Chemical-computational comparison of organometallic complexes in oxygen

carriers and their incidence on blood color

Fabián Santana-Romo

1,*[0000-0002-8753-3349]

, Jessica L. Peñaherrera

1[0009-0009-0732-9494]

, Jorge

S. Sanchez

1[0000-0001-5978-7897]

and Aurora L. Carreño-Otero

2[0000-0003-2357-4798]

Universidad de las Fuerzas Armadas ESPE, Sangolquí 171103, Ecuador

Bucaramanga 680002, Colombia

*fmsantana@espe.edu.ec

Abstract. Blood typically carries small amounts of molecular oxygen (O

) dissolved in

the plasma and large amounts chemically combined with hemoglobin. Partial pressure

depends only on physically dissolved oxygen, which determines how much oxygen will

combine with hemoglobin. The blood needs an O

carrier in humans because this gas is

not sufficiently soluble in the blood plasma to satisfy the body's requirement. At 37 ºC, a

liter of blood only dissolves 2.3 mL of O

. However, one liter of blood contains 150 g of

hemoglobin, and since each gram of hemoglobin dissolves 1.34 mL of O

, 200 mL of O

are transported per liter of blood. This quantity is 87 times more than what plasma could

carry by itself. Without an O

carrier like hemoglobin, blood would have to circulate 87

times faster, requiring a high-pressure pump, turbulent flow, and a considerable

ventilation-perfusion mismatch. This is why other transporter molecules exist in various

organisms, giving each of them a representative color. This is how it is in the human being

that blood is red; in spiders, crustaceans, mollusks, octopuses, and squids, the blood is

blue; in some segmented worms, some leeches, and some marine worms, there is green

blood; finally, in marine worms and brachiopods, the blood is violet.

Keywords: Blood Color, Oxygen, Organometallic Complex, Carriers.

INTRODUCTION

Blood is a tissue mainly responsible for oxygen and carbon dioxide transport. However,

it has other important functions, such as transporting nutrients and hormones or capturing

and dissipation of heat. Reptiles, amphibians, and fish usually transmit heat through their

skin, so they are exposed to sunlight to heat up since they do not have their own source.

When the body needs to conserve body heat, blood flow is reduced, and conversely, when

it needs to dissipate heat, blood flow increases (Arvanitis et al., 2020; Cho et al., 2020;

Fuchs & Whelton, 2020).

Blood is produced in specialized tissues such as the red bone marrow (found in flank

bones) in mammals and the kidneys in fish, in a process known as hematopoiesis ("blood

creation"). Blood comprises a transparent tissue called plasma and cells such as red blood

cells, immune system cells, and platelets, responsible for repairing or healing areas where

the tissue has been damaged. In most cases, inside each red blood cell, a protein captures

oxygen molecules (Brass et al., 2019; Versteeg et al., 2013). This protein is a pigment.

The type of pigment in each group of organisms determines the color of the blood. Some

animals have red blood due to hemoglobin; others may have various pigments that will

change their blood color (blue, green, orange, yellow, purple); others are usually colorless

because they do not have any pigment. Even so, all organisms develop respiration to

oxygenate their tissues (Ji et al., 2022; Noris & Galbusera, 2023; Simpson et al., 2022).

Due to the presence of hemoglobin in vertebrates and some invertebrates, red blood is the

most common for transporting gases. Hemoglobin is a globular protein comprising four

coordinated subunits, differentiated only by their chains: two alpha and the remaining

beta (Ahmed et al., 2020; Ali et al., 2022; Giardina, 2022).

Certain invertebrates, instead of hemoglobin, have hemocyanin, an organometallic

oxygen-carrying complex. The critical difference is that you have a metallic coordinating

agent of copper; it turns blue in the oxygenated form, and after exchanging, it turns

colorless in its deoxygenated form. The organometallic complex owns two copper atoms

that coordinate with O

. Certain mollusks and some arthropods have it in their

hemolymph. (Li et al., 2019; Zhan et al., 2019; Zhao et al., 2022; Zheng et al., 2021)

One genus of skinks (Prasinohaema) has green blood and hemoglobin in its blood, but

they have a higher concentration of biliverdin. The metabolism of precursor hemoglobin

generates this pigment. Thus, being a derivative of it, it is a secretion generated in the

liver secreted with bile. In the green-blooded skink, the biliverdin in its blood reaches

critical levels that would be toxic to other lizards, predators, or organisms such as humans

(Cimini et al., 2022; Mancuso, 2021). Segmented worms of the (Annelida) family and

leeches have chlorocruorin, the green pigment in their blood. This molecule not always

shows their color, since some other annelids have hemoglobin, and this red color masks

the green (Bankar et al., 2021; Benbelkhir & Medjekal, 2022; Clauss & Clauss, 2021).

In the case of brachiopods, they are known to have purple blood, the color provided by

their oxygen carrier. Hemerythrin is a bulky organometallic complex that conjugates a

bidentate ligand and coordinates with the oxygen molecule. They have an open

circulatory system, a small heart suspended above the stomach. Some of the cells in the

coelomic fluid contain hemerythrin. This suggests that the coelomic vessels constitute the

primary transport system for respiratory gases. Some other invertebrate-type marine

organisms have hemerythrin as an organometallic complex that transports oxygen in the

blood. It is pink-violet in color when oxygenated, and in its deoxygenated state, it lacks

color (Kitanishi, 2022; Stenkamp, 1994; Wilkins & Wilkins, 1987).

There are colorless fluids in ice fish that are representatives of the family

(Channichthyidae), they are vertebrates, and they are called white or colorless blood. No

organometallic complex provides color to their blood; due to this characteristic, they have

adapted other physiological functions to live in cold waters (DeVries, 1988; Garofalo et

al., 2009; Padang et al., 2020). Oxygen dissolves better in cold water than in warm water;

this is not enough to keep fish alive. One of the adaptations is having a large heart that

pumps much blood with each beat, with a greater blood volume than fish of comparable

size with red blood. Numerous blood vessels in the skin absorb some oxygen, while the

gill membranes aid gas exchange (Kim et al., 2019).

Organisms with alternating fluids, such as insects, have their hemolymph pale

yellow/green or colorless; hemolymph does not transport O

so it does not require an

organometallic coordination complex. Thus, tubular networks transport O

through a

tracheal system that exchanges gas in aerobic respiration (Blow & Douglas, 2019;

Theopold et al., 2002; Wyatt, 1961).

MATERIALS AND METHODS

Blood Type Contrast in Different Organisms

The literature search will seek various types of organisms that may present notable

variability in their blood fluids. With discrimination of results, it is proposed to observe

mainly the color of the oxygenated blood fluid, compared with the standard molecule for

the human being. Here, the hemoglobin, with its red color, becomes a pattern to be

compared for the multiple species and transporter molecules of oxygen.

It is proposed to compare with the substrate enzyme complexes reported in the RCSB

Protein Data Bank (RCSB PDB) platform, and to obtain the crystalline structures,

exporting the complex for purification, showing only the three-dimensional structure of

each one (Duarte et al., 2022).

Basal Oxygen Carriers and Organometallic Complexes

It is well known that oxygen is transported physically dissolved in the blood and

chemically combined with carrier molecules. Under normal circumstances, more oxygen

is transported with these molecular carriers than is physically dissolved in the blood. In

this way, these molecular targets will be reported as individual structures, that is, it is their

basal state, and it will be contrasted with the internal structures in organometallic

complexes that function as oxygen carriers.

The basal structures will be compared. Some will present a porphyrin structure in

common, looking for the explanation of why they need a metallic agent and how their

coordination number makes possible the conformation of the organometallic complex.

Three-Dimensional Biological Complexes

Knowing that each crystalline complex has protein chains with quaternary structures,

always associated with a ligand, also called exogenous complement structure. The

individual purification of each complex will be carried out using the UCSF Chimera 1.16

Software (Pettersen et al., 2004), eliminating complementary chains and non-compatible

amino acid residues at 4 Angstroms, a length universally considered by default to predict

potential intermolecular interactions (Núñez-Navarro et al., 2016; Santana-Romo, Duarte,

et al., 2020; Santana-Romo, Lagos, et al., 2020).

Each generated image will report the organometallic complex in three dimensions and the

transported oxygen molecule. In the same way, it will be possible to visualize the

coordination links with the metallic agents and how they, through intermolecular

interactions, hold and support the entire complex of oxygen-transporting molecules.

RESULTS AND DISCUSSION

Blood Type in Different Organisms

Red blood. This quantity is 87 times more than what plasma could carry by itself. In the

most developed organisms, blood is red, which takes its color mainly from its content of

red blood cells, as it is in humans (Figure 1). It contains hemoglobin, a protein responsible

for transporting oxygen throughout the body. Hemoglobin is made up of iron, which turns

red when it encounters oxygen. The reddish color refers presence of iron in nature, for

example, in soils with high iron content (Ali et al., 2022; Giardina, 2022).

Now, at certain times the red color of the blood varies. For example, blood that is more

oxygenated and carried from the lungs to the body's organs and muscles through the

arteries tends to be pinker. On the other hand, dark blood has already transported oxygen

to some destination and returns, through the veins, to the heart and lungs, carrying CO

with it to be expelled through respiration to start the cycle again (Ahmed et al., 2020).

Blue blood. Certain organisms such as spiders, crustaceans, some mollusks, octopuses,

and squids have their blood flow blue (Figure 1b). It is important to note that they have

the disadvantage that their oxygen-carrying capacity is three times less than that of red-

blooded organisms. Oxygen is essential to generate energy, and that energy is used to

perform bodily movements or other activities such as growing and searching for food or

a mate. The reduced oxygen-carrying capacity of blue-blooded organisms has led to

cephalopods in the Southern Ocean leading a sedentary lifestyle, marked by little

movement and no bodily changes during their life. (Li et al., 2019; Zheng et al., 2021).

The scientific community is currently concerned that these organisms will not be able to

respond adequately to global warming and will disappear. The main reason is that the

affinity of hemocyanin for oxygen decreases with increasing temperature. What was an

adaptation to extreme cold is a drawback in the current global situation, in which the seas

are warming (Zhan et al., 2019; Zhao et al., 2022).

Green blood. Species of lizards, leeches, some segmented worms, and marine worms

have their blood green (Figure 1c). It is not because they do not have hemoglobin but

rather because of an accumulation of biliverdin, a bile pigment produced by the

catabolism of hemoglobin, which first becomes bilirubin and then, through oxidation,

biliverdin. When it forms its porphyrin conjugation and unites with the iron atom to form

the organometallic complex, this molecule takes the name of chlorocruorin. This

molecule transports oxygen in organisms that have green blood. Green color

predominates on cells and stains blood, bones, muscles, tongue, and mucous membranes

(Cimini et al., 2022; Mancuso, 2021).

In all other vertebrates, this excess bile pigment in the blood causes a pathological

problem known as jaundice, which can cause severe motor and brain damage. The

pigments in certain lizards have a toxic function for their predators; their absorption in

the visible spectrum provides protection against ultraviolet rays. Unlike warm-blooded

organisms, they do not have thermoregulation, so they are exposed to the sun's rays to get

warm (Bankar et al., 2021; Benbelkhir & Medjekal, 2022; Clauss & Clauss, 2021).

Violet blood. Purple blood tends to have this hue due to hemerythrin (Figure 1d); they

are usually present in brachiopods and marine worms, including peanut and penis worms

(Kitanishi, 2022). This complex is another pigment that contains iron, which binds to

oxygen molecules and gives a purple-pink hue. This blood is found in some mollusks,

such as brachiopods (lamp shells), and urochordates or sea squirts (Stenkamp, 1994;

Wilkins & Wilkins, 1987).

Transparent Blood. There is evidence of a particular group of fish whose blood is

transparent. In the Southern Ocean, the so-called "ice fish" lack respiratory pigment in

their blood, which is why it is considered colorless or transparent (DeVries, 1988;

Garofalo et al., 2009). Although it seems illogical not to have respiratory pigment in the

blood, it is likely because the pigment would increase the viscosity of the blood fluid,

which in cold places would make it difficult for the heart to pump. Thus, natural selection

would have favored the removal of pigment in these organisms. On the other hand,

oxygen availability in this habitat is very high, so it is unnecessary to have respiratory

pigment (Kim et al., 2019; Padang et al., 2020).

Other types of fluids. As a complement to the deep search for information, we detail that

other organisms have marked differences in their blood (Figure 1e).

Insect blood. These organisms do not have blood but instead have a fluid called

hemolymph, which is a fluid that carries gases and hormones through the system. Insects

do not absorb oxygen from the blood through openings along their backs and sides.

Hemolymph usually has yellowish or bluish-green pigments. These are often due to these

animals’ diets (Blow & Douglas, 2019; Theopold et al., 2002).

Bloodless animals. In the sea, there is an animal called a flatworm: it looks like a worm;

the oxygenation of the tissues depends on the skin since it lacks a circulatory system. Gas

exchange provides respiration for the body while nutrients go to the intestine. On the

other hand, starfish and sea cucumbers do not have blood, as water is the equivalent, and

it transports nutrients and gases through a water-based vascular system (Herhold et al.,

2023).

Fig 1. Organisms with various types of blood.

Basal Oxygen Carriers and Organometallic Complexes

The complexes reported in Figure 3 clearly show that they are not associated with oxygen;

thus, some are organometallic complexes, such as hemoglobin, chlorocruoryn, and

hemerythrin. On the other hand, in Figure 2, hemocyanin shows two individual

complexes of tris(4-((S)-2-amino-3-oxopropyl)-1H-imidazol-1-yl)copper.

The metallic agent is significant; only in hemocyanin is the copper atom in the two

individual complexes that will form a single organometallic complex by generating their

coordination bonds during oxygen transport.

Fig 2. Hemocyanin organometallic complex subunit

Fig 3. Organometallic no oxygenated complexes in the blood.

Figure 4 shows all the organometallic complexes duly associated with the oxygen

molecule, and it should note that oxygen is generally represented as O

, diatomic,

molecular, or gaseous state. Oxygen transporter complexes show both formal bonds,

sigma σ type, and attractive intermolecular interactions for coordination bonds.

For hemoglobin and chlorocruorin, it is noted that a pair of the nitrogen atoms of the

porphyrin ring has a positive formal charge, indicating that they have a more electrophilic

character when generating false bonds from the coordination of the iron atom.

Regarding the hemocyanin and hemerythrin molecules, no positive or negative formal

charges are shown; this suggests in terms of reactivity that the carrier molecule remains

relatively stable associated with the presence of O