Mass spectrometric analysis of selected radiolyzed amino acids in an astrochemical context

Mass spectrometric analysis of selected radiolyzed amino acidsin an astrochemical context

Cristina Cherubini • Ornella Ursini •

Franco Cataldo • Susana Iglesias-Groth •

Maria Elisa Crestoni

Received: 18 November 2013 / Published online: 9 March 2014

� Akademiai Kiado, Budapest, Hungary 2014

Abstract A selection of amino acids, namely arginine,

proline and tyrosine previously irradiated to 3.2 mega-

Gray in the solid state and analyzed by differential

scanning calorimetry (DSC) and optical rotatory disper-

sion (ORD) were analyzed in the present work by mass

spectrometry with the purpose to identify the radiolysis

products and validate the results obtained previously with

DSC and ORD. The radiolysis of amino acids is a top-

down approach of a research program designed to assess

the radiolysis resistance of these molecules for 4.6 9 109

years once buried in primitive bodies of the Solar

System.

Keywords Amino acids � Radiolysis � Mass

spectrometry � Arginine � Proline � Tyrosine

Introduction

The current research on the solid state radiolysis of amino

acids has been inspired by the work of Urey [1–4] (Nobel

Laureate in 1934) who made an intriguing calculation

which is still valid today. The calculation shows that

organic molecules buried at a depth of[20 m in asteroids,

comets or other primitive bodies of the Solar System are

completely shielded from cosmic rays and are exposed

only to the high energy radiation derived from the decay of

the naturally occurring radionuclides present in the rocks

[2–5]. This implies that, for example, amino acids were

synthesized abiotically before the formation of the Solar

System. Afterwards they were incorporated in asteroids,

comets and large meteorites where they were degraded for

4.6 9 109 years (the age of the Solar System) only by the

radionuclide irradiation, assuming negligible water and

thermal processing inside these bodies.

In order to assess the radiolysis resistance of amino

acids and the preservation of chirality to high energy

radiation, Cataldo and colleagues [6–10] have started a

systematic study on the radiolysis of all proteinogenic

amino acids to a radiation dose of 3.2 mega-Gray (MGy),

which corresponds to 22.8 % of the total dose they should

have received inside asteroids, comets and other bodies of

the Solar System [6–10]. The results were extrapolated to

14 MGy of radiation dose showing that practically all the

proteinogenic amino acids can partially ‘‘survive’’ to such

an enormous dose with preservation of the enantiomeric

excess [10]. Since an important fraction of amino acid

species found in meteorites are non-proteinogenic [11],

Cataldo et al. [12, 13] have irradiated in the solid state to

3.2 MGy also a series of unusual amino acids belonging to

the classes found in carbonaceous chondrites. The con-

clusions were analogous to those made on proteinogenic

C. Cherubini � O. Ursini (&)

CNR-Istituto di Metodologie Chimiche, Area della Ricerca di

Montelibretti, Via Salaria Km 29.300, 00015 Monterotondo,

RM, Italy

e-mail: [email protected]

F. Cataldo (&)

Actinium Chemical Research srl, Via Casilina 1626A,

00133 Rome, RM, Italy

e-mail: [email protected]

S. Iglesias-Groth

Instituto de Astrofisica de Canarias, Via Lactea snc, Tenerife,

Canary Islands, Spain

M. E. Crestoni

Dipartimento di Chimica e Tecnologie del Farmaco, Universita

di Roma Sapienza, P. le A. Moro 5, 00185 Rome, RM, Italy

123

J Radioanal Nucl Chem (2014) 300:1061–1073

DOI 10.1007/s10967-014-3078-1

amino acids: even the non-proteinogenic amino acids are

able to resist to a radiation dose equivalent to 4.6 9 109

years [12, 13].

Therefore, it is not a surprise that some scientists have

confirmed in recent times the presence of amino acids and

other molecules of biochemical interest inside carbona-

ceous chondrites [14–24]. According to certain theories,

these molecules are thought to be abiotically synthesized

before the Solar System formation, for example in the

interstellar medium and in the molecular clouds [25, 26].

Quite unexpected, they were found also in enantiomeric

excess [15–24]. The works of scientists like Bonner and

colleagues [27–31], Kminek and Bada [32] and other

recent works [14–24] have proved experimentally that the

radiolysis resistance of amino acids is accompanied also by

a preservation of the enantiomeric excess, although also the

phenomenon of radioracemization was detected.

In previous works the amino acids were irradiated

[12, 13], the solid state radiolysis causes the formation of

radiolytic products in the crystal structure of each amino

acid. Some of the radiolytic products have low molecular

weight and escape from the crystal by diffusion, while other

products remain trapped in the damaged crystal structure

together with the residual amount of not degraded amino

acid. The melting enthalpy per unit mass of a given organic

compound is a value dependent from the purity of such

compound. Hence, the formation of decomposition products

and the reduction of the amount of pristine molecules in the

crystal after the radiolysis leads inevitably to a reduction of

the melting enthalpy. The residual purity of a chemical

measured by differential scanning calorimetry (DSC) is

linked to the melting enthalpy value which is connected to

the cohesive energy of the molecular crystal. The presence

of foreign molecules in the knots of the crystalline structure

alter the melting enthalpy of the crystal. Furthermore, the

presence of foreign molecules (the radiolysis products) are

source of defects in the crystal of the type of Schottky and

Frenkel and this is another source of melting enthalpy

reduction. As a general rule the reduction of the melting

enthalpy is proportional to the amount of foreign molecules

present in the molecular crystal.

The ratio of the melting enthalpy after radiolysis over

the melting enthalpy before radiolysis gives an indication

of the amount of amino acid that ‘‘survived’’ the radiolysis.

The results of previous studies on amino acid radiolysis

obtained with DSC [6–10] are corroborated by the results

of a similar work where the radiolyzed amino acids were

analyzed by gas chromatography, GC [32]. Indeed, the

decomposition rate constant of certain proteinaceous amino

acids measured with DSC [6–10] are in reasonable agree-

ment with those determined by GC [32].

The effects of radiolysis on proteinaceous and non-

proteinaceous amino acids were previously measured also

by optical rotatory dispersion (ORD) spectrometry, a

polarimetric technique [6–10, 12, 13]. The measurement of

the optical activity by a polarimetric technique, is not a

measurement of pure radioracemization. It is instead a

measurement of the sum of the radioracemization and the

radiolysis of the amino acid. As shown by Bonner and

colleagues [27–31], the radioracemization of a chiral sub-

strate at high radiation dose is, after all, a secondary phe-

nomenon since the primary effect of high energy radiation

is the radiolysis, the degradation of the amino acid into

smaller and generally achiral molecular fragments. How-

ever, in the radiolytic process the radical fragments can

lead to possible formation of different products, some of

which keep a chiral centre.

In the present work we have investigated the irradiation

process of the L-amino acids (L-tyrosine, L-arginine and L-

proline) using the ESI–mass spectrometry (MS). The use of

a MS is important to complete the DSC and ORD data and

to identify the radiolysis products including those which are

able to keep a chiral centre. Moreover, it is possible to make

a quantitative analysis of irradiation products and to deter-

mine the amount of the D-enantiomer formed by radiation.

Experimental

Materials and equipment

The amino acids L-tyrosine, L-arginine and L-proline were

obtained from Sigma-Aldrich (Milan, Italy) and used as

received. The DSC analysis of the amino acids before and

after irradiation was made on a DSC-1 Star System from

Mettler-Toledo. The ORD spectra were obtained on a Jasco

P-2000 spectropolarimeter with a dedicated monochromator.

The chemical structure of reaction products and the

quantitative analysis were performed by MS analysis using

a Finnigan LXQ linear ion trap system equipped with a ESI

ion source. We used the ESI source in either positive or

negative ion polarity mode in order to have the most

complete recognition of products. Operating as MSn scan

mode (n = 1–5) where n is the scan power and where each

stage of mass analysis includes an ion selection step, we

have acquired the maximum structural information about

the single amino acid analyzed.

The HPLC analysis was made using a Shimadzu liquid

chromatograph LC-10AD VP equipped with a chiral

column.

Irradiation procedure with c rays

The irradiation with c rays was made as already detailed in

the previous works [6–10, 12, 13].

1062 J Radioanal Nucl Chem (2014) 300:1061–1073

123

Analysis with DSC and ORD

The irradiated samples were tested for purity by a DSC as

already reported previously [6–10, 12, 13]. The amount %

of residual sample after the solid state radiolysis Nc was

determined from the ratio of the melting enthalpy after the

radiolysis at 3.2 MGy (DHc) and the enthalpy before

radiolysis measured on the pristine sample (DH0):

Nc ð%Þ ¼ 100 DHc=DH0

� �: ð1Þ

Similarly, as already reported previously [6–10, 12, 13]

the ORD was determined from the ratio of the average

specific optical rotation after radiolysis [a]c and before

radiolysis [a]0 the residual optical activity Rc was

determined:

Rc ð%Þ ¼ 100 ½a�c=½a�0n o

: ð2Þ

Mass spectrometric analysis of pristine and radiolyzed

L-arginine, L-proline and L-tyrosine

Each amino acid under study (L-tyrosine = 2.33 mg,

L-arginine hydrochloride = 2.59 mg, L-proline = 2.83 mg)

was dissolved in 1 mL of methanol and 1 mL of ammonium

acetate solution 45 mM.

In order to obtain a more direct comparison between the

pristine and the irradiated samples, we used the same

weight for both of them.

The solutions of amino acids were directly injected

in the ESI source with a syringe pump at a flow rate of

10 lL/min. The acquisition time of every spectra was of

0.3 min. The MS of the irradiated amino acid was com-

pared with that of the pristine amino acid, with the aim of

identifying new irradiation products.

The ionization process was accomplished in ESI source

in positive and negative ion polarity mode, in order to

determine the products deriving from the irradiation. In

fact, the products that retain the acidic groups were better

detectable in negative mode while the products that retain

the basic groups were detectable in positive mode.

In the ion trap analyzer, each new product derived from

the irradiation process was firstly isolated as specific

m/z family ions and then fragmented by collision induced

dissociation (CID) process [33]. The CID process allows

the fragmentation of a selected ion applying a specific

resonance excitation voltage that enhances the ion motion.

As a result, the ions with the same m/z value gain kinetic

energy and through no-reactive collisions with helium

damping gas present in the mass analyzer, they dissociate

to form product ions. This sequential process can be used

again to isolate one of the ion produced by the first CID

process and subject to further collisions (MSn). The frag-

mentation pathway has allowed the determination of the

ion structure and the identification of the chemical nature

of the products.

Coupling ESI–MS with a HPLC equipped by a chiral col-

umn (teicoplanine based stainless steel column,

150 mm 9 4.6 mm) allows us to measure the amount of

D-enantiomer formed by irradiation process [34]. The mobile

phase was 90 % methanol and 10 % ammonium acetate

solution 45 mM.

Results and discussion

To go further insight the radiation chemistry of the radio-

lyzed amino acids at 3.2 MGy, we have selected three

amino acids (L-arginine, L-proline and L-tyrosine) which

were previously studied with DSC and ORD and which

were analyzed by MS. The purpose was the qualitative and

quantitative identification of the radiolytic products,

establishing the amount of the pristine amino acid that

‘‘survived’’ the irradiation process. In particular a special

effort is devoted to elucidate the chemical structure of

radiolytic products, using the possibility to operate in MSn

mode.

The ESI–(MS)n analyses and the possibility to couple

the ESI–ion trap MS with a HPLC instrument equipped

with a suitable chiral column has offered us the possibility

to understand the chemical nature of radiolytic products,

establishing which products lost the chiral centre and

which one was able to keep it. The use of a chiral column it

is also important to determine the radioracemization level

of the residual amino acid, that is to establish the right

amount of D enantiomers formed by the action of c rays

from the starting L-amino acids.

All these factors are key elements useful to understand

the chemical reasons responsible of the difference between

the two values: the amount of residual sample ‘‘survived’’

after the radiolysis (Nc) and residual optical activity (Rc)

studied in our previous investigation [6–10, 12, 13].

Mass spectrometric analysis of pristine and irradiated

arginine

The structure of arginine is reported in Fig. 1.

NH2 NH

NH

NH2

OH

O

Fig. 1 Chemical structure of arginine

J Radioanal Nucl Chem (2014) 300:1061–1073 1063

123

Fig. 2 Mass spectrum of irradiated arginine: positive ion mode. The blue rectangles mark the ions detected in pristine sample. The red

rectangles mark the new ions derived from irradiation products. (Color figure online)

CH2

NH2

OH

O

Fig. 3 Structure of allyl glycine

NH2 NH

NH

NH2

Fig. 4 Structure of 1-(4-aminobutyl)-guanidine, neutral of ion

m/z 131


123

NH2 NH

NH

OH

O

NH

NH2

OH

O

NH

I II

Fig. 5 Neutral structures of ion m/z 160. The structure I derived from deamination process on the chiral centre. The structure II is the neutral

product due to the deamination process on the guanidine terminal group

Fig. 6 Fragmentation spectrum of the ion m/z 160


123

The DSC analysis of pristine and irradiated arginine at 3.2

MGy suggested a high chemical decomposition (Nc = 56.6)

after arginine irradiation [6–10, 12, 13]. However, the residual

optical activity of the irradiated arginine appeared less altered

by radiation according to the ORD measurements since

Rc = 75.3 [6–10, 12, 13]. This difference between the

chemical decomposition value and the higher ORD data could

be rationalized with the formation of radiolytic products that

are able to conserve a chiral centre. The MS of irradiated

arginine in positive ion mode is reported in Fig. 2.

In the spectrum we marked with blue rectangles all

those ions that were already detected in pristine sample.

These ions are:

– ion m/z 175: protonated arginine,

– ion m/z 349: proton bound dimer,

– ion m/z 523: arginine trimer.

NH3+

NH

NH

OH

O

NH

NH3+

OH

O

NH

NH

NH

OH

O

CH2+

OH

O

NH OH

O

NH

CH3

NH3+

OH

O

Structures of ion m/z 160

- guanidine

ion m/z 101

-NH3ion m/z 143

- HN=C=NH

ion m/z 118

+

+

Fig. 7 Fragmentation scheme of ion m/z 160 and plausible products structures

Table 1 Relative percentages

of products formed by arginine

radiolysis

Ions Relative %

Arginine 60.3

116 1.2

131 1.3

160 35.4

319 0.8

334 1.0

NH

OH

OFig. 8 Chemical structure of

proline


123

The ions corresponding to the radiolytic products are

marked by red rectangles and they are the ions m/z 116,

m/z 131, m/z 160, m/z 319 and m/z 334.

Through the MSn analysis the ion m/z 116 was identi-

fied as protonated allyl glycine, that is formed by the

detachment of terminal guanidine induced by irradiation

process.

The allyl glycine (Fig. 3) maintains the a chiral centre

and could be partially responsible of the observed optical

activity retention.

The ion m/z 131 is identified as protonated 1-(4-amin-

obutyl)-guanidine (Fig. 4), formed through the decarbox-

ylation process.

The ion m/z 160 could be represented as the protonated

ion of by two possible structures, shown in Fig. 5, both

derived from deamination processes induced by radiolysis.

The structure on the left of Fig. 5 represents the loss of aamine group of the amino acid, while the structure on the

right of Fig. 5 is obtained removing the amine group from

the guanidine site.

Fig. 9 Proline mass spectrum in positive ion mode. The spectrum mass range is from m/z 50 to m/z 400. The blue rectangles mark the ions

detected in pristine sample. The red rectangles mark the new ions derived from irradiation products. (Color figure online)


123

It should be noted that only the second structure main-

tains the chiral centre.

The fragmentation spectrum (Fig. 6) of the ion m/z 160

confirms the presence of both structures.

Although the loss of ammonia can be justified by both

structures, through MS2 analysis we detected the fragment

m/z 101 due to the loss of a guanidine molecule. This

fragmentation pathway can be only explained with the

presence of the structure I of Fig. 5. On the other hand, the

presence of ion m/z 118, that derives from the loss of

carbodiimide as neutral molecules, could be only ratio-

nalized starting from structure II of Fig. 5.

The fragmentation scheme of the ion m/z 160 is reported

in Fig. 7.

In the Table 1 are reported the relative percentages of

products formed by irradiation process. It is necessary to

mention that these values are calculated on the basis of the

relative abundance of the corresponding mass spectromet-

ric peaks. The amount of arginine ‘‘survived’’ the radiation

dose of 3.2 MGy is 60.3 % as measured by MS, in good

agreement with Nc = 56.6 measured by DSC and sug-

gesting that 56.6 % of arginine was recovered unchanged

after the irradiation. This result is a confirmation of the

validity of the DSC analysis as a rapid method for the

estimation of the purity of compounds.

Due to their low intensities, it was not possible to isolate

the ions m/z 319 and m/z 334. However, they seem to be

proton bound dimer derived from ionization process. The

first one is the proton bound dimer of the ion m/z 160 with

an arginine molecule, while the second one is the proton

bound dimer of the ion m/z 160.

The mass spectrometric analyses allow us to under-

stand the difference between the DSC and ORD data,

previously obtained. In fact, through the ESI–MSn stud-

ies we have identified the chemical structure of two

optically active irradiation products, the allyl glycine

corresponding to the ion m/z 116 and the radiolytic

product corresponding to the structure II of the ion

m/z 160 (Fig. 5).

The estimated relative percentages of these two ions,

could offset the difference between the chemical decom-

position and the optical residual activity values.


proline

Proline with its cyclic structure (Fig. 8) is different from

the other amino acids analyzed, and this difference is dis-

closed by the absence of radiation-induced deamination

process.

The amino group is protected by the cyclic structure and

the high energy radiation essentially causes a cleavage of

the cyclic structure. The positive ion mode MS of proline is

shown in Fig. 9.

The blue rectangles indicate the ions already present

in the MS of pristine amino acid, while the red rectangles

mark the ions corresponding to the radiolysis products.

The ions present in the MS of pristine proline are the

m/z 116 [proline–H?] and the m/z 138 [proline–Na?]

adduct.

The MSn analysis helped us to figure out the structure of

some new ions present in the irradiated sample. The ion

m/z 118 is identified as protonated 5-aminopropionic acid,

formed by the opening of the cyclic structure of proline

induced by irradiation.

By irradiation, it is possible to have some fragmentation

process of proline skeleton. The resulting radical species

could react each other or/and react with the neutral mole-

cules leading to the formation of products that generally

have a larger molecular mass than the starting proline. Two

of these products can be the ion m/z 185 and the ion m/

z 199 that are shown in Fig. 10.

Furthermore, the same radicals could generate smaller

molecules through simple rearrangement processes. In fact,

from the MS obtained operating within a mass range from

15 to 200 m/z (Fig. 11) it is possible to observe the

NH

NH2

O

OH

NH

OH

NOH

O

NH2 OH

O

a

b c

Fig. 10 Possible structure of the neutral species corresponding to the

ions m/z 118 (a), m/z 185 (b) and m/z 199 (c)


123

presence of two additional radiolysis products: the ions

m/z 72 and m/z 100.

The ion m/z 72 corresponds to the decarboxylation

product, while the ion m/z 70 corresponds to the product

formed by the loss of formic acid due the ESI ionization

Fig. 11 Proline positive ion mode mass spectrum at low mass range. The blue rectangles mark the ions detected in pristine sample. The red


NH

H

OFig. 12 Neutral structure of ion

m/z 100, pyrrolidine-2-

carbaldehyde


123

process. Even the ion m/z 100 represents a new ion and its

hypothetical neutral structure is shown in the Fig. 12.

In the Table 2 are reported the relative percentage of

products formed by irradiation process as determined by

the mass spectrometric analysis.

The sum of the relative percentages of the radiation

products is in good agreement with the result obtained by

DSC method (Nc = 83.5).

Furthermore, the presence of some optical active ions,

such as the ions m/z 100, m/z 185 and m/z 199, could justify

the value of Rc (Rc = 86.8) higher than the Nc value.

The chiral HPLC–ESI–MS analysis allowed us to sep-

arate the L and D enantiomers. In order to determinate the

retention time of the enantiomers we used the L and

D proline in the optical pure form. The retention time of

L-proline is 2.85 min, while the retention time of D-proline

is 4.56 min. Analyzing the irradiated proline, we observe

the presence of both enantiomers.

The amount of D-proline, formed by radiation process, is

4.8 % respect to the L-enantiomer. The presence of the

D-enantiomer should affect negatively the Rc value (should

lower the Rc). However, the presence of other chiral pro-

ducts formed after irradiation and detected by MS, com-

pensates the effects of the D-enantiomer on Rc and justify

the difference between the Nc and Rc values.


tyrosine

The structure of tyrosine is reported in Fig. 13.

The MS of irradiated tyrosine in positive ion mode is

reported in Fig. 14.

From the spectrum it is possible to observe the presence

of ions already detected in pristine sample (marked by blue

rectangle), such as

– ion m/z 182: protonated tyrosine,

– ion m/z 136: loss of formic acid due to the ionization

process,

– ion m/z 165: loss of ammonia due to the ionization

process,

– ion m/z 204: Na? adduct,

– ion m/z 363: proton bound dimer,

– ion m/z 385: sodium bound dimer.

The radiolysis products correspond to the ions m/z 138

and m/z 121, marked by a red rectangle. Both these com-

pounds derive from amino acid decarboxylation process.

The 2-(p-hydroxyphenyl)-1-ethanamine is the compound

formed by the direct decarboxylation process (CO2 loss)

and corresponds at the ion m/z 138, while the m/z 121 ion

derives from the m/z 138 ion after loss of ammonia due to

ESI ionization process (Fig. 15).

Moreover, in negative ion mode, it is possible to observe

the ion m/z 165 derived from the radiolytic deamination of

tyrosine, which corresponds to the neutral product 3-(p-

hydroxyphenyl)-1-propanoic acid (Fig. 16).

The DSC and ORD data, show that the tyrosine pos-

sesses an high level of radiolysis resistance (Nc = 92.1)

with a good residual optical rotation (Rc = 98.9). In first

instance, this slight difference between these two value

could be rationalized with a hypothetic formation of

radiolytic products that keep the chiral centre of the amino

acid. The results of mass spectrometric analysis do not

validate this preliminary hypothesis, because the products

that derive from the decarboxylation and deamination

processes have lost the chiral centre. Hence, they cannot be

responsible of the difference between the residual per-

centage of pristine amino acid and the residual percentage

of its optical activity.

In the Table 3 are reported the relative percentage of the

products formed by the irradiation process.

In the case of tyrosine, the radiolysis resistance

assessed by MS has shown that 97.8 % ‘‘survived’’ the

radiation dose of 3.2 MGy. This result is in closer

agreement with the Rc = 98.9 measured with ORD rather

than with the Nc = 92.1 value measured with DSC.

However the standard deviation in the repeatability of

Table 2 Relative percentages

of products formed from proline

radiolysis

Ions Relative %

Residual proline 83.07

72 1.97

100 1.73

118 4.77

185 2.27

199 1.15

206 2.22

277 1.76

300 1.06

OHNH2

OH

O

Fig. 13 Chemical structure of tyrosine


123

the DSC analysis is typical 1.1–1.5 but could be higher

in the worse cases [13]. Considering a 3r, then

Nc = 92.1 ± 4.5 which means that within the variability

of the DSC analysis the Nc value found for tyrosine is in

reasonable agreement with the mass spectrometric ana-

lysis result.

Fig. 14 Mass spectrum of irradiated tyrosine in positive ion mode. The blue rectangles mark the ions detected in pristine sample. The red



123

Conclusions

The mass spectrometric analysis of 3.2 MGy irradiated

arginine has revealed that 60.3 % of it ‘‘survived’’ the

massive radiation dose administered. The result obtained

by MS is in agreement and validates the residual amount of

arginine as measured by DSC: Nc = 56.6 %. The analysis

of the radiolysis products of arginine has permitted the

identification of an optical active product which can justify

the fact that Rc = 75.3. The higher residual optical activity

of arginine in comparison to its purity as measured by DSC

is fully justified by the formation of optically active radi-

olysis products.


proline has revealed that the amount of proline remained

after radiolysis is 83.07 % in good agreement with the DSC

analysis which has given an Nc = 83.5. The slight higher

value of Rc = 86.8 is justified by the detection of optically

active compounds among the radiolysis products. Further-

more, it was possible to quantify the degree of radiorace-

mization of the residual proline: the D-enantiomer was

found to be 4.8 % the amount of the residual L-enantiomer.


tyrosine has shown that 97.8 % of it resisted to radiolytic

degradation. The result is in reasonable agreement both

with Nc = 92.1 ± 4.5 and even more with Rc = 98.9. The

last value combined with the identification of two achiral

radiolysis products suggests a negligible radioracemization

level for tyrosine.

The mass spectrometric analysis of three selected amino

acids has corroborated the validity of the DSC analysis for

the study of the radiolysis resistance of a series of pro-

teinaceous and non-proteinaceous amino acids. The DSC

analysis is very rapid and permits a quick estimation of the

radiolysis resistance of the amino acids but of course has

also limitations due to the variable repeatability. None-

theless it is thanks to the DSC analysis that it was possible

for the first time to monitor the radiolysis stability at 3.2

MGy of all the proteinogenic amino acids as well as a

selection of non-proteinogenic amino acids [6–10, 12, 13].

Thanks to the mass spectrometric analysis performed on

the selected amino acids in the present work, also the ORD

analysis was rationalized in terms of radioracemization

products (wherever possible) and in terms of chiral and

achiral radiolysis fragments identification.

Acknowledgments We wish to thank Professor G. Angelini for

helpful and lively discussion of the results.

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OH

NH2

OH

CH2+

a b

Fig. 15 Neutral structure of the

ion m/z 138 (a) and the

corresponding ammonia loss

ion (b)

OH

OH

O

Fig. 16 Neutral product 3-(p-hydroxyphenyl)-1-propanoic acid of

the ion m/z 165

Table 3 Relative percentages of products formed from tyrosine

radiolysis

Products Relative %

Tyrosine 97.81

2-(p-Hydroxyphenyl)-1-ethanamine 1.97

3-(p-Hydroxyphenyl)-1-propanoic acid 0.22


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Mass spectrometric analysis of selected radiolyzed amino acids in an astrochemical context

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