Sites For Isomerization Research Articles

Revealing Unpredicted Aspartic Acid Isomerization Hotspots by Probing Diagnostic Fragmentation Propensities in Top-Down and Middle-Down Mass Spectrometry.

Isomerization of aspartic acid residues is a relevant degradation pathway of protein biopharmaceuticals as it can impair their biological activity. However, the in silico prediction of isomerization hotspots and their consequences remains ambiguous and misleading. We have previously shown that all ion differential analysis (AiDA) of middle-down spectra can be used to reveal diagnostic terminal and internal fragments with more sensitivity than the conventional fragment ion mass matching methodology. In this study, we use AiDA to characterize the degradation of an antibody fragment at three aspartic acid isomerization sites including a novel DW motif directly with electron-transfer/higher-energy collisional dissociation top-down and middle-down mass spectrometry. We show that AiDA methodology is pivotal to probe diagnostic fragmentation propensities of terminal c and z fragments at the N-terminus and vicinity of isomerization sites in addition to the diagnostic c+57 terminal fragments. Furthermore, AiDA can probe remote structural changes in the loop of an antibody complementarity-determining region induced by isomerization and the succinimide intermediate, revealing interactions between residues in agreement with molecular simulations. This study shows that aspartic acid residues at noncanonical DW and DF motifs can be hotspots for isomerization despite being ranked as false positives in physics-based prediction models. We show that the enzyme-free, fast, and sensitive AiDA methodology can be used as an orthogonal technique to fractionation for online variant characterization.

Facile Synthesis of Silanol Group Rich Sn‐Beta for Highly Efficient Isomerization of Glucose to Fructose

AbstractThe isomerization of glucose to fructose is an important process in the conversion of biomass into valuable fuels and chemicals. Development of efficient catalyst for this reaction has attracted much attention. In this study, Sn‐Beta zeolites with rich silanols were synthesized for glucose isomerization by solid‐state ion‐exchange method, where Si‐Beta zeolites with different amounts of silanols were applied as the precursors. It is confirmed that open Sn sites show much higher catalytic performance than closed Sn sites for glucose isomerization (about 2.1 times). The silanol amount in Si‐Beta not only affects the formation of framework Sn sites in Sn‐Beta but also influences the isomerization reaction. More silanols in the catalyst can promote the 1,2‐H shift in isomerization and reduce the side reactions of formation of mannose and C3 products. Over 3Sn‐Beta‐3h with rich silanols, 64 % of fructose yield was obtained. Furthermore, the prepared Sn‐Beta catalyst is stable and recyclable. High fructose yield and facile synthesis method of Sn‐Beta reported in this work will contribute to the development of biomass and sugar industry.

Localizing Isomerized Residue Sites in Peptides with Tandem Mass Spectrometry.

Isomerized amino acid residues have been identified in many peptides extracted from tissues or excretions of humans and animals. These isomerized residues can play key roles by affecting biological activity or by exerting an influence on the process of aging. Isomerization occurs spontaneously and does not result in a mass shift. Thus, identifying and localizing isomerized residues in biological samples is challenging. Herein, we introduce a fast and efficient method using tandem mass spectrometry (MS) to locate isomerized residues in peptides. Although MS2 spectra are useful for identifying peptides that contain an isomerized residue, they cannot reliably localize isomerization sites. We show that this limitation can be overcome by utilizing MS3 experiments to further evaluate each fragment ion from the MS2 stage. Comparison at the MS3 level, utilizing statistical analyses, reveals which MS2 fragments differ between samples and, therefore, must contain the isomerized sites. The approach is similar to previous work relying on ion mobility to discriminate MS2 product ions by collision cross-section. The MS3 approach can be implemented using either ion-trap or beam-type collisional activation and is compatible with the quantification of isomer mixtures when coupled to a calibration curve. The method can also be implemented in combination with liquid chromatography in a targeted approach. Enabling the identification and localization of isomerized residues in peptides with an MS-only methodology will expand accessibility to this important information.

An in-situ combo Mo-based ionic liquid and SAPO-11 catalyst for efficient biolipids hydrodeoxygenation and isomerization

The design of novel practical catalysts is critical for the blossoming of biodiesel green-energy, in which Mo-based catalysts are of particular interest. In this work, an in-situ synthesis method was proposed to obtain a combo Mo-based ionic liquid and SAPO-11 catalyst. The catalyst presents both highly dispersed MoS2 as hydrodeoxygenation active sites and acidic sites for isomerization. MoS2 with a few fine layers can be identified based on TEM images and the Brønsted acid sites are dominant based on Pyridine-IR analysis. A great performance of MoS2/C8min-15 %SA can be obtained under optimized reaction conditions with 100 % conversion of methyl palmitate, 75.0 mol% hydrodeoxygenation to n-C15-C16, and 20.5 mol % isomerization to i-C15-C16. The structure and activity of the catalyst can be retained at least for 4 cycles. Thin layers and high sulfurization, large amount of Brønsted acid sites, and suitable reaction conditions are key points to achieve a significant performance and avoid over cracking and deactivation. On basis of detected intermediates and kinetic data simulations, a complex reaction pathway was proposed and the specific k values for each elementary reaction were provided for the hydrodeoxygenation and isomerization of methyl palmitate.

Catalytic pyrolysis of harmful plastic waste to alleviate environmental impacts

Wax is a detrimental byproduct of plastic waste pyrolysis causing challenges upon its release into the environment owing to persistence and potential toxicity. In this study, the valorization of wax materials through conversion into BTEX (i.e., benzene, toluene, ethylbenzene, and xylene) was achieved via catalytic pyrolysis using zeolite-based catalysts. The potential of two types of waxes, spent wax (SW), derived from the pyrolysis of plastic waste, and commercial paraffin wax (PW), for BTEX generation, was investigated. Using HZSM-5, higher yields of oil (54.9 wt%) and BTEX (18.2 wt%) were produced from the pyrolysis of SW compared to PW (32.3 and 14.1 wt%, respectively). This is due to the improved accessibility of lighter hydrocarbons in SW to Brønsted and Lewis acid sites in HZSM-5 micropores, promoting cracking, isomerization, cyclization, Diels-Alder, and dehydrogenation reactions. Further, the use of HZSM-5 resulted in significantly larger yields of oil and BTEX from SW pyrolysis compared to Hbeta and HY. This phenomenon is ascribed to the well-balanced distribution of Brønsted and Lewis acid sites and the identical geometric structure of HZSM-5 micropores and BTEX molecules. The addition of Ga to HZSM-5 further led to 2.24% and 28.30% enhancements in oil and BTEX yields, respectively, by adjusting the acidity of the catalyst through the introduction of new Lewis acid sites. The regeneration of the Ga/HZSM-5 catalyst by removing deposited coke on the spent catalyst under air partially recovered catalytic activity. This study not only offers an efficient transformation of undesirable wax into valuable fuels but also provides an environmentally promising solution, mitigating pollution, contributing to carbon capture, and promoting a healthier and more sustainable environment. It also suggests future research directions, including catalyst optimization and deactivation management, feedstock variability exploration, and techno-economic analyses for sustainable wax conversion into BTEX via catalytic pyrolysis.

Competitive double bond isomerization and competitive esterification of C8 to C18 linear 1-alkenes on amberlyst®15 catalyst: Calculation of relative adsorption coefficients in two concurrent reactions

Relative initial reaction rates for both double bond isomerization and direct esterification with heptanoic acid were determined for the series of linear 1-alkenes from 1-octene to 1-octadecene. In competitive reactions between two 1-alkenes on Amberlyst®15 catalyst at 90 °C, rates for both double bond isomerization and esterification decreased with increasing carbon chain length. Data suggested each 1-alkene had two different adsorption coefficients on the catalyst, one for an isomerization site and one for a site for the addition reaction. Results were consistent with previous data suggesting that Amberlyst®15 catalyst has two different types of active sites.

Influence of Asp Isomerization on Trypsin and Trypsin-like Proteolysis.

Long-lived proteins (LLPs), although less common than their short-lived counterparts, are increasingly recognized to play important roles in age-related diseases such as Alzheimer's. In particular, spontaneous chemical modifications can accrue over time that serve as both indicators of and contributors to disrupted autophagy. For example, isomerization in LLPs is common and occurs in the absence of protein turnover while simultaneously interfering with the protein turnover by impeding proteolysis. In addition to the biological implications this creates, isomerization may also interfere with its own analysis. To clarify, bottom-up proteomics experiments rely on protein digestion by proteases, most commonly trypsin, but the extent to which isomerization might interfere with trypsin digestion is unknown. Here, we use a combination of liquid chromatography and mass spectrometry to examine the effect of isomerization on proteolysis by trypsin and chymotrypsin. Isomerized aspartic acid and serine residues (which represent the most common sites of isomerization in LLPs) were placed at various locations relative to the preferred protease cleavage point to evaluate the influence on digestion efficiency. Trypsin was found to be relatively tolerant of isomerization, except when present at the residue immediately C-terminal to Arg/Lys. For chymotrypsin, the influence of isomerization on digestion was less predictable, resulting in long-range interference for some isomer/peptide combinations. Given the trypsin- and chymotrypsin-like behaviors of the 20S proteasome, and to further establish the biological relevance of isomerization in LLPs, substrates with isomerized sites were also tested against proteasomal degradation. Significant disruption of 20S proteolysis was observed, suggesting that if LLPs persist long enough to isomerize, it will be difficult for the cells to digest them.

Relay photo/thermal catalysis enables efficient cascade upgrading of sugars to lactic acid: Mechanism study and life cycle assessment

Chemocatalytic conversion of biomass-based saccharides to lactic acid (LA) is one of the prospective routes for biomass valorization, but generally suffers from harsh reaction conditions with relatively low selectivity toward CC bond cleavage. Herein, a novel ternary heterostructure g-C3N4/N-TiO2/NiFe-LDH (NTCN/LDH) photocatalyst prepared by a simple wet-chemical method was demonstrated to be high-performance for cascade upgrading of various biomass sugars to LA (up to 99.0 % yield) in H2O at 60 °C under visible-light irradiation for a short time (≥15 min). A relay photo/thermal catalytic mechanism was mainly responsible for the superior performance of NTCN/LDH in LA production, as elucidated by DFT calculations, in which NTCN/LDH provided suitable Lewis acid sites (i.e., adsorption sites) for thermocatalytic sugar isomerization (e.g., glucose-to-fructose conversion), while the constructed double type-II heterojunction could regulate the type and count of the formed reactive oxygen species (dominantly ·O2−) to achieve precise photocatalytic C3-C4 bond breaking. The scale-up test could still maintain 92 % LA yield of the first-time experiment, indicating the eximious industrialization potential of the versatile NTCN/LDH catalyst. In addition, the life cycle assessment illustrated that the entire system has less impact on the environment than the current industrial LA production process.

Eco-friendly preparation of phosphated gallia: A tunable dual-acidic catalyst for the efficient 5-hydroxymethylfurfural production from carbohydrates

To develop an efficient catalyst/process for the production of 5-hydroxymethylfurfural (HMF), bifunctional phosphated gallia (GaP) catalysts were prepared by solid-state grinding, an eco-friendly method, and applied for glucose-to-HMF transformation. Strong Lewis (L) acid sites for isomerization of glucose-to-fructose were introduced by unsaturated Ga3+, while Bronsted (B) acid sites for dehydration of fructose-to-HMF were originated from P-OH group. The glucose-to-HMF process was optimized by response surface methodology to maximize the HMF yield. Complete glucose conversion and 65.72 ± 2.3% HMF yield were obtained at 175 °C, 2.75 h in THF/H2O biphasic-system. This catalytic performance can be attributed to the high surface area/porosity, sufficient total acidity, and well-balanced B/L acid sites of the GaP. Reusability/stability of the catalyst was demonstrated in 4 consecutive experiments. This catalyst also proved to be active in transforming other carbohydrates to HMF, obtaining ∼ 57%, ∼ 50%, and ∼ 31% yield with sucrose, starch, and cellulose, respectively.

Toward Rapid Aspartic Acid Isomer Localization in Therapeutic Peptides Using Cyclic Ion Mobility Mass Spectrometry

There is an increasing emphasis on the critical evaluation of interbatch purity and physical stability of therapeutic peptides. This is due to concerns over the impact that product- and process-related impurities may have on safety and efficacy of this class of drug. Aspartic acid isomerization to isoaspartic acid is a common isobaric impurity that can be very difficult to identify without first synthesizing isoAsp peptide standards for comparison by chromatography. As such, analytical tools that can determine if an Asp residue has isomerized, as well as the site of isomerization within the peptide sequence, are highly sought after. Ion mobility-mass spectrometry is a conformation-selective method that has developed rapidly in recent years particularly with the commercialization of traveling wave ion mobility instruments. This study employed a cyclic ion mobility (cIMS) mass spectrometry system to investigate the conformational characteristics of a therapeutic peptide and three synthetic isomeric forms, each with a single Asp residue isomerized to isoAsp. cIMS was able to not only show distinct conformational differences between each peptide but crucially, in conjunction with a simple workflow for comparing ion mobility data, it correctly located which Asp residue in each peptide had isomerized to isoAsp. This work highlights the value of cIMS as a potential screening tool in the analysis of therapeutic peptides prone to the formation of isoAsp impurities.

PIMT-Mediated Labeling of l-Isoaspartic Acid with Tris Facilitates Identification of Isomerization Sites in Long-Lived Proteins.

Isomerization of individual residues in long-lived proteins (LLPs) is a subject of growing interest in connection with many age-related human diseases. When isomerization occurs in LLPs, it can lead to deleterious changes in protein structure, function, and proteolytic degradation. Herein, we present a novel labeling technique for rapid identification of l-isoAsp using the enzyme protein l-isoaspartyl methyltransferase (PIMT) and Tris. The succinimide intermediate formed during reaction of l-isoAsp-containing peptides with PIMT and S-adenosyl methionine (SAM) is reactive with Tris base and results in a Tris-modified aspartic acid residue with a mass shift of +103 Da. Tris-modified aspartic acid exhibits prominent and repeated neutral loss of water when subjected to collisional activation. In addition, another dissociation pathway regenerates the original peptide following loss of a characteristic mass shift. Furthermore, it is demonstrated that Tris modification can be used to identify sites of isomerization in LLPs from biological samples such as the lens of the eye. This approach simplifies identification by labeling isomerization sites with a tag that causes a mass shift and provides characteristic loss during collisional activation.

On the Role of the Conserved Histidine at the Chromophore Isomerization Site in Phytochromes.

Phytochromes are sensory photoreceptors that use light to drive protein structural changes, which in turn trigger physiological reaction cascades. The process starts with a double-bond photoisomerization of the linear methine-bridged tetrapyrrole chromophore in the photosensory core module. The molecular mechanism of the photoconversion depends on the structural and electrostatic properties of the chromophore environment, which are highly conserved in related phytochromes. However, the specific role of individual amino acids is yet not clear. A histidine in the vicinity of the isomerization site is highly conserved and almost invariant among all phytochromes. The present study aimed at analyzing its role by taking advantage of a myxobacterial phytochrome SaBphP1 from Stigmatella aurantiaca, where this histidine is naturally substituted with a threonine (Thr289), and comparing it to its normal, His-containing counterpart from the same organism SaBphP2 (His275). We have carried out a detailed resonance Raman and IR spectroscopic investigation of the wild-type proteins and their respective His- or Thr-substituted variants (SaBphP1-T289H and SaBphP2-H275T) using the well-characterized prototypical phytochrome Agp1 from Agrobacterium fabrum as a reference. The overall mechanism of the photoconversion is insensitive toward the His substitution. However, the chromophore geometry at the isomerization site appears to be affected, with a slightly stronger twist of ring D in the presence of Thr, which is sufficient to cause different light absorption properties in SaBphP1 and SaBphP2. Furthermore, the presence of His allows for multiple hydrogen-bonding interactions with the ring D carbonyl which may be the origin for the geometric differences of the C-D methine bridge compared to the Thr-containing variants. Other structural and mechanistic differences are independent of the presence of His. The most striking finding is the protonation of the ring C propionate in the Pfr states of SaBphP2, which is common among bathy phytochromes but so far has not been reported in prototypical phytochromes.

Catalytic isomerization of dihydroxyacetone to lactic acid by heat treated zeolites

Lactic acid can be prepared by isomerization of renewable dihydroxyacetone over acid catalysts. However, the activities of Lewis acid and Brønsted acid sites in dihydroxyacetone isomerization are poorly understood. We prepared catalysts by heat treatment of ZSM-5. The heat treated ZSM-5 exhibited a greater Lewis acid site density and enhanced selectivity toward lactic acid. Dihydroxyacetone dehydration to the intermediate pyruvaldehyde was readily formed at 140 °C without added catalysts. Lewis acid sites were needed to convert pyruvaldehyde to lactic acid. Moreover, the Lewis acid site density was consistent with the order of catalytic performance, which suggested that the Lewis acid sites were the active sites for pyruvaldehyde rehydration. Conversely, the Brønsted acid sites were key in formation of unwanted product from pyruvaldehyde. These findings highlight the potential use of commercial zeolites as adjustable solid Lewis acid catalysts in biomass conversion reactions in which Lewis acid sites are needed.

Not all acid sites are created equal: Alkene isomerization and dimerization occur on separate sites on Amberlyst®15 catalyst

AbstractLinear 1‐alkenes undergo concurrent dimerization and double bond isomerization on the solid acid catalyst Amberlyst®15. Kinetic data show that double bond isomerization occurs on a site where the alkene adsorption coefficient is small, whereas alkene dimerization occurs on a site where the alkene adsorption coefficient is large. The fraction of isomerization sites occupied by alkene is small, whereas the dimerization sites are practically saturated with alkene. Logically, two different types of acid sites must exist on the surface of Amberlyst®15 catalyst. On the dimerization sites, substantial skeletal rearrangement occurs, likely via rapidly isomerizing carbenium ions, π‐complexes, and/or protonated cyclopropyl intermediates.

Transalkylation of 1,2,4-trimethylbenzene with toluene over large pore zeolites: Role of pore structure and acidity

Effects of pore structure and acidity over Beta and Y zeolites with a range of Brønsted and Lewis acid sites on their catalytic behaviour in the transalkylation reaction were investigated. The tests were conducted for a time-on-stream (TOS) of 50 h in a fixed-bed reactor at 400 °C, 10 bar, H2/HC = 4, and WHSV=5 h−1 with a 50:50 wt. % toluene:124-TMB mixture as feedstock. While initial (TOS ≤3 h) TMBs conversion was similar, wide differences were evident as the effects of partial deactivation became apparent with TOS. Zeolite Beta (Si/Al = 12.5) exhibited highest conversion and xylenes yield. As the Si/Al ratio of zeolite Beta is increased, the performance dropped whereas an opposite trend was observed for Y zeolites underscoring the importance of optimum level of dealumination depending on the zeolite structure. Isomerization of 124-TMB was observed over Beta which was not sustained by Y. The existence of separate active sites for TMB isomerization and its transalkylation with toluene in Beta is suggested.

The controlled synthesis of Pt/Hβ catalysts with intimate metal-acid sites for n-butane isomerization

Abstract Catalytic performance of Pt-promoted zeolite catalysts in alkane isomerization highly depends on the synergistic effect between metal and acid sites. Traditional impregnation synthesis leads to large and nonuniform Pt clusters on external surface of zeolites. An approach based on the Surface Organometallic Chemistry concept was employed to controllably synthesize Pt/Hβ catalysts. The grafting of dimethyl (1,5-cyclooctadiene)platinum (II) at the surface of dehydroxylated Hβ surface and hydrogenolysis of surface organometallic compound result in small Pt clusters (1.73 nm), which are uniformly dispersed on the mesopores of Hβ zeolite. The achieved Pt/Hβ catalyst shows high activity and stability in n-butane isomerization.

Olefin esterification on Amberlyst® 15 catalyst: does the esterification site defy thermodynamics?

Kinetics for the reaction between 1-tetradecene and heptanoic acid to make tetradecyl heptanoates on dry Amberlyst® 15 catalyst at 90 °C are extremely complex. Observed kinetics of simultaneous double bond isomerization and esterification cannot be reconciled with a single type of catalytic site. Esterification and most of the double bond isomerization must occur on separate sites. Relative rates of esterification for tetradecene isomers suggest that adsorption coefficients on the esterification sites follow 1-tetradecene > 2-tetradecene > 3+-tetradecenes. These relative adsorption coefficients imply that, on this esterification site, 2-tetradecene isomerizes to 3-tetradecene faster than 3-tetradecene isomerizes to 2-tetradecene or 4-tetradecene, a truly novel finding not previously reported for any other acidic site. The site where most of the double bond isomerization occurs does not display this behavior, and isomerization of linear tetradecenes proceeds to thermodynamic equilibrium. Kinetic data indicate that esters form when carboxylic acid traps rapidly isomerizing carbenium ions on the catalyst’s surface. Competition is fierce for esterification sites among olefins, carboxylic acid, and esters, and olefins occupy few esterification sites. Contrastingly, olefins triumph in competition for isomerization sites, and olefin isomerization is much faster than esterification. Heptanoic acid exists primarily in dimer form, but the monomer form reacts to make esters. A complex kinetic model fit experimental data for double bond isomerization, conversion of olefin to esters, and the distribution of secondary esters that formed over 22 h.

The Lumi-R Intermediates of Prototypical Phytochromes.

Phytochromes are photoreceptors that upon light absorption initiate a physiological reaction cascade. The starting point is the photoisomerization of the tetrapyrrole cofactor in the parent Pr state, followed by thermal relaxation steps culminating in activation of the physiological signal. Here we have employed resonance Raman (RR) spectroscopy to study the chromophore structure in the primary photoproduct Lumi-R, trapped between 130 and 200 K. The investigations covered phytochromes from plants (phyA) and prokaryotes (Cph1, Agp1, CphB, and RpBphP2) including phytochromobilin (PΦB), phycocyanobilin (PCB), and biliverdin (BV). In PΦB- and PCB-binding phyA and Cph1, two Lumi-R states (Lumi-R1, Lumi-R2) were identified and discussed in terms of sequential and parallel reaction models. In Lumi-R1, the chromophore structural changes are restricted to the C-D methine bridge isomerization site but extended throughout the chromophore in Lumi-R2. Formation and decay kinetics as well as photochemical activity depend on the specific protein-chromophore interactions and thus account for the different distribution between Lumi-R1 and Lumi-R2 in the photostationary mixtures of the various PΦB(PCB)-binding phytochromes. For BV-binding bacteriophytochromes, only a single Lumi-R(BV) state was found. In this state, which is similar for Agp1, CphB, and RpBphP2, the chromophore structural changes comprise major torsions of the C-D methine bridge but also perturbations at the A-B methine bridge remote from the isomerization site. The different structures of the photoproducts in PΦB(PCB)-binding phytochromes and BV-binding bacteriophytochromes are attributed to the different disposition of ring D upon isomerization, which leads to distinct protein-chromophore interactions in the Lumi-R states of these two classes of phytochromes.

Silica supported Sn catalysts with tetrahedral Sn sites for selective isomerization of glucose to fructose

Lewis acid catalyzed isomerization of glucose to fructose is an important reaction for production of renewable chemicals. Here, we show the synthesis of an active and selective Lewis acid catalyst for this reaction by controlling Sn dispersion on SBA15. Sn loading of 1 wt. % over SBA15 (Sn/SBA15) maximized the formation of tetrahedral Sn species on the catalyst surface. Increasing the loading or changing support caused formation of SnO2 clusters which reduced fructose selectivity. A mechanism based on condensation of Sn with silanol group of SBA15 is proposed. The catalyst showed high selectivity of 93 % after 2 h with 57 % fructose yield. The Lewis acid catalyzed isomerization of glucose was proven by isotopic tracer study using d-glucose-2-d. The catalyst deactivated in the third cycle owing to byproduct deposition, but the activity was restored by recalcining the catalyst.

An Alternative Pin1 Binding and Isomerization Site in the N-Terminus Domain of PSD-95.

Phosphorylation-dependent peptidyl-prolyl cis-trans isomerization plays key roles in cell cycle progression, the pathogenesis of cancer, and age-related neurodegeneration. Most of our knowledge about the role of phosphorylation-dependent peptidyl-prolyl cis-trans isomerization and the enzyme catalyzing this reaction, the peptidyl-prolyl isomerase (Pin1), is largely limited to proteins not present in neurons. Only a handful of examples have shown that phosphorylation-dependent peptidyl-prolyl cis-trans isomerization, Pin1 binding, or Pin1-mediated peptidyl-prolyl cis-trans isomerization regulate proteins present at excitatory synapses. In this work, I confirm previous findings showing that Pin1 binds postsynaptic density protein-95 (PSD-95) and identify an alternative binding site in the phosphorylated N-terminus of the PSD-95. Pin1 associates via its WW domain with phosphorylated threonine (T19) and serine (S25) in the N-terminus domain of PSD-95 and this association alters the local conformation of PSD-95. Most importantly, I show that proline-directed phosphorylation of the N-terminus domain of PSD-95 alters the local conformation of this region. Therefore, proline-directed phosphorylation of the N-terminus of PSD-95, Pin1 association, and peptidyl-prolyl cis-trans isomerization may all play a role in excitatory synaptic function and synapse development.