Hydroxylation Regiochemistry Is Robust to Active Site Mutations in Cytochrome P450cam (CYP101A1).

Cytochrome P450cam (CYP101A1) catalyzes the hydroxylation of d-camphor by molecular oxygen. The enzyme-catalyzed hydroxylation exhibits a high degree of regioselectivity and stereoselectivity, with a single major product, d-5-exo-hydroxycamphor, suggesting that the substrate is oriented to facilitate this specificity. In previous work, we used an elastic network model and perturbation response scanning to show that normal deformation modes of the enzyme structure are highly responsive not only to the presence of a substrate but also to the substrate orientation. This work examines the effects of mutations near the active site on substrate localization and orientation. The investigated mutations were designed to promote a change in substrate orientation and/or location that might give rise to different hydroxylation products, while maintaining the same carbon and oxygen atom balances as in the wild type (WT) enzyme. Computational experiments and parallel in vitro site-directed mutations of CYP101A1 were used to examine reaction products and enzyme activity. 1H-15N TROSY-HSQC correlation maps were used to compare the computational results with detectable perturbations in the enzyme structure and dynamics. We found that all of the mutant enzymes retained the same regio- and stereospecificity of hydroxylation as the WT enzyme, with varying degrees of efficiency, which suggests that large portions of the enzyme have been subjected to evolutionary pressure to arrive at the appropriate sequence-structure combination for efficient 5-exo hydroxylation of camphor.

Targeting Pro-Tumoral Macrophages in Early Primary and Metastatic Breast Tumors with the CD206-Binding mUNO Peptide.

M2-like tumor-associated macrophages (M2 TAMs) play important roles in the resistance of tumors to immunotherapies. Selective depletion or reprogramming of M2 TAMs may sensitize the nonresponsive tumors for immune-mediated eradication. However, precision delivery of payloads to M2 TAMs, while sparing healthy tissues, has remained an unresolved challenge. Here, we studied the application of a short linear peptide (CSPGAK, "mUNO") for the delivery of molecular and nanoscale cargoes in M2 TAMs in vitro and the relevance of the peptide for in vivo targeting of early-stage primary breast tumors and metastatic lung foci. First, we performed in silico modeling and found that mUNO interacts with mouse CD206 via a binding site between lectin domains CTLD1 and CTLD2, the same site previously demonstrated to be involved in mUNO binding to human CD206. Second, we showed that cultured M2 macrophages take up fluorescein-labeled (FAM) polymersomes conjugated with mUNO using the sulfhydryl group of its N-terminal cysteine. Pulse/chase studies of FAM-mUNO in M2 macrophages suggested that the peptide avoided lysosomal entrapment and escaped from early endosomes. Third, our in vivo studies with FAM-mUNO demonstrated that intraperitoneal administration results in better pharmacokinetics and higher blood bioavailability than can be achieved with intravenous administration. Intraperitoneal FAM-mUNO, but not FAM-control, showed a robust accumulation in M2-skewed macrophages in mouse models of early primary breast tumor and lung metastasis. This targeting was specific, as no uptake was observed in nonmalignant control organs, including the liver, or other cell types in the tumor, including M1 macrophages. Collectively, our studies support the application of the CD206-binding mUNO peptide for delivery of molecular and nanoscale cargoes to M2 macrophages and manifest the relevance of this mode of targeting primary and metastatic breast tumors.

Targeting Tumors Using Peptides.

To penetrate solid tumors, low molecular weight (Mw < 10 KDa) compounds have an edge over antibodies: their higher penetration because of their small size. Because of the dense stroma and high interstitial fluid pressure of solid tumors, the penetration of higher Mw compounds is unfavored and being small thus becomes an advantage. This review covers a wide range of peptidic ligands-linear, cyclic, macrocyclic and cyclotidic peptides-to target tumors: We describe the main tools to identify peptides experimentally, such as phage display, and the possible chemical modifications to enhance the properties of the identified peptides. We also review in silico identification of peptides and the most salient non-peptidic ligands in clinical stages. We later focus the attention on the current validated ligands available to target different tumor compartments: blood vessels, extracelullar matrix, and tumor associated macrophages. The clinical advances and failures of these ligands and their therapeutic conjugates will be discussed. We aim to present the reader with the state-of-the-art in targeting tumors, by using low Mw molecules, and the tools to identify new ligands.

Insight from atomistic molecular dynamics simulations into the supramolecular assembly of the aldo-keto reductase from Trypanosoma cruzi.

CONTEXT: Currently, Chagas disease represents an important public health problem affecting more than 8 million people worldwide. The vector of this disease is the Trypanosoma cruzi (Tc) parasite. Our research specifically focuses on the structure and aggregation states of the enzyme aldo-keto reductase of Tc (TcAKR) reported in this parasite. TcAKR belongs to the aldo-keto reductase (AKR) superfamily, enzymes that catalyze redox reactions involved in crucial biological processes. While most AKRs are found in monomeric forms, some have been reported to form dimeric and tetrameric structures. This is the case for some TcAKR. To better understand how TcAKR multimers form and remain stable, we conducted a comprehensive computational analysis using molecular dynamics (MD) simulations. Our approach to elucidating the aggregation states of TcAKR involved two strategies. Initially, we explored the dynamic behaviour of pre-assembled TcAKR dimers. Subsequently, we examined the self-aggregation of eight monomers. This investigation led to the identification of crucial residues that contribute to the stabilization of protein-protein interactions. It was also found that TcAKRs can form stable supramolecular assemblies, with each monomer typically surrounded by three first neighbours. These findings align with experimental reports of tetrameric or more complex supramolecular structures. Our computational studies could guide further experimental investigations aiming at drug development and assist in designing strategies to modulate aggregation. METHOD: Atomistic molecular dynamics simulations were carried out. The TcAKR 3D model structure was obtained by homology modelling using the Swiss Model for the TcAKR sequence (GenBank accession no. EU558869). Further, we checked the model with Alphafold2 and found a high degree of similarity between models. Several tools were used to build the dimers including CLUSPRO, GRAMM-Docking, Hdock, and Py-dock. Protein superstructures were built using the PACKMOL package. CHARMM-GUI was used to set up the simulation systems. GROMACS version 2020.5 was used to perform the simulations with the CHARMM36 force field for the protein and ions and the TIP3P model for water. Further analyses were performed using VMD, GROMACS, AMBER tools, MDLovoFit, bio3d, and in-house programs.

A CYCLIC PEPTIDE TARGETS GLIOBLASTOMA BY BINDING TO ABERRANTLY EXPOSED SNAP25.

Disease-specific changes in tumors and other diseased tissues are an important target of research because they provide clues on the pathophysiology of the disease as well as uncovering potentially useful markers for diagnosis and treatment. Here, we report a new cyclic peptide, CESPLLSEC (CES), that specifically accumulated (homed) in intracranial U87MG and the WT-GBM model of glioblastoma from intravenous (IV) injection, associating with the vasculature. Affinity chromatography of U87MG tumor extracts on insolubilized CES peptide identified Synaptosomal Associated Protein 25 (SNAP25) as a candidate target molecule (receptor) for CES. Several results supported the identification of SNAP25 as the CES receptor. IV-injected FAM-CES colocalized with SNAP25 in the tumors, and direct binding studies showed specific CES peptide binding to recombinant human SNAP25. A CES peptide-drug conjugate designed for photodynamic therapy showed selective cytotoxicity to SNAP25+ glioblastoma cell lines. Specific accumulation of systemically injected anti-SNAP25 antibody in U87MG glioblastoma, and labeling of intact U87MG cells with anti-SNAP in flow cytometry showed that SNAP25 is available from the circulation but not in normal tissues and that it is present at the cell surface. Using an array of ECM proteins and surface plasmon resonance revealed that SNAP25 binds moderately to collagen V and strongly to collagen VI. Modeling studies suggested that CES and collagen VI compete for the same binding site on SNAP25. Our results introduce CES as a valuable targeting peptide for drug delivery, and its receptor SNAP25 as a possible molecular marker of interest for glioblastoma.

Solution structural ensembles of substrate-free cytochrome P450(cam).

Removal of substrate (+)-camphor from the active site of cytochrome P450(cam) (CYP101A1) results in nuclear magnetic resonance-detected perturbations in multiple regions of the enzyme. The (1)H-(15)N correlation map of substrate-free diamagnetic Fe(II) CO-bound CYP101A permits these perturbations to be mapped onto the solution structure of the enzyme. Residual dipolar couplings (RDCs) were measured for (15)N-(1)H amide pairs in two independent alignment media for the substrate-free enzyme and used as restraints in solvated molecular dynamics (MD) simulations to generate an ensemble of best-fit structures of the substrate-free enzyme in solution. Nuclear magnetic resonance-detected chemical shift perturbations reflect changes in the electronic environment of the NH pairs, such as hydrogen bonding and ring current shifts, and are observed for residues in the active site as well as in hinge regions between secondary structural features. RDCs provide information about relative orientations of secondary structures, and RDC-restrained MD simulations indicate that portions of a ß-rich region adjacent to the active site shift so as to partially occupy the vacancy left by removal of the substrate. The accessible volume of the active site is reduced in the substrate-free enzyme relative to the substrate-bound structure calculated using the same methods. Both symmetric and asymmetric broadening of multiple resonances observed upon substrate removal as well as localized increased errors in RDC fits suggest that an ensemble of enzyme conformations are present in the substrate-free form.

Depletion of Mannose Receptor-Positive Tumor-associated Macrophages via a Peptide-targeted Star-shaped Polyglutamate Inhibits Breast Cancer Progression in Mice.

Although many studies have explored the depletion of tumor-associated macrophages (TAM) as a therapeutic strategy for solid tumors, currently available compounds suffer from poor efficacy and dose-limiting side effects. Here, we developed a novel TAM-depleting agent ("OximUNO") that specifically targets CD206+ TAMs and demonstrated efficacy in a triple-negative breast cancer (TNBC) mouse model. OximUNO comprises a star-shaped polyglutamate (St-PGA) decorated with the CD206-targeting peptide mUNO that carries the chemotherapeutic drug doxorubicin (DOX). In the TNBC model, a fluorescently labeled mUNO-decorated St-PGA homed to CD206+ TAMs within primary lesions and metastases. OximUNO exhibited no acute liver or kidney toxicity in vivo. Treatment with OximUNO reduced the progression of primary tumor lesions and pulmonary metastases, significantly diminished the number of CD206+ TAMs and increased the CD8/FOXP3 expression ratio (indicating immunomodulation). Our findings suggest the potential benefit of OximUNO as a TAM-depleting agent for TNBC treatment. Importantly, our studies also represent a novel design of a peptide-targeted St-PGA as a targeted therapeutic nanoconjugate. Significance: A peptide-targeted nanoformulation of DOX exclusively eliminates mannose receptor+ TAMs in breast cancer models, generating response without off-target effects (a drawback of many TAM-depleting agents under clinical study).

Experimentally restrained molecular dynamics simulations for characterizing the open states of cytochrome P450cam.

Residual dipolar couplings (RDCs) were used as restraints in fully solvated molecular dynamics simulations of reduced substrate- and carbonmonoxy-bound cytochrome P450(cam) (CYP101A1), a 414-residue soluble monomeric heme-containing camphor monooxygenase from the soil bacterium Pseudomonas putida. The (1)D(NH) residual dipolar couplings used as restraints were measured in two independent alignment media. A soft annealing protocol was used to heat the starting structures while incorporating the RDC restraints. After production dynamics, structures with the lowest total violation energies for RDC restraints were extracted to identify ensembles of conformers accessible to the enzyme in solution. The simulations result in substrate orientations different from that seen in crystallographic structures and a more open and accessible enzyme active site and largely support previously reported differences between the open and closed states of CYP101A1.

Physical interactions driving the activation/inhibition of calcium/calmodulin dependent protein kinase II.

CaMKII is a protein kinase whose function is regulated by the binding of the Calcium/Calmodulin complex (Ca2+/CaM). It is a major player in the Long Term Potentiation process where it acts as a molecular switch, oscillating between inhibited and active conformations. The mechanism for the switching is thought to be initiated by Ca2+/CaM binding, which allows the trans-phosphorylation of a subunit of CaMKII by a neighboring kinase, leading to the active state of the system. A combination of all-atom and coarse-grained MD simulations with free energy calculations, led us to reveal an interplay of electrostatic forces exerted by Ca2+/CaM on CaMKII, which initiate the activation process. The highly electrically charged Ca2+/CaM neutralizes basic regions in the linker domain of CaMKII, facilitating its opening and consequent activation. The emerging picture of CaMKII's behavior highlights the preponderance of electrostatic interactions, which are modulated by the presence of Ca2+/CaM and the phosphorylation of key sites.

PL1 Peptide Engages Acidic Surfaces on Tumor-Associated Fibronectin and Tenascin Isoforms to Trigger Cellular Uptake.

Tumor extracellular matrix (ECM) is a high-capacity target for the precision delivery of affinity ligand-guided drugs and imaging agents. Recently, we developed a PL1 peptide (sequence: PPRRGLIKLKTS) for systemic targeting of malignant ECM. Here, we map the dynamics of PL1 binding to its receptors Fibronectin Extra Domain B (FN-EDB) and Tenascin C C-isoform (TNC-C) by computational modeling and cell-free binding studies on mutated receptor proteins, and study cellular binding and internalization of PL1 nanoparticles in cultured cells. Molecular dynamics simulation and docking analysis suggested that the engagement of PL1 peptide with both receptors is primarily driven by electrostatic interactions. Substituting acidic amino acid residues with neutral amino acids at predicted PL1 binding sites in FN-EDB (D52N-D49N-D12N) and TNC-C (D39N-D45N) resulted in the loss of binding of PL1 nanoparticles. Remarkably, PL1-functionalized nanoparticles (NPs) were not only deposited on the target ECM but bound the cells and initiated a robust cellular uptake via a pathway resembling macropinocytosis. Our studies establish the mode of engagement of the PL1 peptide with its receptors and suggest applications for intracellular delivery of nanoscale payloads. The outcomes of this work can be used for the development of PL1-derived peptides with improved stability, affinity, and specificity for precision targeting of the tumor ECM and malignant cells.

Sodium perchlorate effects on the helical stability of a mainly alanine peptide.

Sodium perchlorate salt (NaClO(4)) is commonly used as an internal intensity standard in ultraviolet resonance Raman (UVRR) spectroscopy experiments. It is well known that NaClO(4) can have profound effects on peptide stability. The impact of NaClO(4) on protein stability in UVRR experiments has not yet been fully investigated. It is well known from experiment that protein stability is strongly affected by the solution composition (water, salts, osmolytes, etc.). Therefore, it is of the utmost importance to understand the physical basis on which the presence of salts and osmolytes in the solution impact protein structure and stability. The aim of this study is to investigate the effects of NaClO(4), on the helical stability of an alanine peptide in water. Based upon replica-exchange molecular dynamics data, it was found that NaClO(4) solution strongly stabilizes the helical state and that the number of pure helical conformations found at room temperature is greater than in pure water. A thorough investigation of the anion effects on the first and second solvation shells of the peptide, along with the Kirkwood-Buff theory for solutions, allows us to explain the physical mechanisms involved in the observed specific ion effects. A direct mechanism was found in which ClO(4)(-) ions are strongly attracted to the folded backbone.

Salt dependence of an alpha-helical peptide folding energy landscapes.

We used CD, UV resonance Raman spectroscopy, and molecular dynamics simulation to examine the impact of salts on the conformational equilibria and the Ramachandran Psi angle (un)folding Gibbs free energy landscape coordinate of a mainly polyalanine alpha-helical peptide, AP of sequence AAAAA(AAARA)(3)A. NaClO(4) stabilizes alpha-helical-like conformations more than does NaCl, which stabilizes more than Na(2)SO(4) at identical ionic strengths. This alpha-helix stabilization ordering is the reverse of the Hofmeister series of anions in their ability to disorder water hydrogen bonding. Much of the NaClO(4) alpha-helix stabilization results from ClO(4)(-) association with the AP terminal -NH(3)(+) groups and Arg side chains. ClO(4)(-) stabilizes 3(10)-helix conformations but destabilizes turn conformations. The decreased Cl(-) and SO(4)(2-) AP alpha-helix stabilization probably results from a decreased association with the Arg and terminal -NH(3)(+) groups. Cl(-) is expected to have a smaller binding affinity and thus stabilizes alpha-helical conformations intermediately between NaClO(4) and Na(2)SO(4). Electrostatic screening stabilizes pi-bulge conformations.

Phage-Display-Derived Peptide Binds to Human CD206 and Modeling Reveals a New Binding Site on the Receptor.

We recently identified a tumor-homing peptide (mUNO, sequence: "CSPGAK") that specifically interacts with mouse CD206 to target CD206/MRC1-expressing tumor-associated macrophages in mice. Here, we report studies on the binding of mUNO to human recombinant CD206 (hCD206) and on modeling the mUNO/hCD206 interaction by computational analysis. Fluorescence anisotropy analysis demonstrated that fluorophore-labeled mUNO interacts with hCD206. Microsecond time-scale molecular dynamics simulations and docking predictions showed that mUNO binds to a newly identified epitope between C-type lectin domains 1 and 2. The physical mechanisms that contribute to the docking interactions of mUNO include electrostatic interactions, aromatic interactions, and hydrogen bonds. We also demonstrate the selectivity of FAM-mUNO for CD206+-cultured human macrophages. The peptide mUNO appears to be the first ligand capable of interacting with this epitope of hCD206, for which no ligands have been reported. Our study has implications for targeting human M2-like tumor-associated macrophages, a subpopulation of immune cells with a major protumoral role.

Structure of aqueous sodium perchlorate solutions.

Salt solutions have been the object of study of many scientists through history, but one of the most important findings came along when the Hofmeister series were discovered. Their importance arises from the fact that they influence the relative solubility of proteins, and solubility is directly related to one of today's holy grails: protein folding. In this work we characterize one of the more-destabilizing salts in the series, sodium perchlorate, by studying it as an aqueous solution at various concentrations ranging from 0.08 to 1.60 mol/L. Molecular dynamics simulations at room temperature permitted a detailed study of the organization of solvent and cosolvent, in terms of its radial distribution functions, along with the study of the structure of hydrogen bonds in the ions' solvation shells. We found that the distribution functions have some variations in their shape as concentration changes, but the position of their peaks is mostly unaffected. Regarding water, the most salient fact is the noticeable (although small) change in the second hydration shell and even beyond, especially for g(O(w)***O(w)), showing that the locality of salt effects should not be restricted to considerations of only the first solvation shell. The perturbation of the second shell also appears in the study of the HB network, where the difference between the number of HBs around a water molecule and around the Na(+) cation gets much smaller as one goes from the first to the second solvation shell, yet the difference is not negligible. Nevertheless, the effect of the ions past their first hydration shell is not enough to make a noticeable change in the global HB network. The Kirkwood-Buff theory of liquids was applied to our system, in order to calculate the activity derivative of the cosolvent. This coefficient, along with a previously calculated preferential binding, allowed us to establish that if a folded AP peptide is immersed in the studied solution, becoming the solute, then increasing the salt concentration will make the helix more stable.

Some Surprising Implications of NMR-directed Simulations of Substrate Recognition and Binding by Cytochrome P450cam (CYP101A1).

Cytochrome P450cam (CYP101A1) catalyzes the stereospecific 5-exo hydroxylation of d-camphor by molecular oxygen. Previously, residual dipolar couplings measured for backbone amide 1H-15N correlations in both substrate-free and bound forms of CYP101A1 were used as restraints in soft annealing molecular dynamic simulations in order to identify average conformations of the enzyme with and without substrate bound. Multiple substrate-dependent conformational changes remote from the enzyme active site were identified, and site-directed mutagenesis and activity assays confirmed the importance of these changes in substrate recognition. The current work makes use of perturbation response scanning (PRS) and umbrella sampling molecular dynamic of the residual dipolar coupling-derived CYP101A1 structures to probe the roles of remote structural features in enforcing the regio- and stereospecific nature of the hydroxylation reaction catalyzed by CYP101A1. An improper dihedral angle Ψ was defined and used to maintain substrate orientation in the CYP101A1 active site, and it was observed that different values of Ψ result in different PRS response maps. Umbrella sampling methods show that the free energy of the system is sensitive to Ψ, and bound substrate forms an important mechanical link in the transmission of mechanical coupling through the enzyme structure. Finally, a qualitative approach to interpreting PRS maps in terms of the roles of secondary structural features is proposed.

Mechanisms of Activation and Subunit Release in Ca2+/Calmodulin-Dependent Protein Kinase II.

Calcium/calmodulin-dependent protein kinase II is an enzyme involved in many different functions, including the so-called long-term potentiation, a mechanism that strengthens synapses in a persistent mode and is believed to be a basic cellular mechanism for memory formation. Here we study the conformational changes of the enzyme due to phosphorylation of some key residues that are believed to drive the transition from an inhibited to an active state; it is this active state the one associated with long-term potentiation. We found that the conformational changes could be explained in terms of three charged regions in the three main subdomains of the enzyme: the hub, linker, and kinase. The role of phosphorylation is to change the charge relation between them, turning on and off their interactions and switching between an attractive state (nonphosphorylated or inhibited) and a not attractive one (phosphorylated or active). We also show that phosphorylated subunits become less stable, and this could favor their release from the multimer, as has been already observed experimentally.

Structure and Dynamics Study of LeuT Using the Markov State Model and Perturbation Response Scanning Reveals Distinct Ion Induced Conformational States.

The bacterial leucine transporter (LeuT), a close homologue of the eukaryote monoamine transporters (MATs), currently serves as a powerful template for computer simulations of MATs. Transport of the amino acid leucine through the membrane is made possible by the sodium electrochemical potential. Recent reports indicate that the substrate transport mechanism is based on structural changes such as hinge movements of key transmembrane domains. In order to further investigate the role of sodium ions in the uptake of leucine, here we present a Markov state model analysis of atomistic simulations of lipid embedded LeuT in different environments, generated by varying the presence of binding pocket sodium ions and substrate. Six metastable conformations are found, and structural differences between them along with transition probabilities are determined. We complete the analysis with the implementation of perturbation response scanning on our system, determining the most sensitive and influential regions of LeuT, in each environment. Our results show that the occupation of sites Na1 and Na2, along with the presence of the substrate, selectively influences the geometry of LeuT. In particular, the occupation of each site Na1/Na2 has strong effects (in terms of changes in influence and/or sensitivity, as compared to the case without ions) in specific regions of LeuT, and the effects are different for simultaneous occupation. Our results strengthen the rationale and provide a conformational mechanism for a putative transport mechanism in which Na2 is necessary, but may not be sufficient, to initiate and stabilize extracellular substrate access to the binding pocket.

Gas-phase fragmentation analysis of nitro-fatty acids.

Nitro-fatty acids are electrophilic signaling mediators formed in increased amounts during inflammation by nitric oxide and nitrite-dependent redox reactions. A more rigorous characterization of endogenously-generated species requires additional understanding of their gas-phase induced fragmentation. Thus, collision induced dissociation (CID) of nitroalkane and nitroalkene groups in fatty acids were studied in the negative ion mode to provide mass spectrometric tools for their structural characterization. Fragmentation of nitroalkanes occurred mainly through loss of the NO(2)(-) anion or neutral loss of HNO(2). The CID of nitroalkenes proceeds via a more complex cyclization, followed by fragmentation to nitrile and aldehyde products. Gas-phase fragmentation of nitroalkene functional groups with additional γ or δ unsaturation occurred through a multiple step cyclization reaction process, leading to 5 and 6 member ring heterocyclic products and carbon chain fragmentation. Cyclization products were not obtained during nitroalkane fragmentation, highlighting the role of double bond π electrons during NO(2)(-) rearrangements, stabilization and heterocycle formation. The proposed structures, mechanisms and products of fragmentation are supported by analysis of (13)C and (15)N labeled parent molecules, 6 different nitroalkene positional isomers, 6 nitroalkane positional isomers, accurate mass determinations at high resolution and quantum mechanics calculations. Multiple key diagnostic ion fragments were obtained through this analysis, allowing for the precise placement of double bonds and sites of fatty acid nitration, thus supporting an ability to predict nitro positions in biological samples.

Structural and dynamic implications of an effector-induced backbone amide cis-trans isomerization in cytochrome P450cam.

Experimental evidence has been provided for a functionally relevant cis-trans isomerization of the Ile88-Pro89 peptide bond in cytochrome P450(cam) (CYP101). The isomerization is proposed to be a key element of the structural reorganization leading to the catalytically competent form of CYP101 upon binding of the effector protein putidaredoxin (Pdx). A detailed comparison of the results of molecular dynamics simulations on the cis and trans conformations of substrate- and carbonmonoxy-bound ferrous CYP101 with sequence-specific Pdx-induced structural perturbations identified by nuclear magnetic resonance is presented, providing insight into the structural and dynamic consequences of the isomerization. The mechanical coupling between the Pdx binding site on the proximal face of CYP101 and the site of isomerization is described.

Computational and experimental determination of the alpha-helix unfolding reaction coordinate.

We demonstrate a calculated alpha-helix peptide folding energy landscape which accurately simulates the first experimentally measured alpha-helix melting energy landscape. We examine a 21-amino acid, mainly polyalanine peptide and calculate the free energy along the Psi Ramachandran angle secondary folding coordinate. The experimental free energy landscape was determined using UV resonance Raman spectroscopy. The relative free energy values are very close as are the equilibrium peptide conformations. We find 2.3 kcal/mol activation barriers between the alpha-helix-like and PPII-like basins. We also find that the alpha-helix-like conformations are quite defective and the alpha-helix-like structure dynamically samples 310-helix and pi-bulges.