Carlsberg Res. Commun. Vol. 46, p. 43-52, 1981 STRUCTURE ELUCIDATION OF SOME PROANTHOCYANIDINS IN BARLEY BY 1H 270 MHz NMR SPECTROSCOPY by HENRIK OUTrRUP Department of Brewing Chemistry, Carlsberg Research Laboratory, Gamle Carlsberg Vej 10, DK-2500 Copenhagen Valby and KJELD SCHAUMBURG Department of Chemical Physics, The H. C. Orsted Institute, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen O Keywords: Anthocyanogens, barley, NMR spectroscopy, proanthocyanidins, structure In the present paper the composition of proanthocyanidins isolated from barley is investigated. Separation by affinity chromatography and HPLC yields a large number of substances. Six of these have been isolated in this work. Based on the chromatographic preparation they are expected to be two dimers DI and D2 and four trimeric substances TI - T4. Studies by means of IH NMR spectroscopy at 270 MHz confirmed DI and D2 as being dimeric compounds, a prodelphinidin and procyanidin B3 respectively. The spatial structure of the prodelphinidin corresponds to procyanidin B3. The peracetates of all six compounds have also been studied by means of IH NMR spectrocopy. By comparison with data for the analogous peracetates of (+)-catechin and (-)-epicatechin and with literature data, T1 - T4 have been identified as trimeric proanthocyanidins and their structures have been elucidated. 1. INTRODUCTION has been shown (10) that they participate in the Proanthocyanidins, in the brewing literature formation of haze in beer through reactions with normally termed anthocyanogens, constitute a proteins. The origin of most of the proanthocya- group of flavonoid compounds of significant nidins in beer is the malt, which contains all the practical interest to the brewing industry since it proanthocyanidins of the barley (11). From Abbreviations: HPLC = high-pressure liquid chromatography, NMR = nuclear magnetic resonance, TMS = tetramethylsilane. 0105-1938/81/0046/0043/$ 02.00 H. OUTTRUP• K. SCHAUMBURG:IH NMR of proanthocyanidins barley and malt three procyanidins and one were fractionated mainly according to molecular prodelphinidin have previously been character- weight (12). Further separation of groups of ized (9, 14). The constitution of the three proanthocyanidins was obtained by preparative procyanidins was deduced by proving their HPLC. identity to authentic species (9, 14). The prodel- In this manner it was possible to separate phinidin could not be identified in this manner prodelphinidins and procyanidins. The two since no reference substance was available, trimeric proanthocyanidins which are at the leaving open the possibilities that the dimeric same time prodelphinidins and procyanidins prodelphinidin had either a procyanidin B 1- or a could not be separated by HPLC. The separation procyanidin B3-1ike structure (9, 14). was accomplished by renewed fractionation on In the present work we have reinvestigated the Sephadex LH-20. composition of the proanthocyanidin fraction of For convenience in the following discussion barley with the primary aim of studying the labels have been defmed for the six compounds. prodelphinidins. The labels D and T refer to the number of The separation of the proanthocyanidins is catechin units, whereas the numbering (e.g. D1 accomplished by a combination of low pressure and D2) refers to the sequence of elution by affinity chromatography (Sephadex LH-20) and HPLC. Thus D1 is more polar than D2. high-pressure liquid chromatography (HPLC) on reversed phase. The identity and structure of the compounds prepared were ascertained by chemi- 2.2. Acetylation of proanthocyanidins cal methods and IH NMR spectroscopy. The proanthocyanidin (about 10 mg) was The reinvestigation showed that the structure dissolved in pyridine (200 ~1). The solution was of the previously mentioned dimeric prodelphini- treated at room temperature with acetic acid din (9, 14) is analogous to that of procyanidin B3. anhydride (200 lal). When deuterated acetates Observations leading to the same conclusion for spectroscopic purposes were needed have been published by PORTER et al. (7). Three (CD3CO)20 (Merck Sharpe & Dohme Canada new trimeric proanthocyanidins were isolated Limited, Montreal) has been used. After 16 from barley (12). hours the reaction mixture was poured on ice, The spectroscopic results and their interpreta- whereby the acetate precipitated. The precipitate tion are discussed in this paper resulting in was washed with cold water until the smell of proposed structures for these compounds. pyridine disappeared. After the washing the precipitate was dried in vacuum at room temperature at 10 -2 torr. 2. MATERIALS AND METHODS 2.1. Isolation of proanthocyanidlns from barley The barley variety investigated was Nordal, 2.3. Hydrolysis of proanthocyanidlns harvest 1978 and 1979. The method of prepara- A few milligrams of proanthocyanidin were tion of pure proanthocyanidins has been descri- treated with 0.1 N HCI in ethanol for 15 min at bed elsewhere (i 2) and will only be stated briefly. 60 ~ according to HASLAM et al. (! 3). By this Barley kernels were decorticated in the Carls- procedure the lower terminal unit was split off as berg Research Mill, the flour fraction being a flavan-3-ol. After oxydative hydrolysis in isolated for further investigations. The fine BuIOH - HC1 (5:1) for half an hour at 100 ~ particles of this decorticated material were the upper units are transformed into anthocyani- separated from coarse proanthocyanidin-free dins. particles by means of bolting, hereby leaving a flour containing 8-9 mg. g-1 of proanthocyani- dins, which were extracted with acetone-water 2.4. Preparation of NMR samples (3:1) (9). An initial fractionation was carried out The proanthocyanidins were dissolved in by low pressure chromatography on a Sephadex deuterioacetone-d6 (Merck, Darmstadt, purity >1 LH-20 column with 96% ethanol-water (3:2). 99.5%) and tetramethylsilane (TMS) (Merck, Following this procedure the proanthocyanidins Darmstadt) was added as internal standard. The 44 Carlsberg Res. Commun. Vol. 46, p. 43-52, 1981 H. OUTTRUP8r K. SCHAUMBURG:IH NMR of proanthocyanidins solutions were placed in 5 mm o.d. sample tubes. 2.6. Nuclear Overhauser experiment (NOE) The acetylated proanthocyanidins were dissolved The proton proton NOE experiment was in CDC13 (Merck, Darmstadt, purity >/ 99 %) performed by alternating selective irradiation at and the solutions were placed in 5 mm o.d. the frequencies of the C4 protons in (+)-catechin sample tubes. The concentrations were in the with a total saturation period of 5 sec. The range 2-5 % w/v. reference spectrum was obtained with the decoupling frequencies outside the spectral re- gion. The total accumulation time was 15 hours. 2.5, tH NMR spectra The two spectra are swopped from disc after 16 The IH NMR spectra have been obtained scans in order to minimize the influence of long using a BRUKER HX270S spectrometer opera- term effects. ting in Fourier transform mode. Spectra were accumulated in 32K data points using a spectral width of 3000 Hz. The repetition time was ~ 5 2.7. Spectral interpretation sec and a pulsewidth of 10 l~sec corresponding to The IH NMR spectra consist in general of a 50 o flip angle was used. The number of large number of lines which may be assigned to accumulated scans varied from 100 to 500 partly overlapping small spin systems, since the depending on concentration and experimental protons in the individual rings do not interfere. conditions. Decoupling experiments have been As a consequence it has been possible to analyse used extensively to confirm the assignments most of the spectra by standard methods. reported in the tables and figures in section 3. The spectrometer was operated at 30 ~ The chemical shift data are referenced to CHC13 (b 3. RESULTS = 7.37 ppm) for CDC13 solutions and to TMS 3.1. Chemical evaluations for solutions in deuterioacetone-d6. In order to According to the Sephadex LH-20 chromatog- improve resolution Gaussian lineshapes have ram (12), the proanthocyanidins were divided been introduced as described by FERmGE and into two groups, the proposed dimers D1 and D2 LINDON (5). and the proposed trimers named T1-T4. Some O Y ct Figure 1. Numbering and labelling in proanthocyanidins straightly C4-Cs-linked. Carlsberg Res. Commun. Vol. 46, p. 43-52, 1981 45 H. OUTTRUP& K. SCHAUMBURG:IH NMR of proanthocyanidins a c 7;0 6'.5 6'.0 plDm -6;2 6",1 6.0 ,S.9 5.8 ppm b d 710 6.5 6".0 ~1 6~2 6.t 6.0 5.9 5.8 ppm Figure 2. The 270 MHz IH spectra of the aromatic regions of two dimeric proanthocyanidins. 2a shows the spectrum of synthetic procyanidin B3, 2b shows the spectrum of prodelphinidin B3 isolated from barley. 2c and 2d show the high field part of 2a and 2b, respectively, on an expanded scale. The impurity observed in the spectrum of B3 is assumed to arise from taxifolin. of their chemical properties were as follows: ring signals and the aliphatic OH signals is also Common to all six components was the found. These effects prevent the use of integra- liberation of (+)-catechin on mild acid hydrolysis tion in the identification beyond the qualitative (2.3). level. The abundance of signals observed in DI On stronger acid hydrolysis (2.3)the identified and D2 arise from the presence of two conforma- anthocyanidins were the following: tions of the molecules under the experimental D1: Delphinidin conditions. D2 : Cyanidin The spectra of the peracetylated compounds in TI : Delphinidin CDCI 3 show only one dominant isomer and the T2: Delphinidin and cyanidin absence of OH signals makes the assignment T3: Delphinidin and cyanidin possible. In Figure 3 the chemical shifts are T4.. Cyanidin reported for the alicyclic parts of the ring systems in the peracetylated proanthocyanidins studied. The corresponding aromatic part of the 3.2. IH NMR analysis chemical shifts is reported in Figure 4. The In Figures 2a and 2b the aromatic region of accuracy of the spectral analysis was better than the IH NMR spectrum of DI and D2 is 10 -2 ppm.