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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5632_Библиотеки_им_академика_М_И_Перельмана.pdf
132 P. Chandra et al.
progressively being utilized to deliver drugs into circulation to their molecular
sites of effect. Protein-based biomaterials with drug-delivery properties have been
shown to have several medical applications, such as the treatment of diabetes,
neuroinflammation, cancer, wound healing, and corneal regeneration. This chapter has given an overview of the features and applications of modern proteinbased materials in biomedicine.
Keywords
Biocompatible materials · Proteins · Protein sequence · Nanoscale · Vaccine ·
Biomedicine
Abbreviations
PEG Poly-Ethylene Glycol
NCAAs Non-Canonical Amino Acids
CRISPR Clustered Regularly Interspaced Short Palindromic Repeats
API Active Pharmaceutical Ingredients
AMP Antimicrobial Peptides
FDA Food and Drug Administration
ELP Elastin-Like Polypeptides
rhTE Recombinant Human Tropoelastin
DNA Deoxyribonucleic Acid
PVA Polyvinyl Alcohol
HIV Human Immunodeficiency Virus
BSA Bovine Serum Albumin
PEE Polyethylene Ether
PLGA Poly Lactic-co-Glycolic Acid
DOX Doxorubicin
HSP90 Heat Shock Protein 90
HEMA 2-hydroxyethyl Methacrylate
C.N. Chitosan
C.P. Collagen Peptide
GLP-1 Glucagon-like Peptide 1
6.1 Introduction
More advanced drug delivery techniques are required in light of medical
advancements and clinical best practices. Historically, the renal clearance process
and short half-lives of proteins and oligonucleotides have hampered the thera peutic
effectiveness of these molecules. Drugs may be quickly filtered in the kidney and
eliminated from circulation by the reticuloendothelial system before reaching their
6 Proteins as Biocompatible Material for Biomedical Applications 133
intended destination. When injecting medications, the vasculature may either guide
the drug directly to the illness site or take a diversion where the drug can be removed.
These drugs must be taken often and in large doses to distribute through the
vasculature and ultimately diffuse into the tissue interstitial, which is necessary to
have the desired therapeutic effect (Hoffman and Falvo
Although taki
ng medications orally is the most practical, secure, and extensively
2004; Layman et al. 2018
used way, these bioactive substances must be insoluble in the stomach to prevent
significant losses from the stomach’s acid, pepsin, and the small intestine’s pancreatic enzymes. Drugs must be soluble and absorbable via the intestinal mucosa once
they reach the gut. For chronic and persistent medical disorders, which call for longterm, non-toxic drug delivery systems that are regu
at the right
time, these conventional techniques mentioned above are not appropriate.
lated to provide the correct dose
Consequently, there is an excellent demand for mechanically steady polymer
materials to be used as drug-delivery vehicles that protect therapies from clearance
and degradation while avoiding over-toxicity to the rest of the body. Treating
polymer materials into various structures and forms, such as films, gels,
microspheres, microcapsules, nanoparticles, and micelles, is simple. Therapeutics
and biomolecular proteins may effectively transport drugs to particular sites while
preventing immunological reactions and serious adverse effects by using polymer
carrier systems to shield them from the harsh gastrointestinal tract environment
(Layman et al.
Polyme
2018).
re a desirable class of materials for use in biomedical applications
rs a
because they provide a great degree of control over their physicochemical
characteristics. Polymeric biomaterials may have mechanical, biodegradable, biocompatible, and processable qualities customized for a particular tissue and application. Among the most helpful and well-researched biologically generated polymeric
materials are proteins. When compared to artificial polymers, proteins have many
benefits. Researchers list four significant drawbacks with synthetic polymers (Austin
and Rosales 2019; Banskota et al. 2019). First, they frequently rely on poly-ethylene
glycol (PEG) to preserve bio-compatibility because it is widely used. Long-term use
of this could result in the development of PEG allergies and anti-PEG antibodies, a
phenomenon that has already been observed in some people. Second, vinyl addition
polymerization produces artificial polymers such as PEG, which are difficult to break
down and may build up inside the body with potentially harmful consequences
(Garay et al. 2012; Delplace and Nicolas 2015; Qi et al. 2016). Third, the level of
polydispersity that results from the polymerization process in the molecular weights
of the polymers will always be present, hence restricting the accuracy with which
physical-chemical parameters may be adjusted. Finally, chemical conjugation
procedures frequently exhibit inadequate site specificity and poorly controlled
stoichiometry during polymer synthesis. However, scalable bacterial, insect, or
mammalian expression systems can make larger recombinant proteins.
In contr
ast, chemical synthesis platforms such as solid-phase peptide synthesis
can produce shorter peptides. Since they don’t need metals, they are also completely
biodegradable and biocompatible in vivo, unlike synthetic polymers. Because of this,
research on protein biomaterials has the potential to benefit human health in the
).
134 P. Chandra et al.
future and has many benefits over other materials for in vivo applications (Boyle
2018).
Improved genetic manipulation methods have significantly enhanced control over
recombinant expression, leading to the development of protein biomaterials. Since
amino acids are the building blocks of proteins, little alterations to their sequence
may be used to alter significant changes in a macromolecule’s characteristics. With
the inclusion of pyrrolysine and seleniocysteine, the number of naturally occurring
amino acids has increased to 22. Artificial non-canonical amino acids (NCAAs) have
been developed for functional versatility, including fluorination and hydroxylation
of peptide backbones. These NCAAs can be included site- or residue-specific,
depending on the needed level of information. The Tirrell group was the first to
introduce vital chemical functions, including azide groups for cycloaddition and
genetically altered microbes that could be added to the NCAA (Voloshchuk and
Montclare
2010).
Furthermore, it is now feasible to carefully and precisely create higher-order
protein structures with specific functional features thanks to more recent
advancements like CRISPR technology. For example, Hernando-Garcia et al.
employed Cas12 as nucleation sites to construct viral capsids that self-assemble to
carry nucleic acids. Protein-based biomaterials have a promising future ahead of
them (Calcines-Cruz et al.
One o
he primary goals of biomaterials has been to improve medication delivery
f t
2021).
because so many various types of molecules have been investigated for their possible
medicinal uses. These substances, also known as active pharmaceutical ingredients,
or APIs, fall into many significant categories, including proteins, peptides, genetic
material, tiny compounds, cells, and cell fragments. Every one of these loads has
benefits of its own and may present different delivery-related difficulties. Small
molecules are considered therapeutic when their molecular weight is less than
900 Da. Thirteen small molecule medications include predictable pharmacodynamics and pharmacokinetics and enhanced diffusion and permeability across tissue and
cell membranes due to their small size. Small molecule medications can interact
unintentionally with untargeted tissues and cells due to their basic chemical
structures, and some drugs may become resistant in cells (Yin et al. 2017)
. G
ene
therapy involves introducing functional copies of genes or manipulating gene
expression in cells, with therapeutic genetic material teaching cells to produce
specific proteins and delivering negatively charged nucleic acids with cationic
materials (Janarthanan et al. 2019).
Additionally,
they frequently require carrier mechanisms to pass across the cell
membrane. Furthermore, proteins are frequently utilized as therapeutic drugs; most
proteins with FDA approval today are antibody therapies. Protein medications like
proapoptotic and antimicrobial peptides (AMPs) disrupt the mitochondrial membrane, forming connections between small molecules and protein antibodies. Also,
these peptides are briefly less than 40 amino acids (Fu et al.
2015;
Janarthanan et al.
2019). Protein supplements frequently work in a concentrated, targeted manner to
reduce the likelihood of unfavourable side effects.
6 Proteins as Biocompatible Material for Biomedical Applications 135
Last, scaffolding is required to lengthen the stability and retention period of cells
and cell derivatives, such as exosomes, employed in regenerative medicine and
wound healing. Various cell types have been used, depending on the specific
needs. These involve primary cells for musculoskeletal tissue engineering and
stem cells that can develop into numerous lineages (Zhang et al.
Protein materials are appealing because their organizational structure can be
carefully controlled to provide various forms with unique physical characteristics.
It is possible to alter a protein’s original amino acid sequence to create specific
secondary structural sequences, such as beta sheet s, helical coils, and even naturally
disorganized structures. Helical coils experience increased mechanical stress due to
hydrogen bonds, while fusion proteins utilize the α-helical peptide linker EAAAK
for increased protein structure rigidity (C hen et al. 2013; Punia et al. 2019). Certain
α-helices with hydrophobic first and fourth residues can form stable coiled coils,
with hydrophilic residues in solvent-exposed areas and hydrophobic residues facing
the interior. Triple helical configurations are standard in collagen and collagen-like
peptides, formed when three left-handed helices combine.
Additionally, proteins can assume the β-strand conformation, which allows them
to form pleated sheets or twists. β-sheets may be connected to other sheets in a
parallel or antiparallel configuration (Cheng et al.
2019). Planar hydrogen bonds are created when the N and C termini of two strands
are positioned adjacent to one other in an antiparallel orientation. This arrangement
is incredibly stable. Every strand is oriented in a parallel pattern with all of its
N-termini next to each other and pointing in the same direction. The conformation is
a little less stable because of the out-of-plane hydrogen bonding that emerges from
this configuration. Some materials, like silk, exhibit exceptional mechanical
properties because of highly ordered secondary structures in their crystalline regions.
Last but not least, as demonstrated by the remarkably elastic and rubbery
characteristics of elastin and elastin-like polypeptides (ELPs), random coils or
disordered secondary structures can also form in proteins (Koh et al. 2015).
2013; Li and Yu 2013; Wang et al.
2018).
6.2 Protein Materials
Drug delivery research has extensively used various fibrous protein materials,
including silk, keratin, collagen, and elastin (Fig. 6.1). Protein materials may be
handled similarly since they have comparable characteristics (Table 6.1) (van
Herwaarden et al. 2011; Brennan et al. 2016).
6.2.1 Keratin
Keratin is found in human and animal integumentary systems, originating from the
outermost layer of skin (Ferraro et al. 2016). Regarding biomedical applications,
keratin offers a practical and affordable substitute for petroleum-based polymers.
About 90% of keratin can be found in wool; the remaining keratins comprise other
136 P. Chandra et al.
Fig. 6.1 Protein-based biomaterials
Table 6.1 Protein biomaterials in biomedical applications
Material Applications Structural design
Collagen To improve the beneficial drug-delivery
Keratin Antimicrobial, medication
Silk Adhesive fillers, wound
Zein Enhanced mechanical strength,
Elastin Wound h
capabilities, cartilage, ocular, corneal,
skin, nerve, and
Collagen tissues are engineered and
combined into a range of composite
materials.
administration, tissue engineerin
medical devices
healing
enzyme immobilization, engineering
cartilage or load-bearing tissues, and
medication administration
microbiological resistance, positive cell
adhesion, biomineralization, controlled
drug release, and osteoblast growth
adjustable thermo-responsive
intracellular functionalized peptide
medicines, engineering of liver, vascular
graft, cartilage, and ocular tissue
tendon/ligament
and trauma, and wound
ealing a
pplications, highly
g,
dressing,
Hydrogels, fibers, films (Sell et al. 2009;
Sionkowska and Kozłowska 2013;
Ghezzi et al. 2017)
Hydrogels, fibers, films, (Apel et al.
2008; Zoccola et al. 2008; Aboushwareb
al. 2009)
et
Hydrogels, microcapsules, films,
microparticles (Gellynck et al. 2008;
Lammel et al. 2011)
Films, nanofibers, microspheres,
nanoparticles (Liu et al. 2005; Li and
Yao 2012; Yao et al. 2013)
Hydrogels, fibers, films (Nowatzki
Tirrell 2004; Srokowski and Woodhouse
2008)
and
hydrocarbons (Cilurzo et al. 2013). Keratin can be categorized into α-, β-, and
γ-keratin, with α-keratin having an α-helix structure containing α-helices forming
a protofibril, low sulphur amino acid concentration, and potential disulfide bridge
formation. It has a molecular weight between forty and seventy kDa. β-keratin shares
a β-sheet shape with other intermediate filament proteins, with a molecular weight
6 Proteins as Biocompatible Material for Biomedical Applications 137
extending from 11 to 22 kDa. Tryptophan, lysine, histidine, serine, glycine, and
alanine comprise a significant portion of its amino acid composition. Due to its high
sulphur amino acid concentration, it can form a high degree of disulfide linkages
inside and between molecules (4–6 w/w). Soft and hard keratins may be produced
based on the cross-linking cysteine residues for mechanical, chemical, and thermal
stability. Wool is an inexpensive source of keratin, a textile i
contrast to collagen
ible, biodegradable, non-toxic, and highly adaptable keratin biomaterials. This
versatile material, shaped into various forms like gels, films, fibres, and sponges,
is utilized for neural tissue regeneration, wound dressings, and medicine administration (Srinivasan et al.
and elastin. Wool and human hair are the sources of biocompat-
2010; C
ilurzo et al. 2013).
ndustry byproduct, in
6.2.2 Collagen
Vertebrates and other species have collagen, an essential protein that promotes cell
development and maintains the structural integrity of tissues. It is a three-chain
polypeptide protein of animal origin present in various connective tissues (Zhang
et al. 2014). Collagen is a very versatile and biocompatible protein substance. It may
be transformed into several drug-delivery-suitable forms, including films, hydrogels,
and microparticles. The heat presents one difficulty when handling collagen
proteins. Collagen must be processed at a somewhat high temperature to become
water-soluble. Drugs and other chemicals should not be kept using this heat in
microparticles, collagen films, or other materials. Therefore, alternative solvents,
including organic acids, may be used. Collagen-based polymers can be used in bone
mending and cancer therapy (Zhang et al. 2014).
Chemical cross-linking of collagen can improve its mechanical strength and
reduce its hydrothermal and enzymatic degradation rate. Two types of coll agen
molecules can be cross-linked chemically: amide-type and bi-functional. Some
examples of bifunctional reagents that bridge the amine groups of the chain of
collagen polypeptide and other naturally occurring protein polymers include
genipin, glutaraldehyde, hexamethylene diisocyanate, and polyethylene glycol
diacrylate (Ghezzi et al. 2017). Chemical cross-linking reagents can have cytotoxic
or chromogenic effects, with genipin and carbodiimide potentially released into the
environment. Safe and well-chosen collagen cross-linking alteration methods can
prolong drug stability and retention despite potential environmental release (Zhao
et al. 2016).
6.2.3 Elastin
Like collagen, elastin is a protein in the extracellular matrix of several flexible
tissues. One of the body’s most stable proteins, it remains insoluble even after
being stretched and relaxed over a billion times. A significant portion of elastic
fibres comprises elastin, a highly cross-linked structure with a secondary beta-spiral
138 P. Chandra et al.
structure. Although soluble elastin derivatives, including human tropoelastin, are
being studied, native elastin is insoluble (Almine et al.
Elastin-like polypeptid
es (ELP) are biopolymers with amino acids like valine,
2010; Barenghi et al. 2014).
glycine, proline, and unknown (Xaa) useful for vascular stents due to their low
critical solution temperature and phase transition characteristics. It is also feasible to
exercise genetic control on the size and sequencing of macromolecular carriers,
microparticles, and functionalized elastin nanoparticles. Among the potential
e I
conditions of interest for elastin materials are typ
et
thritis (Chilkoti
al. 2006; MacEwan and Chilkoti 2014).
I diabetes, cancer, and osteoar-
Elasticin, another thermoresponsive protein, provides shape and elasticity to the
human body’s connective tissues, blood vessels, skin, and lungs. A range of
thermoresponsive elastin forms, including as elastin-like polypeptides (ELPs),
recombinant human tropoelastin (rhTE), and animal-derived soluble elastin, have
been used to create promising synthetic tissue scaffolds. The hydrophobic portions
of the polypeptide are responsible for producing elastin-like polymers (Zhou et al.
2017).
6.2.4 Silk
Elastin-derived structures exhibit a vital phase change at specific transition
temperatures and are commonly used as thermoresponsive units in biomaterials.
These structures can take the form of a star or linear structure. The lower critical
solution temperature can be adjusted by changing the sequence of amino acids.
Elastin-derived structures can also change flow and retention in porous hydrogels,
elastomeric films, and electrospun scaffolds, promoting cell division and impacting
mesenchymal stem cell destiny. The insoluble beta-sheet crystal structures found in
silkworm cocoons and spider threads are the primary sources of silk fibroin protein.
Drug delivery applications have been implemented with several silkworm species,
including Bombyx mori, Tussah, and Eri silks. Various techniques may be used to
create silk proteins, including films, micro and nanoparticles, and three-dimensional
porous scaffolds, all of which have controllable breakdown rates (Pritchard et al.
2013). Due to its moderate processing conditions, silk lowers costs and is a perfect
material for drug delivery applications. Silk-based materials have controlled biodegradability and good biocompatibility, making them ideal for long-term drug-eluting
depots. The solubilization/degradation of the silk material and the drug payload’s
dispersion are the two primary methods used to describe the features of silkcontrolled release. For instance, when a thin silk film degrades in the designated
area, it may release the necess ary material according to a predetermined solubility
rate (Seib and Kaplan 2013)
Spider dragline silks offer numerous desirable physical characteristics, such as far
better tensile strength and flexibility than natively spun silkworm fibres. There is
great interest in producing silk fibres with spider silk qualities. However, since
spiders are naturally very territorial, developing a method for producing spider silk
via spider farming presents significant challenges. For this reason, standard
.
6 Proteins as Biocompatible Material for Biomedical Applications 139
recombinant silk protein manufacturing systems were used. Due to the inability to
properly assembly native-sized recombinant silk protein into spider silk fibres, only
limited amounts of artificial spider silk have been produced (Teule et al.
Although spider silk may be preferentially produced in E. coli that has undergone
metabolic engineering, the recombinant proteins may also produce copies with lower
molecular weights and worse fibre qualities. Researchers used recombinant DNA to
create the novel genetic engineering vector known as “PiggyBac” to get around these
restrictions (Xia et al.
may enter the silkworm’s
transgenic silkworms generated silk fibres that combined the proteins seen in spider
and chimaera silkworms. The resulting genetically modified silk proteins a re a stable
composite material stronger and more elastic than natural dragline spider silk fibres.
Gene modifications into the present commercial silkworm production proces
recombinant DNA may
a wide scale that surpass natural caterpillar silk’s strength and flexibility. This
biotechnological approach offers solid, lightweight textiles with optimized mechanical qualities for drug-delivery applications, including bandages, suture materials,
and tendon and ligament repair scaffolds (Xia et al.
2010). These vectors are transposon-derived bits of DNA that
genetic machinery. They are derived from spiders. These
provide a feasible way to produce modified protein fibres on
2010).
2012).
s by
6.2.5 Resilin
The cuticles of many jumping insects were shown to contain the elastomeric protein
resilin. The cross-linking of di- and tri-tyrosine connections results in an elastic and
flexible network structure due to the coiled amino acid chains. High-frequency
contraction, extension, and deformation do not impair the resilience and longrange reversible elasticity of dityrosine cross-linked protein structures. Resililine,
with its exceptional flexibility, low stiffness, durability, and efficient energy storage,
is ideal for biomedical appli cations like drug delivery and tissue scaffolds due to its
rubber-like properties and exceptional mecha nical and biocompatibility (Qin et al.
2012; Li et al. 2013; McGann et al. 2013).
6.2.6 Zein
Numerous plant-based protein varieties have been integrated into medication delivery syst ems. Maize kernel endosperm contains zein, a significant plant-based storage
protein with a high prolamine concentration. It is the end product of grinding maize
corn into ethanol using wet and dry milling procedures. Ethanol is then used to make
corn gluten meal. Zein, a 40 kDa molecular weight protein, is commonl y found in
glutamine, leucine, alanine, and proline residues (Nonthanum et al.
et al. 2012; Bouman et al. 2016). Zein, a hydrophobic protein, is soluble in ethanol,
acetone, and acetylacetone but insoluble in water. Its non-polar helical cores contain
glutamine-rich twists and loops, allowing self-assemblement into layers and
particles (Sousa et al. 2012). Zein can fold helical segments consecutively in an
2012; Sousa
140 P. Chandra et al.
antiparallel form to exhibit thermoplastic and chemical properties that are pH and
heat-stable. Zein is a desirable material for application in medication administration
and food and pharmaceutical coatings due to its other remarkable properties, including biodegradability, mechanical robustness, and water barrier capacity. Zein is an
ideal matrix material for delayed release since it may also trap and agglomerate
t al.
solutes like amino acids or medicines (Sousa e
2012).
6.3 Proteins as Adaptable and Biocompatible Building Blocks for Biomedical Applications in Biomaterials
Proteins may come from either plants or animals, are widely distributed in nature,
biodegrade, and can often be modified into materials using gentle methods and
environmental circumstances (Zhu et al. 2018; Grigsby et al. 2020). Because they
are quickly produced in vast quantities, emit little greenhouse gases, and originate
from renewable resources, plant-based proteins offer a sustainable and ecologically
benign source of polymers. Plant-based proteins are often free from animal-related
diseases, making them an appealing option for those who choose not to consume
animal products due to ethical, religious, or personal reasons (Nijdam et al. 2012;
Jones et al. 2013; Eshel et al. 2019). Animal-derived proteins, while expensive, can
be obtained from other sources or byproducts of food or agricultural industries,
making them an affordable and environmentally friendly alternative for biomedical
applications. As an alternative to mammalian collagen, collagen derived from
marine waste materials has been utilized to create osteoinductive bio-composite
scaffolds for bone tissue engineering.
more, k
Further
human hair or biological wastes produced by the chicken industry (Barros et al.
2015; Arslan et al. 2017; He et al. 2017; Rapa et al. 2020). The need for more
sustainable, eco-friendly, safe, and green technology and a circular bioeconomy is
now being recognized. Proteins are utilized in biomaterial production, promoting
green chemistry principles such as renewable and biodegradable building blocks,
waste reduction, and finite raw material use (Park et al. 2015)
Regarding
exhibit more strength than those based on plant proteins. Moreover, the spectrum
of suitable solvents for most plant proteins is restricted, which might complicate
producing biomaterials derived from plants. Selecting the suitable solvent, crosslinking, and combining with natural or synthetic bio-polymers like polysaccharides
can enhance the ad aptability and pertinency of plant and animal proteins (Pinheiro
2016;
et al.
eratin, the building block of biomaterials, may be recovered from
.
mechanical strength, biomaterials based on animal proteins often
Samadian et al. 2020; Fiorentini et al. 2021; Kamada et al. 2021).
6 Proteins as Biocompatible Material for Biomedical Applications 141
6.3.1 Building Blocks for Biomaterials: Biocompatible
and Biodegradable Proteins
Proteins are biocompatible and biodegradable, making them ideal for medicinal
applications. Their degradation rate impacts biomaterial performance, with specific
proteases affecting protein susceptibility to degradation (Guo et al.
folding, including secondary and tertiary structures, can obscure recognition motifs
and limit protein accessibility to proteolytic cleavage, making it resistant to protease
digestion. The protein controls degradation speed, but material type also influences
this process. Müller-Herrmann and Scheibel studied the degradation processes of
various materials, including spider silk protein films, nanoparticles, and electrospun
nanofibers (Muller-Herrmann and Scheibel
Interestingly, many companies are already selling goods based on proteins as
alternatives to current materials utilized in a range of applications. Xampla, a spinoff
company, has successfully detangled and reassembled pea protein into a spider silk
protein structure, producing solid and flexible materials like films, microcapsules,
and coatings for various applications (Chandregowda et al. 2009). GelaCellTM, a
Gelatex product, creates nanofibrous scaffolds for tissue engine ering and wound care
using biobased, eco-friendly gelatin and corn-based zein (Kishore et al. 2008).
Sofregen Medical Inc. owns SERI®, a silk-based scaffold used in U.S. surgical
procedures (Hikal et al. 2017). Furthermore, WounDres collagen hydrogel from
Coloplast facilitates wound healing (Boussahel et al. 2015). Lastly, Matriderm®
Dermal MatriX is a cross-linked collagen-elastin template that doesn’t require
chemicals. It is used as a scaffold for dermal replacement to treat ailments related
to the skin, burns, and poorly healing wounds (Pistritto et al. 2016).
2015).
2020). Protein
6.4 Protein Biomaterials as Minimally Invasive
and Non-invasive Approaches for Biomedical Uses
Various preparation techniques may create materials with distinctive physical shapes
starting from particular protein-building components. Protein inherent qualities,
such as bioactivity, may be retained in the substance. The protein’s chemical and
physical characteristics are used in various applications to create materials with
specific qualities. To produce materials with optimized and speci fied qualities,
choosing the suitable protein and having total control over the processing conditions
is essent ial. Table 6.2 showcases protein-based compounds, showcasing their
remarkable adaptability in various applications, which will be further explored in
subsequent sections.
6.4.1 Protein-Based Particle Systems
One of the crucial functions of proteins in vivo is to function as natural transporters
of various compounds, including ions, small molecules, and other macromolecules.