Side Chain Of Molecule Research Articles

Theoretical Insight into Fluorination on Low‐Cost A‐π‐D‐π‐A Type Donor Materials for All‐Small‐Molecule Organic Photovoltaic Cells

Low‐cost organic photovoltaic (OPV) devices have shown enormous potential in large‐scale industrial applications. And it has attracted widespread attention in the past few decades. However, the photophysical characteristics of these budget‐friendly materials haven't been explored much. Here, low‐cost small materials, including small molecule 1 (asm1) with ortho‐fluorinated side chain and small molecule 2 (asm2) with meta‐fluorinated side chain were selected to probe the fluorination effect on the absorption spectra, electrochemical energy levels, electrostatic potential (ESP), etc. The results show that the molecules asm1 and asm2 have good planarity of the backbone. And the meta‐fluorinated side chain of asm2 contributes more to the highest occupied molecular orbital and less to the lowest unoccupied molecular orbital than asm1. Moreover, differences in ESP are found between donor and acceptor materials. Furthermore, strong and broad light absorption in the visible region of these low‐cost molecules is observed, resulting in a better short‐circuit current density for the devices constructed by the donors asm1, asm2, and acceptor Y6. In addition, more charge transfer mechanisms are characterized for the asm1/Y6 system. The introduction of ortho‐fluorination in the conjugated side chain of the molecule is a favorable approach, which will provide theoretical guidance for further molecular design experiments.

Crystallization‐Driven Optimization of Morphology and Performance in Near‐Infrared Organic Photodetectors via Alkyl Side Chain Tuning of Narrow Bandgap Non‐Fullerene Acceptors

AbstractNarrow bandgap non‐fullerene acceptors (NBG NFAs) are crucial in advancing near‐infrared organic photodetectors (NIR OPDs). However, the polymorphous behavior of NFAs introduces energetic disorder and charge‐trapping sites in the bulk heterojunction (BHJ) active layers, thereby hindering the NIR performance of OPDs. This study demonstrates the design and optimization of the molecular structures of alkyl side chains in NFAs to morphologically address these electrical limitations and shed light on how side chains affect the crystallization‐driven control of the morphogenesis process in BHJ layers. The structure, linearity, and length of the alkyl side chain molecules are strategically designed and controlled to synthesize a series of NBG NFAs, namely COT‐R (R = EH, BO, Oct, or Dod). The in‐depth quantitative investigations into the morphological and electrical factors reveal that side‐chain optimization effectively enhances various morphological properties, such as the molecular alignment and compatibility between components in the BHJ layer, resulting in the mitigation of energetic disorders and charge traps in the BHJ layers. Therefore, the optimized BHJ system, comprising COT‐Oct blended with PTB7‐Th, achieves a reduced dark current and increased responsivity, contributing to the development of high‐performance NIR OPDs with an impressive specific detectivity of 1.49 × 1012 Jones at 1000 nm under ‐0.5 V.

Small molecule donor third component incorporating thieno[3,2-b]thiophene unit enables 19.18% efficiency ternary organic solar cells with improved operational stability

The rational design of the third component to enhance the efficiency and stability of organic solar cell (OSC) is a critical and challenge. Herein, a thieno[3,2-b]thiophene (TT) substituted wide-bandgap small molecule donor BTTC, was designed, synthesized and incorporated into the state-of-the-art PM6:L8-BO binary to construct ternary OSC. The rigid side chain and planar molecule skeleton enables strong crystallinity and complementary absorption with binary system. The introduction of BTTC enhanced the molecular packing, fibril-like film and preferred vertical phase separation, yielding balanced and higher charge transport, more efficient exciton dissociation and suppressed charge recombination. Besides, lower energy loss was attained and thus leading to higher open-circuit voltage (VOC). Consequently, the introduction of BTTC achieved the highest efficiency of 18.8% along with extended operational stability. Furthermore, replacing L8-BO to BTP-eC9 further boosted the ternary PCE to 19.18%, standing one of the best charge management in ternary OSC. This work presents efficient rigid side chain substitution strategy to construct the third component of small molecule donor for high performance ternary OSCs with comprehensively improved photovoltaic parameters. This strategy broadens the selection of guest materials toward realizing the actual commercial application of OSCs.

Elaboration of Nanostructured Levan-Based Colloid System as a Biological Alternative with Antimicrobial Activity for Applications in the Management of Pathogenic Microorganisms.

Considering the documented health benefits of bacterial exopolysaccharides (EPSs), specifically of bacterial levan (BL), including its intrinsic antimicrobial activity against certain pathogenic species, the current study concentrated on the development of active pharmaceutical ingredients (APIs) in the form of colloid systems (CoSs) containing silver nanoparticles (AgNPs) employing in-house biosynthesized BL as a reducing and capping agent. The established protocol of fermentation conditions implicating two species of lactic acid bacteria (LAB), i.e., Streptococcus salivarius K12 and Leuconostoc mesenteroides DSM 20343, ensured a yield of up to 25.7 and 13.7 g L-1 of BL within 72 h, respectively. An analytical approach accomplished by Fourier-transform infrared (FT-IR) spectroscopy allowed for the verification of structural features attributed to biosynthesized BL. Furthermore, scanning electron microscopy (SEM) revealed the crystalline morphology of biosynthesized BL with a smooth and glossy surface and highly porous structure. Molecular weight (Mw) estimated by multi-detector size-exclusion chromatography (SEC) indicated that BL biosynthesized using S. salivarius K12 has an impressively high Mw, corresponding to 15.435 × 104 kilodaltons (kDa). In turn, BL isolated from L. mesenteroides DSM 20343 was found to have an Mw of only 26.6 kDa. Polydispersity index estimation (PD = Mw/Mn) of produced BL displayed a monodispersed molecule isolated from S. salivarius K12, corresponding to 1.08, while this was 2.17 for L. mesenteroides DSM 20343 isolate. The presence of fructose as the main backbone and, to a lesser extent, glucose and galactose as side chain molecules in EPS hydrolysates was supported by HPLC-RID detection. In producing CoS-BL@AgNPs within green biosynthesis, the presence of nanostructured objects with a size distribution from 12.67 ± 5.56 nm to 46.97 ± 20.23 was confirmed by SEM and energy-dispersive X-ray spectroscopy (EDX). The prominent inhibitory potency of elaborated CoS-BL@AgNPs against both reference test cultures, i.e., Pseudomonas aeruginosa, Escherichia coli, Enterobacter aerogenes, and Staphylococcus aureus and those of clinical origin with multi-drug resistance (MDR), was confirmed by disc and well diffusion tests and supported by the values of the minimum inhibitory and bactericidal concentrations. CoS-BL@AgNPs can be treated as APIs suitable for designing new antimicrobial agents and modifying therapies in controlling MDR pathogens.

Molecular dynamics simulation of H2 in amorphous polyethylene system: H2 diffusion in various PE matrices and bubbling during rapid depressurization

Polyethylene (PE) is a candidate liner material for Type IV storage devices. In this case, all-atom molecular dynamics simulations are employed to study the properties of Polyethylene with the presence of H2, including tensile properties, glass transition, diffusion in different PE and bubbling during rapid depressurization. The presence of H2 deteriorates the polyethylene matrix's tensile performance and decreases the glass transition temperature. The branch, side chain and small molecules promote the diffusion of H2 in the amorphous region by introducing more free volume below Tg. With a sufficient length, the length of polymer chain has minor effect on the diffusion of H2. Graphene, as a 2D reinforcement, could decrease the diffusion of H2 but suffers from poor interfacial bonding. Finally, H2 bubble(s) formed from the over-saturated H2 molecule and were observed in both the exclusive and free volume and stabilised at low pressure during rapid depressurization. According to the result obtained in this work, branchless HDPE is expected to give superior performance while the viscosity, which is important during processing, could be tailored by molecular weight. Processing technique leads to orientation is preferred, such as injection moulding.

Impact of xanthan gum on gluten microstructure and bread quality during the freeze-thaw storage

Xanthan gum (XG) is commonly used as a hydrocolloid in frozen dough to improve the texture and the sense quality of the products. The effects of XG on the molecular weight, secondary structure, microstructure of gluten and bread quality during freeze-thaw storage were studied. As results showed, XG and gluten formed more compact complexs through electrostatic interactions, which relieved the destruction of side chain molecules for gluten. XG protected the molecular weight of gluten proteins from 2 × 106 Da–108 Da. FTIR showed that XG-gluten mixtures had more α-helices (28.98%) and less intermolecular β-sheets (9.45%) compared with gluten. The secondary structure of XG-gluten remains unchanged under freeze-thaw treatment from 0 to 60 days. However, when the time reached 120 days, there was a decrease in α-helices (11.85%) and β-turns (6.28%) accompanied an increase by β-sheets (19.13%). CLSM revealed that XG gradually migrated to the centre with moisture after freeze-thaw storage, thereby protecting the gluten network. So the bread, prepared from XG-gluten mixtures, possessed superior colour, volume and visible pore proportion than bread without XG. Overall, the freeze-thaw stability of the gluten was enhance by XG under freeze-thaw treatment from 0 to 90 days.

Chirality-Puckering correlation and intermolecular interactions in Sphingosines: Rotational spectroscopy of jaspine B3 and its monohydrate

Chirality is determinant for sphingosine biofunctions and pharmacological activity, yet the reasons for the biological chiral selection are not well understood. Here, we characterized the intra- and intermolecular interactions at the headgroup of the cytotoxic anhydrophytosphingosine jaspine B, revealing chirality-dependent correlations between the puckering of the ring core and the formation of amino-alcohol hydrogen bond networks, both in the monomer and the monohydrate. Following the specific synthesis of a shortened 3-carbon side-chain molecule, denoted jaspine B3, six different isomers were observed in a jet expansion using broadband (chirped-pulsed) rotational spectroscopy. Additionally, a single isomer of the jaspine B3 monohydrate was observed, revealing the insertion of water in between the hydroxy and amino groups and the formation of a network of O-H···N-H···Oring hydrogen bonds. The specific jaspine B3 stereochemistry thus creates a double-faced molecule where the exposed lone-pair electrons may easily catalyze the formation of intermolecular aggregates and determine the sphingosine biological properties.

Evolution of Composting Process in Maize Biomass Revealed by Analytical Pyrolysis (Py-GC/MS) and Pyrolysis Compound Specific Isotope Analysis (Py-CSIA)

An innovative approach based on the combination of analytical pyrolysis coupled with gas chromatography-mass spectrometry (Py-GC/MS) with compound-specific isotope analysis (Py-CSIA) is used to study the composting process of maize biomass. This multidisciplinary approach aims to elucidate the decomposition rate of the main biogenic materials (lignin, cellulose, proteins, lipids, and waxes) responses to the composting process. According to Py-GC/MS data/structural composition, a noticeable and significant decrease during the first stage of the composting process of carbohydrates and aromatic compounds is found, followed by a gradual increase of all compounds till the end of the experiment. This trend, along with an increase of fatty acids methyl-ester at the first composting stage, sustains the microbial activity and its stabilization over time. Py-CSIA data showed a significant enrichment in 13C in all identified compounds over time, supporting the semi-quantitative results and the decomposition of initial biomass throughout the composting process. This trend is also perceptible in lignin moieties, long-chain aliphatic structures, and isoprenoids, as highly recalcitrant compounds, presumably due to depolymerization and carbon translocation of side-chain molecules during the composting process. Compound-specific isotope values showed a good correlation with the bulk isotope data, and this served as validation of the technique. However, bulk values showed higher heterogeneity because those represent an average of all organic compounds in the sample. By combining isotopic and structural information using Py-GC/MS and Py-CSIA, we are able to provide further information and a more detailed approach to the study of the decomposition process of biomass by considering the diverse dynamics of the main biogenic compounds.

The electronic effect of N-protecting groups on Diels–Alder cycloaddition reaction of furan-cored compounds

Furan-derived nitrogen side-chain molecules were used to investigate the effect of nitrogen-protecting groups on intramolecular Diels–Alder cyclization reaction yields. The protection reactions were carried out in both dichloromethane and water. The protected molecules were synthesized in dichloromethane and then cycled in toluene. On the other hand, in water as a reaction media both protection and cyclization were carried out in the same environment which provides advantages in saving time and chemicals. To study the electronic effects of protecting groups, various electron-withdrawing and electron-donating protective groups were used and their effects on cyclization were evaluated. As a result, the mesomeric electron-withdrawing and sterically large protecting groups increased the yield of Diels–Alder cycling reactions effectively. In addition, the iminium ion formed between some protecting groups and the molecule caused the formation of E/Z isomers and this was demonstrated by dynamic NMR experiments.

Surface pre-ignition measurements of fuel components and their mixtures

A closed loop temperature controlled glow-plug has been used to control the incidence of surface pre-ignition of n-pentane, iso-pentane, n-heptane, iso-octane, ethanol, cyclohexane, ethyl-benzene, toluene and xylene, and four representative mixtures of these components. Statistical data on the incidence of pre-ignition compared with spark ignition has been derived from the in-cylinder maximum pressure, and the glow-plug temperature has been characterised on a cycle-by-cycle basis.The alkanes and alcohols were found to be rather susceptible to surface pre-ignition, with the straight chained molecules having a higher pre-ignition tendency than branched chain molecules. The pre-ignition tendency of ring-structured components depends on the length of the attached chain, with shorter side-chain molecules (xylene, toluene) being less susceptible to pre-ignition than longer side-chain molecules like ethyl-benzene; cyclohexane was more susceptible to pre-ignition than any of the aromatic fuels. A higher glow-plug temperature was found to be needed for pre-igniting stoichiometric or rich mixtures, and this was attributed to the weaker mixture having a higher compression index and a lower specific heat capacity.

Bioactive Molecule from Streptomyces sp. Mitigates MDR Klebsiella pneumoniae in Zebrafish Infection Model.

The emergence and spread of multi-drug resistant (MDR) especially carbapenem-resistant Klebsiella pneumoniae is a major emerging threat to public health, leading to excess in mortality rate as high as 50–86%. MDR K. pneumoniae manifests all broad mechanisms of drug resistance, hence development of new drugs to treat MDR K. pneumoniae infection has become a more relevant question in the scientific community. In the present study a potential Streptomyces sp. ASK2 was isolated from rhizosphere soil of medicinal plant. The multistep HPLC purification identified the active principle exhibiting antagonistic activity against MDR K. pneumoniae. The purified compound was found to be an aromatic compound with aliphatic side chain molecule having a molecular weight of 444.43 Da. FT-IR showed the presence of OH and C=O as functional groups. The bioactive compound was further evaluated for drug induced toxicity and efficacy in adult zebrafish infection model. As this is the first study on K. pneumoniae – zebrafish model, the infectious doses to manifest sub-clinical and clinical infection were optimized. Furthermore, the virulence of K. pneumoniae in planktonic and biofilm state was studied in zebrafish. The MTT assay of ex vivo culture of zebrafish liver reveals non-toxic nature of the proposed ASK2 compound at an effective dose. Moreover, significant increase in survival rate of infected zebrafish suggests that ASK2 compound from a new strain of Streptomyces sp. was potent in mitigating MDR K. pneumoniae infection.

Supramolecular Materials Cross-Linked by Host–Guest Inclusion Complexes: The Effect of Side Chain Molecules on Mechanical Properties

Functional polymeric materials constructed by noncovalent bonds have attracted considerable attention due to their beneficial stretching and self-healing properties. We chose host–guest interactions using cyclodextrins (CDs) as host molecules to realize supramolecular materials with stretching and self-healing properties. Notably, an inclusion complex of a CD and a guest molecule functions as a reversible bond in a material. Herein, we studied the relationship between the mechanical properties of the materials and host–guest interactions based on the association constants of CDs with guest molecules and molecular structures of the guest molecules. A chemically cross-linked poly(acrylamide) gel showed high rupture stress, although the rupture strain was noticeably low. However, the host–guest hydrogels (CDAAmMe-R hydrogels) exhibited a higher rupture stress and strain of approximately 1000%. These rupture stress and strain values were related to the association constants of the CDs with guest units on the ...

Effects of carboxylation of the side chains on the order-disorder transition in aqueous solution of schizophyllan, a triple helical polysaccharide

Schizophyllan and scleroglucan are water-soluble polysaccharides having repeating units consisting of three β-1,3-linked glucose residues in the main chain and a single β-1,6-linked glucose residue as the side chain. This polysaccharide dissolves as a triple helix in an aqueous solution and shows a cooperative order-disorder transition between the side chain and solvent molecules while retaining the triple helical conformation. Periodate and subsequent chlorite oxidations selectively modify the side chain glucose to provide the corresponding dicarboxylate units. Optical rotation measurements and differential scanning calorimetry were performed on carboxylated schizophyllan/scleroglucan (‘sclerox’) samples to investigate the effects of the degree of carboxylation on the order-disorder transition in deuterium oxide with 0.1M NaCl. The transition curves for the sclerox samples are strongly dependent on the degree of carboxylation. The modified side chains cannot take the ordered structure, resulting in a reduction of the transition enthalpy. The transition temperature for carboxylated schizophyllan becomes lowered and the transition curve broadens with increasing the degree of carboxylation. The permanent disordered units are included in a trimer by the carboxylation to inhibit a long sequence of the ordered units.

Simulation Study of the Effect of the Side-Chain Structure on the Initial Nucleation Process of Polythiophene Derivatives.

We have investigated the initial nucleation process of polythiophene derivatives by molecular dynamics simulations to elucidate the role of flexible side chains in the crystallization process. We considered poly(3-hexylthiophene) (P3HT), poly(3,3‴-dihexylquarterthiophene) (PQT6), poly(3-dodecylthiophene) (P3DDT), and poly(3,3‴-didodecylquaterthiophene) (PQT12), in which the lengths and densities of the alkyl side chains are different. In the case of the short side-chain molecules (P3HT and PQT6), the initial nucleation process is based on the following steps. At the beginning, the thiophene rings align and ordering of the main chains commences shortly afterward. Ordering of the flexible side chains is induced after formation of a stacked structure of rigid main chains. In the case of a long and dense side-chain molecule (P3DDT), the initial nucleation process shows different features. The ordering process of the thiophene rings or main chains becomes very slow, while that of side chains becomes fast. In addition, the local order of the main chains becomes lower than those of P3HT and PQT6. These simulation results reveal that long and dense side chains suppress the nucleation process, whereas long but low-density side chains lead to formation of stacked nuclei in a different manner from those observed for short side-chain molecules.

A Molecular Dynamics Study of Transport Properties in Degraded Nafion Membranes

Degradation of the polymer membrane is known as one of the important factors to determine the lifetime of polymer electrolyte fuel cells (PEFCs). The membrane electrode assembly of a PEFC is made from various components, e.g. the catalyst layers, the gas diffusion layers, the micro-porous layers and the polymer electrolyte membrane. Generally, the lifetime of a PEFC is specified by the degradation of a polymer electrolyte membrane. Degradation of the membrane can be classified into three categories: thermal, mechanical, and chemical. For Nafion membranes which are often used in PEFCs, the chemical degradation is more significant because Nafion membranes have excellent mechanical and thermal stability. Since the chemical degradation, which is mainly due to the attack of radicals formed during the operation, is inevitable, the knowledge to estimate the performance of the degraded membrane is required. Proton conductivity is one of the typical indicators for chemical degradation of the polymer electrolyte membranes. The proton conductivity in a degraded Nafion membrane becomes lower than the one in the pristine Nafion membrane. The transport properties in the membranes strongly influenced by the molecular morphology of the multicomponent system which includes polymers, water/hydronium molecules, and several fragments generated by the degradation. Although the degraded Nafion membranes had been investigated experimentally and numerically, evaluation of the relationship between transport properties and degradation levels, taking into account several fragments generated by the degradation, had not been considered. In addition, it is difficult to control the degradation experimentally because the degradation occurs in molecular scale. In the present study, we have carried out molecular dynamics simulations of a degraded Nafion membrane system. The effect of a degradation mechanism on transport properties, which is identified by what kind of radical react on, is important because the combination of fragments is considered to determine the water structure. However the degradation mechanism is still under discussion. Here, a proposed mechanism by Ghassemzadeh et al.1 have been adopted to create a degraded Nafion and generated fragments. In their mechanism, the attack of the hydroxyl radical, which is one of the most reactive species in a degraded Nafion membrane, causes unzipping of side chain. Consequently, HF, CO2, CF3, and HOC2F4SO3 molecules are generated in the reaction. A modified DREIDING force field2-4 have been employed for degraded Nafion molecules and fragments. Water/ hydronium molecule interacts with anharmonic potential5 to describe Grothus mechanism with aTS-EVB model6. For side chain molecules of a degraded Nafion monomer and fragment molecules, the equilibrium bond lengths, the equilibrium angles and the partial charges have been determined by DFT calculations. In the simulation systems, there were four ten-unit degraded Nafion molecules, whose degradation level is defined by changing the number of broken side chains, equimolar generated fragments, and water/hydronium molecules with varying water contents. We have conducted simulations in various conditions, such as different water contents and degradation levels. Diffusion coefficient of each molecule have been used as their transport properties. To evaluate molecular structures, radial distribution functions have been calculated in various degradation levels. We have evaluated the relationship between diffusion coefficient and degradation levels.

Enthalpy of sublimation of hydroxyl-containing amino acids: Knudsen’s effusion mass spectrometric study

The sublimation processes of hydroxyl amino acids, L-serine and L-threonine, were investigated by Knudsen’s effusion method with mass spectrometric vapor composition control. Fragmentation of molecules under electron ionization was discussed. For these compounds, the molar enthalpies of sublimation were derived from the Clausius–Clapeyron equation. Using values for the heat capacity differences between the gas and crystal phases of the compounds the molar standard enthalpies of sublimation at T=298.15K, were obtained. The molar enthalpies of solvation in water at infinite dilution (i.d.) were derived from the molar enthalpies of solution of amino acids in water at i.d. and the molar standard enthalpies of sublimation obtained. Correlations between the enthalpies of sublimation/solvation and some molecular descriptors of amino acids were discussed. Conformational analysis of Thr was made by using B3LYP/6-31++G(d,p). Analysis of natural bond orbitals (NBO) has been carried out for the conformers of Ser, Thr and Tyr. It was shown that the side chain of molecule had an influence on formation intramolecular H-bond.

Dielectric Relaxation Spectroscopy of Liquid Crystal in Nematogenic Mesophase

ABSTRACTIn the present study, we report the dielectric behavior of a high temperature nematic liquid crystal (NLC) in both the planar as well as homeotropic alignments. To understand the mechanism of such high temperature NLC, we have performed dielectric spectroscopy in frequency range 100 Hz to 10 MHz. The different dielectric parameters, viz., effective dielectric permittivity and dielectric anisotropy of the sample have been determined with the variation of frequency and temperature. It is found that the dielectric parameters depend on dipole moment and rotation of side chain of molecule. The phase transition of this high temperature NLC has been confirmed by the DSC study.

分子動力学法を用いた劣化高分子電解質膜内部における物質輸送特性の解析

We have conducted molecular dynamics simulations of degraded Nafion membrane systems. The degraded Nafion molecules are made by breaking side chains randomly. Degradation level is controlled by changing the number of broken side chains. For side chain molecules of a degraded Nafion monomer and fragment molecules, the equilibrium bond lengths, the equilibrium angles and the partial charges have been defined originally by DFT calculations. In the simulation systems, there were four ten-unit degraded Nafion molecules and equimolar fragments generated by degradation, in various conditions, such as different water contents and degradation levels. Density, diffusion coefficient, and the number of water clusters have been evaluated. Our findings are, in the degraded Nafion membranes, water molecules and hydronium ions are more dispersed, indicating that Grotthus mechanism are suppressed, and the system becomes much fluidity owing to the existence of various fragments which facilitates Vehicle mechanism. There are two degradation variations of proton diffusion coefficient. In early degradation, the diffusion coefficient decreases because it is harder for the molecules to move freely in the system due to densification. After the maximum density, the diffusion coefficient increases due to fluidity of the system.

Antiferroelectric domains in liquid crystalline methacrylic polymers

The preparation of several liquid crystalline polymers obtained using different AIBN concentration and polymerization times are reported. The compounds were characterized by 1H NMR, differential thermal analysis (DTA), X-ray diffraction and pyroelectric measurements. For all polymers, a smectic C2 phase occurs over broad temperature range, where the molecules adopt an opposite tilt direction for alternate smectic layer. Crystalline polymers show an antiferroelectric behavior, developing an anticlinic phase responsible for the electric behavior. The arrangement of the side chain molecules within the partial crystalline network in these electrically active polymers is related with the kinetic of precipitation.

Protein dynamics viewed by hydrogen exchange

To examine the relationship between protein structural dynamics and measurable hydrogen exchange (HX) data, the detailed exchange behavior of most of the backbone amide hydrogens of Staphylococcal nuclease was compared with that of their neighbors, with their structural environment, and with other information. Results show that H-bonded hydrogens are protected from exchange, with HX rate effectively zero, even when they are directly adjacent to solvent. The transition to exchange competence requires a dynamic structural excursion that removes H-bond protection and allows exposure to solvent HX catalyst. The detailed data often make clear the nature of the dynamic excursion required. These range from whole molecule unfolding, through smaller cooperative unfolding reactions of secondary structural elements, and down to local fluctuations that involve as little as a single peptide group or side chain or water molecule. The particular motion that dominates the exchange of any hydrogen is the one that allows the fastest HX rate. The motion and the rate it produces are determined by surrounding structure and not by nearness to solvent or the strength of the protecting H-bond itself or its acceptor type (main chain, side chain, structurally bound water). Many of these motions occur over time scales that are appropriate for biochemical function.