Bone is a biological composite material comprised primarily of collagen type I and mineral crystals of calcium and phosphate in the form of hydroxyapatite (HA), which together provide its mechanical properties. Bone alkaline phosphatase (ALP), produced by osteoblasts, plays a pivotal role in the mineralization process. Affinity contacts between collagen, mainly type II, and the crown domain of various ALP isozymes were reported in a few in vitro studies in the 1980s and 1990s, but have not attracted much attention since, although such interactions may have important implications for the bone mineralization process. The objective of this study was to investigate the binding properties of human collagen type I to human bone ALP, including the two bone ALP isoforms B1 and B2. ALP from human liver, human placenta and E. coli were also studied. A surface plasmon resonance-based analysis, supported by electrophoresis and blotting, showed that bone ALP binds stronger to collagen type I in comparison with ALPs expressed in non-mineralizing tissues. Further, the B2 isoform binds significantly stronger to collagen type I in comparison with the B1 isoform. Human bone and liver ALP (with identical amino acid composition) displayed pronounced differences in binding, revealing that post-translational glycosylation properties govern these interactions to a large extent. In conclusion, this study presents the first evidence that glycosylation differences in human ALPs are of crucial importance for protein–protein interactions with collagen type I, although the presence of the ALP crown domain may also be necessary. Different binding affinities among the bone ALP isoforms may influence the mineral-collagen interface, mineralization kinetics, and degree of bone matrix mineralization, which are important factors determining the material properties of bone.

Gregarious settlement in barnacle larvae (cyprids) is induced by a contact pheromone, the settlement-inducing protein complex (SIPC). The SIPC has been identified both in the cuticle of adult barnacles and in the temporary adhesive secretion (footprint) of cyprids. Besides acting as a settlement inducer, the presence of the SIPC in footprints points to its additional involvement in the adhesion process. SIPC adsorption behaviour was therefore investigated on a series of self-assembled monolayers (SAMs) by surface plasmon resonance at the pH of seawater (8.3). Fibrinogen and alpha(2)-macroglobulin (A2M) (blood complement protease inhibitors with which the SIPC shares 29% sequence homology) were used in the adsorption experiments as positive and negative standards, respectively. The mass uptake of the SIPC was comparable to that of fibrinogen, with adsorption observed even on the protein-resistant oligo(ethylene glycol) surface. Notably, on the positively charged SAM the SIPC showed a kinetic overshoot, indicating a metastable configuration causing the amount of adsorbed protein to temporarily exceed its equilibrium value. A2M adsorption was low or negligible on all SAMs tested, except for the positively charged surface, indicating that A2M adsorption is mainly driven by electrostatics. Evaluation of SIPC non-specific adsorption kinetics revealed that it adsorbed irreversibly and non-cooperatively on all surfaces tested.

Coiled coils with defined assembly properties and dissociation constants are highly attractive components in synthetic biology and for fabrication of peptide-based hybrid nanomaterials and nanostructures. Complex assemblies based on multiple different peptides typically require orthogonal peptides obtained by negative design. Negative design does not necessarily exclude formation of undesired species and may eventually compromise the stability of the desired coiled coils. This work describe a set of four promiscuous 28-residue de novo designed peptides that heterodimerize and fold into parallel coiled coils. The peptides are non-orthogonal and can form four different heterodimers albeit with large differences in affinities. The peptides display dissociation constants for dimerization spanning from the micromolar to the picomolar range. The significant differences in affinities for dimerization make the peptides prone to thermodynamic social self-sorting as shown by thermal unfolding and fluorescence experiments, and confirmed by simulations. The peptides self-sort with high fidelity to form the two coiled coils with the highest and lowest affinities for heterodimerization. The possibility to exploit self-sorting of mutually complementary peptides could hence be a viable approach to guide the assembly of higher order architectures and a powerful strategy for fabrication of dynamic and tuneable nanostructured materials.

A challenge in the design of plasmonic nanoparticle-based colorimetric assays is that the change in colloidal stability, which generates the colorimetric response, is often directly linked to the biomolecular recognition event. New assay strategies are hence required for every type of substrate and enzyme of interest. Here, a generic strategy for monitoring of phosphatase activity is presented where substrate recognition is completely decoupled from the nanoparticle stability modulation mechanism, which enables detection of a wide range of enzymes using different natural substrates with a single simple detection scheme. Phosphatase activity generates inorganic phosphate that forms an insoluble complex with Zn²⁺. In a sample containing a preset concentration of Zn²⁺, phosphatase activity will markedly reduce the concentration of dissolved Zn²⁺ from the original value, which in turn affects the aggregation of gold nanoparticles functionalized with a designed Zn²⁺ responsive polypeptide. The change in nanoparticle stability thus provides a rapid and sensitive readout of the phosphatase activity. The assay is not limited to a particular enzyme or enzyme substrate, which is demonstrated using three completely different phosphatases and five different substrates, and thus constitutes a highly interesting system for drug screening and diagnostics.

In the context of surface chemistry for affinity biosensor chips, it is widely accepted that uniform orientation of the immobilized recognition element (ligand) is preferred over random orientation. However, this assumption has often been based on studies where differences in ligand immobilization level have not been taken into account. In this contribution, we present a novel two-step method for homogenous orientation and covalent attachment of proteins to sensing surfaces, called Chelation Assisted Photoimmobilization (CAP). Careful quantification of the effect of ligand orientation on analyte responses was performed by comparing this strategy to immobilization by conventional amine coupling.

 In CAP, the chelation agent is nitrilotriacetic acid (NTA) which chelates Ni²⁺. A His-tagged ligand forms an oriented assembly when binding Ni²⁺-NTA and is then covalently bound to the surface via photolabile benzophenone (BP), which attacks C-H bonds upon UV light activation. We relied on a surface chemistry based on self-assembled monolayers (SAMs) of oligo(ethylene glycol) (OEG)-containing alkanethiolates on gold. Alkanethiols terminated with either NTA, BP or OEG were synthesized and mixed SAMs were characterized by infrared reflection absorption spectroscopy (IRAS), ellipsometry and contact angle goniometry. IRAS was also used to quantify ligand immobilization levels obtained either by CAP or by amine coupling via the carboxyl groups of an NTA-presenting surface. The model ligand was human IgG-Fc modified with a C-terminal 6xHis-tag and the analyte was Protein A. The ligand-analyte interaction was quantified by a surface plasmon resonance biosensor.

 Analyte responses were normalized with respect to the ligand amounts obtained by the two immobilization strategies. Interestingly, the normalized analyte response with randomly oriented ligand was >2 times higher than that with ligand immobilized by CAP. This shows that oriented ligand immobilization is not necessarily a means of increasing the sensitivity of a biosensor. Factors that may influence performance include the valency of the ligand and constraints related to the surface chemistry used for orientation.

This report presents a novel method for uniform orientation and covalent attachment of proteins to sensing surfaces, termed Chelation Assisted Photoimmobilization (CAP). Alkanethiols terminated with either nitrilotriacetic acid (NTA), benzophenone (BP) or oligo(ethylene glycol) were synthesized and mixed self-assembled monolayers (SAMs) were prepared on gold and thoroughly characterized by infrared reflection absorption spectroscopy (IRAS), ellipsometry and contact angle goniometry. In the process of CAP, NTA chelates Ni²⁺ and the complex coordinates a His-tagged ligand in an oriented assembly. The ligand is then photoimmobilized via BP, which forms covalent bonds upon UV light activation. The CAP concept was demonstrated using human IgG-Fc modified with C-terminal hexahistidine tags (His-IgGFc) as the ligand and protein A as the analyte.

In the development of affinity biosensors, uniform orientation of ligand molecules where all analyte binding sites are accessible is often preferred to random orientation. In order to monitor the effect of ligand orientation on analyte response, the ligand-analyte interaction was quantified by surface plasmon resonance analysis, both in the case of CAP and when the ligand was attached by conventional amine coupling on surfaces presenting NTA. Responses were adjusted for differences in ligand immobilization level using IRAS. The normalized analyte response with randomly oriented ligand was 2.5 times higher than that with ligand immobilized by CAP, probably due to molecular crowding effects on the surface and the fact that His-IgGFc is bivalent for protein A. This is a reminder that many other factors than orientation alone may play a decisive role in analyte binding on biosensor surfaces.

Bulk and surface refractive index sensitivity for localized surface plasmon resonance (LSPR) sensing based on edge gold-coated silver nanoprisms (GSNPs) and gold nanospheres was investigated and compared with conventional surface plasmon resonance (SPR) sensing based on propagating surface plasmons. The hybrid GSNPs benefit from an improved stability since the gold frame protecting the unstable silver facets located at the silver nanoprisms (SNPs) edges and tips prevents truncation or rounding of their sharp tips or edges, maintaining a high refractive index sensitivity even under harsh conditions. By using layer-by-layer deposition of polyelectrolytes and protein adsorption, we found that GSNPs exhibit 4-fold higher local refractive index sensitivity in close proximity (andlt;10 nm) to the surface compared to a flat gold film in the conventional SPR setup. Moreover, the sensitivity was 8-fold higher with GSNPs than with gold nanospheres. This shows that relatively simple plasmonic nanostructures for LSPR-based sensing can be engineered to outperform conventional SPR, which is particularly interesting in the context of detecting low molecular weight compounds where a small sensing volume, reducing bulk signals, is desired.

A novel strategy for site-specific and covalent attachment of proteins has been developed, intended for robust and controllable immobilization of histidine (His)-tagged ligands in protein microarrays. The method is termed chelation assisted photoimmobilization (CAP) and was demonstrated using human IgG-Fc modified with C-terminal hexahistidines (His-IgGFc) as the ligand and protein A as the analyte. Alkanethiols terminated with either nitrilotriacetic acid (NTA), benzophenone (BP); or oligo(ethylene glycol) were synthesized and mixed self-assembled monolayers (SAMs) were prepared on gold and thoroughly characterized by infrared reflection absorption spectroscopy (IRAS), ellipsometry, and contact angle goniometry. In the process of CAP, NTA chelates Ni2+ and the complex coordinates the His-tagged ligand in an oriented assembly. The ligand is then photoimmobilized via BP, which forms covalent bonds upon UV light activation. In the development of affinity biosensors and protein microarrays, site-specific attachment of ligands in a fashion where analyte binding sites are available is often preferred to random coupling. Analyte binding performance of ligands immobilized either by CAP or by standard amine coupling was characterized by surface plasmon resonance in combination with IRAS. The relative analyte response with randomly coupled ligand was 2.5 times higher than when site-specific attachment was used. This is a reminder that also when immobilizing ligands via residues far from the binding site, there are many other factors influencing availability and activity. Still, CAP provides a valuable expansion of protein immobilization techniques since it offers attractive microarraying possibilities amenable to applications within proteomics.

Rheumatoid factor (RF), i.e. a family of autoantibodies against the Fc part of IgG, is an important seromarker of rheumatoid arthritis (RA). Traditional particle agglutination without disclosing the antibody isotype remains the predominating diagnostic method in clinical routine. Although IgG-RF attracts pathogenic interest, its detection remains technically challenging. The present study aimed at developing a set of tests identifying IgG-RFs directed against the four IgG subclasses. IgG-RF against either subclass of human IgG-Fc were analysed with four novel enzyme-linked immunosorbent assays (ELISAs) utilizing four recombinant human Fc-gamma fragments (hIgG14) as sources of antigen. Sera from 40 patients with recent onset RA (20 seropositive and 20 seronegative by IgM-RF and IgA-RF-isotype-specific ELISA) were analysed. Sera from 20 healthy blood donors served as reference. Among the IgM-/IgA-RF-positive RA-sera, IgG-RF was found directed against hIgG1 and hIgG2, but not against hIgG3 or hIgG4. Significant correlations were seen between IgG-RF against hIgG2-Fc and IgM-RF (r = 0.666) levels. Further prospective studies are warranted to elucidate any correlation to disease course and outcome.

We report the formation of a non-native, folded state of human IgG4-Fc induced by a high temperature at neutral pH and at a physiological salt concentration. This structure is similar to the molten globule state in that it displays a high degree of secondary structure content and surface-exposed hydrophobic residues. However, it is highly resistant to chemical denaturation. The thermally induced state of human IgG4-Fc is thus associated with typical properties of the so-called alternatively folded state previously described for murine IgG, IgG-Fab, and individual antibody domains (V(L), V(H), C(H)1, and C(H)3) under acidic conditions in the presence of anions. Like some of these molecules, human IgG4-Fc in its alternative fold exists as a mixture of different oligomeric structures, dominated by an equilibrium between monomeric and heptameric species. Heating further induces the formation of fibrous structures in the micrometer range.