Lignin from hydrothermally pretreated grass biomass retards enzymatic cellulose degradation by acting as a physical barrier rather than by inducing nonproductive adsorption of enzymes

Table 3 ATR-FTIR assignments of wavenumbers used to measure peak area

Wavenumber (cm⁻¹)	Asssignment^a		Estimated penetration depth^a (μm)
835	Lignin	C–H out-of-plane in all position of H and in positions 2 and 6 of S units [40]	1.99
895	Holocellulose	Anomeric C-groups, C₁-H deformation, ring valence vibration (cellulose, wood, holocellulose) [63]	1.85
1419; 1432	Lignin	Aromatic skeletal vibrations combined with C–H in-plane deformation [40]	1.17; 1.16
1508	Lignin	Aromatic skeletal vibrations; G > S [40, 63]	1.10
1601	Lignin	Aromatic skeletal vibrations plus C=O stretch; S > G [40, 63]	1.04
1732	Hemicellulose	C=O stretch in unconjugated carbonyl groups of carbohydrate origin (side chain acetylation in mannan, carboxylic acid side chain in xylan and ester groups in lignin–carbohydrate complexes) [40, 63]	0.96

^aCalculated based on the formula (Eq. 1): \( d_{\text{p}} = \frac{\lambda }{{2\pi n_{1} \sqrt {\sin^{2} \theta - \left( {n_{2} /n_{1} } \right)^{2} } }} \) (1) where d_p, λ, θ, n₁ and n₂ are penetration depth, wavelength, incident angle, ATR crystal refractive index and sample refractive index respectively. The values of θ and n₁ are specifically known to be 45° and 2.40 respectively for diamond ATR. The refractive index of biomass samples is estimated to be 1.4 which is a common value for an organic polymer, e.g. in wood cell walls [64]

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