Antimicrobial efficacy of cyclic α- and β-peptides incorporated in polyurethane coatings

Microbial growth on surfaces poses health concerns and can accelerate the biodegradation of engineered materials and coatings. Cyclic peptides are promising agents to combat biofouling because they are more resistant to enzymatic degradation than their linear counterparts. They can also be designed to interact with extracellular targets and intracellular targets and/or self-assemble into transmembrane pores. Here, we determine the antimicrobial efficacy of two pore-forming cyclic peptides, α-K3W3 and β-K3W3, against bacterial and fungal liquid cultures and their capacity to inhibit biofilm formation on coated surfaces. These peptides display identical sequences, but the additional methylene group in the peptide backbone of β-amino acids results in a larger diameter and an enhancement in the dipole moment. In liquid cultures, β-K3W3 exhibited lower minimum inhibitory concentration values and greater microbicidal power in reducing the number of colony forming units (CFUs) when exposed to a gram-positive bacterium, Staphylococcus aureus, and two fungal strains, Naganishia albida and Papiliotrema laurentii. To evaluate the efficacy against the formation of fungal biofilms on painted surfaces, cyclic peptides were incorporated into polyester-based thermoplastic polyurethane. The formation of N. albida and P. laurentii microcolonies (105 per inoculation) for cells extracted from coatings containing either peptide could not be detected after a 7-day exposure. Moreover, very few CFUs (∼5) formed after 35 days of repeated depositions of freshly cultured P. laurentii every 7 days. In contrast, the number of CFUs for cells extracted from the coating without cyclic peptides was >8 log CFU.

Differential detection of immune cell activation by label-free radiation pressure force

Label-free radiation pressure force analysis using a microfluidic platform is applied to the differential detection of innate immune cell activation. Murine-derived peritoneal macrophages (IC-21) are used as a model system and the activation of IC-21 cells by lipopolysaccharide (LPS) and interferon gamma (IFN-γ) to M1 pro-inflammatory phenotype is confirmed by RNA gene sequencing and nitric oxide production. The mean cell size determined by radiation pressure force analysis increases slightly after the activation (4 to 6%) and the calculated percentage of population overlaps between the control and the activated group after 14 and 24 h stimulations are at 79% and 77%. Meanwhile the mean cell velocity decreases more significantly after the activation (14% to 15%) and the calculated percentage of population overlaps between the control and the activated group after 14 and 24 h stimulations are only at 14% and 13%. The results demonstrate that the majority of the activated cells acquire a lower velocity than the cells from the control group without changes in cell size. For comparison label-free flow cytometry analysis of living IC-21 cells under the same stimulation conditions are performed and the results show population shifts towards larger values in both forward scatter and side scatter, but the calculated percentage of population overlaps in all case are significant (70% to 83%). Cell images obtained during radiation pressure force analysis by a CCD camera, and by optical microscopy and atomic force microscopy (AFM) reveal correlations between the cell activation by LPS/IFN-γ, the increase in cell complexity and surface roughness, and enhanced back scattered light by the activated cells. The unique relationship predicted by Mie's theory between the radiation pressure force exerted on the cell and the angular distribution of the scattered light by the cell which is influenced by its size, complexity, and surface conditions, endows the cell velocity based measurement by radiation pressure force analysis with high sensitivity in differentiating immune cell activation.

Comparison of two diphenyl polyenes as acid-sensitive additives during the biodegradation of a thermoset polyester polyurethane coating

AIMS

Biochemical hydrolysis and chemical catalysis are involved in the successful biodegradation of polymers. In order to evaluate the potential separation between biochemical and chemical catalysis during the biodegradation process, we report the use of two diphenylpolyenes (DPPs), all trans-1,4-diphenylbutadiene (DPB) and all trans-1,6-diphenylhexatriene (DPH), as potential acid-sensitive indicators in polymers.

METHODS AND RESULTS

1,4-Diphenylbutadiene and DPH (0.1% w/w) were melt-cast successfully with poly(ethylene succinate) hexamethylene (PES-HM) polyurethane (thermoset polyester polyurethane) coatings above 80℃. When these two DPP/PES-HM coatings were exposed to a concentrated supernatant with significant esterase activity resulting from the growth of a recently isolated and identified strain of Tremellomycetes yeast (Naganishia albida 5307AI), the DPB coatings exhibited a measurable and reproducible localized decrease in the blue fluorescence emission in regions below where hydrolytic biodegradation was initiated in contrast with DPH blended coatings. The fluorescence changes observed in the biodegraded DPB coating were similar to exposing them to concentrated acids and not bases.

CONCLUSIONS

Our experiments resulted in (1) a method to blend DPP additives into thermoset coatings, (2) the first report of the biodegradation of polyester polyurethane coating by N. albida, and (3) demonstration that hydrolytic supernatants from this strain generate acidic region within degrading polyester coatings using DPB as the indicator.

SIGNIFICANCE AND IMPACT OF THE STUDY

Our experiments confirm that N. albida is an active polyester degrader and that DPB is a promising acid sensitive polymer coating additive.

Stabilization of reduced copper on ceria aerogels for CO oxidation

Photodeposition of Cu nanoparticles on ceria (CeO₂) aerogels generates a high surface area composite material with sufficient metallic Cu to exhibit an air-stable surface plasmon resonance. We show that balancing the surface area of the aerogel support with the Cu weight loading is a critical factor in retaining stable Cu⁰. At higher Cu weight loadings or with a lower support surface area, Cu aggregation is observed by scanning and transmission electron microscopy. Analysis of Cu/CeO₂ using X-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy finds a mixture of Cu²⁺, Cu⁺, and Cu⁰, with Cu⁺ at the surface. At 5 wt% Cu, Cu/CeO₂ aerogels exhibit high activity for heterogeneous CO oxidation catalysis at low temperatures (94% conversion of CO at 150 °C), substantially out-performing Cu/TiO₂ aerogel catalysts featuring the same weight loading of Cu on TiO₂ (20% conversion of CO at 150 °C). The present study demonstrates an extension of our previous concept of stabilizing catalytic Cu nanoparticles in low oxidation states on reducing, high surface area aerogel supports. Changing the reducing power of the support modulates the catalytic activity of mixed-valent Cu nanoparticles and metal oxide support.

Edge-Localized Biodeterioration and Secondary Microplastic Formation by Papiliotrema laurentii Unsaturated Biofilm Cells on Polyurethane Films

Painted environmental surfaces are prone to microbiological colonization with potential coating deterioration induced by the microorganisms. Accurate mechanistic models of these interactions require an understanding of the heterogeneity in which the deterioration processes proceed. Here, unsaturated biofilms (i.e., at air/solid interfaces) of the yeast Papiliotrema laurentii were prepared on polyether polyurethane (PEUR) and polyester-polyether polyurethane (PEST-PEUR) coatings and incubated for up to 33 days at controlled temperature and humidity with no additional nutrients. Transmission micro-Fourier transform infrared microscopy (μFTIR) confirmed preferential hydrolysis of the ester component by the biofilm. Atomic force microscopy combined with infrared nanospectroscopy (AFM-IR) was used to analyze initial PEST-PEUR coating deterioration processes at the single-cell level, including underlying surfaces that became exposed following cell translocation. The results revealed distinct deterioration features that remained localized within ∼10 μm or less of the edges of individual cells and cell clusters. These features comprised depressions of up to ∼300 nm with locally reduced ester/urethane ratios. They are consistent with a formation process initiated by enzymatic ester hydrolysis followed by erosion from water condensation cycles. Further observations included particle accumulation in the broader biofilm vicinity. AFM-IR spectroscopy indicated these to be secondary microplastics consisting of urethane-rich oligomeric aggregates. Overall, multiple contributing factors have been identified that can facilitate differential deterioration rates across the PEST-PEUR surface. Effects of the imposed nutrient conditions on Papiliotrema laurentii physiology were also apparent, with cells developing the characteristics of starvation response, despite the availability of polyester metabolites as a carbon source. The combined results provide new laboratory insights into field-relevant microbiological polymer deterioration mechanisms and biofilm physiology at polymer coating interfaces.

Flow-Through Optical Chromatography in Combination with Confocal Raman Microspectroscopy: A Novel Label-Free Approach To Detect Responses of Live Macrophages to Environmental Stimuli

Flow-through optical chromatography (FT-OC), an advanced mode of optical chromatography, achieved baseline separation of a mixture of silica microparticles (SiO₂, 1.00 and 2.50 μm) and a mixture of polystyrene microparticles (PS, 1.00, 2.00, and 3.00 μm) based on particle size. Comparisons made between experimentally determined velocities for the microparticles and theoretically derived velocities from Mie theory and Stokes' law validated the data collection setup and the data analysis for FT-OC. A population shift in live macrophages (cell line IC-21, ATCC TIB-186) responding to environmental stimuli was sensitively detected by FT-OC. The average velocity of macrophages stressed by nutritional deprivation was decreased considerably together with a small but statistically significant increase in cell size. Mie scattering calculations demonstrated that the small increase in cell size of macrophages stressed by nutritional deprivation was not entirely responsible for this decrease. Confocal fluorescence microscopy and atomic force microscopy (AFM) studies revealed morphological changes of macrophages induced by nutritional deprivation, and these changes were more likely responsible for the decrease in average velocity detected by FT-OC. Confocal Raman microspectroscopy was used to shed light upon biochemical transformations of macrophages suffering from nutritional deprivation.

Carbon Catabolite Repression and Impranil Polyurethane Degradation in Pseudomonas protegens Strain Pf-5

Polyester polyurethane (PU) coatings are widely used to help protect underlying structural surfaces but are susceptible to biological degradation. PUs are susceptible to degradation by Pseudomonas species, due in part to the degradative activity of secreted hydrolytic enzymes. Microorganisms often respond to environmental cues by secreting enzymes or secondary metabolites to benefit their survival. This study investigated the impact of exposing several Pseudomonas strains to select carbon sources on the degradation of the colloidal polyester polyurethane Impranil DLN (Impranil). The prototypic Pseudomonas protegens strain Pf-5 exhibited Impranil-degrading activities when grown in sodium citrate but not in glucose-containing medium. Glucose also inhibited the induction of Impranil-degrading activity by citrate-fed Pf-5 in a dose-dependent manner. Biochemical and mutational analyses identified two extracellular lipases present in the Pf-5 culture supernatant (PueA and PueB) that were involved in degradation of Impranil. Deletion of the pueA gene reduced Impranil-clearing activities, while pueB deletion exhibited little effect. Removal of both genes was necessary to stop degradation of the polyurethane. Bioinformatic analysis showed that putative Cbr/Hfq/Crc-mediated regulatory elements were present in the intergenic sequences upstream of both pueA and pueB genes. Our results confirmed that both PueA and PueB extracellular enzymes act in concert to degrade Impranil. Furthermore, our data showed that carbon sources in the growth medium directly affected the levels of Impranil-degrading activity but that carbon source effects varied among Pseudomonas strains. This study uncovered an intricate and complicated regulation of P. protegens PU degradation activity controlled by carbon catabolite repression.

IMPORTANCE

Polyurethane (PU) coatings are commonly used to protect metals from corrosion. Microbiologically induced PU degradation might pose a substantial problem for the integrity of these coatings. Microorganisms from diverse genera, including pseudomonads, possess the ability to degrade PUs via various means. This work identified two extracellular lipases, PueA and PueB, secreted by P. protegens strain Pf-5, to be responsible for the degradation of a colloidal polyester PU, Impranil. This study also revealed that the expression of the degradative activity by strain Pf-5 is controlled by glucose carbon catabolite repression. Furthermore, this study showed that the Impranil-degrading activity of many other Pseudomonas strains could be influenced by different carbon sources. This work shed light on the carbon source regulation of PU degradation activity among pseudomonads and identified the polyurethane lipases in P. protegens.

Imaging Active Surface Processes in Barnacle Adhesive Interfaces

Surface plasmon resonance imaging (SPRI) and voltammetry were used simultaneously to monitor Amphibalanus (=Balanus) amphitrite barnacles reattached and grown on gold-coated glass slides in artificial seawater. Upon reattachment, SPRI revealed rapid surface adsorption of material with a higher refractive index than seawater at the barnacle/gold interface. Over longer time periods, SPRI also revealed secretory activity around the perimeter of the barnacle along the seawater/gold interface extending many millimeters beyond the barnacle and varying in shape and region with time. Ex situ experiments using attenuated total reflectance infrared (ATR-IR) spectroscopy confirmed that reattachment of barnacles was accompanied by adsorption of protein to surfaces on similar time scales as those in the SPRI experiments. Barnacles were grown through multiple molting cycles. While the initial reattachment region remained largely unchanged, SPRI revealed the formation of sets of paired concentric rings having alternately darker/lighter appearance (corresponding to lower and higher refractive indices, respectively) at the barnacle/gold interface beneath the region of new growth. Ex situ experiments coupling the SPRI imaging with optical and FTIR microscopy revealed that the paired rings coincide with molt cycles, with the brighter rings associated with regions enriched in amide moieties. The brighter rings were located just beyond orifices of cement ducts, consistent with delivery of amide-rich chemistry from the ducts. The darker rings were associated with newly expanded cuticle. In situ voltammetry using the SPRI gold substrate as the working electrode revealed presence of redox active compounds (oxidation potential approx 0.2 V vs Ag/AgCl) after barnacles were reattached on surfaces. Redox activity persisted during the reattachment period. The results reveal surface adsorption processes coupled to the complex secretory and chemical activity under barnacles as they construct their adhesive interfaces.

The importance of correcting for variable probe–sample interactions in AFM-IR spectroscopy: AFM-IR of dried bacteria on a polyurethane film

Interplay between AFM-IR probe – sample interactions and signal transduction for bacteria – PU bilayer.

Growth and development of the barnacle Amphibalanus amphitrite: time and spatially resolved structure and chemistry of the base plate

The radial growth and advancement of the adhesive interface to the substratum of many species of acorn barnacles occurs underwater and beneath an opaque, calcified shell. Here, the time-dependent growth processes involving various autofluorescent materials within the interface of live barnacles are imaged for the first time using 3D time-lapse confocal microscopy. Key features of the interface development in the striped barnacle, Amphibalanus (= Balanus) amphitrite were resolved in situ and include advancement of the barnacle/substratum interface, epicuticle membrane development, protein secretion, and calcification. Microscopic and spectroscopic techniques provide ex situ material identification of regions imaged by confocal microscopy. In situ and ex situ analysis of the interface support the hypothesis that barnacle interface development is a complex process coupling sequential, timed secretory events and morphological changes. This results in a multi-layered interface that concomitantly fulfills the roles of strongly adhering to a substratum while permitting continuous molting and radial growth at the periphery.

The impact of culture medium on the development and physiology of biofilms of Pseudomonas fluorescens formed on polyurethane paint

Microbial biofilms cause the deterioration of polymeric coatings such as polyurethanes (PUs). In many cases, microbes have been shown to use the PU as a nutrient source. The interaction between biofilms and nutritive substrata is complex, since both the medium and the substratum can provide nutrients that affect biofilm formation and biodeterioration. Historically, studies of PU biodeterioration have monitored the planktonic cells in the medium surrounding the material, not the biofilm. This study monitored planktonic and biofilm cell counts, and biofilm morphology, in long-term growth experiments conducted with Pseudomonas fluorescens under different nutrient conditions. Nutrients affected planktonic and biofilm cell numbers differently, and neither was representative of the system as a whole. Microscopic examination of the biofilm revealed the presence of intracellular storage granules in biofilms grown in M9 but not yeast extract salts medium. These granules are indicative of nutrient limitation and/or entry into stationary phase, which may impact the biodegradative capability of the biofilm.

Barnacle Balanus amphitrite adheres by a stepwise cementing process

Barnacles adhere permanently to surfaces by secreting and curing a thin interfacial adhesive underwater. Here, we show that the acorn barnacle Balanus amphitrite adheres by a two-step fluid secretion process, both contributing to adhesion. We found that, as barnacles grow, the first barnacle cement secretion (BCS1) is released at the periphery of the expanding base plate. Subsequently, a second, autofluorescent fluid (BCS2) is released. We show that secretion of BCS2 into the interface results, on average, in a 2-fold increase in adhesive strength over adhesion by BCS1 alone. The two secretions are distinguishable both spatially and temporally, and differ in morphology, protein conformation, and chemical functionality. The short time window for BCS2 secretion relative to the overall area increase demonstrates that it has a disproportionate, surprisingly powerful, impact on adhesion. The dramatic change in adhesion occurs without measurable changes in interface thickness and total protein content. A fracture mechanics analysis suggests the interfacial material's modulus or work of adhesion, or both, were substantially increased after BCS2 secretion. Addition of BCS2 into the interface generates highly networked amyloid-like fibrils and enhanced phenolic content. Both intertwined fibers and phenolic chemistries may contribute to mechanical stability of the interface through physically or chemically anchoring interface proteins to the substrate and intermolecular interactions. Our experiments point to the need to reexamine the role of phenolic components in barnacle adhesion, long discounted despite their prevalence in structural membranes of arthropods and crustaceans, as they may contribute to chemical processes that strengthen adhesion through intermolecular cross-linking.

High-density amine-terminated monolayers formed on fluorinated CVD-grown graphene

There has been considerable interest in chemically functionalizing graphene films to control their electronic properties, to enhance their binding to other molecules for sensing, and to strengthen their interfaces with matrices in a composite material. Most reports to date have largely focused on noncovalent methods or the use of graphene oxide. Here, we present a method to activate CVD-grown graphene sheets using fluorination followed by reaction with ethylenediamine (EDA) to form covalent bonds. Reacted graphene was characterized via X-ray photoelectron spectroscopy (XPS), attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), and Raman spectroscopy as well as measurements of electrical properties. The functionalization results in stable, densely packed layers, and the unbound amine of EDA was shown to be active toward subsequent chemical reactions.

Optical spectroscopy of marine bioadhesive interfaces

Marine organisms have evolved extraordinarily effective adhesives that cure underwater and resist degradation. These underwater adhesives differ dramatically in structure and function and are composed of multiple proteins assembled into functional composites. The processes by which these bioadhesives cure--conformational changes, dehydration, polymerization, and cross-linking--are challenging to quantify because they occur not only underwater but also in a buried interface between the substrate and the organism. In this review, we highlight interfacial optical spectroscopy approaches that can reveal the biochemical processes and structure of marine bioadhesives, with particular emphasis on macrofoulers such as barnacles and mussels.

Characterization of the adhesive plaque of the barnacle Balanus amphitrite: amyloid-like nanofibrils are a major component

The nanoscale morphology and protein secondary structure of barnacle adhesive plaques were characterized using atomic force microscopy (AFM), far-UV circular dichroism (CD) spectroscopy, transmission Fourier transform infrared (FTIR) spectroscopy, and Thioflavin T (ThT) staining. Both primary cement (original cement laid down by the barnacle) and secondary cement (cement used for reattachment) from the barnacle Balanus amphitrite (= Amphibalanus amphitrite) were analyzed. Results showed that both cements consisted largely of nanofibrillar matrices having similar composition. Of particular significance, the combined results indicate that the nanofibrillar structures are consistent with amyloid, with globular protein components also identified in the cement. Potential properties, functions, and formation mechanisms of the amyloid-like nanofibrils within the adhesive interface are discussed. Our results highlight an emerging trend in structural biology showing that amyloid, historically associated with disease, also has functional roles.

In situ ATR-FTIR characterization of primary cement interfaces of the barnacle Balanus amphitrite

A method is presented for characterizing primary cement interfaces of barnacles using in situ attenuated total reflection-Fourier transform infrared spectroscopy. Primary cement of the barnacle, Balanus amphitrite (Amphibalanus amphitrite), was characterized without any disruption to the original cement interface, after settling and growing barnacles directly on double sided polished germanium wafers. High-quality IR spectra were acquired of live barnacle cement interfaces, providing a spectroscopic fingerprint of cured primary cement in vivo with the barnacle adhered to the substratum. Additional spectra were also acquired of intact cement interfaces for which the upper portion of the barnacle had been removed leaving only the base plate and cement layer attached to the substratum. This allowed further characterization of primary cement interfaces that were dried or placed in D(2)O. The resulting spectra were consistent with the cement being proteinaceous, and allowed analysis of the protein secondary structure and water content in the cement layer. The estimated secondary structure composition was primarily beta-sheet, with additional alpha-helix, turn and unordered components. The cement of live barnacles, freshly removed from seawater, was estimated to have a water content of 20-50% by weight. These results provide new insights into the chemical properties of the undisturbed barnacle adhesive interface.

The assembly of single-layer graphene oxide and graphene using molecular templates

Charged molecular templates were created to direct the placement of single-layer graphite oxide (GO) sheets. The distribution of the GO sheets depended on the surface functionalization, background passivation, pH, and deposition time. Electrostatic attraction guides the templating of the GO sheets and, consequently, templating could be modulated by adjusting the pH of the deposition solution. In contrast to CNT immobilization, we find that the GO sheets do not adhere to the bare Au surface.

Semiconductor surface-induced 1,3-hydrogen shift: the role of covalent vs zwitterionic character

X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) are used to compare the reaction of 1,2-cyclohexanedione (1,2-CHD) with Si(001) and diamond(001) surface dimers under ultra-high-vacuum conditions. 1,2-CHD is known to undergo a keto-enol tautomerization, with the monoenol being the primary equilibrium species in the solid and gas phases. XPS and FTIR data demonstrate that 1,2-CHD reacts with diamond(001) through the OH group of the monoenol, resulting in only one O atom being bonded to the surface. In contrast, XPS and FTIR data suggest that both oxygen atoms in the 1,2-CHD molecule bond via Si-O-C linkages to the Si(001) surface dimer, and that the molecule undergoes an intramolecular 1,3-H shift. While the Si(001) and diamond(001) surfaces are both comprised of surface dimers, the diamond(001) dimer is symmetric, with little charge separation, whereas the Si(001) dimer is tilted and exhibits zwitterionic character. The different reaction products that are observed when clean Si(001) and diamond(001) surfaces are exposed to 1,2-CHD demonstrate the importance of charge separation in promoting a 1,3-H shift and provide new mechanistic insights that may be applicable to a variety of organic reactions.

Chemical Structure and Orientation of Ethylene on Si(114)−(2×1)/c(2×2)

The basic chemical structure and orientation of ethylene chemisorbed on Si(114)-(2 x 1) at submonolayer coverage is characterized in ultrahigh vacuum using transmission Fourier transform infrared (FTIR) spectroscopy. The spectra are consistent with di-sigma bonding of ethylene to the surface with a preferential molecular orientation over macroscopic lengths. These results are supported by density functional theory (DFT) calculations of vibrational frequencies for optimized ethylene-Si(114) structures occupying the dimer and rebonded atom surface sites. A detailed analysis of the strong angular and polarization dependence of the C-H stretching mode intensities is also consistent with the adsorption structures identified by DFT, indicating that ethylene chemisorbs with the C-C bond axis parallel to the structural rows oriented along the [10] direction on the Si(114)-(2 x 1) surface. The results indicate that the unique structure of this surface makes it an excellent template for elucidating relationships between surface structure and organic reaction mechanisms on silicon.

Adsorption of Acrylonitrile on Diamond and Silicon (001)−(2 × 1) Surfaces: Effects of Dimer Structure on Reaction Pathways and Product Distributions

Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) are used to compare the reaction of acrylonitrile with Si(001) and C(001) (diamond) surfaces. Our results show that reaction with Si(001) and C(001) yield very different product distributions that result from fundamental differences in the ionic character of these surfaces. While acrylonitrile reacts with the C(001) surface via a [2 + 2] cycloaddition reaction in a manner similar to nonpolar molecules such as alkenes and disilenes, reaction with the Si(001) surface occurs largely through the nitrile group. This work represents the first experimental example of how differences in dimer structure lead to very different chemistry for C(001) compared to that for Si(001). The fact that Si(001) reacts with the strongly polar nitrile group of acrylonitrile indicates that the zwitterionic character of this surface controls its reactivity. C(001) dimers, on the other hand, behave more like a true molecular double bond, albeit a highly strained one. Consequently, while alternative strategies will be necessary for chemical modification of Si(001), traditional schemes from organic chemistry for functionalization of alkenes and disilenes may be available for building molecular layers on C(001).

Scanning tunneling microscopy, orbital-mediated tunneling spectroscopy, and ultraviolet photoelectron spectroscopy of metal(II) tetraphenylporphyrins deposited from vapor

Thin films of vapor-deposited Ni(II) and Co(II) complexes of tetraphenylporphyrin (NiTPP and CoTPP) were studied supported on gold and embedded in Al-Al(2)O(3)-MTPP-Pb tunnel diodes, where M = Ni or Co. Thin films deposited onto polycrystalline gold were analyzed by ultraviolet photoelectron spectroscopy (UPS) using He I radiation. Scanning tunneling microscopy (STM) and orbital-mediated tunneling spectroscopy (STM-OMTS) were performed on submonolayer films of CoTPP and NiTPP supported on Au(111). Inelastic electron tunneling spectroscopy (IETS) and OMTS were measured in conventional tunnel diode structures. The highest occupied pi molecular orbital of the porphyrin ring was seen in both STM-OMTS and UPS at about 6.4 eV below the vacuum level. The lowest unoccupied pi molecular orbital of the porphyrin ring was observed by STM-OMTS and by IETS-OMTS to be located near 3.4 eV below the vacuum level. The OMTS spectra of CoTPP had a band near 5.2 eV (below the vacuum level) that was attributed to transient oxidation of the central Co(II) ion. That is, it is due to electron OMT via the half-filled d(z)(2) orbital present in Co(II) of CoTPP. The NiTPP OMTS spectra show no such band, consistent with the known difficulty of oxidation of the Ni(II) ion. The STM-based OMTS allowed these two porphyrin complexes to be easily distinguished. The present work is the first report of the observation of STM-OMTS, tunnel junction OMTS, and UPS of the same compounds. Scanning tunneling microscope-based orbital-mediated tunneling provides more information than UPS or tunnel junction-based OMTS and does so with molecular-scale resolution.