Introduction

Lignocellulosic materials are well recognized as a potential sustainable source of mixed sugars for the production of biofuels and biochemicals (Himmel et al. 2007). Due to the complex structure of cellulose, hemicellulose and lignin within the lignocellulosic matrix, there lies a critical challenge for utilization of these major carbohydrates from the biomass. One of the reasons contributing to the barrier of lignocellulosic biomass’ saccharification is the lignin content of the plant cell wall with increased degree of polymerization (Chen et al. 2010; Himmel et al. 2007). The lignin polymers were basically constituted by three types of phenylpropanoid with various proportions: hydroxyphenyl (H), guaiacyl (G) and syringyl (S) (Fig. 1). Evidences point out that lignin embraces and protects cellulose by building physically matrix (Chundawat et al. 2011). Various pretreatment processes, chiefly thermal or chemical, have been developed for delignification to reduce biomass recalcitrance and/or enhance enzymatic hydrolysis efficiency (Mosier et al. 2005; Yang and Wyman 2008). These pretreatments mainly break apart or weaken the lignin and hemicelluloses complex providing the access for cellulases. However, due to the release of heterogenous lignin derivatives and xylooligomers during the thermal or chemical pretreatment processes, the enzyme catalysis becomes inefficient as a result of non-productive hydrophobic absorption of these compounds in cellulases (Kristensen et al. 2007; Yang and Wyman 2006) and subsequent deactivation of cellulases (Pan 2008; Ximenes et al. 2011). Interestingly, recent studies provide evidence that variation in compositional distribution of hydroxyl (H), syringyl (S) and guaiacyl (G) units and functional groups in lignin complex may affect the cellulose hydrolysis more than absolute amount of residual lignin (Li et al. 2010; Studer et al. 2010). It has been revealed that the phenolic hydroxyl groups from lignin complex are found to be more critical than its methoxyl groups during cellulose hydrolysis (Pan 2008). Moreover, a strong negative correlation between sugar release and lignin content was only found for pretreated Populus with an S/G ration <2.0, while for higher S/G ratios, negative influence of lignin content was less pronounced (Studer et al. 2010). However, the detailed mechanism about the effects of lignin units’ distribution on cell wall and interior chemical bondages on cellulase accessibility are not clear right now.

Fig. 1
figure 1

The primary lignin phenylpropanoids: H, G and S

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Unlike thermochemical pretreatment processes, biological degradation essentially overcame the physical and chemical barriers of lignocellulosic complex in order to facilitate the utilization of carbohydrates in lignocellulosic biomass. To unlock that complex, microbes expressed various biological catalysts such as carbohydrate hydrolyzing enzymes (CHE) and ligninolytic enzymes (Chen et al. 2010). The previous studies in Phanerochaete chrysosporium revealed that the majority of the proteins secreted during the wheat straw degradation were related to cellulose digestion in addition to lignin degradation and hemicellulose utilization (Singh et al. 2011). It was found that the lignin degrading enzymes were detected at the beginning and CHE were observed subsequently. It suggested that these enzymes worked in a synergetic strategy: ligninases initially targeted phenolic and non-phenolic lignin; the collapse of lignin further facilitated CHE attacking later; finally polysaccharides converted into monomeric sugars and were utilized as a carbon source by the fungus; the consumption of carbohydrate further accelerated the lignin decomposition (Leonowicz et al. 1999). Basically, the degradation and or modification in the functional groups of lignin entities during the biological lignin degradation and subsequent cellulose hydrolysis may provide an insight to develop a better biomass catalysis technology. The biomass enzyme catalysis is one of the costly processes in the biomass-based biorefineries. Investigation of natural bio-degradation process will provide useful information to researchers for good understanding the relationship among recalcitrance of lignin structure, the lignin degradation and/or modification and cellulose hydrolysis.

The lignin bio-degradation mechanisms of biological system have been mainly focused on white rot basidiomycetes for many years (Bugg et al. 2011; Odier et al. 1992). The cleavage of β-O-4 via breakdown of Cα–Cβ linkages was predominant characteristic in the fungal degradation process as main functional results of lignin peroxidase (LiP; EC 1.11.1.14) and Mn peroxidase (MnP; EC 1.11.1.13). It has been found that the ligninolytic enzymes from Phanerochaete chrysosporium decomposed lignin substructure model compounds as well as lignin (Tien and Kirk 1983, 1984). The reactions in lignin are oxidative, involving demethylation (or demethoxylation), side-chain oxidation at Cα, propyl side-chain cleavage between Cα and Cβ (Chen et al. 1983). Brown-rot fungi are able to remove the hemicellulose and cellulose with only minor modification to the lignin. Consequently, lignin remains a major component of the degraded plant cell wall (Green and Highley 1997). The resulting lignin is demethylated on aryl methoxy groups and contains a greater number of ring hydroxyl groups (Kirk and Highley 1973).

In contrast to white rot and brown-rot fungi, several bacterial species belonged to actinomycetes, α-proteobacteria and γ-proteobacteria also have capability to degrade lignin (Bugg et al. 2011). The catabolic lignin degradation pathway and the corresponding gene clusters were well studied in Sphingomonas paucimobilis SYK-6 using various lignin derived biaryls and monoaryls (Masai et al. 2007). It was clear that bacteria mineralized lignin via protocatechuic acid 4, 5-cleavage pathway and the multiple 3MGA catabolic pathways which may be more significant on lignin degradation than previous thought (Bugg et al. 2011). Considering the other advantages, including fast doubling time and easier gene manipulation, the bacterial system could be a better candidate in bio-conversion of lignocellulosic biomass. However, only a few reports addressed the performance of bacterial degradation on the raw lignocellulosic biomass.

Among lignin degrading bacterial species the genus Streptomyces showed better performance on lignocellulosic biomass degradation (Crawford 1978; Crawford et al. 1983). Streptomyces viridosporus T7A degrades both lignin and carbohydrate. A number of single-ring aromatic intermediates released during the degradation of hardwood, softwood, and grass lignins by S. viridosporus also have been identified (Crawford et al. 1983). S. viridosporus T7A transformed 30 % of the initial lignin of corn lignocellulose into water soluble acid-precipitated polymeric lignin (APPL) (Crawford et al. 1983). It was also found that this species can highly degrade biologically resistant lignosulfonated compounds at certain cultural condition (Hernandez-perez et al. 1998a, b, 1999). Lignin degradation by these bacteria was suggested via the oxidation route of both aromatic rings and propyl side-chain lignin carbons (Phelan et al. 1979). It is proven that S. viridosporus T7A produces a lignin degrading peroxidase (ALiP-P3) (Crawford 1978) and ALiP-P3 may involve a random-binding bi-reactant system, which differs from the ping pong bi-reactant system typically adapted by the lignin peroxidases originating from the fungus P. chrysosporium (Yee and Wood 1997). Based on the model compounds degradation, this bacterial LiP was capable of Cα-oxidation as well as Cβ–Cβ cleavage of lignin and lignin substructure model compounds. The Cβ–Cβ cleavage of Cα carbonyl-containing compounds by the bacterial lignin peroxidase was in direct contrast to the action of lignin peroxidase of P. chrysosporium, which readily cleaved only Cα-hydroxyl containing compounds (Crawford and Ramachandra 1993). Besides that, three peroxide-induced gene homologs were identified from S. viridosporus T7A which may involve in regulating the oxidative lignin biodegradation (Ramachandran et al. 2000). Within the completion of the genome information, its special capability on biodegradation of lignocelluosic biomass will be revealed in the future.

Although the lignin products APPL produced by S. viridosporus T7A was characterized in many aspects, researchers have yet to demonstrate the depolymerization of lignin by S. viridosporus T7A directly on lignocellulosic substrates degradation. Detailed understanding of biomass chemistry after S. viridosporus T7A cultivation can provide crucial information on mimicking an efficient biocatalysis of the lignocellulosic biomass. Therefore, in this study, the biodegradation process of wheat straw by S. viridosporus T7A was investigated. The S. viridosporus T7A spent biomass was subjected to comprehensive chemical compositional analysis, Fourier transform infrared (FTIR), pyrolysis gas chromatography/mass spectrometry (Py-GC/MS) and 13C cross polarization magic angle spinning (CP-MAS) solid state nuclear magnetic resonance (NMR). Results obtained from this study provide insight into the detailed process of lignocellulosic degradation by the bacterial system. Experiments were carried out at Washington state university of Pullman campus in 2011.

Materials and methods

Streptomyces viridosporus T7A cultivation and solid state fermentation (SSF)

Streptomyces viridosporus T7A was provided by Dr. Lee Deobald (University of Idaho). 20 g of wheat straw (Triticum sativum, grown in Moscow, Idaho) in 500-mL Erlenmeyer flasks was autoclaved. Before inoculation, S. viridosporus T7A spores were suspended by ISP 4 medium supplemented with 0.6 % yeast extract. The SSF experiments were conducted at 37 °C for a period from 1 to 3 weeks.

FTIR spectroscopy analysis

Surface chemical analysis was conducted using a FTIR spectrometer (Shimadzu) to compare the changes of the samples during different SSF conditions. 32 scans were taken for each sample from 4,000 to 800 cm−1 at a resolution of 4 cm−1.

Solid state 13C NMR

Solid state NMR was used to study the native form of the substrate without fractionation or isolation of components for determining the associated chemical changes in the structure. Finely ground samples were used for solid state NMR analysis. 200 mg sample were mixed with 100 μL 20 mg/ml 3-(trimethylsilyl) propionic-2,2,3,4-d 4 acid (TSP) and freeze dried. Thereafter, samples were packed in a 5.0-mm rotor and 13C CP-MAS NMR spectra were recorded at ambient temperature in Bruker DMX 400 spectrometer (NMR center, Washington State University). The techniques of proton-carbon cross polarization (CP), high-power proton decoupling, and magnetic angle spinning (MAS) were combined in solid state CP-MAS NMR analysis. The integrals for each peak were normalized with reference to the internal standard.

Analysis of chemical composition of the biomass

The control and pretreated straw samples were water washed and freeze dried then individually ground. 0.5 g of biomass was extracted with toluene:ethanol (2:1) (at room temperature). The resulting samples were characterized by the two-stage acid hydrolysis method described by Standard Biomass Analytical Procedures (NREL) for determination of lignin and carbohydrate content (Zeng et al. 2010).In briefly, the biomass samples were mixed with 72 % w/w H2SO4 at 30 °C for 30 min. After diluting the samples to 4 %, 1 h autoclave was carried out at 121 °C. The solid residue was reported as acid insoluble lignin. The sugar content was determined by ion chromatography using an ion exchange chromatography apparatus (Dionex ICS-300 DC IC). Acid soluble lignin was also measured by UV absorbance at 205 nm with an extinction coefficient of 110 l/g cm (Zimbardi et al. 1999).

Acetyl bromide (AcBr) analysis

The control and S. viridosporus T7A pretreated wheat straw (1 g each) were individually frozen (liq. N2), ground to powder in a Waring blender, with the resulting powder subjected to successive extraction at room temperature for 8 h each with toluene–EtOH (1:1,100 ml g−1), EtOH (100 ml g−1) and H2O (100 ml g−1), respectively, and then freeze dried. The resulting extractive-free freeze-dried cell wall residues (CWR) were ball milled for 2 h individually to fine powder with a Fritsch planetary mill (Pulverisette) using agate bowls and balls, and then subjected to acetyl bromide analysis. The AcBr method was performed as described earlier (Iiyama and Wallis 1990) to estimate the lignin content of extractive-free CWR samples for the control and S. viridosporus pretreated wheat straw (1, 2 and 3 weeks), respectively.

Pyrolysis GC–MS

To determine the compositional changes, samples were subjected to pyrolysis GC–MS (Py-GC–MS). Py-GC–MS was carried out using a CDS pyroprobe 5000 connected in-line to an Agilent GC–MS 6890N. Samples were loaded into a quartz tube and gently packed with quartz wool prior to pyrolysis. The samples were kept briefly in the oven (210 °C) for 1 min to ensure adequate removal of oxygen prior to pyrolysis and were pyrolyzed by heating nearly instantaneously to 600 °C for 1.0 min. The inlet temperature was maintained at 250 °C. The resulting pyrolysis vapors were separated by means of a 30 m × 0.25 μm inner diameter (5 %-phenyl)-methylpolysiloxane non-polar column, with a split ratio of 50:1. The gas flow rate was 1 ml min−1. Linear heating (3 °C min−1) from 40 to 280 °C was designated for the oven program, and to ensure that no residuals were retained, the oven was held at 280 °C for 10 min. The gas was then sent into a mass spectrometer (Agilent Technologies Inert XL MSD) to be analyzed. Carbon dioxide was used as an internal standard. The abundance area (%) of each lignin related compound was referenced against the area (%) of internal standard in each sample (Zeng et al. 2010).

Enzymatic hydrolysis of bio-treated wheat straw

The enzymatic hydrolysis was carried out in 2 % solid loading with 50 mM sodium citrate buffer (pH 5.0) 60 FPU/g cellulase and 120 CBU/g glucosidase were added. The definition of on unit FPU (filter paper unit) is that the dilution of cellulase preparation to a point where 2.0 mg of reducing sugar equivalents is released from filter paper in 1 h at 50 °C and pH 4.8 (Decker et al. 2003). The definition of one unit CBU (cellubiose unit) is based on the international unit: the amount of enzyme to convert 1 μmol/min. The flask was put in a shaking incubator at 50 °C at for 72 h. After hydrolysis, supernatants were collected and used for the sugar analysis. Determination of sugar conversion rate was done by calculating the percentage of released sugars of the samples after enzymatic hydrolysis to respond sugar content in biomass. The enzymes used for enzymatic hydrolysis were cellulase from Trichoderma reesei ATCC 26921 (Sigma) and Novozym 188 (Sigma) as β-glucosidase.

Results and discussion

FTIR spectroscopy

Figure 2 shows the FTIR spectrum of wheat straw after growth with S. viridosporus T7A for 1–3 weeks. The FTIR analysis clearly indicates that significant changes of lignin associated functional groups occurred initially during the first week of solid state fermentation. In this regard, the peaks of the spectrum represent the distribution of functional groups and were assigned to the three major components: hemicelluloses, cellulose and lignin in biomass. Most of the peaks were well defined as compared with previous studies (Buta et al. 1989; Faix and Bottcher 1992; Lin and Dence 1992; Pandey and Pitman 2003). The major peaks during the biological degradation have been marked and listed as: (1) 1,120 cm−1 for cellulose ring stretch, (2) 1,250 cm−1 for vibration of guaiacyl (G) and syringyl ring (S), (3) 1,320–1,330 cm−1 for skeletal of syringyl ring, (4) 1,460 cm−1 for C–H deformation in lignin and carbohydrates, (5) 1,500 cm−1 and (6) 1,595 cm−1 for aromatic skeletal in lignin, (7) 1,650 cm−1 for C=O conjugated ketone stretch, and (8) 1,730 cm−1for unconjugated C=O stretch.

Fig. 2
figure 2

FTIR spectrum of Streptomyces viridosporus T7A bio-degraded wheat straw for 3 weeks

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A visible shoulder at around 1,120 cm−1 was observed in the spectrum of bio-pretreated wheat straw accompanied with the reduced intensity at 1,730 cm−1 of carbonyl groups with increase in pretreatment time. This indicates that the S. viridosporus T7A possibly consumed some hemicellulose in the biomass which could result in increased exposure of the cellulosic counterpart. In addition to the vibration of carbohydrate, the lignin related chemical groups were also observed as expected. The intensity of 1,250, 1,330, 1,460, 1,500, 1,595 and 1650 cm−1 reflected the notable changes on lignin complex. Since the 1,250 cm−1 (2) is attributed to G and S lignin units, the decrease of this peak could signify the unlocking of the lignin cross-links through removal of lignin subunits. Meanwhile, it was also observed that the peak representing the skeleton of the S ring (3) increased which implied the relative increase of S composition in the total lignin complex. Therefore, the vibration at 1,250 cm−1 would mainly result from the changes in G related unit. Furthermore, S. viridosporus T7A degraded wheat straw had relatively lower conjugated carbonyl vibration around 1,650 cm−1 compared to the untreated control which suggested the conversion of C=O at α position of lignin. Thus, it can be speculated that S. viridosporus T7A produces an aromatic aldehyde oxidase to oxidize aldehyde groups in lignin into corresponding acidic groups (Deobald and Crawford 1989).

13C CP-MAS solid state NMR analysis

The normalized 13C CP-MAS NMR spectrum of control and bio-degraded wheat straws are depicted in Fig. 3. The most predominant assignments for each peak have been listed in Table 1 (Almendros et al. 1992; Gilardi et al. 1995; Sun et al. 2005; Zimbardi et al. 1999). In general, the spectrum was composed of two strong signal regions: carbohydrate region (110–162 ppm) and aromatic region (60–110 ppm). The relative fluctuation of carbohydrate/aromatic ratio reflects the three major chemical compositional changes (Cellulose, hemicelluloses and lignin). Besides, the chemical shift resonances of carbonyl and carboxyl group (162–200 ppm), methoxyl group (52–55 ppm) and carbon in etherified and/or non-etherified region (132–152 ppm) acts as indicators to determine the structural characteristics of the biomass. Thus, the solid state NMR spectroscopic analysis of both control and bio-degraded wheat straw suggested that pretreatment with S. viridosporus T7A led to obvious changes in chemical composition and structural characteristics. Specifically, the gradually diminishing shoulder at a chemical shift of 100 ppm refereed to the well ordered hemicelluloses. In addition, the loss of carbonyl and carboxyl group compared to the untreated control sample strongly pointed out the removal of hemicelluloses and side-chain alterations in lignin complex. Meanwhile, the significant changes of C4 carbon at crystalline cellulose/amorphous cellulose (90–80 ppm) and C6 carbon at crystalline cellulose/amorphous cellulose (70–60 ppm) was observed and suggested the deconstruction of cellulose structure which could be explained as a result of the breakdown of β (1–4) linkages and hydrogen bonds in cellulose complex. This indicated that S. viridosporus T7A not only degraded lignin but also released cellulose structures. However, because the background of hemicelluosic carbon widely interfering the spectrum of cellulose, it was very difficult to quantify the content of each component in solid state NMR analysis of biomass. For the lignin part, the chemical shift of 110–130 ppm represented the C2, C6 in syringyl/guaiacyl units and C3, C5 in the p-hydroxyphenyl units. And the C3, C5 in non-etherified and/or etherified lignin contributed to the spectrum of 130–160 ppm. The results about aromatic region of spectrum after S. viridosporus T7A degradation indicated similar distribution compared with control which meant that the bio-treated samples still kept the basic structure of lignin polymers. However, slight changes were observed around 150 ppm suggesting the modification on etherified lignin. Furthermore, in consistence with FTIR analysis, the enhanced signal intensities of methoxyl group supported the hypothesis of preferential G deduction during S. viridosporus T7A-mediated biological degradation process (Table 2). The relative content of carbohydrate component and lignin were measured (Gilardi et al. 1995) and normalized by integration value of internal standard. The decreasing carbohydrate/aromatic ratio of bio-biodegraded sample (Table 2) implied relatively reduced amounts of carbohydrate in comparison to the control wheat straw.

Fig. 3
figure 3

The solid state 13C CP/MAS spectrum of untreated control and Streptomyces viridosporus T7A bio-degraded 1, 2 and 3 week’s wheat straw

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Table 1 The solid state NMR chemical shift assignments
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Table 2 13C CP-MAS NMR quantitative analysis of wheat straw after biodegradation
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Chemical composition analysis

In order to quantitatively determine the extent of lignin degradation in wheat straw on pretreatment by S. viridosporus T7A, acetyl bromide (AcBr) analysis was carried out individually on the control and pretreated wheat straw tissues (1, 2 and 3 weeks). In this regard, AcBr analysis is widely accepted method for the estimation of lignin contents in various plant cell wall residues (CWR) (Iiyama and Wallis 1990). Therefore, extractive-free CWRs of the control and pretreated wheat straw samples (1, 2 and 3 weeks) were individually treated with a reaction mixture consisting of 25 % AcBr by volume in glacial acetic acid containing 4 % of perchloric acid, with the corresponding solubilized materials being individually measured for UV absorptivity (k, 280 nm). An extinction coefficient of 20.09 l g−1 cm−1 (Iiyama and Wallis 1988, 1990) was employed for lignin content estimation. On applying the standard extinction coefficient to the AcBr analyses, the lignin contents were found to be ~17.65 % (176 mg/g of CWR) for the control and ~16.23 % (162 mg/g of CWR), ~14.83 % (148 mg/g of CWR) and ~14.71 % (147 mg/g of CWR) for the 1, 2 and 3 weeks pretreated wheat straw tissues, respectively. These results indicated that in comparison to the control wheat straw (~17.65 % of CWR), a gradual decrease in the lignin content (~16.23 %–14.71 % of CWR) as a result of pretreatment by S. viridosporus. This can only be possible if efficient degradation and utilization of lignin occurred in contrast to cellulosic counterpart during the pretreatment process.

To confirm such a possibility, chemical composition analysis (two-stage acid hydrolysis) on both the control and 3-week pretreated wheat straw tissues was further carried out for evaluating the relative amount of cellulose, hemicellulose and acid soluble and acid insoluble Klason lignin, respectively (Table 3). The estimated/putative monomeric sugar and Klason lignin content for the control and 3 week pretreated wheat straw is summarized in Table 3. As can be seen, the glucose release of the 3-week pretreated sample under acid hydrolysis conditions increased in comparison to the control wheat straw. This elevated level of glucose release indicated the relative increase in cellulose content in the wheat straw samples after pretreatment. The decreased level of corresponding cleavable products obtained from acid hydrolysis i.e., mannose, xylose, arabinose and galactose monomers, which are released from the hemicellulosic counterpart, also provided evidence for removal of hemicellulose as same as the results of FTIR and solid state NMR.

Table 3 Chemical composition (%) of biologically treated and untreated wheat straw samples
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Py-GC/MS analysis

Pyrolysis is a thermochemical technique that can deconstruct lignocellulosic complex into small organic molecules at a certain temperature. GC/MS allows for separation and identification of these organic molecules through selected m/z ratio and retention time. As an advanced analytical technique, Py-GC/MS is widely used to characterize the chemical distribution changes of lignocellulosic biomass (Camarero et al. 1994; Zeng et al. 2011). In this study, Py-GC/MS was applied to analyze the lignin compositional changes in (A) untreated wheat straw control and (B) S. viridosporus T7A spent wheat straw (Fig. 4). The major lignin derivative compounds were generated from hydroxyphenyl (H), guaiacyl (G) and syringyl (S) phenylpropanoid including: phenol; phenol, 2-methyl; phenol, 4-methyl-; phenol, 2-methoxy-; p-cresol, 2-methoxy-; phenol, 4-ethyl-2-methoxyl-; 2-methoxy-4-vinylphenol; phenol, 2,6-dimethoxy-; isoeugenol; 3′,5′-dimethoxyacetophenon; phenol, 2,6-dimethoxy-4-[2-propenyl]-; methoxyeugenol and acetosyringone. The pyrogram showed that the biopretreated wheat straw shared a similar chemical distribution with untreated wheat straw control. However, the quantitative analysis suggested that S. viridosporus caused significant changes in lignin and carbohydrate composition (Table 4). Improvement of the levoglucosan/lignin ratio (from 0.716 to 1.02) was in agreement with chemical composition analysis (Table 3) which supported the idea that relative enhancement of cellulose content resulted from removal of lignin and hemicellulose during S. viridosporus T7A-mediated biopretreatment. In addition, the S/G ratio of the three week sample was increased from 0.358 to 0.620 (>40 %). The degradation of P. chrysosporium on wheat straw also showed a similar trend of G unit reduction (Singh et al. 2011; Zeng et al. 2010). It needed to note that S. viridosporus T7A was proven to produce water soluble acid-precipitable polymeric lignin with increased phenolic G content and decreased methoxy group compared with native lignin (Crawford et al. 1983; Hernández-Coronado et al. 1997; Rodriguez et al. 1997). Therefore, the conversion of G units to soluble lignin derivatives could explain the loss in solid residues after pretreatment. The results showed here were distinguished with popular understandings: (1) S unit were more easily degraded than G unit due to the lower redox potential and (2) The linear arrangement of S units by β-O-4′ linkages were more attackable than G units which provided one more position for connecting. The reasonable explanations could come to the structural differences of lignin on cell wall among various lignocellulosic biomasses. The straw lignin of herbaceous crops contained all three lignin units with certain degree of acetylation which distinguished with the structure of woody biomass (Buranov and Mazza 2008). The quantitative analysis revealed that 20 % β-O-4′ linkage of wheat straw lignin was contributed by G type free phenolic lignin (Camarero et al. 1994; Lapierre et al. 1988). Compared with S units, the phenolic G lignin has less redox potential and was easily oxidized by ligninolytic enzymes. Furthermore, the ALiP-P3 produced by S. viridosporus T7A oxidized phenolic compounds rather than oxidizing non-phenolic substrates (Spiker et al. 1992). In addition, the enzymatic hydrolysis of S. viridosporus T7A treated wheat straw showed that conversion rate of glucose was increased 80 % while the xylose’s conversion rate was similar (Fig. 5). This result, in consistence with FTIR, NMR, demonstrated that the consumption of hemicelluloses and modification on cellulose. It is speculated that the deconstruction of G unit lignin on wheat straw was the results of oxidation phenolic G lignin units which further facilitated the utilization of hemicelluose and celluose. However, due to the compositional complexity in lignocellulosic biomass, the heterogeneously distributed thermal reaction with interaction of cellulose, hemicelluloses and lignin, will definitely influence the accuracy of Py-GC/MS analysis. Current results was also not able to answer the G deconstruction location happened in the lignin complex. Therefore, comprehensive structural analysis at the sub-structural level of isolated lignin therefore needs to be done and is currently underway to further verify the observed critical modification within the lignin assembly.

Fig. 4
figure 4

Pyrograms of untreated control (a) and Streptomyces viridosporus T7A bio-degraded 3 week’s wheat straw (b). The markers of H, G, S represented hydroxylphenyl, guaiacyl and syringyl derivative lignin unit

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Table 4 Py-GC/MS analysis of lignin and carbohydrate derived compounds in Streptomyces viridosporus degraded wheat straw
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Fig. 5
figure 5

The enzymatic hydrolysis of Streptomyces viridosporus T7A bio-degraded 3 week’s wheat straw

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Conclusion

In this study, we investigated the degradation pattern of wheat straw by S. viridosporus T7A. Significant changes on lignin structures after bio-pretreatment were measured by FTIR, Py-GC/MS and Solid state NMR analysis. Besides the relative reduction in content, the degradation and/or modification on lignin units were reflected by an increased S/G ratio, reduction of carbonyl groups and enhancement of methoxyl groups. This study provided new information towards elucidation of the mechanisms involved in biological degradation processes on wheat straw.