news & views living cell. Both designs share some common features, including localization of the active site on the protein–protein interface and that both proteins are tetramers, which is an intriguing detail in view of the fact that the average oligomerization state of proteins in nature is tetrameric2. The designs are not optimized yet: the activity and substrate binding affinity of both proteins clearly need further improvement. Nevertheless, achieving this should be feasible via a combination of the increasingly powerful computational
techniques that are now available, and evolutionary approaches that select for improved function1. These two studies show that a future in which proteins are routinely designed for specific functions in vitro and in vivo is becoming ever more likely. ❐ Arnold J. Boersma is in the Groningen Biomolecular Sciences and Biotechnology Institute and Gerard Roelfes is in the Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747 AG Groningen, the Netherlands. e-mail: [email protected]; [email protected]
References
1. Kiss, G., Çelebi-Ölçüm, N., Moretti, R., Baker, D. & Houk, K. N. Angew. Chem. Int. Ed. 52, 5700–5725 (2013). 2. Ali, M. H. & Imperiali, B. Bioorg. Med. Chem. 13, 5013–5020 (2005). 3. Marianayagam, N. J., Sunde, M. & Matthews, J. M. Trends Biochem. Sci. 29, 618–625 (2004). 4. Bryson, J. W. et al. Science 270, 935–941 (1995). 5. Sontz, P. A., Song, W. J. & Tezcan, F. A. Curr. Opin. Chem. Biol. 19, 42–49 (2014). 6. Zastrow, M. L., Peacock, A. F. A., Stuckey, J. A. & Pecoraro, V. L. Nature Chem. 4, 118–123 (2012). 7. Der, B. S., Edwards, D. R. & Kuhlman, B. Biochemistry 51, 3933–3940 (2012). 8. Faiella, M. et al. Nature Chem. Biol. 5, 882–884 (2009). 9. Joh, N. H. et al. Science, 346, 1520–1524 (2014). 10. Song, W. J. & Tezcan, F. A. Science 346, 1525–1528 (2014).
SURFACE CHEMISTRY
A step in the right direction
Identifying the contribution of different surface sites to the overall kinetics of molecular desorption from solid surfaces is difficult even when using single crystals. A new technique that combines molecular beams with UV−UV double resonance spectroscopy resolves this problem for the case of carbon monoxide on Pt(111).
Francisco Zaera
R
eactions on solid surfaces are ubiquitous in nature and central to many chemical processes such as catalysis, electrochemistry and film deposition, yet the bonding involved is not as well understood as in homogeneous systems. This is not for a lack of trying, it is just that chemistry at interfaces is much more difficult to investigate. First, there are far fewer surface atoms than those in the bulk of a given material, so special techniques are necessary to selectively probe bonds and chemistry at those surface sites. A revolution in surface science in the late twentieth century led to the development of a plethora of new surface-sensitive techniques1, but the information that they have provided remains somewhat limited, particularly when compared with spectroscopies designed to study bulk samples. Second, bonding to solid surfaces is more complex and more difficult to describe than in discrete molecules. This is particularly true on metals, where the electronic structure is delocalized and extends over large atomic distances2. Finally, it is quite difficult to prepare surfaces with only one type of bonding, since adsorbates can coordinate in many different ways to the various ensembles of surface atoms exposed by the solid. Surface scientists deal with this latter problem by using specific facets of single crystals1, which exhibit well defined coordination environments, but even those can feature enough defects —
Ea,111 = 122 kJ mol–1 A111 = 3.5 × 1013 s–1
Ea,step = 101 kJ mol–1 Astep = 1.9 × 1011 s–1
Figure 1 | Schematic of the CO desorption processes identified by Bartels and colleagues. The addition of UV–UV double resonance spectroscopy to molecular beam experiments on surfaces allowed the identification of two desorption processes. The desorption of CO from terrace sites occurs rapidly (brown arrow), yet a slower process involving the diffusion of CO from step sites to terraces sites before subsequent desorption (green arrows) dominates at low coverages (reaction rate constant = Ae(–Ea/RT)).
typically low-coordination, highly-reactive atoms in steps, kinks, and other unique arrangements — to dominate the overall chemistry and skew the results of kinetics measurements. It is this third limitation that has been addressed by Bartels and colleagues writing in the Journal of the American Chemical Society 3. They report a new technique to estimate the binding energy of carbon monoxide on the flat, hexagonal
NATURE CHEMISTRY | VOL 7 | APRIL 2015 | www.nature.com/naturechemistry
© 2015 Macmillan Publishers Limited. All rights reserved
close-packed, (111) facet of platinum — a prototypical system often used by surface scientists as a benchmark. There are three basic ways of measuring binding energies of molecules on surfaces. The most common is temperature programmed desorption, a technique where the temperature at which molecules desorb from a surface as it is heated is used to estimate desorption activation energies. The strength of this technique, and the reason for its wide use, is its simplicity, but its poor accuracy is a significant weakness4. Kinetic data can be better measured isothermally by directing pulsed beams of molecules toward the surface of interest and detecting and analysing the angular and energy distributions of the scattered molecules. However, such molecular beam experiments are difficult to carry out and often suffer from problems of sensitivity and time resolution. Finally, new differential microcalorimeters have been developed recently to directly measure small heat changes, and thus adsorption energies, on single-crystal surfaces. Bartels and colleagues now incorporate UV–UV double resonance spectroscopy into a molecular beam setup to detect the desorbing molecules. This involves an arrangement in which the CO molecules leaving the Pt(111) surface are first excited to a metastable state by means of the adsorption of a UV photon, and then detected following a second excitation, triggered by a second UV photon, at 279
news & views a fixed distance away from the first excitation point. This adds the ability to decouple the residence time of adsorbates on the surface from the temporal profile arising from their velocity distribution after desorption, and with that to an improvement in time resolution and the potential to obtain new insights into chemical kinetics of surface reactions. In this case, the new method afforded the identification of two desorption processes with different rates: a fast rate that Bartels and colleagues associate with CO evolution from the (111) terraces, and a slower process related to the sequential diffusion of CO from steps to terraces followed by desorption (Fig. 1). They argue that previous molecular beam kinetic measurements have erroneously assigned the initial desorption kinetics from clean Pt(111) to CO evolution from the (111) facets, when in fact it is related to the behaviour of adsorbates at surface steps. A fit of their temperature-dependent kinetic data to a bi-exponential rate law ultimately yielded values for the activation energies of CO desorption from (111) terraces and steps of Ea,111 = 122 ± 7 kJ mol−1 and Ea,step = 101 ± 10 kJ mol−1, respectively (Fig. 1). Bartels and colleagues then fit their experimental data to a rate equation developed using transition state theory in order to extract values for the CO binding energies. The approach that they use is undoubtedly interesting, but it also highlights the limitations of using kinetic measurements to calculate thermodynamic properties. A number of assumptions are made in their modelling, and in the end it is not clear why their final reported value for the binding energy of CO to Pt(111) terraces, DPt–CO,111 = 142 ± 5 kJ mol−1, differs so much from their measured activation energies of desorption. It is also worth mentioning that their reported activation energies are accompanied by unexpectedly low pre-exponential factors, suggesting possible errors originating from apparent compensation effects5, a problem common to kinetic studies that use narrow ranges of temperatures (as is the case here). In their discussion, Bartels and colleagues emphasize the shortcomings of previous molecular-beam kinetic measurements, which in their estimation led to erroneous binding energies for CO on Pt(111). However, it is more appropriate to compare their results to microcalorimetric measurements, which yield bond energies directly and have already been used by three different research groups6–8 to estimate DPt–CO,111. Two of those studies6,7 report similar 280
values, around 130 ± 2 kJ mol−1, and although the early work8 did yield a higher value, 187 ± 11 kJ mol−1, the discrepancy was later explained by systematic experimental errors in the measurements. It is not obvious why the new, higher value reported by Bartels and colleagues should be more reliable than the consensus number from the microcalorimetric studies. Also, the differential nature of the microcalorimetry experiments, which measures adsorption energies as a function of surface coverage, should have afforded the direct identification of adsorption on steps and other defects, which saturate first. Instead, no clear distinction between adsorption on such defects versus on terraces was seen in the uptake data versus CO coverage in any of the microcalorimetry reports. In spite of these criticisms, the addition of UV–UV double resonance spectroscopy to molecular beam experiments on surfaces is valuable in that it facilitates the decoupling of the velocity distributions of the desorbing molecules from fundamental kinetic processes. This may open the door for direct studies of the kinetics of surface reactions. It should also be possible to settle the issue of the contribution of steps and other defects to the kinetics of reactions by using crystals cut at specific angles off the (111) plane in order to expose terraces with well-defined widths and control the step density, an approach that has already been used in microcalorimetry studies9. More fundamentally, the new technique can be used to investigate any potential correlations between desorption kinetics and the velocity distributions of the outgoing desorbing molecules. New surface-sensitive techniques designed for kinetic measurements of surface phenomena are certainly always welcomed by the surface-science community. ❐ Francisco Zaera is in the Department of Chemistry, University of California, Riverside, California 92521, USA. e-mail: [email protected] References 1. Somorjai, G. A. & Li, Y. Introduction to Surface Chemistry and Catalysis 2nd Edn (John Wiley & Sons, 2010). 2. Hoffmann, R. Solids and Surfaces: A Chemist’s View of Bonding in Extended Structures (VCH, 1988). 3. Golibrzuch, K. et al. J. Am. Chem. Soc. 137, 1465–1475 (2015). 4. Zaera, F. in Comprehensive Inorganic Chemistry II (eds Reedijk, J. & Poeppelmeier, K.) 39–74 (Elsevier, 2013). 5. Liu, L. & Guo, Q‑X. Chem. Rev. 101, 673–696 (2001). 6. Schieβer, A., Hörtz, P. & Schäfer, R. Surf. Sci. 604, 2098–2105 (2010). 7. Fischer-Wolfarth, J‑H. et al. Rev. Sci. Instrum. 82, 024102 (2011). 8. Yeo, Y. Y., Vattuone, L. & King, D. A. J. Chem. Phys. 106, 392–401 (1997). 9. Karmazyn, A. D., Fiorin, V., Jenkins, S. J. & King, D. A. Surf. Sci. 538, 171–183 (2003).
NATURE CHEMISTRY | VOL 7 | APRIL 2015 | www.nature.com/naturechemistry
© 2015 Macmillan Publishers Limited. All rights reserved
Call for papers
Nature Chemistry invites the submission of high-quality papers that describe the most significant and cutting-edge research in all areas of chemistry. We welcome the submission of manuscripts in the traditional core areas of chemistry, as well as those that describe cross-disciplinary topics between sub-fields. Articles detailing multidisciplinary research performed at the interface of chemistry and other scientific fields of inquiry such as biology, materials science, nanotechnology and physics are also encouraged. Manuscripts should be submitted through the Nature Chemistry online submission system at http://mts-nchem.nature.com and a complete guide to submission can be found at www.nature.com/ nchem/authors/index.html www.nature.com/naturechemistry
techniques that are now available, and evolutionary approaches that select for improved function1. These two studies show that a future in which proteins are routinely designed for specific functions in vitro and in vivo is becoming ever more likely. ❐ Arnold J. Boersma is in the Groningen Biomolecular Sciences and Biotechnology Institute and Gerard Roelfes is in the Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747 AG Groningen, the Netherlands. e-mail: [email protected]; [email protected]
References
1. Kiss, G., Çelebi-Ölçüm, N., Moretti, R., Baker, D. & Houk, K. N. Angew. Chem. Int. Ed. 52, 5700–5725 (2013). 2. Ali, M. H. & Imperiali, B. Bioorg. Med. Chem. 13, 5013–5020 (2005). 3. Marianayagam, N. J., Sunde, M. & Matthews, J. M. Trends Biochem. Sci. 29, 618–625 (2004). 4. Bryson, J. W. et al. Science 270, 935–941 (1995). 5. Sontz, P. A., Song, W. J. & Tezcan, F. A. Curr. Opin. Chem. Biol. 19, 42–49 (2014). 6. Zastrow, M. L., Peacock, A. F. A., Stuckey, J. A. & Pecoraro, V. L. Nature Chem. 4, 118–123 (2012). 7. Der, B. S., Edwards, D. R. & Kuhlman, B. Biochemistry 51, 3933–3940 (2012). 8. Faiella, M. et al. Nature Chem. Biol. 5, 882–884 (2009). 9. Joh, N. H. et al. Science, 346, 1520–1524 (2014). 10. Song, W. J. & Tezcan, F. A. Science 346, 1525–1528 (2014).
SURFACE CHEMISTRY
A step in the right direction
Identifying the contribution of different surface sites to the overall kinetics of molecular desorption from solid surfaces is difficult even when using single crystals. A new technique that combines molecular beams with UV−UV double resonance spectroscopy resolves this problem for the case of carbon monoxide on Pt(111).
Francisco Zaera
R
eactions on solid surfaces are ubiquitous in nature and central to many chemical processes such as catalysis, electrochemistry and film deposition, yet the bonding involved is not as well understood as in homogeneous systems. This is not for a lack of trying, it is just that chemistry at interfaces is much more difficult to investigate. First, there are far fewer surface atoms than those in the bulk of a given material, so special techniques are necessary to selectively probe bonds and chemistry at those surface sites. A revolution in surface science in the late twentieth century led to the development of a plethora of new surface-sensitive techniques1, but the information that they have provided remains somewhat limited, particularly when compared with spectroscopies designed to study bulk samples. Second, bonding to solid surfaces is more complex and more difficult to describe than in discrete molecules. This is particularly true on metals, where the electronic structure is delocalized and extends over large atomic distances2. Finally, it is quite difficult to prepare surfaces with only one type of bonding, since adsorbates can coordinate in many different ways to the various ensembles of surface atoms exposed by the solid. Surface scientists deal with this latter problem by using specific facets of single crystals1, which exhibit well defined coordination environments, but even those can feature enough defects —
Ea,111 = 122 kJ mol–1 A111 = 3.5 × 1013 s–1
Ea,step = 101 kJ mol–1 Astep = 1.9 × 1011 s–1
Figure 1 | Schematic of the CO desorption processes identified by Bartels and colleagues. The addition of UV–UV double resonance spectroscopy to molecular beam experiments on surfaces allowed the identification of two desorption processes. The desorption of CO from terrace sites occurs rapidly (brown arrow), yet a slower process involving the diffusion of CO from step sites to terraces sites before subsequent desorption (green arrows) dominates at low coverages (reaction rate constant = Ae(–Ea/RT)).
typically low-coordination, highly-reactive atoms in steps, kinks, and other unique arrangements — to dominate the overall chemistry and skew the results of kinetics measurements. It is this third limitation that has been addressed by Bartels and colleagues writing in the Journal of the American Chemical Society 3. They report a new technique to estimate the binding energy of carbon monoxide on the flat, hexagonal
NATURE CHEMISTRY | VOL 7 | APRIL 2015 | www.nature.com/naturechemistry
© 2015 Macmillan Publishers Limited. All rights reserved
close-packed, (111) facet of platinum — a prototypical system often used by surface scientists as a benchmark. There are three basic ways of measuring binding energies of molecules on surfaces. The most common is temperature programmed desorption, a technique where the temperature at which molecules desorb from a surface as it is heated is used to estimate desorption activation energies. The strength of this technique, and the reason for its wide use, is its simplicity, but its poor accuracy is a significant weakness4. Kinetic data can be better measured isothermally by directing pulsed beams of molecules toward the surface of interest and detecting and analysing the angular and energy distributions of the scattered molecules. However, such molecular beam experiments are difficult to carry out and often suffer from problems of sensitivity and time resolution. Finally, new differential microcalorimeters have been developed recently to directly measure small heat changes, and thus adsorption energies, on single-crystal surfaces. Bartels and colleagues now incorporate UV–UV double resonance spectroscopy into a molecular beam setup to detect the desorbing molecules. This involves an arrangement in which the CO molecules leaving the Pt(111) surface are first excited to a metastable state by means of the adsorption of a UV photon, and then detected following a second excitation, triggered by a second UV photon, at 279
news & views a fixed distance away from the first excitation point. This adds the ability to decouple the residence time of adsorbates on the surface from the temporal profile arising from their velocity distribution after desorption, and with that to an improvement in time resolution and the potential to obtain new insights into chemical kinetics of surface reactions. In this case, the new method afforded the identification of two desorption processes with different rates: a fast rate that Bartels and colleagues associate with CO evolution from the (111) terraces, and a slower process related to the sequential diffusion of CO from steps to terraces followed by desorption (Fig. 1). They argue that previous molecular beam kinetic measurements have erroneously assigned the initial desorption kinetics from clean Pt(111) to CO evolution from the (111) facets, when in fact it is related to the behaviour of adsorbates at surface steps. A fit of their temperature-dependent kinetic data to a bi-exponential rate law ultimately yielded values for the activation energies of CO desorption from (111) terraces and steps of Ea,111 = 122 ± 7 kJ mol−1 and Ea,step = 101 ± 10 kJ mol−1, respectively (Fig. 1). Bartels and colleagues then fit their experimental data to a rate equation developed using transition state theory in order to extract values for the CO binding energies. The approach that they use is undoubtedly interesting, but it also highlights the limitations of using kinetic measurements to calculate thermodynamic properties. A number of assumptions are made in their modelling, and in the end it is not clear why their final reported value for the binding energy of CO to Pt(111) terraces, DPt–CO,111 = 142 ± 5 kJ mol−1, differs so much from their measured activation energies of desorption. It is also worth mentioning that their reported activation energies are accompanied by unexpectedly low pre-exponential factors, suggesting possible errors originating from apparent compensation effects5, a problem common to kinetic studies that use narrow ranges of temperatures (as is the case here). In their discussion, Bartels and colleagues emphasize the shortcomings of previous molecular-beam kinetic measurements, which in their estimation led to erroneous binding energies for CO on Pt(111). However, it is more appropriate to compare their results to microcalorimetric measurements, which yield bond energies directly and have already been used by three different research groups6–8 to estimate DPt–CO,111. Two of those studies6,7 report similar 280
values, around 130 ± 2 kJ mol−1, and although the early work8 did yield a higher value, 187 ± 11 kJ mol−1, the discrepancy was later explained by systematic experimental errors in the measurements. It is not obvious why the new, higher value reported by Bartels and colleagues should be more reliable than the consensus number from the microcalorimetric studies. Also, the differential nature of the microcalorimetry experiments, which measures adsorption energies as a function of surface coverage, should have afforded the direct identification of adsorption on steps and other defects, which saturate first. Instead, no clear distinction between adsorption on such defects versus on terraces was seen in the uptake data versus CO coverage in any of the microcalorimetry reports. In spite of these criticisms, the addition of UV–UV double resonance spectroscopy to molecular beam experiments on surfaces is valuable in that it facilitates the decoupling of the velocity distributions of the desorbing molecules from fundamental kinetic processes. This may open the door for direct studies of the kinetics of surface reactions. It should also be possible to settle the issue of the contribution of steps and other defects to the kinetics of reactions by using crystals cut at specific angles off the (111) plane in order to expose terraces with well-defined widths and control the step density, an approach that has already been used in microcalorimetry studies9. More fundamentally, the new technique can be used to investigate any potential correlations between desorption kinetics and the velocity distributions of the outgoing desorbing molecules. New surface-sensitive techniques designed for kinetic measurements of surface phenomena are certainly always welcomed by the surface-science community. ❐ Francisco Zaera is in the Department of Chemistry, University of California, Riverside, California 92521, USA. e-mail: [email protected] References 1. Somorjai, G. A. & Li, Y. Introduction to Surface Chemistry and Catalysis 2nd Edn (John Wiley & Sons, 2010). 2. Hoffmann, R. Solids and Surfaces: A Chemist’s View of Bonding in Extended Structures (VCH, 1988). 3. Golibrzuch, K. et al. J. Am. Chem. Soc. 137, 1465–1475 (2015). 4. Zaera, F. in Comprehensive Inorganic Chemistry II (eds Reedijk, J. & Poeppelmeier, K.) 39–74 (Elsevier, 2013). 5. Liu, L. & Guo, Q‑X. Chem. Rev. 101, 673–696 (2001). 6. Schieβer, A., Hörtz, P. & Schäfer, R. Surf. Sci. 604, 2098–2105 (2010). 7. Fischer-Wolfarth, J‑H. et al. Rev. Sci. Instrum. 82, 024102 (2011). 8. Yeo, Y. Y., Vattuone, L. & King, D. A. J. Chem. Phys. 106, 392–401 (1997). 9. Karmazyn, A. D., Fiorin, V., Jenkins, S. J. & King, D. A. Surf. Sci. 538, 171–183 (2003).
NATURE CHEMISTRY | VOL 7 | APRIL 2015 | www.nature.com/naturechemistry
© 2015 Macmillan Publishers Limited. All rights reserved
Call for papers
Nature Chemistry invites the submission of high-quality papers that describe the most significant and cutting-edge research in all areas of chemistry. We welcome the submission of manuscripts in the traditional core areas of chemistry, as well as those that describe cross-disciplinary topics between sub-fields. Articles detailing multidisciplinary research performed at the interface of chemistry and other scientific fields of inquiry such as biology, materials science, nanotechnology and physics are also encouraged. Manuscripts should be submitted through the Nature Chemistry online submission system at http://mts-nchem.nature.com and a complete guide to submission can be found at www.nature.com/ nchem/authors/index.html www.nature.com/naturechemistry
