( 2 ) the diffusion coefficient of solvent within the polymer domain can be represented by an average value; (3) the extension of the side chains can be estimated from virial coefficient measurements; (4) the solvent diffusion coefficient within the domain of the side chain can be
Olayo and Miller assigned the value observed at the concentration where pure solvent domains disappear; and ( 5 ) the mean concentration of the solvent in the domain of the side chain is determined when dnPand dM are assigned, if there is no appreciable volume change on mixing. When this approach was applied to PBLG/DMF using the assumptions above, the computed curves did not match the experimental data quantitatively using the parameters assigned unless a small reduction in the solvent diffusion coefficient in polymer-free domains was considered as the polymer concentration was increased. Although the factor used, 1 - @, is mild, other considerations may be made. The first assumption, that the side chain swelling is independent of polymer concentration, may seem extreme. However, recent analysis of the rotamer side chain distribution of PBLG in the liquid crystalline phase using selective deuteration and deuterium N M R has been made.4* The liquid crystalline phase occurs at concentrations larger than those in the isotropic phase for a given molecular weight. Diameters of 18.0 8, in dioxane, 17.4 8, in chloroform, and 16.6 A in cresol were determined where the diameter is measured from center-to-center of the phenyl group. Inasmuch as the diameter measured to the 'center of the phenyl group" will be less than the diameter expressed to the end of the phenyl group, these results show that even in the liquid crystalline phase the side chains have extensions very similar to those in the isotropic phase studied here, deduced from dilute solution virial coefficients. Thus assumption 1 seems good, and determination of the rod cross section through excluded volume by way of the virial coefficient should have at most a few angstroms uncertainty. Turning to assumption 4, the calculations presented in Figure 4 assign a value of D,, = 0.174Do,based on the 48.5 vol 76 data. This concentration is close to but not likely to be at the space-filling value; i.e., some polymer-free regions still exist. Thus the true value for D, should be less than 0.174. This would reduce the need to use eq instead of eq 4, as a lower value for Dspwould increase the obstruction factor as calculated from eq 4. Although it is highly reasonable that D, becomes less than Do as aP 1, the calculations show that the experimental data can be simulated well with slight adjustments of DM, Dspr or 0,. We conclude that although all parameters in the theoretical model cannot be set precisely, the diffusion of DMF in the presence of PBLG can be accounted for with a solvent penetrable long prolate ellipsoid model. The application of the penetrable ellipsoid model to benzene diffusion in PBIC can also account for the reduction in solvent diffusion measured experimentally. However, both the experimental system and the setting of parameters in the model are not as clean-cut as with PBLG/DMF. Experimentally there is the presence of the isotropic-liquid crystal phase boundary and the existence of metastable isotropic solutions, which have been discussed earlier. The effect of ordering on the diffusion of solvent around spheres as obstructions has been considered and predicted to have only a small effect.I0 A similar result would be expected for the effect of ordering the obstructing rods. Although there are no data to justify assumption 1, an analogy can be made to the PBLG/DMF system discussed above. Probably the area of major concern is the application of a two-domain model to this system. The side chain of PBIC is considerably shorter and smaller than that of PBLG. In fact, the volume of the benzene molecule is larger than the volume of a PBIC side chain. Thus the idea of a side chain-solvent domain seems somewhat contrived, and a lengthy discussion of parameter setting does not seem worthwhile. The simulations as shown in Figure 5 do suggest that a two-domain model can be used to at least rationalize the data. Finally we turn to the application of porous obstruction models to solvent diffusion in the presence of random coil polymers. With swelling taking place in three rather than two dimensions, swell ratios are large compared to rods as the rod swelling is constrained by the attachment of the side chain to the core of the rod. Assumption 1 should be good a t concentrations smaller than that of domain overlap. The finding that literature data can be fit with only a small reduction in solvent diffusion compared to bulk solvent
5
-
(41) Abe, A,; Yamazaki,
T.Macromolecules 1989, 22, 2138.
J . Phys. Chem. 1992, 96, 3157-3162 seems an intuitively reasonable result unless the solvent dynamics is massively affected by the presence of the polymer segments in a manner that is not merely an obstruction. The data shown in Figure 6 are that of toluene in 270 000 molecular weight polystyrene at temperatures in the range from 15 to 115 O C 4 0 a system in which one might expect the solvent to have no unusual specific interaction with the polymer. The fact that the simulation can be extended to polymer concentrations exceeding the domain overlap, Figure 6b, and still match the experimental data with only a modest reduction in solvent motion presents a view of solvent diffusion in random coil polymers a t low to moderate concentrations different than that frequently given.
3157
Acknowledgment. R.O. acknowledges support from CONACYT (Mexico). Support from the Midwest Technology Development Institute, the NSF-Minnesota Center for Interfacial Engineering, and the Graduate School, University of Minnesota, is also acknowledged. The hospitality of Prof. Paul Callaghan and the Department of Physics and Biophysics, Massey University, New Zealand during a sabbatical leave (W.G.M.) and the use of the JEOL PGSE N M R spectrometer are gratefully acknowledged. Registry No. PBLG (homopolymer), 25014-27-1; PBLG (SRU), 25038-53-3; PBIC (homopolymer),25067-04-3;DMF,68-12-2; benzene, 71-43-2.
Peptide Sequence Ions Produced by Postionizatlon of Neutral Molecules Formed during Resonant 266-nm Laser Desorption Gary R. Kinsel, Josef Lindner, and Jiirgen Grotemeyer* Institut fur Physikalische und Theoretische Chemie der Technischen Universitat Munchen, Lichtenbergstrasse 4, 0-8046 Garching, Germany (Received: August 29, 1991; In Final Form: December 3, 1991)
A number of examples of peptide mass spectra obtained using 266-nm laser desorption (LD) followed by 255-nm multiphoton ionization (MUPI) of the laser desorbed neutral molecules are presented which show a variety of structurally significant sequence fragment ions. It is believed that most of these sequence fragment ions result from postionization of neutral fragments produced during 266-nm LD. MUPI postionization of the fragment neutrals produced during LD can provide sequence information which is complementary to the sequence information derived from direct ion desorption experiments. In addition, a variety of immonium fragment ions may be produced at higher ionizing laser power densities which give an overview of the amino acid residues contained in the peptide structure. These fragment ions are formed during MUPI positionization step by multiphoton fragmentation of larger intact ions. Finally, a comparision of the sequence fragment ions produced in these experiments with the sequence ions produced using other direct ion desorption techniques illustrates the analytical usefulness of the sequence ions produced in these experiments.
Introduction One of the goals of analytical mass spectroscopists is the development of routine mass analysis techniques for the structural elucidation of small and medium sized peptides. Ideally, a technique should provide both the molecular weight of the intact peptide and sufficient structural information to allow the determination of the amino acid sequence in the peptide molecule. A variety of approaches for generating this information have been reported in the literature and are summarized in a recent review.' The approach taken in this laboratory involves the combination of laser desorption (LD) followed by multiphoton ionization (MUPI) pitionization of the laser desorbed neutral molecules.* This combined technique has been applied with success to a wide variety of b i m o l e ~ u l e s . ~ ~ ~ In the original experiments performed in this laboratory, a COz laser (10.6 pm) was used to perform the LD step. It was believed that the low energy of the C 0 2 laser photons would help in maximizing the number of intact neutral molecules desorbed. In a recent paper we have shown that much higher energy 266-nm photons can also be used to desorb abundant numbers of intact neutral molecules from many of the samples investigated in the earlier experiment^.^ It was determined in this study that the primary requirement for efficient LD of intact neutral molecules was that the sample under investigation absorb strongly at the desorbing laser wavelength. In these experiments an intense signal from the MUPI postionized peptide molecular ion was always observed, even up to the mass gramicidin D (1881 amu). However, it was also noted in these experiments that the resulting peptide mass spectra always contained abundant signals from a wide variety of sequence type fragment ions. It is believed that the To whom all correspondence should be addressed.
0022-365419212096-3157$03.00/0
majority of the sequence ions observed are produced as fragment neutrals during 266-nm LD which are then transported into the RETOF-MS source region for MUPI postionization.6 In all cases, high quality spectra were obtained and in some cases complete sequence information could be determined from the fragment ion signals in the peptide mass spectra. Resonant 266-nm LD followed by MUPI postionization of the desorbed neutral species, therefore, is an attractive method for investigating small and medium-sized peptides since an abundance of structurally significant fragment ions as well as a strong signal from the molecular ion are all observed in the peptide mass spectrum simultaneously. In addition, the peptide structural information obtained in these experiments is often complementary to the information produced using direct ion desorption techniques such as fast atom bombardment (FAB) or PDMS. In this paper we provide detailed information regarding the types and relative abundances of the sequence fragment ions observed in the 266nm-LD/255-nm-MUPI peptide mass spectra.
Experimental Section The instrument used in performing these experiments is the Bruker TOF-1 reflectron time-of-flight mass spectrometer (RE(1) Biemann, K.; Martin, S.A. Mass Spectrom. Rev. 1987, 6, 1.
(2) Grotemeyer, J.; Lindner, J.; Kbter, C.; Schlag, E. W.J . Mol. Struct. 1990, 21 7, 5 I . (3) Grotemeyer, J.; Schlag, E. W.Biomed. Emiron. Muss Spectrom. 1988, 16, 143. (4) Grotemeyer, J.; Schlag, E. W.Org. Mass Spectrom. 1988, 23, 388. ( 5 ) Kinsel, G. R.; Lindner, J.; Grotemeyer, J.; Schlag, E. W.J. Phys. Chem., 1991, 95, 7824. ( 6 ) Kinsel, G. R.; Lindner, J.; Grotemeyer, J.; Schlag, E. W.J . Phys.
Chem., following paper in this issue.
0 1992 American Chemical Society
