Mass Spectrometry Study of New Polyamine Derivatives of Caffeine

The mass spectrometric fragmentations of ten new 8-alkylaminocaffeine derivatives were investigated. The fragmentation pathways of new polyamine derivatives of caffeine on the basis of low and high-resolution electron ionization (EI) mass spectra were discussed. In the case of new compounds, classical fragmentation of purine skeleton related to the elimination of a neutral molecule CH3N(1)C(2)O from a molecular ion was not observed. Nevertheless, new interesting fragmentation of described caffeine derivatives was observed. Moreover, heats of formation of odd-electron ions of 8-alkylaminocaffeine derivatives were calculated.


Introduction
Alkaloids constitute a large group of naturally occurring chemical compounds with a wide range of pharmacological properties. 1 They are produced by various organisms including bacteria, fungi, plants and animals.Caffeine (1,3,7-trimethylxanthine), the main representative of purine alkaloids, is a bicyclical amine.The most well-known source of this chemical compound are the seeds of coffee (especially Arabica -Coffea arabica, and Robusta -Coffea canephora) or leaves of tea. 2 This alkaloid is a central nervous system (CNS) stimulant and one of the most addicting psychoactive drugs.This is the result of inhibition of adenosine A2A receptor by caffeine and its derivatives. 35][6][7] It enhances the effect of anticancer drugs and the most important is modulation of cis-platinum efficacy induced by caffeine. 8Caffeine is also the lead compound for the development of new drugs.For example, 8-substituted caffeine derivatives are very important chemical compounds that possess pharmacological activity.This kind of derivatives inhibits the monoamine oxidases (MAO) A and B. MAO-A inhibitors can be employed as antidepressant drugs and MAO-B inhibitors can be used in Parkinson's disease therapy. 9,108-aryl-or heteroaryl-substituted xantines are highly potent antago-nists at human adenosine receptors. 118-[(pyrrolidin-1-ylcarbonothioyl)-sulfanyl]caffeine has exceptionally high antioxidant properties and protects human erythrocytes against AAPH-induced oxidative damage. 12ccording to the literature, the caffeine molecule as well as its derivatives were studied by different analytical methods.4][15] There are a few reports found in the literature describing advanced methods studying the caffeine molecule, e.g.photoion mass spectroscopy (PI-MS), 16 thermal and hyperthermal surface ionization (HSI) mass spectrometry. 17In this work we propose the fragmentation pathways of new polyamine derivatives of caffeine on the basis of low and high-resolution EI-MS spectra.This is the new report of mass fragmentation of C(8) substituted derivatives of caffeine.The main goal of our study was to establish how far the presence of di-or polyamino group at C-8 position influences the pathways of fragmentation of this important class of compounds.

Experimental
Low-resolution and high-resolution mass spectra were recorded using a model 432 two-sector mass spectrometer [AMD-IntectraGmbH D-27243 Harpstedt, Ger-Sierakowska et al.: Mass Spectrometry Study of New Polyamine ... many] at ionizing energy 70 eV, accelerating voltage 8 kV, mass resolution 10 000 at 10 % valley.Samples were introduced by a direct insertion probe at a source temperature of ~150 o C. The elemental compositions of the ions were determined by peak matching method relative to perfluorokerosene.The tandem product ion mass spectra in the first field-free region were recorded using linked scans at constant B/E with helium as collision gas at an indicated pressure of 1.75 × 10 -5 Pa with an ion source temperature of 180 o C, ionizing energy of 70 eV and an accelerating voltage of 8 KV.

Results and Discussion
All new di-or polyamine derivatives of caffeine were synthesized according to procedure described in 18 (Scheme 1) with good yields (60-90 %).
The representative samples of set of EI mass spectra of 8-(6-aminohexylamino)caffeine ( 5 In the EI mass spectra of 1-5, molecular ions a were observed in all derivatives and for 2, 4 and 5 its relative abundance is 100 % (Scheme 1, Table 1).Furthermore, the even-electron fragment    3.The molecular ions a were observed with 81 % and 100 % relative abundances for 9 and 10, respectively.The elimination of radicals from the molecular ions a leads to the formation of even-electron bicyclic ions: c, g and tricyclic ions: e, f.Ions e and f may be also formed from the odd-electron ion b, which is created from the molecular ion a by elimination of the 1,3-propanediamine,  but can also be generated from the molecular ion by elimination of adequate radicals (Scheme 3).The ion e is also formed from the even-electron ion c by the elimination of methanimine.
The stability of new caffeine derivatives in gas phase was determined by theoretical method.Heats of formation of odd-electron ions of 8-alkylaminoderivatives were calculated.0][21] The final heats of formation (HOF) for odd-electron ions of all compounds are presented in Table 4.
observed for odd-electron ions of compounds 6-8 as well as 9 and 10.
PM5 semiempirical method is a reliable method for visualization of the structures in the gas phase.Calculated structures of odd-electron ions of: 8-(6-aminohexylamino)caffeine ( 5

Conclusions
The present study has demonstrated that the best method of mass spectrometric analysis of new polyamine derivatives of caffeine is the analysis of their EI mass spectra.Considering the length of the alkyl chain at C(8) position of caffeine molecule, the EI fragmentation pathways of compounds 1-10 were divided into three groups.The substitution of caffeine ring with alkylamino chain in position C(8) causes that the new pathway becomes more competitive than the primary fragmentation of unsubsituted caffeine with the elimination of a neutral molecule CH 3 -N=C=O from purine moiety.Such observation has not been noted previously in the literature in the cases of caffeine derivatives.According to PM5 semiempirical metod, the odd-electron ions of caffeine derivatives in gas phase (under the experimental conditions similar to those in EI measurement) are stable.Moreover, an increase in alkyl chain length at C(8) position of caffeine molecule reduces the values of HOF.As follows from the results, the odd-eletron ions of caffeine derivatives in the gas phase (under the experimental conditions similar to those in EI measurements) are stable.The lowest values of HOF from series 1-5 are observed for odd-electron ion of 8-(6-aminohexylamino) caffeine (5).Simultaneously, an increase of the length of an alkyl chain also reduces HOF.A similar relationship can be

Table 1 .
The

Table 3 .
The

Table 4 .
Heats of formation of odd-electron ions of polyamine derivatives of caffeine (1-10) calculated by PM5 semiempirical method.