Surface-bound polypeptides and proteins are increasingly used to functionalize inorganic interfaces such as electrodes, but their structural characterization is exceedingly difficult with standard technologies. In this paper, we report the first two-dimensional sum-frequency generation (2D SFG) spectra of a peptide monolayer, which is collected by adding a mid-IR pulse shaper to a standard femtosecond SFG spectrometer. On a gold surface, standard FTIR spectroscopy is inconclusive about the peptide structure because of solvation-induced frequency shifts, but the 2D lineshapes, anharmonic shifts, and lifetimes obtained from 2D SFG reveal that the peptide is largely α-helical and upright. Random coil residues are also observed, which do not themselves appear in SFG spectra due to their isotropic structural distribution, but which still absorb infrared light and so can be detected by cross-peaks in 2D SFG spectra. We discuss these results in the context of peptide design. Because of the similar way in which the spectra are collected, these 2D SFG spectra can be directly compared to 2D IR spectra, thereby enabling structural interpretations of surface-bound peptides and biomolecules based on the well-studied structure/2D IR spectra relationships established from soluble proteins.

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A new strategy for rapid evaluation of sequence-stability relationships in the parallel coiled-coil motif is described. The experimental design relies upon thiol-thioester exchange equilibria, an approach that is particularly well suited to examination of heterodimeric systems. Our model system has been benchmarked by demonstrating that it can quantitatively reproduce previously reported trends in interhelical a-a' side chain pairing preferences at the coiled-coil interface. This new tool has been used to explore the role of Coulombic interactions between a core position on one helix and a flanking position on the other helix (a-g'). This type of interhelical contact has received relatively little attention to date. Our results indicate that such interactions can influence coiled-coil partner preferences.Proteins collectively display a broad array of tertiary and quaternary structures, with many different modes of packing between neighboring secondary structure elements. Among the possibilities, the α-helical coiled coil is unusual in that it is both common and regular. 1 In the simplest case, two α-helices associate side-by-side, wrapping around one another with a slight left-handed superhelical twist. A characteristic "knobs-into-holes" interdigitation of sidechains is observed at the helix-helix interface, whether the helices are parallel or antiparallel. 2 The relative simplicity of this architecture has led to extensive exploration of sequencestability relationships, 3 motivated by the prospects of predicting coiled-coil structure from sequence information alone, refining computational tools, and using coiled coils as building blocks in rational protein design and synthetic biology. 4 Although some principles that govern coiled-coil stability have been elucidated, our understanding remains incomplete.Here we introduce a heterodimeric parallel coiled-coil model system designed to provide new insights on the origins of stability and helix-pairing preferences. Our system employs relatively short peptide segments (20 or 21 residues), which facilitates broad exploration of sequence variations. Parallel, two-helix assembly is promoted by a thioester linkage between the Cterminus of one segment and the side-chain of a C-terminal Cys residue on the other; this design enables us to monitor coiled-coil stability under native conditions via thiol-thioester exchange equilibration. 5 Our experimental design (Figure 1) is based on well-known characteristics of sequences that form coiled coils. The segments intended to adopt α-helical conformations feature a heptad sequence repeat pattern (abcdefg), in which side-chains at a and d dominate the helix-helix contacts. Two-helix stoichiometry (rather than alternate three-or four-helix assemblies) is directed by placing Leu at the d sites, Ile at the N-terminal a positions, and Asn at the a sites closest to the covalent connection.6 ,7 The remaining (central) a positions of each segment (designated X and Ψ) are "guest" sites for substitutions that allow us to probe ...

Elucidating relationships between the amino-acid sequences of proteins and their three-dimensional structures, and uncovering non-covalent interactions that underlie polypeptide folding, are major goals in protein science. One approach toward these goals is to study interactions between selected residues, or among constellations of residues, in small folding motifs. The α-helical coiled coil has served as a platform for such studies because this folding unit is relatively simple in terms of both sequence and structure. Amino acid side chains at the helix-helix interface of a coiled coil participate in so-called ‘knobs-into-holes’ (KIH) packing whereby a side chain (the knob) on one helix inserts into a space (the hole) generated by four side chains on a partner helix. The vast majority of sequence-stability studies on coiled-coil dimers have focused on lateral interactions within these KIH arrangements, for example, between an a position on one helix and an a' position of the partner in a parallel coiled-coil dimer, or between a--d' pairs in an antiparallel dimer. More recently, it has been shown that vertical triads (specifically, a'--a--a' triads) in antiparallel dimers exhibit significant impact on pairing preferences. This observation provides impetus for analysis of other complex networks of side-chain interactions at the helix-helix interface. Here, we describe a combination of experimental and bioinformatics studies that show that d'--d--d' triads have much less impact on pairing preference than do a'--a--a' triads in a small, designed antiparallel coiled-coil dimer. However, the influence of the d'--d--d' triad depends on the lateral at a'--d interaction. Taken together, these results strengthen the emerging understanding that simple pair-wise interactions are not sufficient to describe side-chain interactions and overall stability in antiparallel coiled-coil dimers; higher-order interactions must be considered as well.

[chemical reaction: see text]. Incorporation of hydrophilic tetraarylporphyrin phosphoramidites into the 5'-termimus of the DNA as well as noncharged porphyrin-DNA interactions have been studied. Porphyrin-modified oligonucleotides show lower melting temperatures than their unmodified analogues. Single-stranded DNA interacts more strongly with porphyrin and causes more intense chiral disturbance in the porphyrin environment than the corresponding double strand.

The porphyrin chromophore incorporated at the 5'-position of an oligonucleotide allows the simultaneous detection of the B- to Z-DNA transition via the porphyrin Soret band circular dichroism exciton couplet signal around 420 nm and the oligonucleotide CD region below 300 nm, at micromolar concentrations.

Interactions between polypeptide chains containing amino acid residues with opposite absolute configurations have long been a source of interest and speculation, but there is very little structural information for such heterochiral associations. The need to address this lacuna has grown in recent years because of increasing interest in the use of peptides generated from D amino acids (D peptides) as specific ligands for natural proteins, e.g., to inhibit deleterious proteinprotein interactions. Coiled-coil interactions, between or among α-helices, represent the most common tertiary and quaternary packing motif in proteins. Heterochiral coiled-coil interactions were predicted over 50 years ago by Crick, and limited experimental data obtained in solution suggest that such interactions can indeed occur. To address the dearth of atomic-level structural characterization of heterochiral helix pairings, we report two independent crystal structures that elucidate coiled-coil packing between L-and D-peptide helices. Both structures resulted from racemic crystallization of a peptide corresponding to the transmembrane segment of the influenza M2 protein. Networks of canonical knobs-into-holes side-chain packing interactions are observed at each helical interface. However, the underlying patterns for these heterochiral coiled coils seem to deviate from the heptad sequence repeat that is characteristic of most homochiral analogs, with an apparent preference for a hendecad repeat pattern.D peptides | transmembrane peptides | racemic crystallization | racemic detergent | coiled coil P olypeptides comprising D-amino acid residues have been sources of growing interest for biological applications, often for functions that depend on recognition by specific natural proteins (1-3). D peptides offer identical versatility in terms of conformation and side-chain functionality relative to conventional peptides (composed of L-amino acid residues), but D peptides are impervious to the action of proteolytic enzymes, which should improve pharmacokinetic properties in vivo relative to those of conventional peptides. The engineering of D peptides to display defined protein-binding preferences is hindered, however, by the dearth of experimental information available for such complexes. Structural principles that are well-known to govern interactions between two L-polypeptide chains are not directly extensible to pairings between peptides of opposite chirality. Favorable heterochiral interactions (between L-and D peptides) that are analogous to homochiral associations between L peptides were postulated decades ago on the basis of geometrical considerations (4, 5). In a 1953 analysis of structural parameters governing coiled-coil formation between right-handed α-helices formed from L peptides, for example, Crick suggested that analogous assemblies should be accessible to pairs of right-and left-handed helices (4). In the same year, Pauling and Corey postulated that heterochiral peptide mixtures could form "rippled" β-sheet assemblies with backbo...

Pairing preferences in hetero-dimeric coiled coils are determined by complementarities among side chains that pack against one another at the helix-helix interface. However, relationships between dimer stability and interfacial residue identity are not fully understood. In the context of the “knobs-into-holes” (KIH) packing pattern, one can identify two classes of interactions between side chains from different helices: “lateral”, in which a line connecting the adjacent side chains is perpendicular to the helix axes, and “vertical”, in which the connecting line is parallel to the helix axes. We have previously analyzed vertical interactions in antiparallel coiled coils and found that one type of triad constellation (a’--a--a’) exerts a strong effect on pairing preferences, while the other type of triad (d’--d--d’) has relatively little impact on pairing tendencies. Here, we ask whether vertical interactions (d’--a--d’) influence pairing in parallel coiled-coil dimers. Our results indicate that vertical interactions can exert a substantial impact on pairing specificity, and that the influence of the d’--a--d’ triad depends on the lateral a’ contact within the local KIH motif. Structure-informed bioinformatic analyses of protein sequences reveal trends consistent with the thermodynamic data derived from our experimental model system in suggesting that hetero-triads involving Leu and Ile are preferred over homo-triads involving Leu and Ile.