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Mechanisms for the Mutarotation and Hydrolysis of the Glycosylamines and the Mutarotation of the Sugars

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Mechanisms for the Mutarotation and Hydrolysis of the Glycosylamines and the Mutarotation of the Sugars Journal of Research of the National Bureau of Standards Vol. 46 No. 2, February 1951 Research Paper 2186 Mechanisms for the Mutarotation and Hydrolysis of the Glycosylamines and the Mutarotation of the Sugars Horace S. Isbell and Harriet L. Frush A study has been made of t he kinetics of t he mutarot ation and hydrolysis reactions of L-arabinosylamine, and a m echanism has been devised to accoun t for the striking sensit ivity of t he glycosylamines to hydrolysis in a limited pH range. The concepts presented seem applicable for the interpretation of the reactions of other compounds of t he aldehyde ammonia type. 1. Introduction In the development of methods for the preparation of the amides of uronic acids [1] 1 it became necessary to determine conditions whereby the amino group II. of a l-amino-uronic amide could be hydrolyzed without alteration of the amide group. It was L-Arabinosylamine found that the rate of hydrolysis of the amino group of 1-aminomannuronic amide (I ) is extraordinarily OAc OAc H sensitive to the acidity of the reaction medium. Thus, when the compound is mixed quickly with R 2 c - 6 - 6 --6 - C H.NH.Ac one equivalent of a strong acid, hydrolysis requires I ~ 11 I I a period of many hours, but when the acid is added ~O OAc drop wise to the compound in solution, hydrolysis is III. complete in 15 min or less. Further study showed that 1-aminomannuronic amide is surprisin gly T et raacetyl-L- a rabinosylamine stable both to strong acid and to alkali, and that OR OR H hydrolysis can be effected rapidly only in the pH I I I range 4 to 7. B ecause of the presence of the hy­ drolyzable amide group in 1-aminomannuronic H ' P--~ -~;;--rNHA' amide, the study of the hydrolysis was directed to th e l-aminosugars, more properly designated gly­ '---~ O IV. cosylamines. Exploratory experimen ts with glucosylamine, N-Acety l-L- arabinosylamine galactosylamine, and arabinosylamine showed that the glycosylamines in gen eral possess the peculiar II. Structure and Chemical Properties of sensitivity to hydrolysis in a limited pH range that L-Arabinosylamine had been noted for 1-aminomannuronic amide [2]. In this paper, the mutarotation and hydrolysis of L-arabinosylamine (II) was prepared by treating L-arabinosylamine arc consider ed in detail. M echa­ L-arabinose in methanol with ammonia essentially by nisms for the mutarotation of the sugars, comparable the method of de Bruyn and Van Leent [3] . Acetyla­ to those for the mutarotation of the glycosylamines, tion of the compound with acetic anhydride and correlate the properties of the two groups of sub­ pyridine yielded a new tetraacetate (III). Catalytic stances and account for the over-all rate of mutaro­ deacetylation of this substance with barium methy­ tation of both the glycosylamines and the sugars as late in methanol gave a crystalline N -acetyl-L­ a function of pH. arabinosylamine. 2 The latter substance was found to react quickly with 2 moles of sodium periodate, as required for an N -acetylpentosylamine having the H H OH OH pyranose structure IV. Since acetylation and de­ I I I I o = c-c - c --c - c - c H· N H , acetylation were carried out by mild reactions that ordinarily cause no change in ring structure, the hH, ~ ~ II I . original L-arabinosylamine and its tetraacetate arc tentatively classified as pYTanoses. I· In a slow titration of L-arabinosylamine, one l-Aminomannuro nic amide equivalent of acid was required for neutralization ' T his co mpo un d is analogous to the N-acctyl·D-glueosylamine prepared by 1 Figures in brackets ind icate the li terature references at tbe ena of tbis paper. Brigl and Keppler [4] and shown by N iemann and lIays [5] to be a pyranose. 132 8 amine were treated wi th acids and bases in solutians ~ b ufl'cred at vari ous pH values , and the ensuing 7 reactions were followed by op tical rotation. Al­ ~ r- ---- --- ~ though the mu taTotatian and hydrolysis reactions of 6 L-arabinosylamine take place simultaneo usly, they r\ can be studi ed separately because their rates at a given pH value ditrer widely, and because the change 4 \ in optical rotation accompanying each of the two reactions is large. For study, the change in optical 3 u..., rotation was co nsidered to. consist of two periads: a (--"'0 2 short period b eginning at zero time and character­ a 0..1 0.. 2 0..3 0..4 0.5 0..6 0..7 0..8 0..9 1. 0. 1.1 1.2 1.3 iz ed by a decrease in dextrorotation ; and a long EQUIVA LENTS HCL / MaLE L - ARABINaSYL AM INE period begilming when the initial change was com ­ plete, characterized by an increase in dextrorotation 1"1 0 URE 1. Titration of L-arabinosylamine with acid. and extending until the optical rotation reached a value corresponding to complete hydrolysis (+ 10 5.2°). (fi g. 1). Separatian of crystalline L-arabinase in T he rate af the mutarotation reaction "vas obtained nearly quantitative yield from the mixture after from the data for the first period and the rate af the titr atian established the fact that tIlls treatment wi th hydrolysis reaction from the data for the second dilu te acid had caused hydrolysis af the glycosyla­ period . Suitable data. were ob tained for the mu taro­ mine and formation of the free sugar. tation reaction in alkaline solutions, in 'which the ' Vhen dissalved in water, L-arabinasylam ine under­ hydrolysis is extremely slow, and for the hy drolysis goes spon taneous changes that give rise to a decrease reaction in weakly alkaline or acid solutions, in which in optical ratati on and then to an increase. The the mutarotation reaction is almost instan taneous. changes can be ascribed principally to a mutarata­ Satisfactory rate constan ts for the mutarotation of tion rcaction that establishes equilibrium between L-arabinosylamine were ob tained in the pH r ange 7.8 the various modifications of the glycosylam ine, and to 12 by application of the customary formula 4 for hydralysis of the amino group.3 The relative rates a first-order reaction to the data of table l. Ta ble 2 of the two rcactians vary wi th cxperimen tal concE­ gives the mutarotation constants that were obtained tions (fig . 2). Thus, when L-arabinosylamine is for a series of experimen ts used to evaluate the disf;olve d in strong acid, the op tical rotati on drops catalytic effects af the oxo nium, hydroxyl, and am­ almost at once to abou t + 69°, and then incr eases monium ions. The dotted curve of figure 3 represen ts slowl y over a periaJ of several weeks (curve I). these mu tarotatian constants corrected for the cata­ H owevcl", if it is di ssolvcd in weak ac id , fo r example lytic effect of the ammonium ion. The res ul ts in water saturated with carbon dioxide, the op tical clearly show that the mutarotation of L-arabino­ rotation drops immediately to about + 70° and then sylamine is strongly catalyzed by acids bu t not ap­ increascs rapidly to + 105° (curvc II). In a weakly preciably by bases b The mutarotation differs from alkaline solution prepared by dissolving th e sub­ stance in carbon dioxide-free watcr, the specific rota Lion decreases in the co urse of a few hours from + 86 ° to. a m inimum, and thcn slowly riscs (c urve + 110. III). In strong alkali there is almast no changc 10.5 (curve IV). V 1I 100 Thcse qualiLative abservations m.ay be summa­ z rized as fo llows: Q 9 5 1/ ~ Q 0 9 0 a: TIl 0 / 1\ 1 utarotation LL 85 Solution H ydrolys is reaction G reaction w a. 8 0. I ~ .r.. "'-0 '" ~ ill Strongly acid _____ Vcry rapid ________ Slow. 7 5 _____ do ___________ I ' Vcak lyacid ______ Rapid, but mcas- 70. U1·a ble. ' Vcaklyalkaline ___ Rapid, b ut meas- SlolI'. 6 5 L m·able. o 4 8 12 16 20. 24 28 32 36 40 4 4 48 52 56 Strongly alkalin c __ Slow _____________ Do. TIME , HOURS F I r; URE 2. l'o1utw'otation oI L-ambionsylamine. To ob tain quantitative data as to the effect of the I , III 2.5 N H CI; IT, in C 0 2-saturated water; In, in CO,-free water; IV, in 0.01 oxanium ion concentration, samples of L-arabinosyl- NNaOH . •1 'he firs t-ordor formu la is applicable if th e concen tration s of the acid and base 3 Thc process appears to be complicated Wldet· som e conditions by the formation catalysts arc held conslan t. of tile diglycosylamine, and possibly by other reaclions tbat depend on the :I HOWC\'Cf , unpublished work has shown that the mutarotation of a-D-galacto presence of an intermediate imine cation . See pa ge 142. sylallline is weakly but defin itely catalyzed by bases. 133 the mutarotations of the sugars in that the latter are TABLE 1. M.111arolalion and hydl'ol ysis measw'e menis- Con. catalyzed by bases more strongly than by acids. The 0.5 g of L-arabinosylamine dissol ved in suffi.cient acid, base, or buffer to give a reason for this difference will be considered in the volume of 25 ml. Solve n ts are listed opposite experiment n umbers. next section. Mutarotation co n· Hydrolysis rate Time pH stant,a constant,s khydrol (minutes) [al~ 1 Ttl-Tm 1 T I1-TCIl TABLE 1.
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