Scannlng Tunneling Microscopy Studles of the Synthetic Polypeptide

are imaged in the form of a thin liquid crystalline film cast from a dilute N,N-dimethylformamide (DMF) solution onto highly ordered pyrolytic graphit...
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J. Phys. Chem. 1992, 96,6825-6829

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Scannlng Tunneling Microscopy Studles of the Synthetic Polypeptide Pdy( "y-benzyt-L-glutamete) J. J. Breen and G.W.Fly,* Department of Chemistry and Columbia Radiation Laboratories. Columbia University, New York, New York 10027 (Received: April 28, 1992)

Scanning tunneling microscopy (STM) studies of the synthetic polypeptide poly(y-benzyl-L-glutamate)(PBLG) [-NHare reported. These experiments are performed in air, and the molecules CHR-CO-1, where R = CH2CH2COOCH2C6H5, are imaged in the form of a thin liquid crystalline film cast from a dilute N,N-dimethylformamide (DMF) solution onto highly ordered pyrolytic graphite (HOPG). The images reported reveal both the helical molecular structure of the PBLG molecule6 and the highly organized nature of these films due to the strong hydrogen-bonding interactions between the biopolymers. Direct correlation of the image features observed with the STM and the three-dimensional structure of PBLG molecules is made using the molecular modeling program MacroModel and data from reported X-ray diffraction studies. Implications of the observed STM image features for future molecular imaging experiments are discussed.

Introduction The scanning tunneling microsoope (STM) is a powerful instrument with signifhmt potential for imaging molecular structures and changes in these structurea due to chemical reactivity. The STM works on the principle that electrons will tunnel across the gap formed when an atomically sharp tip is brought close to a conducting surface and a suitable potential applied between them.' The flow of current is sensitive to the spatially localized density of states which is dependent on both the surface topography and the chemical identity of the surface atoms? Using piezoelectric positioning elements, the tip can be rastered over the surface with the tip-surface distance under feedback control related to the tunneling current. This often results in an atomic resolution image of the surface topography and local electronic structure. In the case of singlenystal surfaces, the observed details in the STM image can often be correlated to the crystal structure. Unfortunately, this correlation is not as straightforward when imaging molecules adsorbed onto conducting surfaces. Despite a large number of successful imaging studies, the tunneling procegpes occurring when a molecule is located between the tip and the surface are not well understood. In general, the preaence of a molecule on the surface enhances the tunneling probability to an extent which is highly dependent on the substructure of the molecule. A number of attempts have been made to explain the imaging contrast mechanism for such systems. In one model, the local work function of the substrate is considered to be modulated by the polarizable molecular adsorbate.3 A second model employs a resonant tunneling mechanism in which the adsorbate molecules can be considered as a potential well, complete with electronic energy levels due to the adsorbate's electronic structure! Resonances in the tunneling probability can occur for various bias voltages. A feature common to both molecular imaging experiments and the tunneling models is the strong contribution of aromatic substructure to the STM image contrast. The scope of the molecular systems imaged with the STM reflects a widespread interest in obtaining atomic resolution images of adsorbate structures and chemical processes. Molecular resolution images have been reported for systems such as small organic and inorganic molecules, liquid crystals, polymers, and biologically important molecules.s In this work we report images of the readily available synthetic polypeptide poly(ybenzy1-Lglutamate), [-NH-CHR-CO-1, where R = CH2CHzCOOCH2C6HS,hereafter abbreviated as PBLG. These molecules are imaged in air and in a liquid crystalline array which is formed by depositing a few drops of a dilute solution of the polymer onto a highly ordered pyrolytic graphite (HOPG) substrate. Whereas many polymers exist in solution only in a random coil conformation, polypeptides,such as PBLG, can exist with a small number of distinct secondary structures. The main forms of these secondary structures are the a- and whelk, and the @-sheet,and the

cross B-forma6Additionally, a-helices can be wound into either super-coil or coiled-coil tertiary ~tructures.6~~ The appearance of the various secondary and tertiary structures of PBLG, associated or unassociated helices, has a well-established solvent and concentration dependence. For the experiments presented in this paper, the solvent chosen was N,N-dimethylformamide(DMF), a helicogenic solvent which due to solvent-polymer hydrogen bonding supports unassociated helices. In particular, for PBLG, the lyotropic liquid crystal character of this rigid rod polymer has established PBLG as an extremely well-studied polymer in both solutionss and films.g The structural studies of PBLG have involved X-ray diffraction techniques as well as dynamic mechanical and dielectric meas~rements.~~~ This work has resolved the issue of the tertiary structure of PBLG in the complex phase, which was found to be straight a-helices and not coiled-coil helices.sc The ability of PBLG to develop, from a few drops of solution deposited on a surface, into liquid crystalline arrays makes this molecule an attractive candidate for study with the STM. Experimentally isolated individual molecules are difficult to image on a weak binding surface such as H O E . In addition, grain boundaries and other defects in the HOPG surface can be incorrectly interpreted as adsorbed molecules.1° consequently, the most credible images of molecules adsorbed on surfaces, especially on HOPG surfaces, are of those molecular systems, which due to intermolecular interactions have formed two-dimensional crystalline arrays. In such arrays the individual molecules are less likely to move under the influence of the tunneling tip, and the periodic modulations in the adsorbate/surface topography are easily recognized. Previous STM studies focusing on imaging PBLG have yielded some very beautiful results.'l In these experiments films deposited on HOPG were examined, and images revealed molecular scale features, which gave evidence of a screwlike structure, upon the application of two-dimensional Fourier filtering techniques. In the work reported here, higher resolution images have been recorded due either to technical improvements in STM hardware or to more fortuitous imaging conditions. This has resulted in a direct correlation of the image features to a computer-generated model of the polypeptide. A key feature of the present work is the use of the molecular modeling program MacroModel to correlate the observed STM image features to the three-dimensional structure of PBLG.IZ MacroModel is a convenient utility program which can be used to create and visualize models of complex molecular structures. These capabilities and the additional ability to view only selected parts of a molecule are also useful in correlating STM images to molecular structures. Although the STM can operate with subatomic resolution, such high resolution is not generally attainable in imaging molecules adsorbed onto conducting surfaces. In principle, the tunneling probability is modified by the whole

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Breen and Flynn

Figure 1. STM constant current topograph of a 30-nm X 30-nm region of a liquid crystalline film of PBLG on HOPG. Tunneling conditions are -1268 mV (sample bias) and 0.2 nA.

molecule which is present between the tip and surface, but the topographic contributions to the image are a result of those portions of the molecule exposed to the tip. Thus, interpreting the STM image of a nonplanar three-dimensional object is greatly aided by the availability of a structural model. Experimental Section The preparation of liquid crystalline films of PBLG has been well documented. In this study, a few 5-pL drops of a solution (0.5 mg/mL) of PBLG (Sigma, MW 2 85000) in DMF (reagent grade) are deposited on a freshly cleaved piece of H O W (Union Carbide graphite monochromator, Grade ZYB). The solution was then allowed to slowly evaporate under ambient conditions overnight, resulting in an opaque film suitable for imaging. These films are most certainly thicker than a monolayer, but for STM experiments it is expected that only the first layer of molecules adsorbed to the surface will be imaged. STM experiments are conducted using a Nanoscope I1 (Digital Instruments) microscope equipped with a 0.5-pm scanner. Images were obtained by recording variations in the tip position while maintaining the tunneling current constant (Height Mode) using Pt-Ir (80:20) tunneling tips prepared by snipping 0.010-in. wire with a sharp wire cutter. Tunneling conditions yielding molecular resolution consisted of bias voltages in the range 1.2-1.4 V (sample negative) and tunneling current set points in the range 0.1 5-0.25 nA. Experiments are conducted with initial bias voltages greater than 2 V and gradually lowered to values revealing the adsorbed molecules. Image processing, when employed, consisted of the application of a low pass filter, and when used, it is indicated in the figure caption. Results Figure 1 is a 30-nm X 30-nm topographic image of an ordered array of PBLG molecules on graphite. In all of the images which were recorded in our laboratory no clear observation of the absolute end of one of the PBLG polymers was ever made. Molecular dimensions and packing distances characteristic of the films prepared in our laboratory were determined from a large number of measurements averaged for a given film and from five different high-resolution images. The uncertainty in these measurements is indicated in parentheses and is the standard deviation of the distances recorded from the images in units of hundreths of a nanometer. Figure 2A,B shows two different representations of a IO-nm X IO-nm area of the array of molecules which better reveal the detail of the STM images. Figure 2A is a topograph which clearly

Figure 2. (A, top) STM constant current topograph of a IO-nm X IO-nm region of a liquid crystalline film of PBLG on HOPG. This image has been processed using a low pass filtering routine. (B, bottom) 3D surface plot of the same image presented in (A). Tunneling conditions are -1268 mV (sample bias) and 0.20 nA.

shows the interlacing of the molecules with one another, and Figure 2B is a 3D surface plot which better depicts the corrugation of the PBLG molecules on the HOPG surface. Figure 3 is a simplified graphic representation of the surface features appearing in the STM images, which is useful in relating the measured spacings and angles to the PBLG images. The molecules in these films are arranged in a close-packed, somewhat intertwined, array with a diameter of 2.26 (9) nm and a molecule-molecule center spacing of 2.26 (8) nm. The measurement of the molecular width is less accurate than that of the center-to-centerspacing because of the difficulty in establishing where one molecule ends and another begins. Each molecular column is offset with respect to the adjacent column by 0.34 (4) nm. The long diagonal bright band is 2.84 (4) nm in length and is at an angle of 52 (2)' with respect to the axis of the molecular column. Along the axis of the molecule are three characteristic spacings which, when looking at the image from left to right, are 0.99 (2) nm (the distance from the continuous long band to the broken diagonal band), 0.88 (2) nm (the distance from the broken diagonal band to the continuous long band), and 1.87 (1) nm (the distance between two continuous

STM Studies of Poly(y-benzyl-L-glutamate)

B = 2.26(8) nm C

=

E

i

2.84(4) nm 0.88(2) nm a

D = O W 2 ) nm F = 0.34(4) nm i

52(2)'

Figure 3. A graphic representation of the surface structure as revealed by the STM. Part A depicts four adjacent PBLG molecules, and part B is a smaller scale sketch which better depicts the partial intertwining of two adjacent PBLG molecules. The distances indicated in the figure are average values of a large number of measurements taken from five

high-resolution STM images. long bands or two broken bands). The surface corrugation measures about 0.6 nm.

Discussion It is apparent from Figures 1 and 2 that the films cast for these experiments lead to ordered arrays of molecular scale features, which can be attributed to an ordered array of PBLG molecules over at least the region of the surface which is sampled with the STM. Most noticeable in Figure 2B is the tight packing of the individual molecules and the appearance of some intertwining between adjacent molecules. The observed STM image features are composed of alternating long continuous and shorter broken bands which are arranged along the long axis of the molecules and make an angle of 52 (2)O with this axis. These PBLG molecules arranged on the HOPG surface do indeed reveal a helical, screwlike conformation although we will give strong evidence below that this apparent helical structure is a result of the a-helical conformation of the polypeptide but not the hydrogenbonded amide linkages of the peptide. PBLG has been examined with the STM prior to this work. In the studies of McMaster et al., arrays of features quite similar to those reported here were observed." The molecules in the previously reported images were either 2.3 or 2.9 nm wide and were spaced either 4.12 or 3.30 nm apart from one another. In addition, a repeating band spaced by either 1.61 or 1.65 nm and oriented at an angle of approximately 60° with respect to the long axis of the feature was observed. The authors refer to this band as a helical repeat and suggest that this helical structure might arise from the benzyl groups on the circumference of the PBLG molecule. The principal differences between these earlier results and those of the present study are the observation of the features between the long bands, the much tighter packing of the molecules in the liquid crystalline array, and consequently the observed partial intertwining of the individual molecules with adjacent molecules. In addition, when compared to the earlier STM study," the images reported here employ either no filtering or a minimal amount of filtering performed using a single application of a low pass filtering routine.

The Journal of Physical Chemistry, Vol. 96, No. 16, 1992 6%21 A number of earlier studies of thin films of PBLG cast from various solvents over a wide range of conditions have been camed out by Watanabe, Uematsu, and ~ o - w o r k e r s .These ~ ~ ~ reports include measurements of the visooelastic and dielectric properties of the films as well as X-ray diffraction measurements. Two distinct phases have been observed in these films. The one resulting from DMF solutions leads to helices with a repeat unit involving 18 rcpidues and five turns (repeat length 2.7 nm). The phase which results from solutions of PBLG in chloroform, very slowly cast over a period of months, results in a helical structure with a repeat unit of seven residues and two turns (repeat length 1.05 nm). This 7/2 form of the PBLG structure is metastable and undergoes an irreversible phase transition to the 18/5 conformation upon heating to 84 OC. Because of the importance of PBLG in the area of polymer chemistry and in particular because of its relationship to biopalymers, numerous concentrated solution studies of PBLG have been reported. An extensive X-ray diffraction study of concentrated solutions of PBLG in DMF, resulting in a paracrystalline phase, has been reported by Elliot and mw0rkers.6'~ T h e studia were the first to reveal that the molecules of PBLG were straight helices as opposed to coiled coils as was previously thought. The individual helices were shown to be in the 18/5 conformation and to have a helical pitch of 0.54 nm with a repeat unit along the helix of 2.7 nm. Additionally, a model for the organization of the molecules in solution was proposed using a diameter of 2.3 nm for PBLG and a slight interlacing of the benzyl groups between adjacent molecules such that one helix was offset from the other helix by 0.34 nm, the assumed thickness for a planar benzyl group. In order to better understand the features observed in both the images reported in this paper and those previously reported, we have employed the molecular program MacroModel. Depicted in Figure 4A-C are a series of computer images of a calculated, but not energy minimized, structure of 40 residues which would comprise a portion of a PBLG polymer. In this molecular model, the polypeptide fragment was forced into an a-helical conformation and intermolecular packing fonxs were neglected. This molecular model provides a low-resolution template to better understand the STM image features. The more subtle structural details present in a completely energy-minimized structure are not expected to be apparent in the STM images at the resolution presently available in adsorbate studies. In this sequence the ability of the program to pass a plane through the molecule, moving in a direction perpendicular to the molecular axis, and to display only that portion of the molecule above the plane is used. In Figure 4C, which is the topmost cut presented, one clearly sees lines of benzyl groups which make an angle of approximately 50' with respect to the molecular axis as measured from a photograph. This angle corresponds closely to the 52 (2)' measured in the STM images. The length of these lines of benzyl groups is about 2.6 nm, which also matches well with the length of the bright bands observed with the STM, which is 2.84 (4) nm. The spacing between the lines of benzyl groups is about 1.7 nm (distance G? as indicated in Figure 5. The corresponding measurement taken from the STM images is 1.87 (1) nm (the sum of distances D and E in Figure 3). In Figure 4B, more of the molecule is revealed in the MacroModel screen display, and the origin of the broken bands in the STM images becomes apparent. The structure which appears between the bright bands observed with the STM is asymmetrically located between the bands with spacings of 0.99 (2) and 0.88 (2) nm. In the computer-generated images the additional benzyl groups which appear between the lines of benzyl groups depicted in Figures 4C and 5 are also asymmetrically situated with spacings of 1.1 and 0.8 nm as measured along the outermost portion of the molecular model. Figure 6 shows this additional structure in more detail. The arrows in the figure point to the intermediate structural features. Finally, in Figure 4A the full structure of the 40 residues is revealed in the screen display. The diameter of the computergenerated model is about 2.1 nm. The value of the molecular diameter obtained from the STM images is 2.26 (8) nm. The

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Figure 5. The same view of the MacroModel-generated structure of a 40-residue section of PBLG which appears in Figure 4C. Lines have been drawn to indicate the benzyl groups of Figure 4C.

Figure 6. The same view of the MacroModel-generated structure of a 40-residue section of PBLG which appears in Figure 4B. Lines have bcen drawn to indicate the benzyl groups of Figure 4C and 5 while arrows show the intermediate structure discussed in the text.

Figure 4. A series of images of a computer-generated structure of a 40-residue section of PBLG done with MacroModel. In the series A-C (top-bottom), a plane is translated through the molecule moving in the direction perpendicular to the helical axis and only portions of the molecule above the plane appear in the image.

polypeptide backbone, which forms the right-handed a-helix with a typical pitch of 0.5-0.6 nm and a diameter of about 0.5 nm, is largely hidden by the individual benzyl groups substituted on each of the peptide residues. The corrugation observed with the STM, 0.6 nm, is substantially less than the 2.3-nm diameter of the polypeptide molecule. This may reflect the tight packing of the molecules on the surface, which along with the shape and size of the tunneling tip might prohibit imaging the full molecular height or it may reflect the mechanism for imaging such adsorbates with the STM. Immediately apparent from this comparison of the STM images with the molecular model is the reality that the STM images are dominated by the outermost portions of the molecules. The screwlike structure of the images is not the same helical screw of the polypeptide backbone but rather a lower resolution and less

detailed view of the molecule. This most certainly is a result of the contrast mechanism responsible for molecular imaging and the domination of the image by the r-bonded benzyl groups. Previous STM imaging studies of molecules adsorbed on HOPG and other surfaces, especially those involving the n-alkylbiphenyl liquid crystals, have established this strong influence of the T bonded substructure on the STM image as the norm.13 In these STM images there are a number of factors which could prevent the resolution of individual benzyl groups. These include principally the tunneling tip condition, thermal drift, vibrational motion of the molecules, and rotation of the long flexible side chain about the a-carbon. The principal result of this domination of the image contrast by the benzyl groups is a lack of detail available about the inner helical core of the molecules. Further extension of the structure in the STM images to the more subtle details of the helical conformation is not obvious. Conclusion

STM images of PBLG molecules in liquid crystalline arrays on H O E are reported. These molecular resolution images reveal details of the molecular structure consistent with the a-helical conformation expected for PBLG molecules in solution and films made with DMF. Measurements of the molecular diameter and packing geometry are consistent with previous structural determinations of PBLG. Comparisons of the details contained in the

J. Phys. Chem. 19!22,96, 6829-6834 STM images with computer-generated structures using MacroModel support the conclusion that the image contrast is dominated by the benzyl groups on the circumference of the polypeptide molecules.

Acknowledgment. We thank Professor W.Clark Still for both helpful discussions and the use of his molecular modeling program and Professor Koji Nakanishi for the use of his computing facilities. This work was supported by The Joint Service8 Electronics Program (U.S.Army, US. Navy, and U.S. Air Force; DAAL03-91-C-0016), The Office of Naval Research, and The IBM Materials Research Program. Equipment support was provided by the National Science Foundation (CHE-88-16581).

References and Notes (1) (a) Binnig, G.; Roher, H.; Gerber, Ch.; Weibel, E. Phys. Rev. Lett. 1982,49, 57. (b) Binnig, G.; Roher, H. Surf. Sci. 1983,126, 236. (c) Binnig, G.;Roher, H.;Gerber, Ch.; Weibel, E. Phys. Rev. Lett. 1982, 50, 120. (2) (a) Tersoff, J.; Hamann, D. R. Phys. Rev. B 1985,31,805. (b) Tersoff, J. Phys. RN. krr.1986,57,440. (c) Tekman, E.; C i i , S.Phys. RN. B 1989, 40, 10286. (3) Spong, J. K.; Mizes, H. A.; Lacomb, Jr., L. J.; Dovek, M. M.; Frommer, J. E.; Foster, J. S.Nature 1989, 338, 137. (4) (a) Mizutani, W.;Shigeno, M.; Ono, M.; Kajimura, K. Appl. Phys. Lett. 1990,56, 1974. (b) Mizutani, W.; Shigeno, M.; Ohmi, M.; Suginoya, M.; Kajimura, K.; Ono, M. J . Vac. Sci. Technol. B 1991, 9, 1102.

(5) For a recent reviews see: (a) Engle, A. Annu. Rev. Biophys. Biophys. Chem. 1991,20,79. (b) Rocccdings of the Annual International Conference on Scanning Tunneling Microscopy/Spectropy published each year in J. Yac. Sci. Technol. (c) Frommer, J. Angew. Chem., to be published. (6) (a) Bamford, C. H.; Elliot, A,; Hanby, W. E. Synthetic Polypeptides; Academic Press: New York, 1956. (b) Elliot, A. In Poly-a-amino Acids; Fasman, G., Ed.; Marcel Dekker: New York, 1967; pp 1-67. (c) Squire, I.

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Photophyslcs and Photochemistry of Quantized ZnO Colloids Prashant V. Kamat* and Brian Patrick? Radiation hboratory, University of Notre Dame, Notre Dame, Indiana 46556 (Received: January 23, 1992; In Final Form: April 28, 1992)

The photophysical and photochemical behavior of quantized ZnO colloids in ethanol has been investigated by time-resolved transient absorption and emission measurements. Trapping of electrons at the ZnO surface resulted in broad absorption in the red region. The green emission of ZnO colloids was readily quenched by hole scavengers such as SCN- and I-. The photoinduced charge transfer to these hole scavengers was studied by laser flash photolysis. The yield of oxidized product increased considerably when ZnO colloids were coupled with ZnSe.

Introduction Ultrasmall semiconductor particles prepared in aqueous, nonaqueous, polymeric, zeolite, and other organized media have drawn considerable interest in recent years because of their interesting photophysical and photochemical properties (see, for example, ref 1-4). Of particular interest are the size quantization effects and the surface modification of semiconductor colloids with suitable redox couples and sensitizers. The efficiency of photoinduced charge separation in a colloidal semiconductor system can be improved by coupling two semiconductor particles with different energy level^.^*^ The emission properties of ZnO single crystals and films have been extensively studied (see, for example, refs 7-9), yet there have been only a few studies on ZnO colloids. The initial effort to characterize the absorption and emission properties of ZnO colloids was made by Henglein and co-workersloJ1and Bahnemann et al.’* These researchers observed size quantization effects as Address correspondence to this author. ‘Visiting student from University of Waterloo under Co-op program.

the onset of absorption of these colloids was shifted to shorter wavelengths as the particle size was decreased. Spanhel and Anderson13succeeded in preparing high concentrations of ZnO colloids in ethanol without adding any stabilizers. Spanhel et al. and RabaniI4 have shown that ZnO colloids can be coupled with another semiconductor such as CdS or ZnS. Recently, we have shownl5that the films of ZnO colloids coated on Sn02films exhibit excellent photoelectrochemical properties, and these colloidal semiconductor films can be sensitized to the visible region by depositing CdS or chlorophyll (1.’’ Such systems are potentially useful in the direct conversion of solar energy into electricity. In view of these various important features of the ZnO system, we have now investigated photoinduced charge-transfer processes in colloidal ZnO suspension in ethanol.

Experimental Section Materials. ZnO colloidal suspension in ethanol was prepared by the method described by Spanhel and Ander~0n.l~ The zinc complex precursor was prepared by refluxing an ethanol solution containing 0.1 M zinc acetate for 2-3 h. This precursor solution

0022-3654/92/2096-6829$03.00/00 1992 American Chemical Society