Mass spectrometry of 4N-pyrimidinyl amino acids

ORGANIC MASS SPECTROMETRY, VOL. 24, 55-58 (1989)

Mass Spectrometry of 4N-Pyrimidinyl Amino Acids Adam S. P4aziak, Lech Celewicz, Jardaw Spychda and Krzysztof Golankiewicz Department of Synthesis and Structures of Organic Compounds, Faculty of Chemistry, Adam Mickiewin University, 60-780 Pozna6, Poland

The electron impact m a s spectra of N-(lH-hxo-4pyrimidinyl) amino acids are discussed. It was established that thermal elimination of water took place prior to mass fragmentation of all thee compounds with the formation of bicyclic structures. The fragmentation of bicyclic ions led to lH-hxo-pyrimidinyl species.

INTRODUCTION The mass spectrum of cytosine, known since 1965,' is characterized by the high stability of its molecular ion as well as electron impact (EI) induced decomposition occurring primarily in the heterocyclic ring. Substitution of cytosine by an alkyl group in the 4N position extends the possibilities of mass fragmentation, especially by formation of bicyclic even-electron ions with involvement of a non-bonding electron pair of 3N- or 4~-nitrogen.~ In continuation of our studies of mass fragmentation of the basic components of nucleic acid^^.^ we have investigated N-( 1H-2-oxo-4-pyrimidinyl) amino acids which can be regarded as 4N-substituted derivatives of cytosine : R

I

HOOC-CH-NH

J.3

0+;

made by Jeol, Japan. Metastable ions were recorded on the same instrument equipped with MS MT-01 metastable ion detector (defocusing technique). Compounds were introduced by direct insertion probe at ionizing energy 75 eV, ionizing current 0.3 mA, accelerating voltage 3 kV, source temperature 200°C and vaporization temperature 200-260 "C. Synthesis of compounds 1-6 was described previ~usly.~.'

RESULTS AND DISCUSSION Mass spectra of compounds 1-3 and 46 are shown in Figs 1 and 2, respectively. Mass fragmentation of the pyrimidinyl amino acids is characterized by three processes appearing successively: (i) the loss of H,O species from molecules 1-6; (ii) fragmentation of R-substituents leading to formation of ions detected at m/z = 151 and 138; and (iii) further decomposition of above ions to fragment ions observed at low m/z values.

1-6

1: R = H 2: R=CH, 3: R = CH(CH,)CH, 4: R = CH(CH,)CH,CH, 5: R = CH,CH(CH,)CH, 6 : R=CH,C,HS

The compounds of such structure have attracted interest on account of their potential biological a~tivity.~ We were fully aware of the difficulties connected with the proper interpretation of mass fragmentation of these compounds on account of their low volatility and thermal decompositions taking place prior to the EI ionization phenomenon.6

The loss of H,O species In all spectra of investigated compounds instead of molecular ions the peaks [M - H,O]+'of considerable intensity (100-12%) appeared. Metastable ions corresponding to transition M+' + [M - H,O]+' were undetectable. Also in low-energy mass spectra of 1-6, M+' ions were undetectable. This finding is in accord with the supposition that the water elimination is a thermal process. The loss of H,O species from pyrimidinyl amino acids 1-6 is shown in Scheme 1.

EXPERIMENTAL 1-6

Low- and high-resolution mass spectra were obtained on a double-focusing mass spectrometer JMS D-100 0030-493)3/89/010055-04 $05.00 0 1989 by John Wiley & Sons, Ltd.

"I

'-I

I

Scheme 1. The loss of H,O species from pyrimidinyl amino acids 1-6.

Received 19 April 1988 Accepted (revised) 1 July 1988

56

A. PXAZIAK ET AL.

a,

1

50

100

200

150

1 r mlz

r

l

i

1

250

Figure 1. Low-resolution mass spectra of compunds 1-3.

x

-

u?

t 50-

96

0

$ 1

L

95i e

t I

1

138

122 123

1 Ils1

q

1

178

Il6&

I Compound 5

l l

E 50

Figure 2. Low-resolution mass spectra of compounds 4-6.

In opposition to the above, the series of methyl ester derivatives of pyrimidinyl amino acids have given molecular ions at 75 and 15 eV as well.*

The loss of R-substituents The ions of formula [M - HzO]+' after expulsion of R-substituents yielded the odd-electron ion at m/z = 151 (compounds 3-6) and even-electron ion at m/z = 138 (compounds 2-6). Above ions have been very representative for all studied pyrimidinyl amino acids. The ion at m/z = 151 was formed as a result of an elimination of unsaturated hydrocarbons from ions [M -H,O]+' (3: m / z = 193, 4, 5: m/z=207, 6: m/z = 241). In our opinion, alkene elimination from the last derivative (6) may be realized by the loss of the unsaturated seven-membered hydrocarbon ring formed by multistep rearrangement. In such case the lack of

4 100 5 6

83 11

Scheme 2. Formation of the ion at m/z = 151 appearing in mass fragmentation of pyrimidinyl amino acids M .

MASS SPECTROMETRY OF 4N-PYRIMIDINYL AMINO ACIDS

I

u

I"

H m/z=178

* m/z=207

Scheme 3. Formation of the ion at m/z = 138 appearing in mass fragmentation of pyrimidinyl amino acids 2-6.

metastable transition m/z = 241 m/z = 151 is not surprising. Formation of the ion at m/z = 138 can be explained by the following pathway: the loss of hydrocarbon radicals from ions [M - HzO]+', then skeletal rearrangements of even-electron ions at m/z = 178 and 164, and elimination of alkyne species from rearranged ions, as shown in Scheme 3.

is not relevant to fragmentation of the pyrimidinyl amino acids and has been described in papers reporting fragmentation pathways of cytosines.','

Furtker decomposition of the ions at m/z = 151 and 138

In the mass spectrum of the pyrimidinyl glycine (compound 1) the strong ions at m/z = 111 (51%) and 112 (49%) appeared, whose origin was not included in Schemes 1-4. On the basis of high-resolution measurements, their elemental compositions have been proposed: m/z = 11lGcytosine, m/z = 112-protonated cytosine. Metastable ions corresponding to the formation of these ions were undetectable. Probably, fragmentation of N-( lH-2-oxo-4-pyrimidinyl) glycine is more destructive than other pyrimidinyl amino acids and leads to formation of cytosine ions. The mass spectra of the remaining derivatives had cytosine ions of very low (0-7%) abundance. In the case of mass fragmentation of the alanine derivative (2), on account of the simple amino acid substituent instead of elimination of the bi-radical CH;., ketene loss was observed:

--f

The ions at m/z = 151 and 138 undergo decomposition by loss of CO, CHO', HCN or H' species up to the lhl-2-0x0-pyrimidinyl ion at m/z = 95, as shown in Scheme 4. Further fragmentation of the pyrimidinyl ion

Formation of the specific ions for the series of the pyrimidinyl amino acids

2: mlz = 165

2: m/z = 164 0-

7

Y"

T

-CH

-2-0

-CH

-C-0

mlz = 123

m/z = 122

61 1 4

I

Scheme 4. Decomposition of the ions at m/z = 151 and 138 to fragment ions observed at low values of m/z.

In the mass spectrum of the pyrimidinyl phenylalanine (compound 6) the ions at m/z = 77 (9%), 91 (100%)and 104 (31%) are derived from the aromatic substituent. Stabilization of the charge on a benzyl group is commonly known and expected in the above pyrimidinyl amino acid.

58

A. PZAZIAK ET AL.

pyrimidinyl substituent. Mass fragmentation of these ions leads to the lH-Zoxo-pyrimidinyl cation at mfz = 95.

CONCLUSIONS The primary loss of H,O from the pyrimidinyl amino acids is the thermal process which leads to bicyclic compounds. The abundant [M - H,O]+' ions are wellstabilized bicyclic structures formed by the condensation of carboxylic group with 3N-nitrogen of

Acknowledgement This work was supported by the project RP.II.13.2.3.

REFERENCES 1. J. M. Rice, G. 0. Dudek and M. Barber, ./.Am. Chem. SOC.87, 4569 (1965). 2. A. S. Ptaziak, L. Celewicz, J. Spychda and K. Golankiewicz, Org. Mass Spectrom. 23,654 (1 988). 3. J. Jankowska, L. Celewicz and K. Golankiewicz, Org. Mass Spectrom. 22, 52 (1987). 4. M. Dezor-Mazur and K. Golankiewicz,Org. Mass Spectrom. 22, 197 (1987).

5. T. Ueda and J. J. Fox, J. Med. Chem. 6,697 (1963). 6. H. Budzikiewicz, C. Djerassi and D. H. Williams, Structure Elucidation of Natural Products by Mass Spectrometry, Vol. 2. Holden-Day, San Francisco (1964). 7. L. Celewicz, J. Spychala and K. Golankiewicz, Synthetic Commun. 17,1939 (1987). 8. A. S. Ptaziak, L. Celewicz, J. Spychala and K. Golankiewicz, unpublished results.