Spectroscopic probes of molecular recognition

In all domains of molecular science, it is of paramount importance to understand how molecules ‘recognize’ each other. A wide range of spectroscopic probes can provide detailed information and insight, even where diffraction and imaging techniques are inapplicable or inconclusive. That applies to arbitrary states of aggregation and includes dynamical and energetic aspects of the recognition process. This issue of PCCP presents a cross section of the state of the art and it hints at remaining challenges. It was made possible by the outstanding support of the PCCP staff. All contributions^1–18 use quantum chemical modelling to assist the construction of molecular pictures from spectroscopic data, thus underscoring the importance and viability of an intimate contact between experiment and simulation in this field. This is particularly true for small model systems, which can be prepared in well defined conformations and sizes by gas expansion techniques.^{1–6,8–11,14,16,17} Quite often, the isolated molecules still need to be characterized in detail,^{3–6,9,13,16,17} before their interaction with solvent molecules^{1,2,4,5,8,11,14} and their recognition behaviour in solution^{5,12,13,15,18} or in the solid state^7,11,12 can be fully understood. Regularly, isolated chain-like monomers themselves exhibit fascinating intramolecular recognition phenomena via folding.^3–6,9,17 Among the available gas phase techniques, micro- and mm-wave spectroscopy^3,8,9 as well as high resolution optical spectroscopy¹⁴ provide the most accurate structural information. UV spectroscopy, either fluorescence-^2,4,14 or ion-detected^1,6,16,17via a suitable chromophore,^{1,2,4,6,14,16,17} can achieve conformational selectivity. It reveals a wealth of dynamical information in combination with vibrational spectroscopy,^{1,2,4,6,15–17} which can also be used as a stand-alone technique.^5,10,11,18 In condensed phases, nuclear magnetic resonance is probably most powerful,^7,12,13 but fast processes and low concentrations invite complementary approaches.^15,18 Important auxiliary tools to characterize molecular recognition are proton transfer and tunneling processes^{5,7,9–11,14,16} as well as isotope^5–7,10 and fluorine^5,15 substitution. A large fraction of the studied systems involves aromatic rings.^{1–4,6,7,9,12–14,16–18} This is in part due to methodology,^{1,2,4,6,12–14,16–18} but it also reflects the importance of X–H⋯π interactions in molecular recognition.^{1,3,4,6,9,13,17} Important elements and sensitive probes of molecular interactions are the N–H^{4,6,7,9,13,14,16–18} and O–H bonds.^{1–5,8,10–12,14–16} As a consequence, hydrogen bonding is discussed in almost all studies.^1–15,17,18 The carboxylic acid group receives much attention,^10,11,15,18 as do planarity constraints in general.^{7,10,11,14–16} Limitations are quite visible in the accuracy of density functional theory for weak interactions.^{1,4–6,8,13,17} On the experimental side, the characterization of conformational relaxation phenomena often remains incompletely understood.^{3–5,8,11,17} The delicate interplay of conformational flexibility and molecular recognition^{1–9,11,12,15,17,18} in guest–host interactions^{1,2,4,8,12,13,18} is at the very heart of this prospering field of spectroscopic characterization. Chirality recognition represents another important and often addressed aspect in this context.^{1,3–6,8,9,17,18} Alcohols,^3–5,8 amines and amidines,^4,7,9 acids,^10,11,15 nitriles,¹⁴ sugars,¹ purines,¹⁶ peptides,^6,17,18 crowns, calixes and tweezers^2,12,13 reveal their interaction preferences. In macromolecules, many of these individually subtle interactions may accumulate into formidable selectivities. Of course, the work presented in this issue is nothing but a small excerpt of how spectroscopy can contribute to the understanding of molecular cooperation in the life and materials sciences. Together with other recent work, it illustrates how diverse, powerful and detailed these contributions can be. Enjoy some fine molecular detectives at work!

M. A. Suhm, Göttingen

Papers in this issue
1	J. P. Simons et al., DOI: 10.1039/b704792d
2	T. Ebata et al., DOI: 10.1039/b704750a
3	W. Caminati et al., DOI: 10.1039/b705114j
4	A. Zehnacker-Rentien et al., DOI: 10.1039/b705650h
5	M. Suhm et al., DOI: 10.1039/b705498j
6	M. Mons et al., DOI: 10.1039/b704573e
7	H.-H. Limbach et al., DOI: 10.1039/b704384h
8	N. Borho and Y. Xu, DOI: 10.1039/b705746f
9	J. L. Alonso et al., DOI: 10.1039/b705614a
10	P. Zielke and M. A. Suhm, DOI: 10.1039/b706094g
11	R. Signorell et al., DOI: 10.1039/b704600f
12	H. W. Spiess et al., DOI: 10.1039/b704269h
13	C. Ochsenfeld et al., DOI: 10.1039/b706045a
14	D. W. Pratt et al., DOI: 10.1039/b705679f
15	B. H. Pate et al., DOI: 10.1039/b704900e
16	M. S. de Vries et al., DOI: 10.1039/b705042a
17	M. Gerhards et al., DOI: 10.1039/b706519a
18	C. Schmuck et al., DOI: 10.1039/b709142g

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